Dealloying corrosion

High temperature/stress and stop/start operation effects on 70 30 cupronickel tubes (as found in some FW heaters). A chain of cause and effect including Oxygen corrosion Dealloying Exfoliation corrosion... 

Corrosion, temperature Liquid metal embrittlement, hot corrosion, dealloying... 

Corrosion within pump and piping systems is another problem, and general uniform attack is common. Pitting, crevice corrosion, intergranular corrosion, dealloying, galvanic corrosion, and cavitation corrosion are also possible depending on the environment. 

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. ... 

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. 

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. 

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. 

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. 

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. 

Dealloying selective removal by corrosion of a constituent of an alloy. 

These galvanic corrosion processes take place when one or more elemental constituents of an alloy is leached, often leaving a weak, porous structure, although the component dimensions often are unchanged. Dealloying particularly affects equipment constructed of cupronickels, bronzes, brasses, and gunmetal, such as FW heaters, strainers, valves, and pump impellers. 

Certain pre-boiler cupronickels, such as 70 30 alloy tubes, subjected to high temperature, stress, and stop-start operation may suffer oxygen corrosion-induced dealloying followed by exfoliation corrosion, in which oxidized sheets peel away from the solid metal. 

As noted previously, dealloying also affects pre-boiler components manufactured of cast iron, such as FW pumps and valves. These components may, under certain (long-term) circumstances, suffer a selective form of preferential leaching galvanic corrosion. Specifically, graphitic corrosion may take place when the (anodic) iron matrix con-... 

Additional reference material came from Dealloying Corrosion Basics, Materials Performance, vol. 33, no. 5, p. 62, May 2006, adapted by NACE from Corrosion Basics—An Introduction, by L. S. Van Dellinder (ed), NACE, Houston, Tex., 1984, pp. 105-107. 

---