Effects of high-pressure torsion on the pitting corrosion resistance of aluminumiron alloys

The effects of reducing the grain size by high-pressure torsion (HPT) on the pitting corrosion resistance of AlFe alloys with Fe contents of 0.5, 2 and 5 mass% were investigated by means of polarization curves in solutions containing 0.1 mol̇dm^-3 Na₂SO₄ and 8.46 mmol̇dm^-3 NaCl (300 ppm Cl^-) at 298K and by surface analysis. The potentials for pitting corrosion of the AlFe alloys were clearly shifted to the noble direction by HPT, leading to an improvement in pitting corrosion resistance. This improvement was larger in the Al0.5% Fe and Al2% Fe alloys and smaller in the Al5% Fe alloy. The AlFe alloys contained precipitates of AlFe intermetallic compounds, around which pitting corrosion occurred. The Al5% Fe alloy, in particular, contained large precipitates tens of micrometers in size, and pitting corrosion was significant around these large precipitates. It is evident from the time-dependence of the corrosion potential and the polarization resistance of the corrosion reaction that the formation rate of Al oxide films increases as a result of HPT. It was therefore concluded that the improvement in pitting corrosion resistance of the AlFe alloys with HPT is caused by increasing the oxidation rate of Al.