Fe-18Cr-20Ni-2Mo alloy prepared in vacuum induction furnace by melting electrolytic iron, chromium, nickel and metallic molybdenum showed an unusually high corrosion rate (i.e. 100-200g/m²·hr), compared with the commercially produced molybdenum stainless steels, in 5 per cent sulphuric acid test designated by JIS.
Among thirty kinds of elements added to the above alloy up to 0.1 per cent, the most beneficial in reducing the corrosion rate were rhodium, palladium, platinum, copper and lead, followed by cerium, hafnium, thorium, uranium, indium and tin. Titanium, niobium, tungsten and silver were also beneficial if added more than 0.1 per cent. No minor alloying elements tested increased the corrosion rate of the base alloy. The mechanism of the beneficial effects were discussed. It was pointed out that minor additions of tin and lead had a beneficial effect although these elements did not influence the corrosion potential and anodic polarization of the base alloy.
By considering the characteristics of elements in melting operations and by analysing the impurities in the commercial austenitic stainless steels containing molybdenum, it is concluded that the relatively low corrosion rate of the commercially produced stainless steel in 5 per cent sulphuric acid test is attributed to the beneficial effect of residual elements, especially copper and tin.