In response to recent environmental regulation, e.g., RoHS Directives, chromate-free coatings have been developed and applied to electrical appliances. However, thinner chromate-free coatings are necessary to obtain better electroconductivity, which influences the electromagnetic shielding performance of digital electrical appliances. We investigated the corrosion resistance and film structure of thiol self-assembly ultra-thin layers on zinc coated steel sheets. The thiol layers' corrosion prevention mechanism was discussed to develop a new design concept of thinner chromate-free coatings with high corrosion resistance. The remarkably thin triazinethiol layers with three thiol groups per molecular showed excellent corrosion resistance despite their thickness of a few molecules. Their corrosion resistance derives from their suppression of oxygen reduction reaction. Nevertheless, identifying the layer structure using XPS analysis was difficult because of the high surface roughness of the zinc-coated steel sheets. We investigated the structure of thiol self-assembly ultra-thin layers on zinc single crystals. Results showed that triazinethiol molecules were partially decomposed by dissolution to water and drying. A new molecular assembly was formed by combination to Zn. Thinner chromate-free coatings with better corrosion resistance can be produced by applying compounds that can form a molecular assembly and which can be combined to Zn.
It is possible to form highly ordered porous type anodic oxide film by anodizing in acid solutions including oxalic acid, phosphoric acid, and sulfuric acid, and both pore size and thickness are controlled by the anodizing condition. Therefore, porous type anodic oxide films are widely used and studied. However, generally research on area-selected anodizing has been carried out with mask-processes. The solution flow type micro-droplet cell (Sf-MDC) has been applied to form thick porous type aluminum anodic oxide films locally. By coating a side wall Pt wire electrode, it is possible to reduce the width of the formed anodic oxide line without changes in the film formation reaction. Up to ten repeated passes, there is no change in the oxide film growth rate; the widths of the oxide lines were observed by a coated Sf-MDC electrode.
Ni-W alloy plating has been considered for application to various industrial fields as a substitute for hard Cr plating. However, by plating conditions, the plating crack on Ni-W alloy film might occur and cause difficulties in industrial use. In this study, we elucidated the plating crack behavior of Ni-W alloy films with various W contents. We also investigated the influence of the internal stress of Ni-W alloy film on the plating cracks. Results show that the number of the plating cracks changed by W content in the range of 27.3-51 wt%W. At approximately 40 wt%W, plating cracks did not occur. Moreover, the change of the internal stress of Ni-W alloy film in the range of 30-46 wt%W showed a convex-downward curve against the W content, which indicated the least internal stress in approximately 40 wt%W. Furthermore, the influence of the misfit between the Ni-W alloy film and that substrate was small, which suggests that the internal stress of the film itself changed. Therefore, the internal stress was regarded as a direct factor affecting the plating crack. Furthermore, from results of X-ray diffraction patterns of Ni-W alloy films with various W contents, the change of the internal stress was inferred to have resulted from the change of the crystalline state.