長時間電解によるアルミニウム陽極酸化皮膜の局部腐食

Abstract

Local corrosion occurred in a long-period anodizing of aluminum specimens (Al P3-1/2H) in 100g/l H2SO4 containing 15g/l of H2C2O4.After anodizing, the cross-sections of specimens were observed by microscopy. The normal layer of the film was referred to as the 1st layer. An abnormal layer (the 2nd layer), having a considerably lower hardness value, was observed on the side of the base metal by microscopy.The results of the experiments were as follows:1) When the anodizing time was constant, the abnormal layer was likely to occur at lower bath temperature and under high current density (or in either of the both cases).2) When the local corrosion occurred, fluctuation appeared in V-T (bath voltage-anodizing time) curve. Then, a new layer (the 3rd layer) was produced and the bath voltage tended to be increased again.3) It was shown by electron microscopy that the cell size was the largest in the 2nd layer and was smaller in the 3rd layer approximate to the normal layer.4) Several V-T curves were studied at various bath temperatures and under various current densities. The bath voltage was rapidly dropped down from the maximum value under such a high current density as 8amp./dm2, independent of the bath temperature. Four peaks appeared on the V-T curve at 20°C under 4amp./dm2 and the layers corresponding to the peaks were also observed in the cross-section of the film.5) It was effective in preventing local corrosion of the anode, when the current density was decreased at the bath voltage approximate to the maximum value.