Pitting Corrosion of Powder Metallurgy AlZnMg Alloys

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Abstract
Pitting susceptibility in 3.5% NaCl was compared for three AlZnMg alloys containing 2.5% Mg and 8.8, 9.0, and 12.5% Zn. Pit nucleation potentials were determined using electrochemical techniques. The distribution, size, and density of second-phase precipitates in the alloys’ microstructures were evaluated by scanning electron microscopy (SEM). The chemical composition and topography of corrosion products on the surfaces of alloys after their exposure to NaCl were studied by SEM and energy dispersive spectroscopy (EDS). Pits nucleated in the vicinity of small particles of second-phase precipitates (probably MgZn2). A correlation was found between the number of pits occurring on the specimen surface and the number of precipitated particles present in the microstructure of the alloy. When there were fewer precipitates, fewer pits formed. The alloy with 12.5% Zn was more susceptible to pitting than the two remaining alloys. The 8.8% Zn alloy was less susceptible to pitting after a T65 heat treatment than after additional aging at 150 to 160 C. The corrosion products in pits were enriched in Zn, whereas those on the outer surface of specimens were enriched in chloride ions.