15. Dealloying

15.1. General Description

An alloy has been dened as "a substance having metallic properties andbeing composed of two or more chemical elements of which at least one is anelemental metal." Dealloying is a corrosion process in which one or morealloy components are removed preferentially. The corroded region usuallyhas a markedly dierent structure than the original alloy. However,macroscopic dimensions of the corroded part often remain unchanged. Theprocess is also referred to as selective leaching or parting. Obviously,dealloying can occur only in alloys containing two or more elements.Particularly susceptible alloys that may be used in the boiler water circuit arebrasses (in which zinc and aluminum are leached from copper-zinc andcopper-aluminum alloys, respectively) and cupronickels (in which nickel isremoved). The name given to a particular dealloying process derives from theleached element. For example, in common brasses where zinc is removed,dealloying is referred to as dezincication ; where nickel is removed fromcupronickels, dealloying is referred to as denickelication.

A number of mechanisms have been proposed for dealloying of binary alloys,one of which involves dissolving both the constituents and redepositing ofthe more noble element. Other proposed mechanisms involve dissolving onlythe less noble element. The details of the possible mechanisms are beyondthe scope of this book and are reviewed elsewhere. Dierent conditions(i.e., electrochemical potential, pH) likely result in dierent predominantdealloying mechanisms.

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Dealloying

In the case of dezincication, brass alloys containing more thanapproximately 15 wt% zinc are susceptible to attack. Brasses containing upto 32 wt% zinc only contain alpha phase. Alloys containing more thanapproximately 32 wt% zinc are particularly vulnerable to dezincication dueto the presence of the beta phase, which is richer in zinc than the alphaphase. Two-phase alloys (containing, in the case of brass, both alpha andbeta phases) are called duplex alloys . Since zinc is less noble than copper,galvanic eects promote preferential removal of zinc from the zinc-rich betaphase rst, leaving behind the alpha phase and porous, weak, copper-richcorrosion products (Fig. 15.1). Subsequent dezincication of the alpha phasemay still occur.

There are two commonly recognized forms of dezincication: plug and layertypes. In plug-type dezincication, small, localized areas of metal loss in theform of deep, narrow "plugs" occur in the tube wall. Such plugs may be blownout of pressurized tubes (Fig. 15.2). More general attack is called layer-typedezincication (Fig. 15.3). It is likely that fundamental mechanisms of plug-type and layer-type attack are similar. However, plug-type attack can producelocalized wastage rates of up to several hundred mils per year (mpy), while

Figure 15.1. Micrograph of brass containing both alpha and betaphases, showing copper-rich corrosion products in the former betaphase. Etchant: Potassium dichromate.

more uniform, shallower metal loss is common with layer-type dealloying.

Cupronickels are generally more prone to layer-type denickelication than toplug-type deterioration. However, cupronickel wastage is usually slightcompared to attack in brasses. Low-ow conditions and high temperatureslikely promote denickelication.

Figure 15.2. Plug-type dezincication beneath a deposit layer in anadmiralty brass tube.

Figure 15.3. Layer-type dezincication of a brass pump component.

Destannication (loss of tin) has been observed in gunmetal, which can beused for steam valves, and tin bronze, which is sometimes used in pumpimpellers. In boiler systems, steam environments and hot feedwaters areknown to promote destannication.

Dealuminication of duplex aluminum bronzes (greater than approximately 9wt% aluminum) has been reported in waters having both high and low pH.

15.2. Locations

Dealloying most commonly occurs in copper-containing alloys. Corrosion isconned primarily to feedwater systems and afterboiler regions. Componentsthat can suer attack include bronze pump impellers and boiler peripheralssuch as pressure-gauge ttings and valves. Monel (a family of nickel-copperalloys) steam strainers have been subject to dealloying corrosion whenexposed to steam containing sulfur compounds at elevated temperatures(Fig. 15.4).

Condensers and heat exchangers are also frequently aected; however,dealloying generally occurs on the cooling water side (see Case History 15.5).Failures in such systems can result in contamination of the boiler water withcooling water. For more in-depth discussion of dealloying in cooling water

Figure 15.4. Cross section through a corroded Monel metal steamstrainer. The dark area consists of oxides, suldes, and elementalcopper. Unetched.

environments, see The Nalco Guide to Cooling Water Systems FailureAnalysis .

15.3. Critical Factors

Using a susceptible alloy in an environment conducive to dealloying promotesattack. Deposits, soft waters (especially those containing carbon dioxide),high heat transfer, stagnant conditions, either high- or low-pH waters andhigh-chloride waters accelerate most forms of dealloying in copper-containing alloys. Addition of small amounts of arsenic, antimony, orphosphorus to some brass alloys, such as admiralty brasses, can reduce thelikelihood of dezincication, possibly due to the redeposition of a protectivelayer on the metal surface. Such alloys are referred to as inhibited brasses .However, attack may still occur under extreme conditions.

15.4. Identication

In copper-based alloys, copper is almost never selectively removed. Rather,other elements are leached, leaving behind a comparatively soft, porous massof copper. Dealloying may be discovered through visual examination ifdamage is severe. However, dealloying is most eectively diagnosed throughmetallographic examination. Attacked metal is relatively weak and can bebroken easily by stresses from impact or bending. Frequently, surfaces willbe riddled with cracks, but will retain original dimensions (Fig. 15.5).Attacked areas will usually change to a deep red or salmon color indicative ofelemental copper (Figs. 15.2 and 15.3). In brass alloys that have a two-phasemicrostructure consisting of alpha and beta brass, the beta brass will bepreferentially attacked (Fig. 15.1). Light-colored, zinc-rich corrosion productscommonly overlie regions in which dezincication has occurred (Fig. 15.6).

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Figure 15.5. Micrograph showing dealloying in a threaded region ofthe housing of a brass valve. Cracks developed in the dealloyedareas due to the weak nature of the corrosion products. Unetched.

Figure 15.6. Light-colored, zinc-rich corrosion products overlyingareas in which dealloying has occurred in the duplex brass housingof a ball valve.

15.5. Elimination

Surfaces should be kept free of deposits. In general, outages should be asshort as is practical. Air contact should be prevented through the use ofsteam or nitrogen blanketing. Water and steam quality must be controlled sothat concentrations of chloride and sulfur compounds are minimized.Environmental factors that can accelerate dezincication include hightemperatures, deposition, and stagnant conditions. Dezincication is alsopromoted by exposure to soft water, high-pH conditions, and highconcentrations of carbon dioxide in the water.

Substitution of alternative alloys may be necessary. Use of inhibited gradesof admiralty brass (all alpha phase), which contain small concentrations ofphosphorus, arsenic, or antimony, should be insisted on when conditionsdictate.

15.6. Related Problems

Graphitic corrosion (see Chap. 14, Graphitic Corrosion) is sometimesconsidered to be a form of dealloying, although the mechanisms are actuallyquite dierent. In graphitic corrosion, microgalvanic cells are formedbetween the phases of more-noble graphite and less-noble iron in gray andnodular or malleable cast irons, resulting in corrosion of the iron phase. Intrue dealloying, one of the alloying elements is chemically removed from anintimate mixture of metals. Formation of microgalvanic cells may contributeto true dealloying, particularly in duplex brasses, although it is not the onlydriving force for such corrosion.

Exfoliation is a unique form of wastage that may occur in highpressure utilityboiler feedwater heater tubes made of cupronickel. Exfoliation in cupronickelis unrelated to the types of exfoliation that may occur in ferrous boilerreheater and superheater tubes (see Chap. 3, Long-Term Overheating) or inaluminum alloys. In the presence of high-pressure oxygenated steam, distinctlayers of oxide material may form on the cupronickel tube surfaces.Eventually, sheets of oxide corrosion products can peel (exfoliate) o surfaces(Fig. 15.7). Microscopic examinations may reveal nely stratied corrosionproducts (Fig. 15.8). The exfoliated layers can then be swept downstreaminto the boiler, where copper- and nickel-containing deposits may form ontubes within the boiler water circuit as well as on turbines.

Although exfoliation in cupronickel is sometimes associated with dealloying,

Figure 15.7. Peeling layers of oxide on a cupronickel high-pressurefeedwater heater tube.

Figure 15.8. Micrograph showing nely stratied, oxide-containingcorrosion products on the external surface (steam side) of acupronickel utility boiler feedwater heater that experiencedexfoliation. Unetched.

some studies suggest that such attack is not actually a form of dealloying.Chemical analysis of the oxide layers has indicated that they can containcopper and nickel in nearly the same proportion as that of the original alloy,indicating no preferential removal of either of the main alloying elements. Insome cases, evidence has suggested that alternating periods of oxidizing andreducing conditions result in alternating oxide and metallic copper layers(see Case History 15.2).

Feedwater heaters in peaking and/or intermittent service tend to be moresusceptible to exfoliation, compared to those in base-loaded (continuous)service. High pH, high temperature, and the presence of ammonia also tendto promote attack. The likelihood of exfoliation increases with increasingnickel content. In terms of common commercial alloys, 70:30 cupronickels areparticularly vulnerable to exfoliation. Alloys such as 80:20 cupronickels aresusceptible to a lesser extent. Such wastage is rare in 90:10 cupronickels.Additions of iron and manganese to the alloy have been shown to increaseresistance to exfoliation. Since exfoliation in high-pressure utility boilerfeedwater heaters provides contaminants that can produce deposits onturbines, austenitic stainless steel is an alternative to cupronickel that hasbecome the predominant feedwater heater tube material. However, austeniticstainless steels may be subject to stress corrosion cracking if even smallamounts of chloride are present in the steam (see Chap. 13, Stress CorrosionCracking).

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A Monel turbine steam strainer was discovered to contain manybroken elements. The metal was converted to oxide, sulde, andelemental copper (Fig. 15.4). The deteriorated metal consisted ofnumerous elemental copper particles embedded in an oxide-suldematrix. Cracks in the wasted metal were lined with elementalcopper.

Failure was attributed to carryover of sulfur-containing compoundsin the steam. The system had a history of poor steam purity andcarryover of boiler water into the superheater.

Case History 15.1

Industry: Utility

Specimen location: Turbine steam strainer

Specimen orientation: Vertical

Years in service: 16

Water treatment program: Phosphate

Drum pressure: 1500 psi (10.3 MPa)

Specications: Monel wire, 0.15-in. (0.4-cm)diameter

Fuel: Coal

Feedwater heater tubes were thinned by cyclic oxidation followedby reduction of oxides in service (Fig. 15.7). Wall thickness wasreduced by as much as 15%.

During the previous 2 years, the boiler had experienced frequentoutages in which air leaked into the heater shell and causedsurface oxidation. Conversion of oxidized copper to elementalcopper occurred during normal operation, as evidenced by layers ofmetallic copper-colored particles within the corrosion products.

Case History 15.2

Industry: Utility

Specimen location: Inlet rst pass of high-pressurefeedwater heater

Specimen orientation: Horizontal

Years in service: 7

Water treatment program: All volatile treatment

Drum pressure: 400 psi (2.8 MPa)

Tube specications: 5/8-in. (1.6-cm) outer diameter,70:30 cupronickel

Chemical analysis of the valve housing alloy indicated that it was anuninhibited brass alloy with a zinc content of approximately 38wt%. The internal surfaces of the valve housing and ball werecovered with mounds of light-colored corrosion products (Fig. 15.6).Chemical analysis of the corrosion product mounds by SEM-EDSindicated zinc contents greater than 45 wt% in many areas.Beneath the corrosion products, areas of the metal were copper-colored. Microscopic examinations indicated a duplexmicrostructure consisting of alpha and beta brass, typical of brassalloys containing greater than 32 wt% zinc (Fig. 15.1). Along theinternal surface, porous copper corrosion products lined thesurface in many places, indicating that dezincication occurred.Characteristics of both plug- and layer-type attack were observed.Cracks penetrated along the porous corrosion products in threadedregions (Fig. 15.5). It was recommended that inhibited single-phasebrass alloys with less than 32 wt% zinc be considered to limit thelikelihood of dezincication. The pH of the water was reportedlyabove 10, which most likely promoted dezincication.

Case History 15.3

Industry: Institutional

Specimen location: Low-pressure boiler feedwaterreturn piping

Specimen type: Ball valve

Metallurgy: Brass

Years in service: 2

The pump supplied feedwater to a boiler that was used for heating.As such, the system experienced extended idle periods. Theimpeller exhibited smoothened metal loss in most locations. Thesurface was generally covered with a thin layer of deposits orcorrosion products. In a few scattered locations, white corrosionproduct mounds containing an average of 38 wt% zinc (as analyzedby SEM-EDS) were observed. Trace amounts of sulfur and chlorinewere also detected on the surface by SEM-EDS. In some places, thesurface was bare and revealed the typical dendritic structure thatresults from solidication during the casting process (Fig. 15.9).Chemical analysis of the impeller alloy indicated that it containedapproximately 13 wt% zinc and 5% silicon.

Case History 15.4

Industry: Institutional

Specimen location: Boiler feedwater system

Specimen type: Pump impeller

Years in service: 5

Specications: 4-in. (10.2-cm) diameter, die-cast silicon brass

Microscopic examinations indicated that the microstructureconsisted of dendrites of the alpha solid solution phase, withsignicant coring (compositional segregation) evident in thedendrites. Porous, copper-rich corrosion products indicative ofdezincication were present on all surfaces, althoughdezincication penetrated preferentially along zinc-rich areas in thecored microstructure (Fig. 15.10). Elongated depressions weresuperimposed on the dezincied areas.

Figure 15.9. Bare-surfaced metal loss areas on the surfaceof a die-cast silicon brass feedwater pump impeller,showing the normal dendritic structure that resulted fromthe casting process.

Dezincication was most likely promoted by stagnant conditionsduring idle periods and the presence of sulfates, suldes, and/orchlorides. Erosive forces easily removed the soft, copper-richcorrosion products produced by dezincication. It wasrecommended that pumps be drained and their components driedprior to extended idle periods. Alternative metallurgies were alsosuggested, including aluminum bronze, leaded nickel bronze, andcast 400-series stainless steels. Materials containing secondaryphases that are particularly susceptible to dealloying (such as betaphase in brass) or that exhibit segregation of alloying elementswere not recommended.

Figure 15.10. Preferential dezincication along coring in theall-alpha-phase dendritic microstructure of a silicon brassboiler feedwater pump impeller. Etchant: Potassiumdichromate.

The failure in this case history occurred due to dealloying on thecooling water side; however, it is included due to the relativelycommon nature of dealloying in condensers, which are part of theboiler water circuit. For more in-depth information regardingdealloying in cooling water environments, see The Nalco Guide toCooling Water Systems Failure Analysis.

Eddy current testing indicated both general corrosion and pittingon the internal surface of the condenser tubes (cooling water side).A few perforations occurred in deep internal surface depressions(Fig. 15.11). The depressions were generally covered with hard,light-colored deposits and corrosion products. Material close to andaway from the depressions was found to contain a highconcentration of silicon, in addition to numerous other elements.Chemical analysis of material within a depression by SEM-EDSindicated that it contained an elevated concentration of zinc,relative to material elsewhere on the surface. The deep depressionswere lled with porous, copper-based corrosion products, indicativeof plug-type dezincication (Fig. 15.12). The patches of depositmaterial on the surface most likely promoted plug-type attack dueto the formation of concentration cells beneath the material.

Case History 15.5

Industry: Utility

Specimen location: Condenser, inlet side

Metallurgy: Brass

Years in service: 29

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Figure 15.11. Internal surface (cooling water side) of abrass condenser tube, showing a deep depression causedby plug-type dezincication.

The internal surface was generally covered with a layer of porouscopper-based corrosion products, indicating that layer-typedezincication had occurred in addition to plug-type attack. High-salt content of the water and high temperatures likely promotedlayer-type dezincication. Water analyses indicated highconductivity and sulfate concentration in the water. The system hadreportedly experienced biological fouling several years before themetallurgical analysis was conducted; occlusive material in general,including biological material, promotes dezincication.

Figure 15.12. Micrograph showing deep, plug-typedezincication on the cooling water side of a brasscondenser tube. Unetched.

Microscopic examinations indicated that the microstructure wassingle-phase, consisting of twinned grains of alpha brass. Chemicalanalysis of the tube alloy indicated that it was consistent withadmiralty brass. Small concentrations of arsenic and phosphoruswere detected in the metal, but they were below the levels typicallyused in inhibited admiralty brass alloys. The addition of anadequate concentration of a corrosion-inhibiting element may haveprevented the plug-type dezincication and limited layer-typedezincication. It was recommended that tube surfaces be keptclean to minimize the possibility of concentration of aggressivespecies beneath occlusive material. The reported service life of 29years was considered to be long for uninhibited admiralty brassexposed to water with a high conductivity and sulfateconcentration.

15.7. References

1. "Denitions Relating to Metals and Metalworking," ASM Metals Handbook ,vol. 1: Properties and Selection of Metals , American Society for Metals,Metals Park, Ohio, 1961, pp. 141.

2. S. G. Corcoran,"Eects of Metallurgical Variables on Dealloying Corrosion,"ASM Handbook, vol. 13A: Corrosion: Fundamentals, Testing, andProtection , ASM International, Metals Park, Ohio, 2003, pp. 287293.

3. H. M. Herro and R. D. Port, The Nalco Guide to Cooling Water SystemsFailure Analysis , McGraw-Hill, New York, 1993.

4. J. A. Beavers, A.K. Agrawal, and W. E. Berry, Corrosion-Related Failures inFeedwater Heaters , Electric Power Research Institute, Palo Alto, CA, 1983,CS-3184, pp. 51 to 53.

5. T. J. Muldoon, "Stress Corrosion Cracking (SCC) of 304SS Tubes at Outlet ofthe Desuperheating Zone in Feedwater Heaters,"Feedwater HeaterTechnology Symposium, Electric Power Research Institute, Palo Alto, CA,2004.

6. Herro and Port, The Nalco Guide to Cooling Water Systems FailureAnalysis , McGraw-Hill, New York.

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

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