Aluminum alloys corrosion intergranular forms

Susceptibility to intergranular corrosion dep>ends primarily on the typ>e of alloy and fabrication process and can occur in most environments. The six-hour ASTM G 110 test (Practice for Evaluating Intergranular Corrosion Resistance of Heat Treatable Aluminum Alloys by Immersion in Sodium Chloride + Hydrogen Peroxide Solution) is most often used to determine inherent susceptibility to this form of corrosion. However, users are cautioned that the degree of susceptibility in this accelerated test may not be representative of p>erformance in outdoor atmospheres [22]. 

For instance, when Al-Cu alloys are heat-treated by delayed quenching after solid-solution treatment, copper will diffuse to the grain boundaries. The copper-poor regions formed adjacent the grain boundaries play an anodic role relative to the other regions which have more noble potentials (Galvele and Micheli, 1970). Some aluminum alloys subjected to inadequate quench treatment become susceptible to intergranular corrosion. 

SCC is usually nucleated by some form of localized corrosion in chloride-SCC of stainless steels [68] or aluminum alloys [54], cracks start from areas of pitting, intergranular eorrosion, or crevice eorrosion that create die stress concentration and the acidity required for cracking (Fig. 7). In SCC of C-Mn or low-alloy steels, intergranular corrosion occurs along segregated zones rieh in earbon, nitrogen, or phosphorous and provides a stress concentration (helps to achieve A iscc) [ 7]... 

Raising the temperature increases die dissolution of zinc and the ZA alloys (Belisle and DuFresne, 1986) in water. A marked increase occurs up to about 60°C, followed by a decrease at higher temperatures. Intergranular corrosion of the zinc-aluminum casting alloys is a risk above about 70 C in wet or humid conditions (e.g., in steam), when no protective layer can form and selective dissolution of the structure occurs. A steam test at 95°C is indeed a standard method of checking whether alloys have been correctly produced. 

---