金属表面技術

The specific conductivity of oxalic acid baths was measured as functions of composition, pH and temperature, by using a Wheatstone bridge. The specific conductivity of oxalic acid bath changes with concentrations in total chromium, Cr⁶⁺ and (NH₄₎₂SO₄, and with bath pH and temperature; for a typical bath with total Cr 100g/l, Cr⁶⁺ 12-38g/l and (NH₄₎₂SO₄ 75g/l at 60°C, it was about 0.08Ω^-1cm^-1. The bath voltage was controlled by the solution resistance and expressed by V=((-0.038logK-0.0273)L+0.09)J+2, in which K is the specific conductivity, J is the cathode current density and L is the distance between the anode and cathode electrodes. The covering power of the bath evaluated from the appearance of chromium plating on bent cathodes (plates crooked at an angle of 78°) at 100A/dm² was found to be superior to that of Sergent's baths. This is attributed to smaller current efficiencies at higher current densities and the extremely low conductivity of the oxalic acid baths.

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Etch-pits of various shapes were obtained when aluminum was anodically dissolved in chloride solutions. Studies were therefore conducted on predicting by computer simulation the shape of the etch-pit formed under various conditions. Aluminum of 99.99% purity was electrolytically etched in 0.85M HCl and it was found that: a) etch-pit diameters were smaller at 90°C than at 25°C, at 2, 000 Am^-2 than at 500Am^-2, and 5, 000Cm^-2 than at 30, 000Cm^-2; b) pit depth increased with increasing the electricity of etching; c) pits were square in shape under all conditions. A theoretical equation was derived from anodic dissolution theory for simulation purposes, and simulation for the etching morphology was carried out on the HITAC M-280H computer of the University of Tokyo. Constants in the literature were used for the digital simulation, and results were output in the form of contour line plots and cross-sectional profiles. The surface profile after a given anodization time was plotted by three-dimensional contour lines and in cross-section. Comparison with observations a), b), and c) of actual etching showed good agreement.

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