Corrosion behavior of rapidly solidified magnesium-aluminum-zinc alloys

Rapidly solidified magnesium alloys with 8 at%, 15 at%, and 20 at% Al and 1 at% and 3 at% Zn were fabricated by centrifugal atomization followed by hot extrusion. Microstructure of the alloys was composed of a fine-grain magnesium matrix (0.5 mu m) with beta-Mg17Al12 precipitates. Electrochemical and weight-loss tests were performed in borate and ASTM D 1384 solution (chloride, carbonate, and sulfate). In both media, corrosion current of the alloys decreased with increases in aluminum or zinc content. In borate solution a passivating plateau was observed from the corrosion potential (E-corr) to E-corr + 1,200 mV. Current density decreased with aluminum and zinc concentrations. Electrochemical behavior of the synthesized matrix and precipitates was characterized. Zinc increased E-corr of the two phases, with a corresponding decrease of corrosion current. The same trend was noticed for aluminum but with a less dramatic effect. The corrosion mechanism was suggested result from galvanic coupling of the matrix and the second phase. The galvanic corrosion, however, was reduced strongly by passivation of the matrix as a result of the surrounding precipitates. The positive influence of rapid solidification (corrosion rate decreased 1 order of magnitude) was the creation of a fine, highly homogeneous microstructure through this fabrication process.