Effects of alloying elements on the pitting corrosion resistance of 17Cr-16Ni steels with and without 4% Mo were evaluated by the corrosion rate in 10% FeCl3.6H2O and 4% NaCl added with hydrogen peroxide and by the polarization measurement in 0.1N NaCl+0.1-0.25N Na2SO4 at 40C.The elements, the content of which was varied, were carbon, nitrogen, silicon, phosphorus, sulfur, manganese and nickel. The supplementary alloying elements were aluminum, titanium, vanadium, cobalt, copper, zirconium, niobium, tin and tungsten. Tungsten, like silicon, was found beneficial in minimizing the pitting corrosion if added together with molybdenum.Carbon was favorable if no chromium carbides were formed. Copper added to the Mo-free steel was beneficial, but not effective in the Mo-bearing steel. Manganese was proved to be harmful in both Mo-free and Mo-bearing steels, but increasing its content more than that of ordinary stainless steels did not enhance its harmful effect. The very favorable effect of nitrogen was also recognized. The effects of the other alloying elements are summarized in a table. The more noble the electrode potential at a relatively high c.d., i.e. 10mA/cm2, in anodic polarization curves measured in sodium chloride solution, the lower was the corrosion rate in the ferric chloride test if a second phase was absent. The presence of a second phase increases especially the corrosion rate in ferric chloride solution. It was indicated that the alloying to increase the passivating ability of the steel would shift the pitting potential to the more noble direction by passivating the depassivated sites before they can grow as ordinary pits.

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