The chromate alternative conversion coatings were formed on zinc plated steel by immersing it into a chemical conversion bath containing sodium tungstate and phosphoric acid at room temperature. The effects of additives in the bath, such as inorganic and organic acids, on the properties of the coatings were investigated. As a result, it was found that the addition of sulfuric acid made the corrosion resistance of the coatings remarkably improve and that the further addition of borate made the surface of the coatings smooth, fine, and glossy. The bath compositions determined as optimum for both corrosion resistance and the surface appearance of the coatings were 15.0g of Na2WO4·2H2O, 44mL of 85%H3PO4, 2.1mL of H2SO4, and 8.51-26.7g of Na2B4O7·10H2O dissolved in 400mL of water. Also, the optimum pH range of the bath turned out to be 0.9-1.1. The corrosion resistance of the coating was evaluated by the neutral salt spray test, on which the first appearance time of white corrosion products on the surface of the coating was improved to 160-170h, indicating comparatively higher corrosion resistance, suitable as an alternative to chromate conversion coatings formed on zinc plating. The coatings were also examined by glow discharge optical emission spectroscopy and X-ray photoelectron spectroscopy. Their thicknesses were found to be around 0.5μm. It was suggested that the coatings were composed of zinc ions eluted from the zinc plating, tungstate ions consisting of W6+, W5+, and W4+, and phosphate ions.
Boric acid has been an essential substance in plating technology for obtaining good film properties and for improving bath life. However, there are restrict ions against Boron in waste water ; therefore the development of a boric acid free nickel electroplating bath was in demanded.In our previous paper, we attempted to develop a boric acid-free sulfamate Ni plating bath. We examined over 20 kinds of plating solutions with alternative chemicals, and found that acetic acid, propionic acid, Ni acetate, and citric acid were recognized as potential candidates to obtain similar film properties and appearance. This paper reports the evaluation of deposited film properties obtained using these four kinds of organic acid baths. Acetic acid, propionic acid, and Ni acetate, as replacements for boric acid, exhibited similar properties as those obtained by boric acid. The propionic acid bath showed less pit defects even after thick film formation without surfactants. Accordingly, it appears to be a favorable boric acid replacement for those applications that require thicker require films.
Using TOF-SIMS, we found that the NiO2/NiSO/NiS multi-layers are formed at the surface of an FeNi plated film in nanometer scale by adding sodium saccharin into the plating bath, and that these multi-layers reduce oxygen inclusion into the plated film while plating, preventing natural oxidation of the surface of the film. Also, using AES, we found the presence of a Ni rich layer just under the NiO2/NiSO/NiS multi-layers. Next, we plated an FeNi film in a plating bath without sodium saccharin onto an FeNi film which had been previously plated in another plating bath with sodium saccharin, and found that NiS which had existed at the surface of the lower film moved to the surface of the upper film. We also found that the surface structure of these double films is the same as that of a single FeNi film plated in a bath with sodium saccharin, when the upper film is thin enough.