4. Caustic Corrosion

4.1. General Description

Caustic (sodium hydroxide) is intentionally added at controlledconcentrations to boiler water in many treatment programs to maintainproper pH. This ensures the formation and retention of passive, protectivemagnetite layers on boiler tube surfaces. Alkaline-producing salts may alsobe introduced to the boiler water unintentionally. If caustic or alkaline saltconcentration becomes excessive, corrosion may result.

The susceptibility of steel to attack by sodium hydroxide is based on theamphoteric nature of iron oxides. That is, oxides of iron are corroded by bothlow-pH and high-pH environments (Fig. 4.1).

Caustic Corrosion

High-pH substances, such as sodium hydroxide, which is the most commonalkaline salt in boiler water solution, may dissolve magnetite [Eq. (4.1)]:

4NaOH + Fe O → 2NaFeO + Na FeO + 2H O

(4.1)

When magnetite is removed, the sodium hydroxide may react directly withthe iron, as in Eq. (4.2):

Fe + 2NaOH → Na FeO + H ↑

(4.2)

Caustic corrosion may also be referred to as caustic gouging and ductilegouging. All terms refer to the corrosive interaction of sufficientlyconcentrated sodium hydroxide, with boiler tube steel, to produce distincthemispherical or elliptical craters or gouges. The term gouging when appliedto caustic corrosion may be considered to be somewhat misleading, since it

Figure 4.1. Attack on steel at 310°C (590°F) by water of varyingdegrees of acidity and alkalinity. (Curve by Partridge and Hall,based on data of Berl and van Taack. Courtesy of Herbert H. Uhlig,The Corrosion Handbook, John Wiley & Sons, New York, 1948.)

3 4 2 2 2 2

2 2 2

tends to imply a mechanical form of damage. Caustic corrosion is rarelyassociated with mechanical damage. The craters may be filled with densecorrosion products that sometimes contain sparkling crystals of the ironoxide magnetite (Fe O ). Frequently, a crust of hard deposits and corrosionproducts containing magnetite crystals will surround and/or overlie theattacked region. The affected metal surface within the crater generally has asmooth, rolling contour.

4.2. Locations

Generally, caustic corrosion is most common in boilers that operate above1000 psi (6.9 MPa), or at lower pressures where high-purity makeup water isused. Higher operating pressures provide both higher saturated steamtemperatures and higher tube metal temperatures, which promoteconcentrating mechanisms for caustic. In addition, corrosion rates by causticincrease as a function of increasing metal temperatures. Lower-pressureboilers that require high-purity boiler feedwater typically are of achallenging design that includes zones of high heat flux, poor circulation, orboth. Challenging designs include externally finned tubes, which are used inheat recovery steam generators (HRSGs). Certain types of waste heat boilersprovide challenging designs, particularly those that employ shell-and-tubedesign, where steam generation occurs on the shell side of the tubes.Crevices at tube-to-tubesheet joints in such boilers are also susceptible tocorrosion. When high-purity water is used, metal oxides are the primarysource of deposits. Such deposits are conducive to caustic corrosion. Causticcorrosion rarely occurs in lower-pressure boilers that use low-quality makeupwater. Here deposits tend to consist of hardness scales. Caustic corrosiondamage is confined to localized areas on water-cooled tubes in

1. Regions of high heat flux

2. Slanted or horizontal tubes

3. Locations beneath heavy deposits

4. Heat-transfer regions at or downstream from features that disrupt flowsuch as intruding welds, weld backing rings, or other devices

In some boiler designs, devices are installed within portions of tubes with theintention of directing or channeling flow. In some cases, such devices may fail

3 4

to perform the intended function adequately and/or may even promotecaustic corrosion. Superheaters may experience caustic corrosion if there issubstantial boiler water carryover.

4.3. Critical Factors

Two critical factors are required for caustic corrosion. The first is theavailability of sodium hydroxide or of suitable alkaline-producing salts (i.e.,salts whose solution in water may produce a base and are capable ofreacting with protective iron oxide layers and boiler tube steels). As statedpreviously, sodium hydroxide may be intentionally added to boiler water atnoncorrosive levels. Sodium hydroxide is produced in boiler water solution bycomponents of phosphate-based pH control programs, which are commonlyused in high-pressure boilers. Detailed descriptions of typical phosphatechemistries are presented in The Nalco Water Handbook , third edition, page33.23. Alkaline-producing salts may also be introduced unintentionally ifchemicals from a caustically regenerated demineralizer, or condensatepolisher, are inadvertently released into makeup water. Alkaline-producingsalts may also contaminate condensate that is returned to the boiler.Contamination may result from in-leakage of cooling water through failedsurface condensers, or from process streams into the steam and condensatesystem. Poorly controlled or malfunctioning chemical feed equipment mayalso contribute excessive concentrations of alkaline salts. Boilers that usepotassium hydroxide or phosphates that produce potassium hydroxide willexperience higher corrosion rates than at equivalent concentrations ofsodium hydroxide. As a consequence, the use of potassium salts is notrecommended for boilers that operate at pressures over 580 psi (4 MPa) or inboilers that include superheaters and steam turbines.

The second contributing factor is the mechanism of concentration. Becausesodium hydroxide and alkaline-producing salts are not present at corrosivelevels in the bulk boiler water under proper control guidelines, a means ofconcentrating them must be present. Three basic concentration mechanismsexist:

1. Departure from nucleate boiling (DNB): The term nucleate boiling refersto a condition in which discrete bubbles of steam nucleate at points on ametal surface. Normally, as these steam bubbles form, minute amounts of

boiler water solids will deposit at the metal surface, usually at theinterface of the bubble and the water. As the bubble separates from themetal surface, the water will redissolve or rinse soluble solids such assodium hydroxide (Fig. 1.1).At the onset of DNB, the rate of bubble formation exceeds the rinsing rateof the soluble solids. Under these conditions, sodium hydroxide, as well asother less soluble dissolved solids or suspended solids, will begin todeposit (Figs. 1.3 and 4.2). The presence of sufficiently concentratedsodium hydroxide and other concentrated corrosives will both compromisethe thin film of protective magnetite and cause metal loss.

Under the conditions of fully developed DNB, a stable layer of steam willform. Corrosives concentrate at the edges of the steam layer to causecorrosion.

2. Deposition: Another concentrating mechanism may occur when depositsshield the metal surface from the bulk water. Steam that forms underdeposits escapes, leaving behind and entrapping a corrosive residue that

Click to loadinteractive graph

Figure 4.2. Sodium hydroxide content attainable in concentratingfilm of boiler water. [Based on data from International CriticalTables, 3:370(1928). Courtesy of Herbert H. Uhlig, The CorrosionHandbook, John Wiley & Sons, New York, 1948.]

can deeply corrode the metal surface (Fig. 4.3). Insulation provided bydeposit layers may promote or cause the production of steam. Thisphenomenon is commonly referred to as wick boiling (Fig. 4.4). Whencaustic corrosion occurs in this manner, it is sometimes referred to asunder-deposit corrosion . However, caustic corrosion is only one type ofunder-deposit corrosion. Other types of under-deposit corrosion will bedescribed in Chap. 5, Low-pH Corrosion during Service, and Chap. 7,Phosphate Corrosion.

Figure 4.3. Deep caustic corrosion beneath insulating internaldeposits. (Courtesy of National Association of CorrosionEngineers.)

3. Evaporation along a steam/water interface: Horizontal or slanted tubesare more susceptible to steam/water stratification since the critical heatflux for DNB is significantly lower than in vertically oriented tubes. Underunfavorable conditions of heat flux relative to boiler water flow rate,steam/water stratification may occur. In addition, boiler operation atexcessively low water levels, or excessive blowdown rates, may createwaterlines. Waterlines may also be created by excessive load reductionwhen pressure remains constant. In this situation, water velocity in theboiler tubes is reduced to a fraction of its full-load value. If velocitybecomes low enough in vertical, horizontal, or slanted tubing, steam/waterstratification occurs, creating stable or metastable waterlines. Damaged ormissing refractory layers on floor tubes may allow higher fireside heatinput to cause steam/water stratification. Blowdown lines may also besusceptible to stratification under certain operating conditions. Atlocations of steam/water stratification, corrosives may concentrate byevaporation, resulting in corrosion.Intruding welds, or welds employing backing rings, can create localizedpressure drops immediately downstream. The pressure drop may promote

Figure 4.4. Wick boiling. (Courtesy of McGraw-Hill, The NalcoWater Handbook, 3d ed., 2009.)

localized DNB, which may cause deposition and under-deposit corrosion.

4.4. Identification

If affected waterside surfaces are accessible, visual examinations that revealcraters filled with hard corrosion products are potential sites of causticcorrosion damage. The extent of corrosion damage may be determined byremoving the corrosion products.

In horizontal or slanted tubes, a pair of parallel or converging, longitudinaltrenches may form (Fig. 4.5). If the tube is nearly full, the parallel trencheswill coalesce into a single elongated, longitudinally oriented crater, along thecrown of the tube (Fig. 4.6). In vertically oriented tubes, corrosiveconcentration at a steam/water interface will yield a circumferentiallyoriented crater (Fig. 4.7).

Figure 4.5. Caustic corrosion along a longitudinal waterline.(Courtesy of National Association of Corrosion Engineers.)

Figure 4.6. Caustic corrosion resulting from evaporation at awaterline riding along the crown of the tube. (Courtesy of NationalAssociation of Corrosion Engineers.)

Figure 4.7. Metal loss on a vertically oriented tube that penetrates

If the waterside surface is not accessible, nondestructive testing techniquessuch as ultrasonic testing may be required. Steam studies using a hydrogenanalyzer may also function as an alert for the possibility of caustic corrosion.

Once possible caustic corrosion damage is identified, it is necessary formetallographic analysis to be conducted to confirm that caustic or alkalinesalt corrosion occurred, since acid and phosphate corrosion during servicemay cause similar damage (see Chaps. 5 and 7). Supporting evidence forcaustic corrosion may include the presence of alkaline material in depositsthat cover the corroded surfaces. X-ray fluorescence and diffraction may beused to identify compounds that may cause corrosion, particularly sodiumcompounds. However, since such compounds are highly water soluble, theymay wash away subsequent to the corrosion events. Magnetite needles mayalso be present in the corrosion products that form on the corroded surface,as shown in Fig. 4.8. Though the presence of magnetite needles providesstrong supporting evidence for caustic and alkaline salt corrosion, suchevidence should be used with caution since the needles may be formed byother environments as well. Metallic copper particles may also deposit in thecrater. This is due to the evolution of hydrogen in the caustic corrosionreaction, which reduces dissolved copper in the water. Metallic coppertypically will not promote galvanic corrosion of the boiler steel, since depositand corrosion product layers provide an electrically insulating barrier layer.The chemical treatment program and treatment historical data should bereviewed to determine if conditions may have supported caustic or alkalinesalt corrosion.

around the entire circumference.

4.5. Elimination

Corrosion may occur when the boiler water contains dissolved sodiumhydroxide or alkaline-producing salts and a concentrating mechanism existssimultaneously at a particular location. The following remedies may be usedto significantly control or eliminate corrosion by concentrated sodiumhydroxide or alkaline-producing salts:

Reduce the amount of available free sodium hydroxide: This is theunderlying concept for various phosphate and caustic treatment programsimplemented in high-pressure boilers (see The Nalco Water Handbook ,third edition, pages 33.22 to 33.28). If a phosphate treatment program isused, it should be appropriate for the specific conditions of boileroperation. For instance, the congruent phosphate program was designedto prevent caustic corrosion. Below the congruent point, free sodiumhydroxide is not produced, thus preventing caustic corrosion. However, thetreatment program must consider the possible effects of not only caustic

Figure 4.8. Metallographic cross section of a surface thatexperienced caustic corrosion. The surface is covered with corrosionproducts that contain magnetite needles and metallic copperparticles.

corrosion, but also other types of corrosion. It has been found that undercertain conditions, the use of a congruent phosphate program maypromote phosphate corrosion (see Chap. 7). The program must bedesigned to control not only caustic corrosion, but also phosphate and acidcorrosion (see Chap. 5).

Prevent in-leakage of alkaline-producing salts by cooling water andprocess streams: Contamination of steam and condensate by coolingwater leaks at condensers, or by process streams that return to the boilermust be eliminated.For most industrial and utility boilers, the chemical treatment programsemploy caustic or alkaline salts for pH and alkalinity control. Contaminantsmay contribute sodium hydroxide and alkaline salts that are outside of thetreatment program control parameters. This may require significantadjustments to the program to control corrosion. Because of the powerfulconcentrating mechanisms that may operate in a boiler, the addition ofonly a few parts per million of contaminants to the boiler water may besufficient to initiate or accelerate corrosion. In treatment programs that donot use caustic or alkaline salts, such as all volatile treatment (AVT), oroxygenated treatment (OT), contamination by alkaline salts cannot betolerated. They may cause corrosion, even when present at smallconcentrations.

Prevent inadvertent release of caustic regeneration chemicals frommakeup water demineralizers and condensate polishers.

Prevention of localized concentration is the most effective means of avoidingcaustic corrosion. However, in practice it is also the most difficult to achieve.The methods for preventing localized concentration include:

Prevent DNB: This usually requires the elimination of hot spots, achievedby controlling the boiler's operating parameters. Hot spots are caused byexcessive overfiring, misdirected burners, change of fuel, and gaschanneling. Conditions that reduce flow below appropriate levels, such asreduced firing rates or excessive blowdown, should be avoided. In heatrecovery steam generators (HRSGs), excessive heat input due to the use ofduct burners may promote deposition and under-deposit corrosion.

Prevent excessive waterside deposition: As mentioned previously,deposition should be controlled by maintaining consistently good

feedwater quality. Tube sampling on a periodic basis may be performed tomeasure the relative amount of deposit buildup on tubes. This sampling isreferred to as a deposit weight determination , deposit loading , or gramloading . Deposit weight determination practice is described in Chap. 1,Water-Formed and Steam-Formed Deposits. Susceptibility to under-depositcorrosion increases as a function of increasing deposit weight and boilerpressure. The boiler manufacturer should be consulted for chemicalcleaning recommendations. Aside from boiler manufacturer's guidelines,associations such as the National Association of Corrosion Engineers(NACE) and Electric Power Research Institute (EPRI) provide cleaningguidelines. Proper monitoring will ensure that corrosion is controlled inthe preboiler system, in order to reduce the amount of corrosion productsthat are introduced to the boiler.

Prevent steam/water stratification in tubes: Proper boiler operatingparameters must be followed to ensure proper steam production rates andboiler water circulation. If the design and operating conditions cannot bealtered practically to prevent poor circulation, then consistent periodicinspection and cleaning practices may be required for certain tubes.

Ensure proper component design: Intruding welds and other componentswithin the fluid flow path on the water side should not promote DNB,entrap steam, or cause caustic and alkaline salts to concentrate.

4.6. Cautions

It is very difficult to distinguish localized attack by high-pH substances fromlocalized attack by low-pH corrosion during service simply by visualexamination. A formal metallographic examination is required. Evaluating thetypes of concentrateable corrosives that may be contaminating the boilerwater will aid in the determination.

Because corrosion products may fill the depressions caused by causticcorrosion, the extent and depth of metal loss in the affected area—and eventhe existence of a corrosion site—may be overlooked. Probing a suspect areawith a hard, pointed instrument may aid in the determination, but becausethe corrosion products are often very hard, a corrosion site may remainundetected.

4.7. Related Problems

See also Chap. 1, Water-Formed and Steam-Formed Deposits; Chap. 5, Low-pHCorrosion during Service; and Chap. 7, Phosphate Corrosion.

Visual examinations disclosed a thickened patch of hard corrosionproducts covering a crater adjacent to one bend (Fig. 4.9).Perforation of the wall had not occurred, but transverse crosssections cut through the site revealed substantial metal loss (Fig.4.10).

Case History 4.1

Industry: Utility boiler

Specimen location: Camera port, waterwall

Orientation of specimen: Vertical and slanted, S shaped

Years in service: 25

Water treatment program: Coordinated phosphate

Drum pressure: 2000 psi (13.8 MPa)

Tube specifications: 3-in. (7.6-cm) outer diameter

Fuel: Ground coal

Figure 4.9. Patch of hard iron oxides on internal surface.

The crater was caused by sodium hydroxide that concentrated tocorrosive levels due to wick boiling beneath thermally insulatingdeposits. Previous failures of this type had not occurred in thisregion of the boiler. The boiler had been cleaned 4 years previously,and was in peaking service. Closer control over the water treatmentprogram was recommended. If control under the coordinatedphosphate program could not be improved, alternate programs maybe used to minimize the corrosion.

Figure 4.10. Cratered region beneath patch of iron oxides.

Many transfer line exchangers have a shell-and-tube design, wherehot process gases are cooled by boiler water that flows across theshell sides. They may be either vertically or horizontally oriented.

In this case, the unit was vertically oriented, with heat flux beinghighest at the bottom tubesheet where the hot process gases wereintroduced. Deep metal loss penetrated around the entirecircumference of several tubes at the bottom tubesheet interface,beneath thick layers of deposits and corrosion products (Figs. 4.11and 4.12). Above about 7 in. (17.8 cm) from the tubesheet interface,no significant external surface deposition occurred. The depositlayers within the corroded area contained components that werealkaline in a distilled water solution. Metallographic analysisrevealed the presence of substantial amounts of magnetite needlesin the corrosion products that covered the wasted surface (Fig.4.13). The metal loss was caused by caustic corrosion, primarilyresulting from the concentration of alkaline salts beneath depositlayers. Suspended material in the water tends to settle along thebottom tubesheet. High heat flux also promotes deposition at thatlocation. The deposit layers gradually build in thickness. Thisreduces heat transfer and promotes wick boiling, resulting infurther deposition and corrosion. It was determined that periodiccleaning of deposits at the bottom tubesheet was not conducted ona timely basis.

Case History 4.2

Industry: Ethylene production

Specimen location: Transfer line exchanger (TLE)

Orientation of specimen: Vertical

Years in service: 25

Water treatment program: Coordinated phosphate

Drum pressure: 1750 psi (12.1 MPa)

Tube specifications: 1¼-in. (3.2-cm) outer diameter

Fuel: Ethylene process gas(tubeside)

Figure 4.11. Deep metal loss on the external (waterside)surface of a vertically oriented tube from a shell-and-tubewaste heat boiler at the bottom tubesheet interface.

Figure 4.12. Higher magnification view of wastage attubesheet interface.

Deposition and corrosion may also happen at top tubesheets inshell-and-tube design boilers, if design and operating conditionspromote steam stratification. In some cases, steam becomesentrapped along the top tubesheet. Proper design changes havebeen made to minimize or prevent steam entrapment in some units.If steam entrapment cannot be adequately controlled along the toptubesheet, periodic cleaning is recommended.

Figure 4.13. Metallographic cross section of corrosionproducts covering the corroded surface, which containmagnetite needles and metallic copper particles.

A growing number of small leaks were occurring in lower slag-screen tubes of this boiler. One of the leaking tubes was removedfor examination.

Figure 4.14 illustrates the appearance of the internal surface in thearea of leakage. A small perforation of this rifled tube was observedin the center of a large elliptical area, or crater, of metal loss (Fig.4.15). The crater was covered with a thick, irregular mound ofcoarsely stratified iron oxides. Removing the mound revealed thatthe underlying crater had a smooth, rolling metal-surface contour(Fig. 4.16). The rest of the internal surface had suffered no metalloss.

Case History 4.3

Industry: Utility

Specimen location: Bottom slag-screen tube

Orientation of specimen: Slanted, 15° slope

Water treatment program: Coordinated phosphate

Drum pressure: 2200 psi (15.2 MPa)

Tube specifications: 3-in. (7.6-cm) outer diameter

Figure 4.14. Thick, irregular mound of hard iron oxidescovering perforation.

Figure 4.15. Perforation at bottom of crater.

Figure 4.16. Corrosion products removed to reveal cratersurface that has a smooth, rolling contour.

Since deposits were not present and evidence of a waterline wasnot observed, it can be assumed that the concentration of thecaustic material was caused by highly localized nonnucleate boiling(DNB). The rifling of the internal surface is designed to induceswirling of the water to prevent nonnucleate boiling andsteam/water phase stratification. It is surprising, therefore, to findsevere caustic gouging in this tube design. However, this boiler wasidle on weekends. It is possible that highly localized nonnucleateboiling occurred during start-up, before normal boiler watercirculation was fully established.

Case History 4.4

Industry: Ore refining

Boiler type: Circulating fluidized-bed boiler

Specimen location: Bottom leg of U bend in bed

Orientation of specimen: Horizontal

Years in service: 0.75

Water treatment program: Polymer

Drum pressure: 1280 psi (8.8 MPa)

Tube specifications: 2½-in. (6.4-cm) outer diameter

Fuel: Coal

Failures recurred within the horizontally oriented bottom legs of Utubes at the 12 o'clock position. The bottom legs were located onthe upstream end. No leaks occurred within the U bends or top legsof the tubes. Narrow strips of sheet steel that were bent into ahelical shape were welded in place within the bottom legs of thetubes (Fig. 4.17). These strips reportedly were used to promote apreferred fluid flow pattern through the tubes to control or preventsteam/water stratification. The bottom legs of several different Utubes were longitudinally saw-cut to reveal their internal surfaces.This revealed repeating patterns of deposition and under-depositcorrosion at the 12 o'clock positions every 4 in. (10.2 cm),corresponding to locations where the sheet steel strips contactedthe tube internal surface in a helical pattern (Figs. 4.18 and 4.19).Metallographic, corrosion product, and deposit analyses revealedthat the metal loss was caused by caustic corrosion that resultedfrom steam blanketing. Steam blankets became entrapped at thetops of the tubes along the helical strips due to insufficient flow.This allowed alkaline salts, which were dissolved at low andnoncorrosive levels in the bulk water, to concentrate to corrosivelevels. Methods to increase boiler water flow, such as installation ofbooster pumps, were considered. Increased flow would moreeffectively control steam blanketing in the lower legs of the U bend,since circulation under normal operating conditions wasinsufficient.

Figure 4.17. Helically shaped strip of sheet steel that waswelded inside the horizontally oriented bottom leg of a Ubend.

Figure 4.18. Internal surface deposition and metal losswithin localized areas at 12 o'clock position where thehelical strip shown in F. 4.17 was installed.

Figure 4.19. Close-up of crater shown in F. 4.18.

Case History 4.5

Industry: Soap and detergentmanufacturing

Specimen location: Riser tube at tubesheetinterface

Orientation of specimen: Slanted

Water treatment program: Polymer

Drum pressure: 1000 psi (6.9 MPa)

Tube specifications: 3-in. (7.6-cm) outer diameter

Fuel: Natural gas

A substantial tube leak in a package boiler required anunscheduled shutdown for repair. On-site inspection of riser tubeswithin a steam drum revealed a single perforation in one of thetubes (Fig. 4.20). The perforation, which was located about 4½ in.(11.4 cm) from the tube end, was irregularly shaped andtransversely oriented. A section containing the tube end was torch-cut for examination. Close visual inspection revealed that nosignificant metal loss occurred on the internal waterside surface inareas surrounding the perforation (Fig. 4.21). However, theperforation occurred at the base of a deep groove on the externalsurface at the interface of the rolled portion of the tube and theexternal surface of the steam drum (Fig. 4.22). Shallow metal lossand deposition occurred on the external surface within the adjacentrolled portion. Metallographic examination revealed the presenceof corrosion product, deposit layers containing alkaline material,and agglomerated magnetite needles. The deposits and corrosionproducts reveal that the attack was not caused by erosion fromescaping steam. Maintenance work was conducted a few monthsprior to the failure. During that time, the tube was improperlyrolled into the tubesheet, allowing boiler water to seep through thejoint and into the firebox. The water evaporated upon escape. Thiscaused the dissolved caustic in the boiler water, which was atproper concentration, to concentrate to corrosive levels, eventuallyperforating the tube wall. Proper surface cleaning and tube rollingpractice must be followed to prevent leaks that can result in suchdamage.

Figure 4.20. Perforation in tube found during internalinspection of steam drum. (© NACE International, 2005.)

Figure 4.21. No metal loss on internal surface surroundingthe perforation. (© NACE International, 2005.)

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Figure 4.22. Perforation at base of deep groove on externalsurface at tube/tubesheet interface. (© NACE International,2005.)

Citation

Nalco Company: Nalco Guide to Boiler Failure Analysis, Second Edition. CausticCorrosion, Chapter (McGraw-Hill Professional, 2011), AccessEngineering

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