Extremely high corrosion resistance of thin film stainless steels deposited by ion beam sputtering

Electrochemical behaviour of thin film stainless steels was analysed in terms of its localised corrosion resistance. Type 304 stainless steel and a sheet of additional element, if necessary, were simultaneously bombarded by Ar+ ion beams to obtain Type 304 steel films with additive element (∼10 at.% Mo, W, Ti, Al). The transmission electron microscopy of the deposited film revealed that the film consists of fine crystalline grains of 25∼30 nm without any preferential orientation. The crystal structure of Type 304 steel film, however, was proved to be body-centred-cubic (bcc). Polarisation curves of the films with or without additives do not indicate pitting corrosion in 0.6 kmol m−3 NaCl solution. Even in the more severe environment, that was 1.7 kmol m−3 NaCl+0.1 kmol m−3 HCl solution at 70°C, the sputtered Type 304 stainless films with Mo, W, or Ti addition reveal no pitting corrosion. The extremely high corrosion resistance of the film cannot be attributed to the microstructure. Localised corrosion usually initiates at some defects, such as grain boundaries, segregation sites, precipitates. Type 304 steel used as target in this study was not high purity material but commercial grade steel. Therefore, the deposited film in this work should contain impurities similar to those of the target material. However, the sputtered thin film might be free of precipitates, because the non-metallic inclusions such as oxides and sulphides, that were contained in the target, could have been dissociated during sputtering and dissolved uniformly into the film. It is noticeable that the thin films whose composition corresponds to Type 304 stainless steel, reveal extremely high corrosion resistance like that of super stainless steels which contains large amounts of alloying elements. The results obtained in this work suggest the essentially high corrosion resistance of the passive film on stainless steel and reveal the fatal role of defects, such as non-metallic inclusions, on the localised corrosion of stainless steels.