Resistance to Forms of Corrosive Attack

Despite its excellent corrosion resistance in general, copper and its alloys are susceptible to several forms of corrosion. Copper is susceptible, to some extent, to uniform corrosion, galvanic corrosion, dealloying (selective leaching), pitting, SCC, erosion corrosion, fretting, intergranular corrosion, and corrosion fatigue. These forms of corrosion with respect to copper and its alloys are described in the following sections. (They are also described in more general terms in Section 1.0.)

Uniform Corrosion

Copper and its alloys have a strong resistance to uniform corrosion under normal conditions, but over long periods of exposure they will exhibit to some extent this form of non-localized corrosion. Immersion in or uniform exposure to aerated or oxidizing acids or sulfur containing compounds, etc., will accelerate the process of uniform corrosion on copper and its alloys.

Galvanic Corrosion

Copper has a relatively noble (cathodic) position on the Galvanic Series compared to many structural metals and alloys, thus it will most likely not corrode preferentially when electrically coupled with one of them. When coupled with more noble metals, however, such as nickel, titanium, and some stainless steels, copper will corrode preferentially by galvanic action.

Dealloying (Dezincification)

When considering a copper alloy it is very important to realize its potential for dezincification, if it has a significant zinc content (>15% in Cu-Zn alloys). Dezincification is a process which leaches out the zinc leaving behind a less ductile, porous copper structure that is more susceptible to fracture. This often occurs in ambient waters or salt solutions. Dealloying also occurs in some copper-aluminum alloys, where the aluminum is selectively leached from the alloy. This has a greater occurrence in alloys with more than 8% aluminum.

Pitting

Most often, pitting does not occur to a significant extent in copper, that is, not enough to cause any considerable damage. If very thin copper or copper alloys are used, however, perforation is possible by pitting. Moreover, if copper is used in low flow velocity or stagnant seawater, it does have a slightly higher propensity for pitting.

Crevice Corrosion

Copper and its alloys are generally resistant to crevice corrosion, although a few specific alloys may have a tendency to experience a form of crevice corrosion. Typically, copper alloys containing aluminum or chromium have a higher susceptibility to crevice corrosion.

Erosion Corrosion

Copper and its alloys are susceptible to erosion corrosion, which is also characterized as impingement attack. This is especially the case for copper when immersed in polluted water, contaminated water, seawater, or water containing sulfur compounds. Erosion corrosion from cavitation also has a tendency to occur on copper alloys.

SCC

Copper and its alloys are susceptible to stress corrosion cracking, especially in the presence of ammonia and ammonium compounds. Stress corrosion cracking of copper alloys is presumed to be integrally related to dealloying. Table 32 presents some copper alloys and their corresponding resistance to SCC.