Corroison Failures ARAMCO COE10602

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Chapter : Materials & Corrosion Control For additional information on this subject, contactFile Reference: COE10602 S.B. Jones on 874-1969 or S.P. Cox on 874-2488

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Corrosion Failures

Engineering Encyclopedia Materials & Corrosion Control

Corrosion Failures

Saudi Aramco DeskTop Standards

CONTENTS PAGES

INTRODUCTION....................................................................................................... 1

Corrosion Failures ........................................................................................... 1

FEATURES OF UNIFORM (GENERAL) CORROSION ......................................... 3

Uniform Corrosion Rates................................................................................. 4

Corrosion Allowance....................................................................................... 4

NACE Corrosion Data Survey ........................................................................ 5

TYPES AND FEATURES OF LOCALIZED CORROSION..................................... 6

Galvanic (Two-Metal) Corrosion .................................................................... 6

Stray Current Corrosion....................................................................... 9

Inspection for Galvanic Corrosion Failures ......................................... 9

Pitting ............................................................................................................ 10

Inspection for Pitting Corrosion Failures........................................... 13

Intergranular Attack....................................................................................... 15

Sensitization....................................................................................... 16

Weld Decay ....................................................................................... 17

Knife-Line Attack .............................................................................. 18

Inspection for IGA Failures ............................................................... 18

Crevice Corrosion.......................................................................................... 18

Under-deposit Corrosion.................................................................... 19

Corrosion Under Insulation................................................................ 19

Inspection for Crevice Corrosion....................................................... 19

Selective Leaching ........................................................................................ 20

Inspection for Selective Leaching...................................................... 21

FEATURES OF CORROSION THAT INVOLVES MULTIPLEVARIABLES ............................................................................................................ 22

Environmental Cracking................................................................................ 22

Stress Corrosion Cracking............................................................................. 24


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Material Factors ................................................................................. 25

Environmental Factors ....................................................................... 25

Mechanical Factors ............................................................................ 26

Time................................................................................................... 26

General Features of Stress Corrosion Cracks..................................... 26

Stress Corrosion Cracking in Specific Materials ............................... 27

Inspection for SCC Failures............................................................... 38

Hydrogen-Induced Damage .......................................................................... 38

Hydrogen Embrittlement.................................................................... 39

Sulfide-Stress Cracking ..................................................................... 40

Hydrogen Blistering........................................................................... 41

Hydrogen-Induced Cracking.............................................................. 41

Inspection for Hydrogen Damage...................................................... 42

Liquid Metal Embrittlement .......................................................................... 43

Solid Metal Embrittlement................................................................. 43

Inspection for Liquid/Solid Metal Embrittlement Failures ................ 43

Microbiologically-Influenced Corrosion ....................................................... 44

Anaerobic Bacteria............................................................................. 44

Aerobic Bacteria ................................................................................ 44

Additional Forms of MIC .................................................................. 44

Inspection for MIC Damage .............................................................. 45

Erosion-Corrosion ......................................................................................... 45

Inspection for Erosion-Corrosion....................................................... 45

Corrosion Fatigue .......................................................................................... 46

Inspection for Corrosion Fatigue ....................................................... 46

Elevated Temperature Attack ........................................................................ 47

Oxidation ........................................................................................... 47

Inspection for Oxidation .................................................................... 49

Sulfidation ..................................................................................................... 50

Cyanide-Induced Corrosion ............................................................... 52


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Inspection for Sulfidation .................................................................. 52

Vanadium Attack........................................................................................... 52

Inspection for Vanadium Attack ........................................................ 53

Carburization ................................................................................................. 53

Metal Dusting .................................................................................... 53

Decarburization.................................................................................. 54

Inspection for Carburization .............................................................. 54

Hydrogen Attack ........................................................................................... 55

Inspection for Hydrogen Attack......................................................... 56

GLOSSARY ............................................................................................................. 57

ADDENDUM A: COMMON ENGINEERING MATERIALS................................ 62

ADDENDUM B: EXCERPT FROM NACE CORROSION DATASURVEY................................................................................................................... 63

REFERENCES.......................................................................................................... 68


Corrosion Failures

Saudi Aramco DeskTop Standards 1

INTRODUCTION

Corrosion Failures

Corrosion is the material deterioration that is caused by a chemical or electrochemicalreaction with the environment. Corrosion occurs in all phases of oil and gas production aswell as in refinery and petrochemical plant operations. When corrosion is uncontrolled,failures occur. Corrosion is a leading cause of materials-related failures in thepetroleum/petrochemical industry. The purpose of this module is to discuss, with emphasis onthe features and characteristics of actual failures, the types of metallic corrosion that areencountered in the petroleum/petrochemical industry.

Corrosion experts typically cite eight forms of corrosion. Additional forms are included insubgroups of these eight forms of corrosion. Many of the forms are interrelated, and the eightforms differ in various textbooks. In the analysis of corrosion failures, it is more useful todivide the forms of corrosion into three major categories that can often be distinguishedduring the initial field examination of failures:

• Uniform Corrosion: Corrosion that occurs uniformly over all exposed surfacesof the material.

• Localized Corrosion: Corrosion that occurs preferentially in specific areas ofthe material.

• Corrosion that Involves Multiple Variables: Deterioration that results from theinteraction of corrosion with one or more factors, such as stress, velocityeffects, temperature, or bacteria. This category includes various forms ofenvironmental cracking.

A survey of the forms of corrosion that were experienced over a nine-year period revealed thedistribution that is shown in Figure 1.


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Uniform35

Corrosion that Involves Multiple Variables

68

Localized 32

Figure 1. Distribution of Corrosion Failures by Major Category(Mobil Experience, 1980-1988)


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FEATURES OF UNIFORM (GENERAL) CORROSION

The most common example of uniform (general) corrosion is the atmospheric rusting ofordinary steel. Constantly dry climates are not extremely corrosive; however, atmosphericcorrosion is accelerated in industrial environments where moisture and corrosive gases aremore likely to be found. The green patina on copper and bronze statues is an example ofcorrosion where sulfur in the atmosphere formed copper sulfide. Other locations whereuniform corrosion is commonly found include marine, soil, boiler/heater (high temperature),and process (chemical) environments.

The term “uniform” implies that corrosion is perfectly even along the entire metal surface.Since some irregularity in the corrosion almost always occurs, many people prefer to use theterm “general corrosion” to describe this form of attack.

Uniform (or general) corrosion is illustrated schematically in Figure 2. In this case, the metalis thinned, and although it is slightly roughened, the corroded surface is clean. Clean surfacesare typical of acid forms of corrosion attack in carbon steels. In other materials, such asaluminum alloys, clean surfaces suggest alkaline attack. Clean, uniformly thinned surfaces,however, are not always indicative of general corrosion. As will be discussed later in thismodule, mechanisms of corrosion that involve multiple variables such as erosion corrosionalso exhibit smooth, thinned surfaces.

Figure 2. Uniform (General) Corrosion

Uniformly corroded metals often retain a thick corrosion product layer on the exposedsurfaces, as illustrated in Figure 3. In this course, we will define “corrosion product” as thematerial that is produced by the corrosion reaction. Corrosion products typically involveoxidized forms of the metal or alloy.

Figure 3. Uniform Corrosion with Remaining Corrosion Product


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High temperature oxides [found above approximately 300 °C (572 °F)], or other corrosionproducts, are commonly referred to as “scale.” “Deposits” are foreign materials that settle orattach in the region that is under study. Deposits on the surface do not necessarily mean thatthe underlying surface is corroded; however, in most cases deposits are a major accelerant ofcorrosion. (Under-deposit corrosion will be covered later in this module.) Deposits may becorrosion products from elsewhere in the system where corrosion is occurring, or they simplymay be the intrusion of foreign material such as sand. Where the deposits are not corrosionproducts from the specific area of interest, their presence may be an indicator of corrosion inanother area, and such an indication would be important in the overall failure analysis.

The examination and analysis of corrosion products, scales, and deposits provide importantinformation for use in failure analyses. As will be demonstrated later in this course, chemicalcomposition, thickness, and morphological features of the corrosion product can be used toidentify the corrosion mechanism, rate of corrosion, and variations in in-service conditions.

Uniform Corrosion Rates

Uniform corrosion is one of the forms of corrosion attack with which it is most easy to deal.Since corrosion rates are conveniently measured in the field or laboratory, it is easy to identifyproblems early and implement solutions.

Metal loss is described in terms of mils per year (mpy) according to the following equation:

3445 WDAT

mpy =(1)

where W = weight loss, mg

D = density of metal, g/cm3

A = area of specimen, cm2

T = exposure time, hr

Corrosion Allowance

Uniform corrosion rates are used to set the corrosion allowance (CA) for equipment incorrosive service. As an example, consider a vessel with a typical design life of 10 years. Forcarbon steel, the expected corrosion rate in the vessel environment is 10 mpy. A corrosionallowance of2.5 mm (0.1 in) is chosen since 10 mpy X 10 years = 2.5 mm (0.1 in).


Corrosion Failures


In the performance of failure analyses, it is often prudent to back-calculate the expectedcorrosion for comparison with actual corrosion. Often, it is found that the design conditionswere exceeded in actual operation due to increases in temperature, pressure, concentration ofcorrodant, or throughput. Corrosion rates that are adjusted for these changes, may enable oneto determine the reasons for failure.

Example: Exchanger Shell Failure

An exchanger in mildly corrosive service (10 mpy) was designed for 10 years of operationwith a 12 mm (0.5 in) shell thickness and a CA of 2.5 mm (0.1 in). Failure of the shelloccurred after seven years. A review of the service history revealed that for the first two yearsoperating conditions followed the design conditions, but after this period the shell-sideproduct was changed to one with a corrosion rate of 75 mpy. Calculated thickness loss was asfollows:

2 years X 10 mpy = 0.020 in5 years X 75 mpy = 0.375 in

7 years = 0.395 in (10.0 mm)

The calculated corrosion of the vessel thus was found to be approximately two-thirds of thewall thickness. A stress analysis of the vessel confirmed that failure at the operatingtemperature and pressure would occur at approximately two-thirds of the original wallthickness.

NACE Corrosion Data Survey

An extensive compilation of uniform corrosion rates is available in text and computerizedform from the National Association of Corrosion Engineers (NACE). In the absence ofspecific corrosion test data, this compilation can be used in the analysis of failures to estimatecorrosion rates.

A sample selection and keys to the table are shown in the Addendum (Reference No. 6). Theformat is semi-graphical; that is, corrosion rates are classified into four ranges and arrayed ona temperature-composition plot. Selective attack such as pitting or stress corrosion cracking isindicated by footnotes. The example that is shown indicates the increased corrosion resistanceof steels in dilute carbonic acid with increasing chromium content.

Because general corrosion usually is a slow process which can be detected easily, there arefew equipment failures caused by general corrosion. By comparison, the majority ofequipment failures from corrosion are due to localized corrosion.


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TYPES AND FEATURES OF LOCALIZED CORROSION

Galvanic (Two-Metal) Corrosion

Galvanic corrosion is an accelerated type of corrosion that occurs when dissimilar metalscontact each other to form an electrical couple. An electrolyte (water) environment isnecessary to complete the circuit. The electrochemical cell that is formed between the twometals has a current flow that is similar to the current flow in a battery. In general, the lessnoble metal is corroded, as shown schematically in Figure 4. Metal loss is typically moresevere near the metal-to-metal couple, with the result that there is a localized form of attack atthe junction.

Figure 4. Galvanic (Two-Metal) Corrosion


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The galvanic series (Figure 5) of engineering alloys is often used to predict galvaniccorrosion. The materials are listed according to their activity (electrical potential) when theyare immersed in seawater. The less noble, or more negative alloy in the series will corrodefaster. Greater distances between alloys result in a greater difference in their electricalpotential and an increased possibility of galvanic corrosion.

Chlorimet 2Copper alloy C27000 (yellow brass. 65 % Cu)Copper alloys C44300, C44400, C44500 (admiralty brass)Copper alloys C60800, C61400 (aluminum bronze)Copper alloy C23000 (red brass, 85 % Cu)Copper C11000 (ETP copper)Copper alloys C65100, C65500 (silicon bronze)Copper alloy C71500 (copper nickel, 30 % Ni)Copper alloy C92300, cast (leaded tin bronze G)Copper alloy C92200, cast (leaded tin bronze M)Inconel alloy 600 (passive)Monel alloy 400Type 410 stainless steel (passive)Type 304 stainless steel (passive)Type 316 stainless steel (passive)Incoloy alloy 825Inconel alloy 625Hastelloy alloy CChlorimet 3SilverTitaniumGraphiteGoldPlatinum

Protected end (cathodic, or most noble)

Galvanic Series in Seawater at 25 °C (77 °F)

Corroded end (anodic, or least noble)

MagnesiumMagnesium alloysZincGalvanized steel or galvanized wrought ironAluminum alloys5052, 3004, 3003, 1100, 6053, in this orderCadmiumAluminum alloys2117, 2017, 2024, in this orderLow-carbon steelWrought ironCast ironNi-Resist (high-nickel cast iron)Type 410 stainless steel (active)Type 316 stainless steel (active)LeadTinCopper alloy C28000 (Muntz metal. 60% Cu)Copper alloy C67500 (manganese bronze A)Copper alloys C46400, C46500, C46600, C46700 (naval brass)Nickel 200 (active)Inconel alloy 600 (active)Hastelloy alloy B

Figure 5


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Galvanic corrosion is often the result of poor design and selection of materials. Theappearance of galvanic corrosion includes localized corroded areas that are relatively free ofcorrosion products. The amount of corrosion lessens with increasing distance from the metalcouple as illustrated in Figure 6. The extent of the corrosion from the couple depends on thedifference between the metals in the galvanic series and the conductivity of the electrolyte.

Corroded Zone

AluminumBrass Bolt

Copper

Copper

Corroded Zone

Steel

Figure 6. Examples of Galvanic Corrosion(Reference No. 8)

Surface areas of the anodic and cathodic materials are an important factor in galvaniccorrosion. A large anode (active metal) coupled to a small cathode will result in only minorcorrosion; however, a small anode will corrode rapidly if it is surrounded by a large cathode.Depending on their relative positions in the galvanic series, coated metals exhibit specificgalvanic-corrosion characteristics. Figure 7 shows that tin, which is cathodic to steel, protectssteel as long as the coating is intact. If perforation occurs, the underlying steel is galvanicallyattacked. By comparison, the zinc on galvanized steel is anodic, and thereby results in a largeanode that sacrificially protects steel.


Corrosion Failures


Tin

Steel

Zinc

Steel

Figure 7. Galvanic Corrosion of Tin and Zinc-Coated SteelArrows indicate corrosive attack.

(Reference No. 9)

Stray Current Corrosion

This special form of galvanic corrosion occurs when stray currents from nearby electricalequipment pass through conductive liquids or soil. The current tends to jump back and forthfrom the corroding object, and it essentially forms a cathode where the current enters and ananode where the current leaves. The result is accelerated corrosion at the anode.

Inspection for Galvanic Corrosion Failures

Do the following when inspecting possible cases of galvanic corrosion:

• Look for localized attack that is adjacent to material interfaces.

• Examine all parts of equipment that are in contact with the failed region.Different metals should not be mixed.

• Look for contamination from melted droplets, steel wool, or other foreignmaterials that are in contact with corroded surfaces.

• Check for possible stray currents.

• Look for perforations in protective coatings, especially coatings that arecathodic.

• Check for a small anode-to-cathode ratio.

• Check for dissimilar weld and base metals.

• Determine the environment. Remember that the galvanic series pertains to aseawater environment and that the galvanic series may not be useful in somepetroleum/petrochemical environments.

Dissimilar metal couples also can result in cracking due to hydrogen embrittlement. Thisphenomenon will be covered later in this module.


Corrosion Failures


Pitting

Pitting is a localized corrosion process that results in sharply defined surface cavities or holesin a metal as illustrated in Figure 8. The rate of attack inside a pit can be many times the rateof general corrosion. Hence, pitting is often described as an autocatalytic process whereby thereactions within the pit induce conditions that make the pitting process self-sustaining oraccelerating in rate. Since through-wall penetrations can occur in a relatively short time,failures that are caused by pitting corrosion are more unexpected than are those failures thatare caused by general corrosion.

Figure 8. Pitting Corrosion


Corrosion Failures


A common mechanism that is used to describe pitting is the localized electrochemical cell. Asshown in Figure 9, the bottom of the pit is anodic, and forms metal ions. If the adjacentsurface is cathodic, the cell will produce hydroxyl ions. The increasing metal ionconcentration in the pit attracts cations in solution such as the chlorine ion, which reacts toform acids that lower the pH and accelerate corrosion. Since reactions within the pit are oftenindependent of the surface conditions, pitting is often hard to stop. For example, chemicalcleaning or neutralizing the surface may not halt the acid formation and corrosion that is deepwithin the pit.

Figure 9. Electrochemical Nature of Pitting Corrosion(Reference No. 1)

Pitting corrosion begins at defects or discontinuities in the surface films on metals. Often, themore passive metals, such as stainless steels, are more prone to pitting than are the lesspassive metals, which tend to corrode more uniformly. Sometimes general corrosion andpitting occur simultaneously, as illustrated in Figure 10. When numerous pits in closeproximity overlap, the condition is described as interconnected pitting or wastage.

Figure 10. Pitting and General Corrosion


Corrosion Failures



Corrosion Failures


Pitting of a metal is environment specific. For example, pitting of carbon and alloy steelsoccurs in boiler feedwater piping due to dissolved oxygen and in oil field equipment due todissolved oxygen, carbon dioxide, or hydrogen sulfide. Aluminum alloys pit in chlorideenvironments, and the copper alloys (aluminum, brass, and cupronickel), pit in seawater thatis contaminated with sulfides. As a class, stainless steels are very susceptible to pitting inchloride solutions; however, resistance varies somewhat with composition. The alloyingelement molybdenum, added to make Type 316 stainless steel, is probably the most effectiveaddition for pitting resistance.

Inspection for Pitting Corrosion Failures

The American Society for Testing and Materials (ASTM) includes a Standard Practice (G-46)for Examination and Evaluation of Pitting Corrosion. In addition to pit depth and densitymeasurements, variations in cross-sectional shapes of pits (Figure 11) are useful in analyzingfailures. Different environments produce different shapes of pits in different materials. Forexample, stainless steels exposed to chlorides tend to develop sharp, deep pits, while cast ironunderground piping tends to develop shallow pits. Subsurface and undercut pits may explainwhy large pits may have been missed during inspections. Preferential pitting that is related tomicrostructural orientation can significantly influence the performance or failure of a part inservice. As will be discussed later, cross-sectional variations can also be used to distinguishpitting corrosion from erosion or erosion-corrosion.

Figure 11. Variations in the Cross-Sectional Shape of Pits


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Depending on the material, the environment, and other conditions of exposure, such as flow,pits may or may not contain corrosion products. Features that should be noted in analyzing afailure include pits that are filled with corrosion product that sometimes exhibit pitcaps ornodules. For example, pitting of steels is accompanied by tubercles of increasing size as thepH increases. Filled pits also provide well-preserved samples for chemical analyses. In certaincases, thermal cycling of equipment containing tightly filled pits has caused cracks to initiatefrom the bottoms of sharp pits and propagate to the point where the metal fails. Such caseswill be further explored under the subjects of stress corrosion cracking and thermal fatigue.

Intergranular Attack

Intergranular attack (IGA), or intergranular corrosion (IGC), occurs preferentially at oradjacent to the grain boundaries of a metal. Usually, there will be only a slight attack on theremainder of the grains. Intergranular attack is shown schematically in Figure 12. Asillustrated, IGA can occur uniformly or, more likely, in localized regions such as welds. Insome cases of IGA, metal loss occurs as the metal grains fall completely out of the sample;however, in other cases, the grain boundaries are attacked deep below the surface withoutdrop-out. In the latter case although the material appears to be acceptable, the mechanicalproperties (strength and ductility) of intergranularly attacked regions are severely degraded,which results in unexpected failures.

Figure 12. Intergranular Attack

Intergranular attack can occur by either of two mechanisms. In some alloys, such as brass andaluminum, precipitation of active second phases with poor corrosion resistance results in IGA.Failures in this case could be attributed to an incorrect material heat-treated condition or to anunexpectedly corrosive environment.

In the case of austenitic materials (stainless steels and some nickel alloys), IGA occurs whenthe materials are sensitized. In this case, it is not the second phase (chromium carbide) that isattacked, but instead the adjacent region, which is depleted of chromium, is attached. Theprocess of sensitization and resulting IGA are discussed below.


Corrosion Failures


Sensitization

Austenitic alloys, such as Type 304 stainless steel (Fe - 18Cr - 8Ni), depend largely onchromium for corrosion resistance. In the normally used solution annealed condition,chromium is homogeneously dispersed in the single-phase austenite solution. If the alloy isheated in the range of approximately 427 °C (800 °F) to 816 °C (1500 °F), chromium carbideprecipitation occurs at the grain boundaries. As shown in Figure 13, the region that is adjacentto the chromium-rich precipitates is depleted of chromium, and the depleted region is nolonger corrosion resistant or “stainless.” The material is “sensitized.”

Figure 13. Sensitized Grain Boundaries in Austenitic Stainless Steel(Reference No. 1)

Sensitization of stainless steels does not significantly affect the mechanical properties orcorrosion resistance at elevated temperatures; however, environments that contain suchsubstances as nitric acid, which normally does not attack austenitic stainless steel, rapidlycorrode the depleted zones and leave deep ditches.


Corrosion Failures


Sensitization can be eliminated by the following:

• Heating sensitized material to approximately 1050 °C (1925 °F) to dissolve thechromium carbides, and cooling rapidly to avoid re-precipitation.

• Lowering the carbon content of the alloy during melting so that less carbon willbe available to precipitate as chromium carbide. Examples include Types 304Land 316L.

• Adding carbide forming (stabilizing) elements during melting to tie up thecarbon so that it is unable to form chromium carbide. Stabilized stainless steelsinclude Type 321 (titanium stabilized) and Type 347 (niobium stabilized).

Weld Decay

When stainless steels are welded, a heat-affected zone (HAZ) forms adjacent to the weld.Within the zone, temperatures pass through the sensitizing temperature range. Depending onmetal thickness and welding heat input, sensitization may occur if the temperature remains inthe range long enough. Subsequent service exposure can result in localized intergranularattack known as “weld decay.” The IGA occurs in the HAZ, parallel to the weld as illustratedin Figure 14.

Figure 14. Weld Decay of Austenitic Stainless Steel


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Knife-Line Attack

Stabilized stainless steels can undergo, as a result of welding, a special case of IGA that isknown as “knife-line attack.” In a region of the HAZ that is closer to the weld, the metaltemperatures can exceed 1230 °C (2250 °F), and thereby cause the stabilizing carbides (NbCor TiC) to dissolve. If rapid cooling occurs and is followed by heating in the sensitizing range,the material may be susceptible to IGA. The IGA occurs in a thin band that is close to theweld.

Inspection for IGA Failures

Do the following when inspecting possible cases of IGA:

• Inspect for localized attack at welds.

• Look for a shiny, crystalline appearance in regions with metal loss. In somecases, small crystals that dropped out of the corroded region may be found atthe bottom of tanks, etc.

• Look for surface etching in regions without grain dropout.

• Check fracture surfaces for brittle, intergranular appearance.

Crevice Corrosion

Crevice corrosion, which sometimes is referred to as gasket corrosion or concentration cellcorrosion, is a localized attack that occurs at crevice regions such as joints or gasket seals(Figure 15). Corrosion occurs by a concentration cell mechanism. Stagnant conditions withinthe crevice prevent the interchange of corrosive with the bulk environment outside thecrevice. As a result, oxygen in the crevice is depleted due to the corrosion reaction, and aconcentration gradient forms. The low-oxygen crevice region becomes the anode of theconcentration cell, and accelerated corrosion (typically 10 - 100 times the general corrosionrate) occurs.

Figure 15. Crevice Corrosion


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Crevices, like galvanic couples, are design-related and account for a significant number ofequipment failures. Crevice corrosion occurs under washers, gaskets, nuts, bolt heads, and atpoorly fitted joints. In tubular heat exchangers, crevice corrosion occurs at the tube totubesheet crevice and between tubes and baffles.

Under-deposit Corrosion

This specific form of crevice corrosion occurs under deposits where corrosive contaminantstend to concentrate and oxygen depletion occurs. Examples of these deposits are sludge ontank bottoms, silt that results from untreated water and is deposited on cooler tubes duringlow flow conditions, and scale that is deposited in the dead legs of piping. In steamgenerators, deposits insulate tube surfaces and cause overheating and dryout. Whenevaporation occurs, water contaminants such as caustic precipitate. Subsequent wetting resultsin a highly concentrated solution that can be very corrosive.

Corrosion Under Insulation

This related form of corrosion is especially critical in the petroleum and petrochemicalindustry. While corrosion under insulation does resemble crevice corrosion, the mechanismfor corrosion is usually somewhat different. Moisture and an adequate oxygen supply arerequired for corrosion of steel under insulation. Failures under insulation result from use ofthe wrong insulation materials for the application, poor weatherproofing, as well as improperdesign for drainage. Since it is often inconvenient to remove insulation for inspections,corrosion often continues until failure results. It is worth noting that in addition to localizedcorrosion under insulation, austenitic materials experience a significant number of chloride-stress corrosion cracking failures as discussed later in the course.

Inspection for Crevice Corrosion

Do the following when inspecting possible cases of crevice corrosion:

• Inspect crevices and dead-leg regions.

• Inspect under insulation, loose coatings, and packing materials.

• Look for damage to, or absence of, vapor barriers and weatherproofing ininsulated components.

• Determine if the service conditions exceed the insulating material limits.

• Inspect under deposits and scale. Check localized areas where deposits arelikely to accumulate.


Corrosion Failures


Selective Leaching

The preferential removal (corrosion) of one element in an alloy is called selective leaching,dealloying, or parting. The alloy constituents that are more corrosion resistant remain in aporous structure with essentially the same geometry, but with severe loss of strength,hardness, and ductility. The two general types of selective leaching, layer and plug, areillustrated in Figure 16.

Layer Plug

Figure 16. Selective Leaching

Common types of selective leaching that are found in the petroleum and petrochemicalindustry are as follows:

• Dezincification: Selective leaching of zinc in copper-zinc (brass) alloys.

• Denickelification: Selective leaching of nickel in copper-nickel alloys.

• Dealuminization: Selective leaching of aluminum in aluminum brasses orbronzes.

• Graphitization (Graphitic Corrosion): Selective leaching of iron in gray castirons.

Selective leaching mechanisms vary with material. One mechanism that is commonly used fordezincification is the dissolution of copper and zinc, with the copper subsequently redepositedat the site of attack in metallic form. A suggested mechanism for other alloys is the removal ofone alloying element, which selectively leaves behind all others.

Failures that are caused by selective leaching are often sudden and unexpected since thedimensions of the failures do not change significantly. A color change is one visible indicatorof a selective leaching problem. Copper-base alloys where zinc, nickel, or aluminum haveleached away often display a dark orange metallic copper appearance.

Graphitic corrosion or graphitization of gray cast irons is a unique form of selective leaching.The iron is leached away, leaving a porous structure of graphite, with some iron oxide,silicon, and phosphorous. The porous structure is soft and easily cut with a knife.Underground cast iron piping is particularly prone to graphitic corrosion.


Corrosion Failures


Inspection for Selective Leaching

Do the following when inspecting possible cases of selective leaching:

• Examine for color changes in the area of the failure when copper alloys areinvolved.

• Test the hardness of gray cast iron that is suspected of graphitic corrosion.

• Remember that selective leaching can occur at ambient temperatures in somecases, and, in other cases, it can occur above an upper limit.

An example of a high temperature selective leaching failure is the denickelification anddezincification of a bronze labyrinth seal for a high pressure steam turbine. The result of theinitial failure was extensive foreign object damage to the turbine rotor. The steam at the seallocation was approximately 482 °C (900 °F). Handbook data revealed that the Cu-Ni-Zn alloycorrosion and strength loss were both excessive at the operating temperature. The inherentloss of strength at temperature combined with weakness due to selective leaching resulted inthe failure. The bronze seal was replaced with a stronger, more corrosion-resistant material(Ni-Resist: Fe-21Ni-2Cr ductile cast iron).


Corrosion Failures


FEATURES OF CORROSION THAT INVOLVES MULTIPLE VARIABLES

Corrosion that involves multiple variables include stress corrosion cracking (SCC), hydrogen-induced damage, liquid metal embrittlement (LME), corrosion fatigue, elevated temperatureattack, erosion corrosion, and microbiologically-influenced corrosion (MIC). For these typesof corrosion, a second variable (stress, bacteria, or high temperature) in addition to a corrosiveenvironment is necessary for failure to occur.

Corrosion that involves multiple variables includes environmentally-induced cracking, whichcan be the most devastating form of failure for a vessel, pipeline, structure, or piece ofequipment. In most cases, the reaction rates for other forms of corrosion permit detection byinspection. Environmental crack propagation, however, occurs at a relatively high rate, and itoften precludes detection. Furthermore, while other forms of corrosion commonly result inlocalized failures such as pinhole leaks, environmental cracking is often catastrophic, withlarge cracks that are analogous to brittle fracture. Since higher alloy materials are often themost prone to environmental cracking, this form of corrosion can occur when it is leastexpected in otherwise corrosion-resistant equipment.

Environmental Cracking

Environmental cracking may be defined as a cracking process that is caused by the synergisticeffects of stress and environment on a specific material. The phenomenon is best illustrated inthe popular Venn Diagram as shown in Figure 17. All three factors—stress, aggressiveenvironment, and susceptible material—are necessary for environmental cracking. If initiationhas not occurred, the removal of any one factor will often negate cracking.

Figure 17. Interaction of Material, Stress, and Environment


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Stress corrosion cracking, hydrogen-induced damage, and liquid metal embrittlement fit thegeneral definition for environmental cracking. Corrosion fatigue also fits the definition insome cases.

Several theories have been advanced to describe environmental cracking, but none of thetheories can explain all the types. From an electrochemical point of view, stress corrosioncracking is generally assumed to be an anodic process, while hydrogen-induced cracking is acathodic process.

Some of the popular environmental-cracking mechanisms are discussed below and illustratedin Figure 18.

Figure 18. Schematic Illustrations of SCC Mechanisms

• Slip-Dissolution (Film Rupture): Localized plastic deformation (slip) rupturesthe passive film and exposes bare metal that dissolves and results in crackextension.

• Film-Induced Cleavage: Stress causes a brittle crack to initiate in a brittlesurface film. The brittle crack continues to propagate into the ductile base metaluntil arrest occurs.

• Stress-Sorption: Weakening of the cohesive bonds between surface metalatoms through adsorption of damaging species in the environment. The surfaceenergy of the metal is assumed to be reduced, and this reduction increases thetendency for crack formation under tensile stress.


Corrosion Failures


Stress Corrosion Cracking

As mentioned previously, complex forms of corrosion typically account for one-half of thetotal failures in the petroleum and petrochemical industry. Stress corrosion cracking usuallyaccounts for over one-half of the complex failures.

Stress corrosion cracking (Figure 19) is a form of environmental cracking that is caused bythe synergistic effects of a corrosive environment and sustained stress on a specific material.

Figure 19. Stress Corrosion Cracking

Conditions that are necessary for initiation and propagation of SCC are as follows:

• Susceptible Material

• Specific Aggressive Environment

• Sustained Tensile Stress

• Time.

The factors that are involved with the above conditions are discussed on the following pages.


Corrosion Failures


Material Factors

High purity metals are generally more resistant to SCC than are commercial metals andalloys. The addition of even small quantities of solute or interstitial elements can significantlyincrease the potential for SCC. Impurities that are concentrated in grain boundaries promoteintergranular cracking. Microstructures that are produced by various heat treatments alsoinfluence the susceptibility to SCC. It is interesting to note that transitions in crack modes(intergranular versus transgranular) can occur in the same environment, depending on the heattreated condition for iron-chromium-nickel, high nickel, and brass alloys.

Environmental Factors

In most practical cases, water (moisture) is required for SCC. When exposure to water can beabsolutely ruled out for a given cracking problem, the possibility of SCC (but not other formsof embrittlement) sometimes can be dismissed. When water is present, the possibilities ofstress corrosion cracking increase, especially when temperatures are above ambient and thewater contains contaminants.

Chloride solutions at the ppm level are often sufficient to crack austenitic stainless steels,particularly when crevices and/or dryout promote concentration of the contaminating species.Dissolved oxygen alone can also crack sensitized stainless steels at temperatures ofapproximately 300 °C (570 °F). Some of the more interesting research in the field of SCC ison the synergistic effects of contaminants such as chlorides, sulfates, and oxygen. A uniqueform of intergranular SCC is pure water or “Coriou” cracking of nickel-base alloys in hightemperature, 315 °C (600 °F), deaerated, contaminant- free water. Surprisingly, a sensitizedmicrostructure is more resistant than an unsensitized microstructure for this form of SCC.

Temperature at the environment/material interface is another important factor for SCC.General temperature ranges exist for different types of SCC. Cracking commonly occurs atambient temperatures. While chloride SCC of austenitic stainless steels has a thresholdtemperature of approximately 60 °C (140 °F), SCC is often found at ambient temperatures inequipment that is radiantly heated.

Additional environmental parameters that are known to affect SCC are as follows:

• Pressure

• pH

• Electrochemical Potential

• Solution Viscosity

• Stirring or Mixing


Corrosion Failures


Mechanical Factors

Tensile stress, either applied or residual, is a requirement for SCC. For SCC to occur,materials need not be stressed above the yield strength. In some situations, very low thresholdstresses (e. g., 10 % of the yield strength) are sufficient to cause cracking. While compressivestresses from shot peening have been used to avoid SCC in some cases, control of applied(operating) stress levels generally is not considered an effective method of preventing SCC.Stresses that are sufficient to cause cracking can result from thermal processing, fabrication,assembly, surface finishing, and localized stress risers. Surfaces with pits, defects, or otherstress risers tend to provide initiation sites for SCC.

Time

Another factor that is common to the various SCC mechanisms is time. Since an incubationperiod is necessary before cracking initiates, SCC is sometimes referred to as delayedcracking. The incubation period is dependent on the material/environment couple as well asstress. Generally, the length of time for cracking to initiate is shorter for higher stresses. Oncecracks are initiated, propagation velocities for SCC are relatively slow as compared to LME.Nevertheless, from a practical viewpoint, the speed of crack growth can be devastatingly fast.

General Features of Stress Corrosion Cracks

Various macroscopic and microscopic features that are related to SCC are used in failureanalyses. These features are as follows:

Macroscopic Features

• Stress corrosion cracks are usually perpendicular to principal stresses in regionsof stress concentration; for example, cracking of steel vessels in caustic serviceis typically perpendicular to (across) girth welds and parallel to longitudinalweld seams.

• Stress corrosion cracks are more likely on hardened surfaces or in heat affectedzones.

• Stress corrosion cracks are often tighter than other failure modes.

Microscopic Features

• Crystallographic features include transgranular and intergranular crackpropagation.

• Other morphological features include singular and multiple cracks that may bebranched or unbranched. Some stress corrosion cracks typically initiate at thebases of corrosion pits.


Corrosion Failures


• Some forms of SCC characteristically exhibit cracks that are filled with oxide(example: caustic cracking of steel) or corrosion products. An SCC failureanalysis is more conclusive if the causative contaminant can be identified, bymicroanalysis, deep within the crack.

• Fractographic indications of crack-propagation lines that resemble fatiguestriations are observed for SCC in some alloy/environment systems.

Stress Corrosion Cracking in Specific Materials

The environmental cracking susceptibility of various engineering materials in more commonenvironments is summarized in the table in Figure 20. Included is information for hydrogenembrittlement (HE) and liquid metal embrittlement (LME), which are covered later in thismodule. Due to the complex nature and uncertainties of environmental cracking, the table inFigure 20 should not be considered all-inclusive. In some cases, an indication of susceptibilityto cracking in one ionic species (e.g., chlorides) may indicate susceptibility to other membersof the elemental group (e.g., halogens). Discussions on SCC characteristics for each materialfollow.

Material SCC HE LME

Steel

Annealed OH-, NO3, CN-, CO3, NH3 Cd, Cu, Pb, Sn, Zn

Hardened OH-, Cl-, NH3 Ho, H2S Cd, Cu, Pb, Sn, Zn, In, Li

Stainless Steel

Austenitic OH-, Cl- - Al, Cd, Cu, Pb, Zn

Austenitic (Sens.) OH-, Cl-, S2O6, steam - Al, Cd, Cu, Pb, Zn, Hg

Ferritic OH-, Cl- -

Martensitic OH- Ho, H2S

Copper Alloys NH3, NO3, steam, SO2 Ho Hg, Sn, Pb, Bi, Na

Nickel Alloys OH-, HF + O H2S Hg, Pb, S

Aluminum Alloys Cl-, Ox, agents - Hg, Na, Sn, Zn, Ga, In

Titanium Alloys Cl-, HNO3 Methanol Cd, Pb, Sn, Zn

* Based on various empirical data.

=

Figure 20. Environmental Cracking of Engineering Materials*


Corrosion Failures


Carbon and Low Alloy Steels - The most common cases of SCC in carbon and low alloysteels result from caustics, nitrates, and ammonia. Stress corrosion cracking in sodiumhydroxide (NaOH) solutions occurs at temperatures above approximately 150 °C (300 °F) inapplications such as boilers, where concentration or dryout occurs. This type of SCChistorically was called caustic embrittlement. Caustic cracking is ordinarily intergranular andbranched. Traces of oxide are often observed deep within the cracks. Low carbon steel alsohas been reported to crack transgranularly in potassium hydroxide (KOH) at temperatures aslow as 33 °C (91 °F). Caustic SCC can be mitigated by the lowering of the pH of the solution,the addition of inhibitors, the anodic or cathodic protection of the steel, and stress relief ofwelds.

The intergranular SCC of mild steels in various nitrate solutions is well documented. Crackinghas been reported at temperatures as low as ambient after several months exposure. In someways, nitrate SCC is the converse of caustic cracking; as the environment becomes moreacidic, the potential for SCC increases. The addition of caustic to nitrate solutions can retardcracking, just as the addition of nitrates to caustic solutions can retard caustic SCC. Inhibitoradditions, the increase of the pH of the environment, and welds that relieve stress are threeapproaches for mitigating nitrate SCC. While cathodic protection may be beneficial for theretarding of nitrate SCC, anodic polarization is harmful.

Stress corrosion cracking of steels in ambient temperature ammonia (NH3) is an anomaly withrespect to other types of aqueous SCC. Failures occur when air is mixed with anhydrousammonia. Cracking is intergranular in carbon steels and both intergranular and transgranularin high strength steels. The addition of a small quantity of water to the ammonia is a widely-used method for eliminating ammonia cracking of steels. Stress relieving is also beneficial.

Austenitic Stainless Steels - Austenitic stainless steels are subject to SCC in chloride,polythionic acid, and caustic solutions. Optical micrographs of the three types of SCC areshown in Figure 21.

Figure 21. Stress Corrosion Cracking in Austenitic Stainless Steels


Corrosion Failures



Corrosion Failures


Chloride Stress Corrosion Cracking - The susceptibility of austenitic stainless steels to SCCin chloride solutions is widely known. Nevertheless, numerous failures occur each year,particularly in the case of the 300-Series grades. The role of nickel in the chloride SCC ofaustenitic stainless steel is illustrated in Figure 22. Minimum SCC resistance occurs atapproximately 8 % nickel, which makes Type 304 (8 - 10.5 % Ni) the type that is most proneto attack. Other 300-Series stainless steels such as Type 321 (9 - 12 % Ni), Type 347 (9 -13 % Ni), Type 316 (10 - 14 % Ni), and Type 309 (12 - 15 % Ni) offer very marginalimprovement in chloride SCC resistance. Ferritic (Fe-Cr) stainless steels, as well as highernickel alloys, generally are resistant to chloride SCC.

Cracking

Minimum Time to Cracking

No Cracking

- Did Not Crack in 30 Days

Nickel, (%)

20 40 60 80

1

10

100

1000

Time to Failure

(hr)

Figure 22. The Effect of Nickel Content on SCC of Austenitic Materials(Tests conducted in boiling MgCl2 . )


Corrosion Failures


Chloride SCC occurs when the five following conditions are present:

• Chloride ion

• Oxygen

• Tensile stress

• Moisture at a pH less than 7.

• Temperature above 60 °C (140 °F).

The results of a survey that included 136 cases of chloride SCC at 109 locations is shown inFigure 23. Interestingly, some failures were reported at or below 1 ppm chloride.

Figure 23. Chloride SCC of Austenitic Stainless Steels - MTI Survey Data (ReferenceNo. 12)


Corrosion Failures


Chloride stress corrosion cracks are typically transgranular and branched; however, sensitizedmaterial can crack intergranularly in high-temperature water that contains oxygen. Thesynergistic role of oxygen and chloride is illustrated in Figure 24. In the complete absence ofoxygen, chloride levels as high as 1000 ppm may be tolerable, but in highly oxygenatedwater, 0.0l ppm chloride could cause SCC. The type of metallic cation in the chloride solutionalso affects SCC susceptibility; for example, magnesium and ferric chlorides are moreaggressive than sodium chloride.

304

SCC - All Heat Treatments SCC No SCC

SCC Sensitized

Only

Tentative SCC - Safe Area

1000

10010 10.1

0.01

0.0010.01

0.1

100

1,000 10,000

Sensitized

Annealed

10

1

250 - 300 C

Dis

solv

ed O

, g/

m (

ppm

)2

CI Concentration, g/m (ppm)3

Figure 24. Synergistic Effect of Chlorides and Oxygen on theSCC of Type 304 Stainless Steel

(M. O. Speidel, ARPA Handbook on Stress Corrosion Cracking, to be published.)

The control of chloride SCC is often very difficult, especially on heat transfer surfaces or increvices where chloride concentration can occur. Localized boiling can cause concentrationand cracking at practically any bulk water chloride concentration. Since very low stresses cancause SCC, often the only recourse is to change materials. In some cases, removal of oxygenand/or the increase of the solution pH can effectively mitigate cracking.


Corrosion Failures


Polythionic acid stress corrosion cracking (PSCC) - Cracking that is due to sulfurcompounds was first recognized as a problem in catalytic reformers in the 1950s. This form ofattack involves intergranular cracking of sensitized austenitic materials at ambient conditions.Three environmental conditions are necessary to form polythionic acids — oxygen (air),moisture, and a source of sulfur (H2S, SO2, S, or FeS).

Austenitic stainless steels are often used because of their resistance to high-temperaturesulfidation; however, thin iron sulfide films do form in service. The film that is formed at hightemperatures is porous and tends to be more susceptible to polythionic acid formation thansulfide films that are formed at lower temperatures or in aqueous media. When equipment isopened to the atmosphere, formation of polythionic acid (H2SxO6 in Eq. 2) can result in PSCCdue to residual stresses.

FeS + H2O H2SxO6 + FeO (2)

Austenitic stainless steel equipment often is neutralized before exposure to the atmosphere.NACE Standard RP-01-70, “Protection of Austenitic Steel in Refineries Against StressCorrosion Cracking by Use of Neutralizing Solutions During Shutdown” covers alkalinewashing of stainless steel equipment.

Usually only sensitized materials are susceptible to PSCC. Unfortunately, servicetemperatures are often high enough to sensitize unstabilized grades of stainless steel. The useof properly heat treated stabilized grades (Types 321 and 347) can prevent PSCC. It is worthnoting that higher nickel alloys such as Alloy 800 are very susceptible to PSCC when thealloys are sensitized.


Corrosion Failures


Caustic Stress Corrosion Cracking - Continual concerns over chloride SCC often leavecaustic SCC to be overlooked. All austenitic stainless steels are susceptible to causticcracking. Both transgranular and intergranular cracking have been observed. The temperatureand concentration limits for caustic SCC of various alloys are shown in Figure 25.

350

300

250

200

150

100

50

00 10 20 30 40 50 60

100

200

300

400

500

600

Failure (One Day) Unsensitized

Unsensitized 304

Sensitized 304

Failure (100-300 Days)

Tentative Safe SCC Limit

Tem

pera

ture

, ÞC

ÞF

Concentration of Sodium Hydroxide, Wt. %

Figure 25. Temperature and Concentration Limits for Caustic SCCof Types 304, 316, 321, and 347 Stainless Steel

(Reference No. 12)

Ferritic, Martensitic, and Duplex Stainless Steels - Ferritic stainless steels are susceptibleto SCC in chloride and caustic solutions, but these steels are affected to a lesser degree thanare austenitics. Unless the alloy contains significant amounts of nickel or copper, SCC inservice occurs very rarely.

Martensitic, maraging, and precipitation hardening stainless steels are susceptible to SCC inthe hardened condition. These alloys are especially susceptible to hydrogen embrittlement,and distinguishing SCC from HE is usually difficult. Cracking can occur in nonspecificenvironments. For example, cracking has been reported in ambient temperature fresh water.Any environment that is corrosive enough to cause hydrogen evolution can cause crackingwhen the alloys are in the fully hardened condition. Cracking of these materials is usuallyintergranular.


Corrosion Failures


Duplex stainless steels are mildly susceptible to chloride stress corrosion cracking. The factthat cast austenitic stainless steels contain ferrite, which makes them essentially duplex,accounts for their improved assistance to chloride SCC as compared to wrought materials.Hardenable duplex grades are susceptible to hydrogen embrittlement.

Copper Alloys - Copper alloys are susceptible to SCC in ammonia, nitrates, steam, and moistsulfur dioxide. Stress Corrosion Cracking may also occur in amines at low concentrations,and in caustic in the case of high zinc alloys.

Ammonia SCC is the most common cause of copper alloy heat exchanger tube failures inmany plants. The number of ammonia SCC failures has actually decreased in recent years,mostly because copper alloys have been replaced by more resistant materials such as titaniumalloys.

A classic example of ammonia SCC is the season cracking of brass as illustrated in Figure 26.As shown in the accompanying optical micrograph, cracking is typically intergranular andbranched. Cracking can be transgranular in alloys that contain more than 40 % zinc and insome other copper alloys.

Figure 26. Season Cracking of BrassTube 1 - Cracked in Mercurous Nitrate Test

Tube 2 - Cracked in 3 Days in Ammonia Vapor


Corrosion Failures


The conditions that are necessary for ammonia SCC of copper alloys include the following:

• Ammonia

• Oxygen

• Tensile stress

• Moisture.

Carbon dioxide especially accelerates the cracking process. Anhydrous ammonia does notcrack copper alloys; however, it is almost impossible to keep ammonia free of air andmoisture during shipment and storage.

The corrosion products that form under the above conditions frequently are dark blue, darkbrown, or black with only a small amount of the metal being corroded. The visual corrosion isoften associated with a special SCC mechanism (the tarnish rupture model) that is related tofilm-induced cleavage. Depending on the stress level, solution concentration, and alloysusceptibility, failures may develop within a few hours or after several years. Some of thevariables affecting ammonia SCC of copper alloys are as follows:

• Susceptibility increases with increased cold work

• Time to failure decreases with increased temperature

• Time to failure decreases with increased stress

• Time to failure decreases with increasing grain size

• Time to failure is a minimum at pH = 7

• Threshold stresses are usually very low.

The possibility of ammonia SCC in brass can be minimized by eliminating any of the fourconditions that are necessary for attack to occur. Although not often achievable, the removalof ammonia, oxygen, or moisture from the environment will mitigate cracking. Annealing thematerial will also mitigate cracking.


Corrosion Failures


Changing the alloy content is another method for reducing the susceptibility to ammoniaSCC. As the zinc content of brass is reduced, the SCC susceptibility is also decreased. If alower zinc content alloy is otherwise acceptable for a given application, the possibility of SCCfailure can be greatly reduced by its use. The table in Figure 27 ranks the SCC resistance ofseveral copper alloys.

Relative Susceptibility of Copper Alloys to Ammonia SCC

Class 1Very Low Susceptibility

Cupro-Nickel 90-10Cupro-Nickel 70-30ETP Copper

Class 2Low SusceptibilityDLP CopperDHP Copper

Class 3Intermediate Susceptibility

Red BrassCommercial BronzeAluminum BronzeSilicon BronzePhospor BronzeNickel Silver

Class 4High SusceptibilityLeaded BrassNaval BrassAdmiralty BrassYellow BrassManganese BronzeAluminum BrassMuntz MetalCartridge Brass

Figure 27. Relative Susceptibility of Copper Alloys to Ammonia SCC

Nickel Alloys - Nickel and its alloys are relatively resistant to SCC; Alloy 600 is virtuallyimmune to chloride SCC. Intergranular caustic SCC can occur in high temperature [300 °C(575 °F)] water. Fluorides crack Monel Alloy 400 in the presence of oxygen or oxidizingspecies, and various cases have been reported in HF Alkylation Units. Nickel-chromium-molybdenum alloys such as Hastelloy C-276 and Alloy 625 are often used for their SCCresistance in various chemical environments.

Aluminum Alloys - High-strength aluminum alloys with greater than 6 % magnesium or12 % zinc are susceptible to SCC in ordinary atmospheric and aqueous environments.Moisture, temperature, chlorides, and other contaminants accelerate cracking. Cracking isusually intergranular. Commercially pure and lower-strength alloys are not susceptible toSCC. The corrosion rate of aluminum alloys in caustic is so great that SCC is precluded.

Titanium Alloys - Titanium alloys are resistant in most cases to SCC in environments such asboiling magnesium chloride and sodium hydroxide solutions. Stress corrosion cracking doesoccur in fuming nitric acid and hot, dry chloride salts. In chlorine and hydrogen chlorideenvironments, SCC can occur when both oxygen and water are present. When sharp cracks ornotches are present, cracking of some titanium alloys can occur in seawater; however,commercially pure grades of titanium that are generally used in industrial applications arequite resistant to SCC in chloride solutions.


Corrosion Failures


Inspection for SCC Failures

When inspecting possible SCC failures, it is helpful to visualize the Venn Diagram inFigure 17 and ask “are the conditions for SCC satisfied?” In other words, is the specificmaterial known to be susceptible to SCC in the given environment, and does the failed regioncontain sufficient tensile stresses?

Related factors that often play roles in failure analyses are as follows:

• An aqueous phase must be present for SCC. Process streams that are free ofwater will not stress corrosion crack unless moisture enters during a shutdown,for example.

• Specific material/environment couples have threshold temperature limits forSCC. SCC failures are less likely to occur below the threshold temperature.

• Specific material/environment couples have pH ranges for SCC. For example,mild acid solutions are necessary for chloride SCC of austenitic stainless steel,while caustic SCC requires strongly alkaline solutions.

Stress corrosion cracking failures typically have a brittle appearance. When the failed partcontains a multitude of cracks, stress corrosion cracking is the most probable failure mode.

During the inspection of possible SCC failures, it is most important to look for consistencies.For example, macroscopic features, microscopic features, fractographic features, temperatureranges, chemical analyses, and pH should all be indicative of the specific type of SCCcracking.

Hydrogen-Induced Damage

Hydrogen causes several types of metal degradation. Hydrogen embrittlement, sulfide stresscracking, hydrogen blistering, and hydrogen-induced cracking are lower temperature typesthat are covered in this section. Hydrogen attack, which occurs at high temperatures in steels,is covered in the Elevated Temperature Attack section. The types of hydrogen-induceddamage that are covered in this section pertain primarily to steels, but other engineering alloysthat form hydrides, such as titanium, also experience hydrogen embrittlement.


Corrosion Failures


Hydrogen Embrittlement

Hydrogen embrittlement, which is caused by the absorption of atomic (nacent) hydrogen, isthe loss of toughness or ductility in a metal. When carbon and low-alloy steels are exposed incorrosive environments, atomic hydrogen is generated by cathodic reduction. While hydrogenmolecules are too large to pass into the metal, atomic hydrogen readily diffuses through themetal lattice, becomes entrapped, and interferes with the normal process of deformation underload. This form of embrittlement typically occurs at temperatures of -100 °C to +120 °C (-148 °F to 248 °F). In addition to corrosion processes, acid cleaning (pickling), electroplatingprocesses, and poor welding procedures can embrittle steels.

Hydrogen gas environments do not embrittle steels at low temperatures because hydrogen isin molecular form. At temperatures above approximately 150 °C (300 °F), appreciabledissociation to atomic hydrogen occurs and diffusion into the metal can occur. Hydrogen thatleaves the metal is approximately the same as hydrogen that enters; however, when the steel iscooled rapidly, some atomic hydrogen remains trapped in the steel and this entrappedhydrogen results in embrittlement. Baking hydrogen out of steels at temperatures of 175 °C to200 °C (347 °F to 392 °F) or higher is commonly performed for de-embrittlement.

Hydrogen embrittlement results in degraded, but not necessarily failed metal. For example,baking can restore ductile properties before failure occurs. Failures occur by cracking whenthe embrittled metal is subjected to stress. Shock loading almost always results in cracking;however, residual stresses can sometimes be sufficient to cause cracking. An example of thelatter is the delayed hydrogen cracking failures that result in welds that are made through theuse of moist weld rods. In this case, the water that is heated during welding dissociates andforms atomic hydrogen that enters the metal. Upon cooling, residual stresses cause hydrogen-embrittlement cracking.

Hydrogen-embrittlement cracking is also referred to as hydrogen-assisted cracking, hydrogen-stress cracking, and hydrogen-stress corrosion cracking. Different terms are used by differentexperts to denote specific characteristics. For example, hydrogen-assisted cracking andhydrogen-stress cracking are often used to describe hydrogen embrittlement in the absence ofa corrosion reaction, and hydrogen-stress corrosion cracking refers to cracking with activecorrosion.

Generally, higher strength (hardenable) steels and weldments are most prone to embrittlementcracking failures. Cracking may be either intergranular or mixed mode. Most often, failuresoccur from a singular crack with no branching.


Corrosion Failures


Sulfide-Stress Cracking

Sulfide-stress cracking (SSC) is a special case of hydrogen embrittlement in which highstrength or high hardness steels crack under stress. Sulfide-stress cracking results fromcathodic charging that is caused by corrosion in sulfide environments. Sulfide-stress crackingcan occur in materials with over 200 BHN hardness, and is sometimes considered as a type ofstress corrosion cracking. This type of cracking is found most commonly in weld heataffected zones.

NACE standard MR0175-93 presents the material requirements for resistance to SSC.Parameters considered to affect SSC are as follows:

1) chemical composition, strength, heat treatment, and microstructure of thematerial

2) hydrogen ion concentration (pH) of the environment

3) hydrogen sulfide concentration and total pressure

4) total tensile stress (applied plus residual)

5) temperature

6) time

Sour environments commonly are defined as fluids that contain water and greater than0.34kPa (0.05 psia) hydrogen sulfide partial pressure. More specifically, MR0175-93 definescritical H2S levels as follows:

• Sour Gas: SSC susceptibility if the gas being handled is at a total pressure of448 kPa (65 psia) or greater and if the partial pressure of H2S in the gas isgreater than 0.34 kPa (0.05 psia). Systems operating below 65 psia totalpressure or below 0.05 psia H2S partial pressure are outside the scope of thisstandard.

• Sour oil and multiphases: SSC susceptibility when

1) the gas:oil ratio is over 5,000 SCF:bbl (barrel of oil)

2) the gas phase contains a over of 15% H2S

3) the partial pressure of H2S in the gas phase is over of 69 kPa (10 psia)

4) the surface operating pressure is over of 1.8 MPa (265 psia)

5) when pressure exceeds 265 psi, refer back to sour gas rules


Corrosion Failures


Figures for determination of critical SSC environments are provided in Addendum B. When itis determined that a critical environment exists, the requirements for materials must befollowed according to MR0175-93.

Hydrogen Blistering

Hydrogen blistering occurs when atomic hydrogen diffuses into steel and recombines to formmolecular hydrogen (H2) at metallurgical defects such as planar inclusions or laminations.The molecular hydrogen that is formed cannot diffuse, and it creates extremely high pressuresas more and more atomic hydrogen diffuses to the defect. The blister that forms eventuallybecomes visible on the metal surface. When the blister forms close to the surface, the surfaceexhibits a typical blister. When the blister forms more deeply below the surface, near themiddle of the thickness, the surface may exhibit a bulged appearance.

Blistering typically occurs in carbon and other low alloy, unhardened steels. Hardened steelsusually crack instead of blistering. Blistering is often found at the interface in clad vessels.

Hydrogen-Induced Cracking

Hydrogen-induced cracking (HIC), which is also referred to as stepwise cracking, occurs incarbon and low alloy steels that are exposed to wet hydrogen sulfide environments. Themechanism for HIC is related to the mechanism for blistering. Small blisters form at planarinclusions and grow at varying depths through the thickness of the steel. Cracking initiates atthe tips of the blisters, and stepwise crack propagation between blisters on adjacent parallelplanes occurs, as shown in Figure 28. A series of steps eventually reaches the surface of thesteel, and pressure is relieved.

Figure 28. Hydrogen-Induced Cracking (HIC)

Hydrogen-induced cracking appears visually as small surface blisters with associated cracks.The mode of cracking is usually transgranular, but it can be intergranular or mixed mode. HICis typically found in older, dirtier steels. Elongated manganese sulfide inclusions arecommonly associated with the cracking. Newer, cleaner steels, and steels with sulfide shapecontrol are less prone to HIC.


Corrosion Failures


Since hydrogen-induced cracking is driven by internal pressure in blisters, high pressures inthe vessel or pipeline are not required. Stress oriented hydrogen-induced cracking (SOHIC) isa related form of HIC that occurs when the steel is stressed significantly. In this case, theblisters are stacked perpendicularly to the applied stress (Figure 29). The end result of SOHICis through-thickness cracking. Whereas HIC may be found by inspection before leakage orfailure occurs, SOHIC more often results in failure before surface evidence is apparent.

Figure 29. Stress-Oriented Hydrogen-Induced Cracking(Reference No. 17)

Inspection for Hydrogen Damage

Do the following when inspecting possible cases of hydrogen embrittlement:

• Check for previous chemical cleaning, electroplating, or welding operationswhere atomic hydrogen may have been generated.

• Inspect for surface blisters.

• Check for delaminations in bonded steel.

• Determine the environment and the likelihood of atomic hydrogen existing atthe service temperature.

• Check for galvanic couples or nearby corroding surfaces. Cases have beenobserved where high strength stainless steel fittings were embrittled by nearbycarbon steel corrosion.


Corrosion Failures


Liquid Metal Embrittlement

Liquid metal embrittlement (LME), which is also called liquid metal cracking, is a form ofenvironmental cracking in which a normally ductile solid metal under stress cracks in a brittlemanner due to contact with a specific liquid metal. Cracking may be spontaneous or delayed.Several characteristics of LME are similar to SCC, and variations of the stress-sorptioncracking model often are used to describe the LME mechanism. As in the case of SCC, LMEcracking can be intergranular or transgranular, depending on the embrittled metal and liquidmetal embrittler. Whereas SCC usually occurs in an aqueous environment, LME does notnecessarily involve moisture.

The number of liquid metal/solid metal couples that are susceptible to LME are numerous.Some of the common couples are included in Figure 20. At elevated temperatures, moltencadmium, copper, lead, tin, and zinc have caused problems in steel process equipment. Forexample, welding on copper clad vessels has led to cracking. Another example occurred whenan alloy steel motor shaft was overheated and caused tin babbit sleeve bearings to melt. Shaftfailure resulted from LME.

Stainless steels are particularly susceptible to LME that is caused by molten aluminum. Nickelalloys can be embrittled by molten nickel sulfide formed from sulfur contamination. Copperand aluminum alloys are especially susceptible to LME by mercury. Mercury in the processstreams at LNG plants has caused several cases of cracking of aluminum process equipment.

Solid Metal Embrittlement

Cracking of a metal that is exposed to specific embrittlers at temperatures below the meltingpoint of the embrittler is called solid metal embrittlement (SME). Although this form ofembrittlement is not recognized as a problem in most industrial processes, it can play a role incertain failures. For example, cadmium-plated high strength steel bolts can fail attemperatures above 230 °C (450 °F), which is below the melting point for cadmium (321 °C[610 °F]). Titanium alloys are also susceptible to SME due to the presence of solid cadmium.Leaded steel parts have failed below the melting point for lead due to SME.

Inspection for Liquid/Solid Metal Embrittlement Failures

Do the following when inspecting possible cases of liquid or solid metal embrittlement:

• Examine for brittle fracture appearance.

• Check fasteners for plating with embrittlers such as cadmium.

• Determine if temperatures at the failure were sufficient to cause melting(LME), or high enough to otherwise embrittle by SME. Also, determine ifmolten metal may have dripped from another hot location.


Corrosion Failures


Microbiologically-Influenced Corrosion

Microbiologically-influenced corrosion (MIC), which is also referred to as biologicalcorrosion or microbiological corrosion, is considered a complex form of corrosion, sinceattack is the indirect consequence of the presence of microbes. Corrosion can occur due tocorrodants that are produced by microbes or due to under-deposit attack that is related to thepresence of microbes. Subsurface regions and water systems are prone to MIC. Most often,MIC causes localized attack such as pitting or under-deposit corrosion.

Anaerobic Bacteria

Anaerobic bacteria are sulfate-reducing types of bacteria that cause corrosion, but the bacteriado not require oxygen to exist. They reduce relatively noncorrosive sulfate ions to corrosivesulfide ions according to the equation:

SO4-2 + 4H2 S-2 + 4H2O (3)

Sulfate-reducing bacteria are commonly found underground in oxygen-starved regions suchas wet clay. The sandy soils of Saudi Arabia generally provide better resistance to anaerobicbacteria than do soils in other parts of the world. Steel that is corroded as a result of anaerobicbacteria produces iron sulfide as a corrosion product.

Aerobic Bacteria

Bacteria that exist in aerobic conditions oxidize sulfur to corrosive sulfuric acid according tothe equation:

2S + 3O2 + 2H2O 2 H2SO4 (4)

Sulfur oxidizing bacteria are found in low pH environments where sulfur is present. Locationsinclude oil fields and regions with sewage.

Additional Forms of MIC

Iron bacteria form ferric hydroxide and cause localized attack of steels. Corrosion productsfrequently appear as tubercles on steel surfaces, with localized corrosion underneath. Otherbacteria oxidize ammonia and carbon dioxide respectively to corrosive nitric acid or carbonicacid. Several microorganisms simply attach to exposed metal surfaces and result in under-deposit corrosion. A special group of “slime formers” are commonly found in oil fields.


Corrosion Failures


Inspection for MIC Damage

MIC failures resemble other localized corrosion failures, such as pitting and under-depositattack. Probable locations are stagnant regions of systems such as tank bottoms and filterbeds. Unusual smells, slime, sludge, and black water, all of which are indicative of bacteria,may suggest MIC. Sulfur species that are detected in deposit or water analyses may alsoindicate MIC. The acquisition and biological analysis of actual bacterial samples requiresspecial procedures that will be further covered in COE 106.06 and COE 106.07. This type ofcorrosion, even with laboratory investigation, is extremely difficult to verify. For the mostpart, MIC damage is identified by default when no other corrosion mechanism can bedefinitely determined.

Erosion-Corrosion

Erosion is surface wear that is caused by the impingement of particles or fluids. Erosion willbe covered along with other forms of wear in COE 106.03. Erosion corrosion is acceleratederosion that is caused by corrosive fluid. The total metal loss from erosion-corrosion can beseveral times the metal loss from erosion or corrosion alone. Most metals are susceptible toerosion-corrosion under specific conditions. Passive films often play an important role inerosion-corrosion. As long as the film remains intact, resistance to erosion-corrosion ismaintained. When the film is broken, rapid attack can occur. Carbon and low-alloy steelsgenerally perform acceptably in hydrocarbon streams that contain hydrogen sulfide becauseof the tenacious iron sulfide film that is formed. If the film is broken down by cyanide or byanother corrodant that attacks the film, rapid corrosion occurs.

Liquid erosion-corrosion failures exhibit grooves, ditches, rounded holes, horseshoe-shapedattack, and other forms of metal loss that are associated with flow patterns. It is often difficultto distinguish erosion from erosion-corrosion, since both of them result in clean surfaces inregions of high velocity, impingement, or turbulence. In many cases, conclusions areintuitive, based on prior experience of, for example, the corrosion rates under stagnantconditions or the erosion rates in noncorrosive streams with similar flow conditions.

Inspection for Erosion-Corrosion

Do the following when inspecting possible erosion-corrosion failures:

• Examine failed region for flow patterns such as grooving and lack of adherentcorrosion product.

• Compare corrosion rate data from stagnant tests and general erosion resistanceof materials, if available.

• Examine the failed region for excessive fluid velocities, impingement, andturbulence.


Corrosion Failures


Corrosion Fatigue

Corrosion fatigue occurs as a result of the combined action of a cyclical stress and a corrosiveenvironment. Purely mechanical fatigue failures will be examined in COE 106.03. The criticalparameter for fatigue failure is fatigue strength, which is the maximum stress that can besustained by a metal for a specified number of cycles without failure. The fatigue strength of ametal can decrease when cycled in the presence of a corrodant. An aggressive environmentcan affect crack initiation, crack propagation, or both. It is worth noting that LME cracking isalso accelerated by cyclical stress.

Corrosion pitting and intergranular attack are sometimes initiation sites for corrosion fatigue.Subsequent corrosion-fatigue cracking may be intergranular or transgranular. Sometimescorrosion products cause a wedge-opening effect, which causes crack growth. When crackpropagation is caused by a wedge-opening, straight singular cracks are observed. In othercases, some crack branching may be evident.

As with erosion-corrosion, corrosion fatigue is often difficult to discern from purelymechanical fatigue. The surfaces of cracks that are produced by mechanical fatigue oftenexhibit fatigue striations, which will be examined later. Corrosion often destroys telltalefatigue striations; however, corrosion often occurs after cracking rather than as part of thecracking process.

Inspection for Corrosion Fatigue

Do the following when inspecting possible corrosion fatigue failures:

• Examine for corrosion products within the cracks.

• Check for pits, intergranular attack, or other localized forms of corrosion thatmay have contributed to crack initiation.

• Look for singular, or widely spaced cracks that are unbranched or only slightlybranched.

• Investigate for possible cyclical stressing.


Corrosion Failures


Elevated Temperature Attack

The types of corrosion that are covered in this section are those types that occur attemperatures above approximately 260 °C (500 °F). Most of the types actually occur at muchhigher temperatures.

Oxidation

Generally, oxidation refers to any corrosion process that results in an oxide corrosion product.High temperature forms of oxidation that occur in petroleum and petrochemical facilitiestypically involve oxidation by hot gases that contain air, oxygen, or other oxygen sourcessuch as carbon dioxide, as well as oxidation in boiler water and steam.

Much of the processing of oil occurs at elevated temperatures. While the hydrocarbon processstreams are not oxidizing environments, heating is performed by combustion with air, andexternal surfaces oxidize. The engineering alloys that are used for processing equipment havemaximum temperatures above which they readily oxidize in service. Approximate maximummetal temperatures for selected engineering alloys are given in the table in Figure 30. Notethat steam oxidizes more severely than air.


Corrosion Failures


Maximum Metal Temperature, °C (°F)

Alloy Composition Clean Combustion Atmosphere Steam

Carbon Steel 425 (800)

Brass 70 Cu - 30 Zn 700 (1290)

Fe - Cr Alloys 1.25 Cr - 0.5 Mo 595 (1100) 565 (1050)

2.25 Cr - 1.0 Mo 635 (1175) 610 (1125)

5.0 Cr - 0.5 Mo 650 (1200) 620 (1150)

7.0 Cr - 0.5 Mo 675 (1250)

9.0 Cr - 1.0 Mo 705 (1300) 650 (1200)

Type 410 SS 12.0 Cr 760 (1400)

Type 304 SS 18.0 Cr - 8.0 Ni 845 (1550) 815 (1500)

Type 310 SS 25.0 Cr - 20.0 Ni 1040 (1900)

Alloy 800 20.0 Cr - 32.0 Ni 1095 (2000)

Figure 30. Maximum Metal Temperatures for Resistance to Oxidation

The ability of metals to resist oxidation depends largely on the added alloying elements andconsequently on the formed oxides. Dense, tightly adhering oxides, such as chromium oxide(Cr2O3), are protective, while voluminous, incoherent oxides offer no protection from furtheroxidation. In failure analyses, the oxides are often studied in detail to determine variouscharacteristics and compositions as well as to distinguish oxides from other forms ofcorrosion such as sulfidation or carburization. Internal oxidation occurs when oxygen diffusesbelow the surface of the metal and preferentially oxidizes specific alloy constituents. Thisform of attack is less predictable or detectable, and it often results in premature failures.


Corrosion Failures


Overheating is probably the most common cause of failure for heater tubes. Failure modes areeither oxidation or, more likely, creep (covered in COE 106.03). Oxidation rate curves asshown in Figure 31 are useful for determining tube temperatures. For example, if 25 mils ofscale (oxide) are found on a tube after one year of operation, and the volume of oxide isassumed to equal that of metal loss (corrosion), the operating temperature for 25 mpy can beread from the chart.

Figure 31. Oxidation Rate Curves for Selected Alloys(Oxidation rates in Steam)

(Reference No. 19)

Thermal cycling can cause oxides that normally adhere tightly to crack and spall off of heatertubes. Each time spalling occurs, bare metal oxidizes until a sufficient protective scale isformed. Exfoliation is a term that is used to describe preferential corrosion that is generallyalong grain boundaries and parallel to the metal surface. Differential expansion of corrosionproducts at grain boundaries causes spalling of layers of metal and creates a leafy appearance.Exfoliation is often associated with cyclical operation.

Oxides are usually more voluminous that the base metal. Extremely high compressive forcescan occur when oxides form in confined spaces. Corroding carbon steel tubesheets have beenknown to crush tubes or crack tubesheets due to the oxide growth in crevices that is formedby rolled-in tubes.

Inspection for Oxidation

Do the following when inspecting possible oxidation failures:


Corrosion Failures


• Determine, if possible, operating temperature. It is possible that temperatureswere simply too high for the alloy (Figure 30).

• Determine, if possible, if unusual cycling has repeatedly occurred duringoperation.

• Check for excessive scaling and spalling.

• Analyze scale to confirm oxide. Note that magnetite (Fe3O4) is magnetic;therefore, a magnet test can provide a simple clue to scale identity.

Sulfidation

Sulfidation is the high temperature reaction of a metal with sulfur species to form metallicsulfides. Whereas oxidation occurs in combustion and boiler environments, sulfidation occurson the process side. The common source of sulfur in petroleum processing is hydrogen sulfide(H2S). Sulfidation proceeds according to the following equation:

Fe + H2S FeS + H2 (5)


Corrosion Failures


Sulfidation begins at approximately 260 °C (500 °F) and reaches a maximum atapproximately 427 °C (800 °F). The effect of temperature and hydrogen sulfide concentrationon the sulfidation rate is exemplified in Figure 32 for a low alloy steel and an austeniticstainless steel. Lower corrosion rates for the austenitic stainless steel indicate why stainlesssteels are often used for sulfidation resistance.

Figure 32. Sulfidation rates of low alloy (0.5 Cr) steeland austenitic stainless (18 Cr - 8 Ni) steel.

In some ways, sulfidation is analogous to oxidation. Under certain conditions both scales areprotective; as the scale thickens, the corrosion rate decreases. Sulfidation is generally moredestructive than oxidation because the protective sulfide scales are more friable and tend tospall off more than do oxide scales. Sulfidation is also sensitive to erosion effects in highvelocity streams. Nickel alloys have poor sulfidation resistance; therefore, corrosion of nickelalloys by sulfidation is much more likely to occur than is corrosion by oxidation.


Corrosion Failures


Cyanide-Induced Corrosion

A special case of accelerated sulfidation attack occurs in refineries where cyanides are formedfrom nitrogen in the crude. Cyanides destroy the sulfide scale, and expose the base metal torapid sulfidation attack.

Inspection for Sulfidation

Do the following when inspecting possible sulfidation failures:

• Determine if operating temperatures were within the expected range foraccelerated sulfidation.

• Check for sulfur and sulfide attacking agents such as cyanides in the processstream.

• Check for excessive spalling of sulfide layers.

• Analyze to confirm sulfide scale.

Note that some sulfides are similar to oxides in that they are mildly magnetic.

Vanadium Attack

Many crudes contain a stable organic compound of vanadium that remains in residual fuel oil.When burned, the vanadium is oxidized to vanadium pentoxide, V2O5. Aided by sulfur in thefuel oil as well as by other impurities such as sodium and potassium, the vanadium pentoxideforms low melting phases on iron or nickel surfaces. As a result, severe metal loss can occur.In steels, liquid phases can form at temperatures as low as 566 °C (1050 °F).

Vanadium attack exhibits thick scale layers on components such as heater tubes. The scalemay have a beaded external surface appearance. Often the scale spalls to reveal localizedregions of metal loss with features similar to pitting or acid attack. Sometimes the pittedregions will exhibit features that resemble melting. Analysis of the scale usually will revealsignificant quantities of sulfur and vanadium, and the analysis possibly will reveal traces ofsodium or potassium.

It is not unusual for heaters to operate for several years without corrosion problems and thensuddenly experience accelerated corrosion due to introduction of vanadium-containing fuel.Slight temperature increases can result in exceeding the melting temperature for the vanadiumpentoxide phase. All iron-base alloys are susceptible to vanadium attack. A cast 50 Cr - 50 Nialloy offers the best resistance to this type of attack.


Corrosion Failures


Inspection for Vanadium Attack

Do the following when inspecting possible cases of vanadium attack:

• In the case of steels, check metal temperatures to determine if temperatures areabove approximately 566 °C (1050 °F). Vanadium attack of nickel alloysoccurs at higher temperatures. Heater and boiler components such as tubesupports operate at higher temperatures than do tubes, and, therefore, thesecomponents are particularly prone to vanadium attack.

• Confirm the presence of vanadium in the scale and/or the fuel oil.

Carburization

Carburization is an environmentally-related problem in high temperature petrochemical andrefinery equipment. Carbon in the process stream diffuses at high temperatures into the metaland results in carbide formation. Iron carbide (Fe3C) forms in carbon and low alloy steels. Theresult is low temperature embrittlement and enhanced creep fissuring in regions such aswelds. (Embrittlement and creep will be discussed in COE 106.03.) Chromium carbides formin stainless steels and result in embrittlement and increased oxidation. Oxidation attack is alsoaccelerated due to the depletion of chromium from the matrix by the preferential formation ofchromium carbides. The greatest attack occurs when metal surfaces are simultaneously orintermittently exposed to carburizing and oxidizing environments. The resulting metal lossvisually resembles oxidation failures. Carburization-related corrosion attack of stainless steelheating elements and heater tubes that is caused by simultaneous carburization and oxidationis referred to as “green rot.”

Internal carburization can be determined metallographically. As a general rule, tubes that arecarburized through one-half the wall thickness are prone to failure from thermal shock. Thecarburized regions of the tube are magnetic in some cast high alloy austenitic steels.

Metal Dusting

Metal dusting is an accelerated form of carburization that occurs in austenitic alloys attemperatures that range from 482 to 816 °C (900 to 1500 °F) in strongly reducingatmospheres, such as those that contain large amounts of hydrocarbon gasses. Alternating(oxidizing/reducing) atmospheres promote metal dusting.

The specific mechanism by which metal dusting occurs is not well understood. Under normalconditions, the attack is highly localized and occurs in apparently random areas. Metaldusting often appears as random pitting on the ID surfaces of heater tubes. Rates of attack canexceed1 mil/hr (0.6 mm/day).


Corrosion Failures


Metal dusting is an intergranular form of attack. Carburization at grain boundaries results ingrain fallout. The term “metal dusting” originated from the fine granular corrosion productthat falls out of the attacked regions. The attack often appears as random pitting. Loose,magnetic coke is often associated with the attacked regions. Metallographic cross sectionstypically show a thin layer of intergranular carbides near the surface and intergranularcarbides that penetrate deeper into the metal.

Decarburization

Decarburization can occur when carbon diffuses out of steel on the steam side of boiler tubes.Decarburized regions have lower mechanical properties when compared to normal steel.Fortunately, steam-induced decarburization only occurs in thin surface layers and does notgenerally result in failures. Internal decarburization is a consequence of hydrogen attack,which will be covered in the next section.

Inspection for Carburization

Do the following when inspecting possible carburization failures:

• Examine apparent brittle failures for possible carburization, since carburizedtubes often fail due to thermal shock. Metallographic examination is required toconfirm carburization.

• Check for cyclic oxidizing/reducing operating conditions.

• Examine for metal dusting features in austenitic materials in stagnant,carbonaceous gas atmospheres.

• Determine if operating temperatures are within ranges for carburization attack.


Corrosion Failures


Hydrogen Attack

Hydrogen attack is the form of hydrogen-induced damage that occurs at temperatures above221 °C (430 °F). As previously discussed in the subsection on hydrogen embrittlement,dissociated hydrogen atoms can diffuse through the metal lattice at high temperatures. Whendiffusing atomic hydrogen reaches defects (voids, inclusions, and oxides), it can react withoxygen, in the case of copper alloys, or with carbon, in the case of steels, to form water ormethane, respectively:

Copper: 2H2 + O2 2H2O (6)

Steel: 2H2 + C CH4 (7)

or

2H2 + Fe3C CH4 + 3Fe (8)

Both water and methane formations create high pressures that cause internal fissures orcracks. The internal fissures result in degraded mechanical properties. This phenomenon iscalled hydrogen sickness in the case of copper alloys, and, more commonly, hydrogen attack,in the case of steels. Since the water or methane molecules are too large to diffuse out of themetal, the resulting damage is permanent.

An illustration of the reaction sites for methane formation in steel is shown in Figure 33. Thefissures and cracks decrease mechanical properties of the steel. In addition, the carbon that isinvolved in methane formation is removed from the steel and this removal of carbon results ina decarburized metal matrix with very poor mechanical properties.

Figure 33. Sites for Methane Formation (Hydrogen Attack) in Steel


Corrosion Failures


Hydrogen attack is dependent on temperature, the amount of hydrogen that is present, and thecarbon content of the steel. Alloying steel is the best means of preventing hydrogen attack.Molybdenum is the principal alloying element since molybdenum carbide is more stable thaniron carbide and, therefore, more resistant to methane formation. Temperature limits forvarious alloys are plotted versus hydrogen partial pressure on Nelson Curves. These curvesare based on data that was tabulated by G. A. Nelson in 1949, and they are illustrated in theAddendum at the end of this module.

Inspection for Hydrogen Attack

Do the following when inspecting possible hydrogen attack failures:

• Examine for cracking that suggests mechanical property loss (e.g., overload).Extensive surface corrosion is not expected in hydrogen environments.

• Determine the temperature and the hydrogen partial pressure for the failedmaterial.

• Use nondestructive ultrasonic attenuation or radiography to test the failed areafor indications of hydrogen attack. Hydrogen attack often requires opticalmetallographic (destructive) examination for positive confirmation. Detectionof both fissures and associated decarburization will conclusively indicatehydrogen attack.


Corrosion Failures


GLOSSARY

aerobic bacteria Bacteria that oxidize sulfur species to sulfuric acid to causecorrosion.

anaerobic bacteria Bacteria that reduce sulfate to sulfide, do not require air oroxygen in order to exist, and cause corrosion.

atmospheric corrosion Corrosion of unprotected materials that are exposed toatmospheric conditions, primarily moist air.

atomic hydrogen Uncharged atoms of hydrogen that can diffuse throughmetals. Sometimes called nascent (newly formed) hydrogen.

carburization Elevated temperature diffusion of carbon into metals, whichusually results in the formation of carbides. Metal loss andembrittlement are typical results of carburization.

caustic embrittlement An obsolete term that is used to describe the caustic stresscorrosion cracking of carbon steel.

corrodant Chemical (environmental) species that cause corrosion of amaterial.

corrosion that involvesmultiple variables

Deterioration that results from the interaction of corrosionwith one or more factors, such as stress, velocity effects,temperature, or bacteria.

corrosion allowance Extra thickness that is added to equipment to compensate forexpected corrosion over design life.

corrosion attack (or simply “Attack”)

A term that is often used to describe severe or acutecorrosion.

corrosion fatigue Accelerated fatigue that results from the combined action of acyclical stress and a corrosive environment.

corrosion product Material that is produced by a corrosion reaction. Corrosionproducts are often found on or in the vicinity of the corrodedsurface.

corrosion under insulation Corrosion of equipment surfaces that are covered byinsulation. Common corrosion problems include localizedcorrosion of carbon steels and stress corrosion cracking ofaustenitic materials.


Corrosion Failures


cyanide-induced corrosion A complex corrosion process wherein cyanides attackprotective sulfide scales on metals, and thereby accleratesulfidation.

dealuminization Selective leaching of aluminum in aluminum brasses orbronzes.

decarburization Diffusion of carbon from steel, that results in a carbon-free(ferrite) matrix with low mechanical properties.

denickelification Selective leaching of nickel in copper-nickel alloys.

deposits Foreign materials, which may be either corrosion productsfrom elsewhere in the system or contaminants, that settle orattach in the area of interest.

dezincification Selective leaching of zinc in copper-zinc (brass) alloys.

environmental cracking Cracking processes that are caused by the synergistic effectsof stress and environment on a specific material.Environmental cracking includes stress corrosion cracking,hydrogen-induced damage, and liquid metal embrittlement.

erosion-corrosion Accelerated erosion that is caused by corrosive fluid.

erosion Surface wear that is caused by the impingement of particlesor fluids.

exfoliation Preferential corrosion that is parallel to the metal surface.Differential expansion of corrosion products at grainboundaries causes spalling of layers of metal.

fatigue Metal failure that results from cyclical stresses below theyield strength.

galvanic corrosion(two-metal corrosion)

Accelerated corrosion of a less noble metal in contact with amore noble metal.

graphitic corrosion Selective leaching of iron in gray cast irons.

green rot Carburization-related corrosion attack of stainless steelheating elements that is caused by simultaneous carburizationand oxidation.

hydrogen-assisted cracking Stepwise cracking of carbon and alloy steels that are exposed


Corrosion Failures


(HAC) to wet hydrogen sulfide environments.

hydrogen attack High temperature damage that is caused by atomic hydrogendiffusion into the metal lattice and the resulting formation ofmethane gas at defects. This damage can result in internalfissuring.

hydrogen blistering Low temperature blistering or bulging of steel that is causedby high pressure molecular hydrogen. The high pressuremolecular hydrogen is a result of the recombination of atomichydrogen and internal defects.

hydrogen embrittlement The loss of toughness or ductility in a metal caused byabsorption of atomic hydrogen.

hydrogen-induced cracking(HIC) (hydrogen blistercracking [HBC], stepwisecracking [SWC])

Stepwise cracking of steels that are exposed to wet hydrogensulfide.

hydrogen-induced damage Degradation of a metal or alloy (usually steel) by exposure tohydrogen-containing environments.

hydrogen sickness Hydrogen attack in copper alloys whereby water formationcauses fissuring.

interconnected pits Surface condition where multiple pits grow together, andresult in a corrosion attack that resembles general corrosion.

intergranular attack (IGA)(intergranular corrosion[IGC])

Preferential corrosion at, or adjacent to, the grain boundariesof a metal.

internal oxidation Preferential oxidation below the surface of a metal.

knife-line attack A narrow band of intergranular corrosion that is adjacent to aweld in stabilized stainless steel. High heat from weldingdissolves carbides and subsequently results in sensitization ina thin band during cooling.

liquid metal embrittlement(liquid metal cracking)

Spontaneous or delayed cracking of a stressed metal that isexposed to specific embrittling liquid metals.

localized corrosion Corrosion that occurs preferentially in specific areas of thematerial.


Corrosion Failures


metal dusting Accelerated intergranular carburization of austeniticmaterials.

microbiologically-influencedcorrosion (MIC) (biologicalcorrosion, microbiologicalcorrosion)

Forms of corrosion that involve multiple variables and thatare associated with the presence of microbes.

molecular hydrogen Molecules of hydrogen (H2) that are too large to diffusethrough metals.

morphological features Usually microscopic features that are related to the structureand form of a material.

Nelson Curves Curves that show the operating limits for various steels toavoid decarburization and fissuring in hydrogen service.

noble metal A metal whose potential is positive relative to the hydrogenelectrode. In the galvanic series, noble metals are those metalsthat are closer to the cathodic, or protected, end.

oxidation A corrosion process that generally occurs at elevatedtemperatures and results in an oxide corrosion product.

patina Corrosion product that is found on materials after long-termatmospheric exposure. Most notably, the green film that isexhibited on copper and bronze statues.

pitting corrosion Localized corrosion that is characterized by sharply definedsurface cavities or holes through a metal.

scale High temperature corrosion products, such as oxides, that arefound on boiler tubes.

selective leaching(dealloying; parting)

Preferential corrosion of one element in an alloy.

sensitization Precipitation of chromium carbides in the grain boundaries ofaustenitic materials. The adjacent regions are depleted ofchromium, and they corrode intergranularly in an otherwisenoncorrosive environment.

solid metal embrittlement Cracking of a stressed metal that is exposed to specificembrittlers at temperatures below the melting point of theembrittler.


Corrosion Failures


spalling Breaking or flaking away of surface layers of oxides, sulfides,or other surface coatings. Differential stresses that are createdduring cooling often cause the surface layers to “pop” off.

stray current corrosion Corrosion that is caused by stray currents from nearbyelectrical equipment.

stress-corrosion cracking A form of environmental cracking that is caused by thesynergistic effects of a corrosive environment and sustainedstress on a specific material.

stress-oriented hydrogen-induced cracking (SOHIC)

Cracking of steels that is perpendicular to an applied stressand is caused by wet hydrogen sulfide.

sulfidation The high temperature reaction of a metal with sulfur speciesto form metallic sulfides.

sulfide-stress cracking(hydrogen-stress cracking,hydrogen-embrittlementcracking)

Special case of hydrogen embrittlement where high strengthor high hardness steels crack under stress in sulfide solutions.

tubercles Corrosion products that appear as mounds over regions oflocalized attack.

under-deposit corrosion Accelerated corrosion which occurs under deposits. Thedeposits provide crevices for crevice corrosion.

uniform corrosion (generalcorrosion)

Corrosion that occurs uniformly over all exposed surfaces ofthe material.

vanadium attack(fuel ash,hot ash, oil ash corrosion)

High temperature corrosion that occurs as a result of lowmelting phases and is formed from vanadium in fuel oils.

wastage Localized corrosion that usually extends deeply beneath thesurface of the sample over a specific area. Wastage issometimes used to describe uniform corrosion orinterconnected pits.

weld decay Intergranular corrosion in the heat affected (sensitized) regionof welded stainless steel.


Corrosion Failures


ADDENDUM A: COMMON ENGINEERING MATERIALS

Fe Alloys

• Carbon Steel (10XX, A516)

• Alloy Steels (13XX-98XX, Cr-Mo Steels)

• Stainless Steels (12 Cr, 400-Series)

(18 Cr-8 Ni, 300-Series)

(Duplex Alloys; PH Grades)

• Cast Irons (2.0 - 6.7 wt.% Carbon)

Cu Alloys

• Brasses (Cu-Zn)

• Bronzes (Cu-Sn, Cu-Si, Cu-Al)

• Cupronickels (90 Cu - 10 Ni, 70 Cu - 30 Ni)

Ni Alloys

• Inconels (Alloys 600, 625, X-750)

• Monels (Alloys 400, K-500)

• Hastelloys (Alloys B-2, C-22, C-276, G-3, G-30, X)

Al Alloys

• > 99 wt.% Aluminum (1XXX)

• Alloys (2XXX - 9XXX)

Ti Alloys

• Unalloyed (Grades 1, 2, 3, 4, 7)

• Low-Alloyed (Grades 7, 11, 12)

• Alloys (Alpha, Beta, Alpha-Beta)


Corrosion Failures


ADDENDUM B: EXCERPT FROM NACE CORROSION DATA SURVEY

Reference No. 6


Corrosion Failures


Key to Data Tables

° F 500 400 300 200 100

0

° C 260 204 149

93 38

-18

Throughout the data tables in this book, data points representing average penetration per year (key below) are plotted on the matrix enlarged here. The horizontal grid represents temperature. The vertical grid represents concentration.

20 40 60 80 100

% concentration in H2O

Matrix Key

Key to Footnotes

Key to Data Points

1. Pitting 2. Stress Corrosion Cracking 3. Crevice Attack

Average Penetration Rate Per Year

Code Mils Inches µm

<2 0.002 50

<20 0.020 508

20-50 0.020-0.050 508-1270

>50 0.050 1270X


Corrosion Failures



Corrosion Failures


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Corrosion Failures


REFERENCES

1. Fontana, M. G. Corrosion Engineering, Third Edition, McGraw-Hill BookCompany, New York, NY, USA, 1986.

2. Dillon, C. P. Corrosion Control in the Chemical Process Industries, McGraw-Hill Book Company, New York, NY, USA, 1986.

3. American Society for Metals (ASM), “Corrosion,” Metals Handbook, Vol. 13,Ninth Edition, Metals Park, Ohio, USA, 1987.

4. E. D. D. During, comp. Corrosion Atlas - A Collection of Illustrated CaseHistories, Vols. 1 and 2, Second Edition, Elsevier Science Publishers B. V.,Amsterdam, The Netherlands, 1988.

5. Wyatt, L. M., Bagley, D. S., Moore, M. A., and Baxter, D. C., An Atlas ofCorrosion and Related Failures, MTI Publication No. 18, Materials TechnologyInstitute of the Chemical Process Industries, Incorporated, St. Louis, Missouri,USA, 1987.

6. National Association of Corrosion Engineers (NACE), Corrosion Data Survey,Metals Section, Sixth Edition, Houston, Texas, USA, 1985.

7. National Association of Corrosion Engineers (NACE), NACE Basic CorrosionCourse, Fourth Printing, Houston, Texas, USA, 1973.

8. H. H. Uhlig, ed., The Corrosion Handbook, John Wiley and Sons, Inc., NewYork, NY, USA, 1948.

9. National Association of Corrosion Engineers (NACE), Process IndustriesCorrosion, Houston, Texas, USA, 1975.

10. American Society for Testing and Materials (ASTM), “Metals Test Methodsand Analytical Procedures,” 1986 Annual Book of ASTM Standards, Sect. 3,Vol. 03.02, Wear and Erosion; Metal Corrosion, Philadelphia, Pennsylvania,USA, 1991.

11. O. Bauer and O. Vogel, “Mitt. Mat. Pruf. Amt.,” Berlin, Germany, 1918.

12. D. R. McIntyre, “Experience Survey Stress Corrosion Cracking of AusteniticStainless Steels in Water,” MTI Publication No. 27, Materials TechnologyInstitute of the Chemical Process Industries, Inc., Columbus, OH, USA, 1987.

13. J. A. Sedriks, Corrosion of Stainless Steels, John Wiley & Sons, New York,NY, USA, 1979.


Corrosion Failures


14. D. R. McIntyre and C. P. Dillon, “Guidelines for Preventing Stress CorrosionCracking in the Chemical Process Industries,” MTI Publication No. 15,Materials Technology Institute of the Chemical Process Industries, Inc.,Columbus, OH, USA, 1985.

15. C. P. Dillon, “Guidelines for Control of Stress Corrosion Cracking of Nickel-Bearing Stainless Steels and Nickel-Base Alloys,” MTI Manual No. 1,Materials Technology Institute of the Chemical Process Industries, Inc.,Columbus, OH, USA, 1979.

16. D. Warren, Materials Performance, Vol. 26, No. 1, p. 38, 1987.

17. R. D. Merrick, Materials Performance, Vol 28, No. 2, p. 53, 1989.

18. W. Rostoker, J. M. McCaughey, and H. Marcus, Embrittlement by LiquidMetals, Reinhold Publishing Corp., New York, NY, USA, 1960.

19. P. M. Brister and G. N. Emmanuel, Steels for Elevated Temperature Service,United States Steel Corporation, Pittsburgh, PA, USA, 1965.

20. D. N. French, Metallurgical Failures in Fossil Fired Boilers, John Wiley &Sons, New York, NY, USA, 1983.

21. National Association of Corrosion Engineers (NACE), NACE StandardMR0175-93, Item No. 53024, "Standard Material Requirements - Sulfide StressCracking Resistant Metallic Materials for Oilfield Equipment," Houston, Texas,USA, 1993.

22. American Petroleum Institute (API), “Steels for Hydrogen Service at ElevatedTemperatures and Pressures in Petroleum Refineries and PetrochemicalPlants”, API Publication No. 941, Washington, DC, USA, 1983.

23. H. M. Herro and R. D. Port, The Nalco Guide to Cooling Water System FailureAnalysis, McGraw-Hill, Inc., New York, NY, USA, 1993.

Corroison Failures ARAMCO COE10602

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