Graphitic Corrosion

7/23/2019 Graphitic Corrosion

1/12

14. Graphitic Corrosion

14.1. General Description

Graphitic corrosion is possible only in structures composed of cast irons

containing graphite particles. Gray, nodular, and malleable cast irons, which

contain graphite particles of different morphologies, are susceptible to

graphitic corrosion. Although frequently considered immune to graphitic

corrosion, nodular cast iron and malleable iron (which contain rounded forms

of graphite) are often attacked. Gray cast iron, which contains akes of

graphite, is more widely used and has more dramatic and recognizable

corrosion characteristics than other cast irons.

Although graphitic corrosion is often considered to be a form of dealloying or

selective leaching, it actually has more in common with galvanic corrosion.

Microscopic galvanic cells form between the metal matrix and graphite

particles embedded in the casting when the same mildly aggressive water

contacts both materials (Fig. 14.1). The graphite is cathodic to t he adjacentm etal, causing the anodic metal portion of the casting to corrode . Note the

large distance between graphite and iron in the galvanic series (Table 14.1).

Although the series is for a seawater environment, iron is also anodic relative

to graphite in a freshwater environment. Graphitic corrosion converts the

casting to mechanically weak corrosion products containing graphite

particles. Graphitic corrosion usually progresses s lowly, taking many months

or even years to produce signicant attack. Corrosion is accelerated if watersare mildly acidic, soft, of high conductivity, and/or contain high

concentrations of aggressive anions such as sulfate and chloride.

Graphitic Corrosion
http://accessengineeringlibrary.com/http://accessengineeringlibrary.com/http://accessengineeringlibrary.com/http://accessengineeringlibrary.com/browse/nalco-guide-to-boiler-failure-analysis-second-edition/c9780071743006ch14#ch14table01http://accessengineeringlibrary.com/browse/nalco-guide-to-boiler-failure-analysis-second-edition/c9780071743006ch14#ch14fig01http://accessengineeringlibrary.com/browse/nalco-guide-to-boiler-failure-analysis-second-edition/c9780071743006ch14#p13n-login-panelhttp://accessengineeringlibrary.com/


2/12

Table 14.1. A Galvanic Series, Showing Relative Reactivities of

Assorted Pure Metals and Commercial Alloys in a Seawater

Environment

Figure 14.1. Mechanistic correlation between a dry cell battery and

graphitic corrosion. (Courtesy of McGraw-Hill, The Nalco Guide to

Cooling Water Systems Failure Analysis, New York, 1993.)

Cathodic (inert) Platinum

Gold

Graphite

Graphite

Silver

316 Stainless steel (passive)

304 Stainless steel (passive)

Inconel (passive)

Nickel (passive)


3/12

14.2. Locations

Feedwater pump impellers, water supply lines, valves, and other components

made of graphitic cast irons may experience graphitic corrosion. Because

cast irons are used mainly in preboiler regions, attack occurs primarily in

water pretreatment and transport equipment. However, graphitic cast iron

components in other parts of the boiler water and steam circuit, such as

steam trap bodies, may also experience graphitic corrosion.

14.3. Critical Factors

Graphitic corrosion requires a susceptible graphite-containing cast iron

Monel

Copper-nickel alloys

Bronzes

Copper

Brasses

Inconel (active)

Nickel (active)

Tin

Lead

316 Stainless (active)

304 Stainless (active)

Cast iron

Iron and steel

Aluminum alloys

Cadmium

Zinc

Anodic (active) Magnesium and magnesium alloys


4/12

microstructure and a common electrolyte that contacts both the graphite and

iron phases.

Environmental factors that promote graphitic corrosion include soft waters,

stagnant conditions, slightly acidic pH, high conductivity, and the presence of

chlorides, sulfates, or low concentrations of hydrogen sulde. Graphitic

corrosion tends to occur in relatively mild environments. In more aggress iveenvironments, corrosion may be more general, rather than graphitic in

nature.

Attack often occurs predominantly during idle periods under stagnant

conditions. If turbulence is pronounced (e.g., in pumps), the mechanically

weak corrosion products may be dislodged, accelerating wastage.

The ake morphology of the graphite in gray cast iron provides a largercathode-to-anode surface area ratio compared to nodular graphite.

Consequently, gray cast iron is more susceptible to attack than nodular and

malleable cast irons.

14.4. Identication

Cast iron is converted to a soft mixture of iron oxide corrosion products andgraphite (Figs. 14.2 and 14.3). Pieces of corrosion product tend to smudge

hands and can be used to mark paper, just as if the corroded material were

lead in a pencil (Fig. 14.4). Attack is often uniform, with all exposed surfaces

corroded to roughly the same depth (Figs. 14.5 and 14.6). Fractures that

result from graphitic corrosion are thick-walled due to the brittleness of the

corrosion products and the alloy. If localized deposits are present, especially

those containing sulfate, chloride, or other acid-producing species, corrosionmay be conned to distinct regions beneath such deposits. When attack is

severe or prolonged, the entire component is converted to corrosion product.

Surface contour and appearance are often preserved, although the

graphitically corroded material may appear black, rather than the original

metallic gray. Attack is usually not apparent until surfaces are probed or

stressed. Craters can be dug in the soft, black corrosion products to reveal

the depth of penetration. Corroded areas can be broken with bare hands or

by gentle taps with a hard implement.
http://accessengineeringlibrary.com/browse/nalco-guide-to-boiler-failure-analysis-second-edition/c9780071743006ch14#ch14fig06http://accessengineeringlibrary.com/browse/nalco-guide-to-boiler-failure-analysis-second-edition/c9780071743006ch14#ch14fig05http://accessengineeringlibrary.com/browse/nalco-guide-to-boiler-failure-analysis-second-edition/c9780071743006ch14#ch14fig04http://accessengineeringlibrary.com/browse/nalco-guide-to-boiler-failure-analysis-second-edition/c9780071743006ch14#ch14fig03http://accessengineeringlibrary.com/browse/nalco-guide-to-boiler-failure-analysis-second-edition/c9780071743006ch14#ch14fig02


5/12

Figure 14.2. Pump impeller severely attacked by graphitic corrosion.

Material of the impeller is gray cast iron. Note the dark gray areas

on both internal and external surfaces where the metal has been

converted to iron oxidebased corrosion products and graphite.

Figure 14.3. Note how uniformly metal is converted to corrosion

product. Impeller vanes were lost in service because the

graphitically corroded metal is brittle and weak.


6/12

Figure 14.4. Graphitic corrosion products can be used to write on

paper, similar to lead in a pencil. (Courtesy of McGraw-Hill, The

Nalco Guide to Cooling Water Systems Failure Analysis, New York,

1993.)

Figure 14.5. Graphitically corroded valve buttery. Original surface

contours are preserved. Edges, which were completely converted to

brittle corrosion product, have broken.


7/12

Metallographic examination will reveal graphite particles from the

microstructure (akes or nodules, depending on the type of cast iron)

surrounded by corrosion products and embedded in corrosion product layers

(Figs. 14.7 and 14.8).

Figure 14.6. Severe graphitic corrosion of a gray cast iron feedwater

pump impeller. Patches of corrosion product have cracked and

spalled, revealing uniform general wastage. Note the fracture in the

impeller vane.
http://accessengineeringlibrary.com/browse/nalco-guide-to-boiler-failure-analysis-second-edition/c9780071743006ch14#ch14fig08http://accessengineeringlibrary.com/browse/nalco-guide-to-boiler-failure-analysis-second-edition/c9780071743006ch14#ch14fig07


8/12

14.5. Elimination

Attack is reduced b allo substitution, chemical treatment, and/or

Figure 14.7. Nodular cast iron that sustained graphitic corrosion,

showing graphite nodules embedded in the corrosion product layer.

Etchant: Picral.

Figure 14.8. Gray cast iron, showing graphite akes surrounded by

iron oxidebased corrosion products. Etchant: Picral.


9/12

operational changes. Alloy substitution can entirely eliminate graphitic

corrosion if the replacement alloy does not contain graphite. The substitute

alloy choice is dictated by requirements unique to each environment. Alloys

that may be suitable substitutes include austenitic cast iron; corrosion-

resistant cast irons containing chromium, nickel, or silicon; and cast steels

because they are essentially immune to graphitic corrosion. When changingalloys, care should be taken to avoid the formation of galvanic couples

between components in electrical contact with each other.

Raising water pH to neutral or slightly alkaline levels decreases attack,

especially if relatively high concentrations of aggressive anions such as

chloride and sulfate are present. When water ow is slight, such as during

prolonged shutdowns and lengthy idle periods, attack increases. Stagnant

conditions promote graphitic corrosion and should be avoided.

14.6. Cautions

Only cast irons containing graphite can corrode graphitically. Due to

microstructural differences in graphite particle size, morphology, and

distribution, as well as other differences in alloy composition, attack is

usually worst in gray cast irons.

Although pipes and other components may be severely corroded, they may

not fail. The corrosion product has some mechanical strength, but is brittle.

If corroded components are stressed, failure may occur catastrophically.

Graphitic corrosion should not be confused with graphitization, which is a

microstructural change that can occur in carbon-containing irons and steels

at temperatures from approximately 800 to 1025F (427 to 552C). Ingraphitization, iron carbide is converted to elemental iron and graphite. See

Chap. 3, Long-Term Overheating, for more in-depth information about

graphitization.

14.7. Related Topics

Chapter 3, Long-Term Overheating, and Chap. 15, Dealloying.
http://accessengineeringlibrary.com/browse/nalco-guide-to-boiler-failure-analysis-second-edition/c9780071743006ch15#c9780071743006ch15http://accessengineeringlibrary.com/browse/nalco-guide-to-boiler-failure-analysis-second-edition/c9780071743006ch03#c9780071743006ch03http://accessengineeringlibrary.com/browse/nalco-guide-to-boiler-failure-analysis-second-edition/c9780071743006ch03#c9780071743006ch03


10/12

A small feedwater pump impeller was removed during a scheduled

maintenance outage. The entire impeller surface was converted

into soft, black corrosion products. In some areas, no trace of the

original impeller alloy was left. Vane tips were totally converted to

corrosion product. Vanes were cracked, and pieces of the corrosion

product were dislodged, producing irregular contours (Fig. 14.6).

The impeller had been used intermittently, with greater than 50%

idle time in the previous 2 years.

Case History 14.1

Industry: Food processing

Specimen location: Feedwater pump

Specimen orientation: Horizontal

Years in service: 5

Water treatment program: Phosphate

Drum pressure: 600 psi (4.1 MPa)

Tube specications: 8-in. (20.3-cm) outer diameter,

ve vanes of gray cast iron

Fuel: Natural gas
http://accessengineeringlibrary.com/browse/nalco-guide-to-boiler-failure-analysis-second-edition/c9780071743006ch14#ch14fig06


11/12

Gray cast iron boiler feedwater pump components from various

different pumps were analyzed, including a large casing and two

suction head rings. Reportedly, the feedwater source was changed

from softened lake water to second-effect condensate from an

evaporator process approximately 3 years before the components

were removed from service. It was reported that pump service life

had decreased dramatically after the change in feedwater source.

Reportedly, the pump internals had changed in color from red to

gray. When the gray surfaces were wiped, a slippery, powdery gray

material rubbed off. Flow-oriented, smooth-surfaced grooves

interrupted the surfaces of the suction head anges of the large

casing. In microscopic examinations, the surfaces that visually

appeared to have experienced metal loss were bare or covered witha very thin layer of corrosion products. In comparison, the

apparently unattacked surfaces were covered with iron oxide

corrosion products containing graphite ake particles and skeletal

remnants of pearlite colonies.

The grooves were primarily caused by erosion; however, evidence

of graphitic corrosion away from the grooves suggested that

mechanically weak graphitic corrosion products somewhat

promoted erosive metal loss. The condensate was most likely lower

in pH than the lake water that was previously used as the

feedwater source. Graphitic corrosion is promoted by slightly acidic

conditions. Intermittent contamination of the condensate that was

used as feedwater probably also contributed to graphitic corrosion.

Case History 14.2

Industry: Food processing

Specimen location: Boiler feedwater pump

Years in service: 1.5 or less

Citation

Nalco Company: Nalco Guide to Boiler Failure Analysis, Second Edition. Graphitic

Corrosion, Chapter (McGraw-Hill Professional, 2011), AccessEngineering

EXPORT
http://accessengineeringlibrary.com/ris/nalco-guide-to-boiler-failure-analysis-second-edition/c9780071743006ch14http://accessengineeringlibrary.com/browse/nalco-guide-to-boiler-failure-analysis-second-edition/c9780071743006ch14


12/12

Copyright McGraw-Hill Global Education Holdings, LLC. All rights reserved.

Any use is subject to the Terms of Use. Privacy Notice and copyright information .

For further information about this site, contact us.

Designed and built using Scolaris by Semantico.

This product incorporates part of the open source Protg system. Protg is

available at http://protege.stanford.edu//
http://protege.stanford.edu/http://www.theiet.org/inspechttp://www.semantico.com/http://accessengineeringlibrary.com/contacthttp://accessengineeringlibrary.com/privacy-noticehttp://accessengineeringlibrary.com/terms-and-conditionshttp://accessengineeringlibrary.com/browse/nalco-guide-to-boiler-failure-analysis-second-edition/c9780071743006ch14#

Graphitic Corrosion

Documents

Transcript of Graphitic Corrosion