The phases and microstructure of the alloy layer formed during hot dipping of carbon steels in molten Zn-Al bath were investigated. Pure iron, 0.2%C and 0.82%C steels and 2.45%C cast iron were used. A plate shape specimen of 300μm in thickness was firstly hot dipped in a molten Zn bath for 3s and then hot dipped in a molten Zn-6mass%Al alloy bath for certain times, and quenched with water. The microstructure of the quenched specimen was observed by using a SEM, and change in thickness of the specimen was measured by using a laser microscope. The pure iron specimen was entirely changed to an intermetallic compound of Fe2Al5(Zn) phase after 1.2ks hot dipping. The thickness of 0.2%C steel specimen was decreased linearly with the dipping time after an early non-reaction period, and 2/3 of the specimen was changed to the Fe2Al5(Zn) phase after 3.6ks hot dipping. It was observed that primarily precipitated ferrite phase preferentially reacted and pearlite structure scarcely reacted. The reacted thickness of the 0.82%C steel and cast iron specimens were only several μm after 86.4Ks hot dipping. In the cast iron specimen, cementite reacted with Al and an Al3C4 phase was formed. The growth of the alloy layer was restricted by the Al3C4 phase. A heat-treated 0.2%C steel specimen with sorbite structure also exhibited the same low reaction behavior as the 0.82% C steel due to homogenized ferrite + cementite structure.
Aluminum was anodized in diluted KOH solutions galvanostatically to examine the effect of KOH-concentration on the growth and breakdown of anodic oxide films. Time-variations in the anode potential, Ea, the amount of dissolved Al3+ ions, and electroluminescence intensity were measured during anodizing, and the film structure was observed by transmission electron microscopy and scanning electron microscopy. At 5×10-5 and 5×10-4M, Ea increased with time linearly at the initial stage, and the rate of increase in Ea decreased at 400V. The slow increase in Ea stopped at about 600V, and beyond the potential Ea increased at a high rate until the film breakdown started at 1600V and 1100V in 5×10-5 and 5×10-4 M solution, respectively. In both solutions, the growth of the oxide film was accompanied by the formation and growth of relatively large voids between 400V and 600V, and beyond 600V small numerous voids developed on the entire surface. The film growth can be explained by the local oxide dissolution and precipitation of hydroxide. At 5×10-3 M, Ea remained zero for the initial 1500s of anodizing, and then increased linearly with time before film breakdown started at 600V. The anodic oxide films consisted of two layers: an outer porous layer and an inner dense layer.
Aluminum covered with porous anodic oxide films was re-anodized in 0.5 kmolm-3-H3BO3 and 5×10-5kmolm-3-KOH solutions to examine the formation and breakdown of the oxide film. In H3BO3-solution, the pores were filled with new oxide during re-anodizing, resulting in the uniform thickening of the barrier layer until film breakdown started. The breakdown potential of the film formed by the “pore-filling” method was 200V higher at maximum than that formed by anodizing after electropolishing. In KOH solution, the pore-filling with new oxide was accompanied by local dissolution of oxide film, leading to the formation of rough interphase between oxide film and the substrate. The film breakdown potential of the film formed by the “pore-filling” was as high as that the the oxide film formed after electropolishing. Effects of the structure of porous anodic oxide films on the film breakdown potential during re-anodizing in H3BO3 solution are discussed.
We investigated a recycling process for electroless nickel plating bath using nickel hypophosphite. Phosphite ion accumulated in aged bath were separated as calcium phosphite precipitated by addition of calcium hydroxide, then pH of the phosphite ion separated mother liquor was adjusted to plating condition by way of electrodialysis (method I), which was compared with sulfuric acid addition (method II). The deposition rate kept constant till 30 MTO in method I, but decreased slightly in method II. The internal stress of deposit tended to be almost 0 constantly in method I, but shifted strongly to the tensile side in method II. In addition, concentration of ingredients in the bath remained almost constant in method I, but sulfuric ion concentration increased by accumulation of sulfuric acid added to adjust pH in method II. We found that method I was superior on the pH adjustment to method II from standpoint of keeping bath life long. Corrosion-resistance of film plated by recycled bath remained good (larger than 9 of rating score in a salt spray test) and the surface of the films stayed clean till 30 MTO. We found that there was little variation of the recycling bath composition by the treatment in this condition and constant aging state of the bath was maintained and good plating film was obtained.