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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In a glass moulding press (GMP) for refractive/diffractive hybrid lenses, to improve the service life of nickel-phosphorus (Ni-P) plated moulds, it is necessary to control the diffusion of constituent elements from the mould into the release agent coating. In this study, diffusion phenomena of constituents of NiP plating are investigated for two types of release agent coatings, iridium-platinum (Ir-Pt) and iridium-rhenium (Ir-Re), by cross-sectional observation, compositional analysis and stress measurements. The results show that Ni atoms in the plating layer flow from regions of compressive stress to regions of tensile stress. In the case of the Ir-Pt coated mould, the diffusion of Ni is promoted from the grain boundaries between the Ni and Ni 3 P phases in the plating towards the surface of the Ir-Pt coating. However, in the Ir-Re coated mould, the diffusion of Ni is suppressed because the diffusion coefficient of Ni in the Ir-Re alloy is smaller than that in the Ir-Pt alloy, although the stress state is similar in both cases. By controlling the diffusion of Ni atoms, the use of Ir-Re alloy as a release agent coating for NiP plated moulds is expected to lead to a high degree of durability.

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