Effect of Cold Rolling on Passive Film on Pure Iron in pH 8.4 Borate Buffer Solution

摘要

Industrially important metallic materials such as steels and aluminum alloys usually undergo modification of metallographic texture by the working process of cold rolling. Although a number of metallographical studies have revealed the mechanical characteristics of materials deformed by cold rolling, it has not been clarified that corrosion resistance of the material's surface is degraded by the rolling. The degradation of corrosion resistance seems to be mainly caused by a phase transformation and alloy segregation (1-7), though dislocation and/or strain of the material's substrate have also been shown to be detrimental to the corrosion resistance (8, 9). However, there are no consistent changes in pitting and corrosion potentials (4, 6, 10, 11) and in the dissolution rate for the worked steels (3, 12-14). These inconsistent results seem to be due to the differences in the working degree and the materials' compositions and phases. It is very difficult to investigate only the effect of individual metallographic texture induced by working on corrosion behavior of alloys because the alloys easily cause the phase transformation and/or alloy segregation at grain boundaries. Thus, it is important to investigate each metallographic parameter with pure metals causing no phase transformation and no precipitation.rnThe corrosion resistance of metals strongly depends on the passive film formed on the surface. The characteristics of a passive film formed on pure iron in pH 8.4 borate buffer solution have been investigated in detail using various techniques such as coulometry (15, 16), electrochemical impedance spectroscopy (EIS) (17-19), ellipsometry (16, 20), surface-enhanced Raman spectroscopy (21, 22), extended X-ray absorption fine structure (23, 24), X-ray diffraction (25) and X-ray photoelectron spectroscopy (26). Although details are still not clear completely, the iron passive film is composed of a spinel structure like γ-Fe_2O_3 and/or Fe_3O_4, having the property of an n-