Dealloying

Dealloying is a prominent mode of decay of copper alloys. It is possible that one of the components of the alloy is preferentially leached out. Dealloying has been observed in the case of copper alloys in the form of preferential leaching of zinc from brasses, aluminum from aluminum bronzes and nickel from cupronickels. Brasses containing 15% zinc are susceptible to dezincification in aqueous medium. 

Dealuminification occurs in aluminum bronzes containing the y-2 phase microstructure and the process is more severe when the y-2 phase forms a continuous grain boundary network. Dealuminification can be averted by rapid cooling from 600°C, or by addition of 1-2% iron or more than 4.5% nickel. Microstmctural changes can still occur during welding and lead to corrosion in the heat-affected zone. 

Dezincification of brass fittings occurs when the /1-phase is present. Dezincification is accelerated by high temperature, high chloride concentration, low flow-rates and differential aeration. Dezincification can be inhibited by the addition of 1% tin and 0.04% arsenic. 

Another type of localized corrosion involves the selective removal by corrosion of one of the elements of an alloy by either preferential attack or by dissolution of the matrix material. The various kinds of selective dissolution have been named after the alloy family that has been affected, usually on the basis of the element dissolved (except in the case of graphitic corrosion). 

Dezincification refers to the selective leaching of the zinc phase in alloys such as brasses that contain more than 15 percent Zn. The gross appearance and size of a part that has suffered dezincification is often unchanged except for the formation of a copper hue. The part, however, will have become weak and embrittled, and therefore subject to failure without warning. To the trained observer, dezincification is readily recognized under the microscope, and even with the unaided eye, because the red copper color is easily distinguished from the yellow of brass. 

0 percent, Zn 0.7 percent Pb 4.1 percent. (Courtesy of Defence R D Canada-Atiantic) 

Graphitic corrosion usually occurs in three ways. Sometimes just the surface of the pipe graphitizes, forming a graphitic coating on the exterior. Frequently, this protects the pipe very well, leading 

---

Moffat T P, Fan FRF and Bard A 1991 Electrochemical and scanning tunneling microscopic study of dealloying of CUjAu J. Electrochem. Soc. 138 3224... 

Table 8. Relative Susceptibility to Stress Corrosion and Dealloying of Commercial Copper Alloys...

Parting, or Dealloying, Corrosion This type of corrosion occurs when only one component of an alloy is removed by corrosion. The most common type is dezincification of brass. 

See also Chap. 2, Crevice Corrosion Chap. 3, Tuherculation Chap. 5, Oxygen Corrosion Chap. 6, Biologically Influenced Corrosion and Chap. 13, Dealloying. ... 

Dealloyed areas are structurally altered. Corroded areas are weak and porous, causing fracture and weeping leaks. 

Dealloying is influenced by many factors. In general, any process that increases general corrosion will promote dealloying. However, specific acceleration factors may be further classified into one of three categories metallurgy, environment, and water chemistry. 

Zinc brasses containing phosphorus, antimony, and/or arsenic dezinc-ify much less readily than brasses free of these elements. Brasses containing less than 15% zinc are virtually immune to attack, and those containing more than 32% zinc are readily dealloyed. 

Because wastage is usually slight, identification by visual observation alone is difficult microscopic examination is usually required. Layer-type dealloying commonly occurs. Plug-type denickelification attack has never been observed at this laboratory. Surfaces have a reddish color due to the accumulation of denickelified metal. 

Dealloying can be reduced (as can any other form of corrosion) by good system operation and the judicious use of appropriate materials and chemical treatment. Specific categories needing attention follow. 

The propensity toward dealloying decreases as surface cleanliness increases. Flow should be maintained at rates high enough to prevent... 

Chemical corrosion inhibition can reduce all forms of corrosion including dealloying. In particular, filmers such as tolyltriazole are effective in reducing corrosion of yellow metals. 

Any deposit can increase dealloying. Biological accumulations, such as slime layers, are no exception to this rule. In the age of increasing public concern regarding pollution, biological control via chemical treatment can be difficult. However, good biological control is always beneficial. 

Removal of deposits and corrosion products from internal surfaces revealed irregular metal loss. Additionally, surfaces in wasted areas showed patches of elemental copper (later confirmed by energy-dispersive spectroscopy) (Fig. 13.12). These denickelified areas were confined to regions showing metal loss. Microscopic analysis confirmed that dealloying, not just redeposition of copper onto the cupronickel from the acid bath used during deposit removal, had occurred. 

Surfaces of plugged tubes were carefully compared to surfaces of tubes from the same condenser that were never plugged. Only plugged tubes showed denickelification. Stagnant conditions, as well as the deposits and chloride, caused the dealloying. The deepest dealloying was up to 5% of the remaining wall thickness. 

Figure 13.14 A generally dezincified bronze pump impeller. Surfaces are dealloyed to a depth of about 0.020 in. (0.050 cm).

The basic mechanisms involved in graphitic corrosion are familiar and easily understood. Hence, remedial and preventive measures are relatively simple to implement. Although commonly categorized as a form of dealloying, graphitic corrosion has much in common with galvanic corrosion. 

---