Mechanism of localized corrosion of 7075 alloy plate
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MOSThigh strength, heat treatable aluminum alloys are, depending on metallurgical practices, susceptible to localized corrosion. The mechanisms of these corrosion processes have been investigated in detail. It is generally accepted that these processes can be related to local variations in the microstructure, usually in the vicinity of grain boundaries, that result from heat treatment and aging. These variations have a large influence on the nature and distribution of the electrochemical reactions that control these localized corrosion processes. Most mechanisms proposed to explain the phenomenon of intergranular corrosion (or intergranular stress-corrosion cracking in the presence of tensile stress) are based on the development of a preferential anodic path along grain boundaries resulting from localized precipitation. Originally, susceptibility was attributed to differences in corrosion potentials between matrix and grain boundary regions. However, this mechanism did not explain why halide ions (chloride ions, in particular) were necessary for intergranular corrosion, since galvanic corrosion should occur in any electrolyte of comparable ionic conductivity. Based on these observations, Galvele and De Micheli (') proposed that intergranular corrosion resulted not from differences in corrosion potentials but from differences in breakdown potentials of these regions. The existence of an active region in the vicinity of grain boundaries has been attributed either to anodic precipitates or to solute-depleted regions resulting from precipitation. More recently, investigators have shown the effects on the electrochemical properties of the grain boundary regions arising from solid solution composition fluctuations brought about by precipitation. These investigators attribute the anodic nature of the grain boundary region to a high concentration of solute atoms in these regions as compared to grain interiors. 2,3 The age hardening high strength 7075 alloy provides a good example of the influence of heat treatment on S. MAITRA and G.C. ENGLISH are Staff Scientist and Technologist, respectively,Alloy Technology Division, Alcoa Technical Center, Alcoa Center, Pa. 15069. Manuscript submitted August 22, 1979. METALLURGICALTRANSACTIONSA
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intergranular corrosion susceptibility. Overaging beyond maximum hardness, i.e. to T7X temper, is known to reduce susceptibility. 4 At present, however, there is no satisfactory explanation for this reduced susceptibility. This paper summarizes the effect of artificial aging of cold water quenched 7075-W plate (to T651 and T7351 tempers) on the anodic polarization behavior and demonstrates how corrosion performance, and the corrosion mechanism, may be inferred from changes occurring in the anodic polarization curves. MATERIAL The 7075 alloy used in this investigation had the following composition: si Fe 0.10 0.26
Cu Mn Mg Cr Ni Zn Ti Be 1.70 0.03 2.54 0.20 0.00 6.08 0.05 0.001
The material was in the form of plate, 19 mm thick. The W temper was obtained by solution heat treating for 30 rai
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