The effects of fine silica particles on dry-in-place chromate coating system were examined in terms of the amount of silica addition, the drying temperature in treatment, and the corrosive environment on corrosion resistance. The mechanism behind corrosion resistance was analyzed in the terms of the corrosion prevention of silica particles in the chromate coatings by AC impedance and XPS. When silica is added to a solution of dry-in-place chromate, the anticorrosion effect is clearly improved due to the chemical and physical effect of silica particles, i. e., the chemical adsorption-desorption action of chromium ion and the physical resistance against the transmission of corrosive factors through the coating. We found that the optimum amount of silica addition and the optimum drying temperature for obtaining high corrosion resistance differed with the environment. These results indicate that the coating properties of silica contained in dry-in-place chromate are extrinsically determined by the corrosive environment.
We attempted to nitride titanium using radio frequency plasma. We studied the relationship between nitriding conditions and characteristics of the nitrided layer have been investigated, then tested stationary fracture strength and the fracture mechanism of the nitrided layer using inverse acoustic emission (AE) analysis. We obtained the following results. 1) The composition of the nitrided layer depends greatly on the substrate temperature. Stoichiometric TiN were maintained as long as temperature was high enough. Nitriding was governed by the internal diffusion of nitrogen. The hardness of the nitrided layer decreases exponentially with depth. The nitriding rate showed the activating dominance of temperature dependence. 2) Fracture mode and dynamics under stationary loaded conditions was evaluated by inverse AE signal analysis. A nitrided layer dominated by Ti2N, which is treated at high temperatures, has more excellent mechanical properties than that of TiN-dominated layers treated at high temperatures. 3) The wear loss in the nitrided layer, which has considerable hardness and is dominated by TiN, is small compared to that of Ti2N. Nevertheless, cracks occurred in the TiN layer under sliding wear. Exfoliation and fracture of the nitrided layer under sliding wear could be evaluated by AE source wave analysis if an exact analysis of stress in the nitrided layer could be conducted..
The anodic oxidation of reductant H2PO2- related to electroless nickel plating was studied at various pH, reductant concentrations, temperatures, and additives using electrochemical methods. The oxidation rate of H2PO2- decreases with the decrease in pH and, when pH is in the range of 4-6, the oxidation rate can be increased by adding malic acid. Malic acid decreases the reaction order of H2PO2- from 0.54 to 0.19. The existence of free Ni2+ in the solution decreases the H2PO2- oxidation rate because insoluble materials are formed between free Ni2+ and HPO32+ coming from the oxidation product of H2PO2-. The detrimental effect of free Ni2+ can be eliminated by the addition of malic acid through the complex reaction between free Ni2+ and malic acid. Experimental results are better explained by the electroless reaction presented by Van Der Meerakker.
The objective of this study is to examine the mechanical strength of ARE-deposited Ti (C, N) film by using acoustic emission (AE) source characterization under tensile and compressive loading. Threshold stress in the substrate steel causes Mode-I fractures in the film as determined by quantitative source inversion of AE signals emitted from the limited Ti (C, N) area with a uniform stress distribution. Film fractures were invariably associated with the emergence of slip steps in the substrate steel. The threshold stress for film fractures under tensile loading was almost 9times greater than the stress under compressive loading. A simple method for estimating internal stresses in film from threshold stresses under different loading was also proposed. The internal stress in the film, estimated from the maximum strain criterion, was -3.8GPa which coincided with the reported internal stress measured using X rays.
The cross-sectional structure of a silica-added chromate film on electrogalvanized sheet steel was studied by TEM, XPS, and GDS. The results were as follows: 1) the chromate film thickness was about 100nm; 2) the chromate film consisted of aggregated silica and chromate which consisted of Cr3+ and Cr6+; and 3) the composition of silica and chromate was constant in the cross-sectional direction except at the surface where bare silica was exposed.