Resistance to Forms of Corrosive Attack

Although aluminum and its alloys have a good resistance to various forms of corrosive attack, they are still susceptible to some mechanisms including galvanic, pitting, SCC, intergranular, crevice corrosion, corrosion fatigue, and occasionally filiform corrosion. Susceptibility to other forms of corrosion is often dependent on alloy composition and heat treatment.

Pitting

Pitting is one of the most common forms of corrosion found in aluminum and its alloys. Chloride containing environments pose one of the biggest threats to aluminum in terms of corrosion, since pitting corrosion tends to occur in salt water and marine environments. Seawater flowing at a high velocity relative to the aluminum is especially corrosive in the form of pitting since it will inhibit the protective oxide layer from automatically healing itself.

SCC

Additions of copper, magnesium and zinc in sufficient amounts can lead to SCC of aluminum alloys. SCC is dependent on the environment that the aluminum alloy is exposed to. For instance, chloride, bromide and iodide environments are particularly dangerous to aluminum since SCC tends to occur in such environments. Aluminum alloys tend to resist SCC in hydrogen, argon and air with no moisture content. Marine environments, which are commonly encountered in applications of aluminum alloys, typically promote SCC of aluminum alloys. Increasing the pH in chloride environments, however, works to inhibit SCC in aluminum and its alloys.

Furthermore, SCC can also be dependent on the heat treatment and grain orientation. For example, 7075-T6 (or 2024-T4) is most susceptible to SCC when the tensile stress is applied in the short transverse direction, is less susceptible to SCC when it is applied in the long transverse direction, and is least susceptible to SCC when it is applied in the longitudinal direction. (This is only the case for thick specimens; thin aluminum sheets and castings are typically not affected by this.) Shot peening can be used to improve resistance of aluminum alloy structural forgings, machined plates and extrusions to SCC and corrosion fatigue. Some environments that are known induce or retard SCC in aluminum alloys are given in Table 30. Table 31 provides a relative comparison of various aluminum alloys and their resistance to SCC.

Crevice Corrosion and Exfoliation

Aluminum is also susceptible to crevice corrosion, and since it is often used in components where joining and fastening is required, crevices must be eliminated to avoid this particular form of corrosion. Exfoliation in aluminum is commonly a consequence of crevice or galvanic corrosion. Aluminum alloys that have elongated grain structures are susceptible to exfoliation.