塗膜下腐食の交流インピーダンス特性と物質移動
1988 年 39 巻 1 号 p. 2-9
詳細
1988 年 39 巻 1 号 p. 2-9
Impedance characteristics and mass transport of underfilm corrosion were studied in detail under anodic and cathodic polarization. Cathodic blisters were found to be 20 to 100 times larger than anodic blisters. Chemical analysis of the solution under the films revealed that the anodic current through the film was carried by chloride and ferrous ions, and the cathodic current was carried mainly by sodium ion in the initial stage of blistering. As a result of the charge transport, the pH of the anolyte decreased through hydrolysis of ferrous ion, while that of the catholyte increased to 12.5 or more due to the concentrated solution of sodium ion.
At the initial stage of delamination, the impedance characteristics of coated steels showed an impedance arrest in the very low frequency region. This arrest split into two-in the very low and medium frequencies-and the impedance values at both arrests decreased with time. In cathodic polarization, the impedance decreased very rapidly and the impedance spectra were extremely similar to those observed on naturally delaminated coated steels. An equivalent circuit for the degraded coated steels was derived from a physicochemical model of delamination. It was easily shown that the break point frequency which appeared in the Bode diagram as the onset frequency of the impedance arrest (phase shift =45°) was proportional to the area of delamination. Based on this, a break frequency method was proposed and examined to evaluate the degree of degradation of coated steels.
