EXTRAORDINARILY THICK PASSIVE FILM FORMATION ON Ni-Cr ALLOY WITH SQUARE WAVE POTENTIAL PULSE POLARIZATION

摘要

A thick passive film showing interference color is found to form on Ni-Cr alloys undergoing a square wave potential pulse polarization. A Ni-18Cr alloy is polarized in 5 kmolm~(-3) H_2SO_4 solution at 60℃ with the potential being modulated as square wave. The thickness of the film, which can be estimated from interference color spectra, varies with the modulation potential, frequency and duration time of the process. The films obtained consist mainly of chromium hydroxide and/or oxide, and have a porous structure which permits their further growth with almost no decrease in the growth rate. Two mechanisms for film formation are found. First, chromium ions are dissolved at the active potential region provided at the lower potential of the square wave, leaving a porous oxide/hydroxide layer under the high field provided at the passive potential region applied at the higher potential of the modulation. Alternatively, if the higher potential is in the transpassive region and the lower in the passive region of chromium, the Cr(Ⅵ) ions dissolved at the higher potential can be reduced to create the thick film, which consists mainly of oxide/hydroxide of Cr(Ⅲ).