Abstract
The corrosion potential of many passive metals (e.g., Mo, Ta, Zr, Nb, Hf, Ti, Al, and Fe) was found to vary linearly towards more noble values with logarithm of time in various passivating solutions. Potential and capacitance measurements indicate formation and thickening of barrier films. A general theoretical treatment of the kinetics of barrier film formation and thickening was introduced. According to this treatment, the barrier film may grow in 2 or 3 dimensions. The 3-dimensional barrier film may grow via the dissolution-precipitation mechanism or the solid-state mechanism. In all cases, the following equations were theoretically derived: for potential, Eh=A+B log (t+t°), where A, B, and t° are constants; and for capacitance, Cm−1=A′+B′ log (t+t°). where A′ and B′ are constants. The thickness measurements may help to distinguish between 2- and 3-dimensional barrier films. Also, the variation of some experimental conditions may be considered to identify the mechanism for 3-dimensional barrier film formation.
