Thermodynamic Evaluation of the Effects of Amorphism on Film Breakdown and Pitting Initiation

Maximum differences in the chemical free energy Δ G a - c between a crystalline oxide and its fully amorphous counterpart at 298 K are estimated from melting point T m and enthalpy of fusion H F data for oxides in ten different metal systems (Be, Zn, Cu, Al, Cr, Fe, Ni, Zr, Sn, and Ti). The results are used to determine the relative thermodynamic stability of crystalline and amorphous phases in aqueous solutions in order to understand the possible influence of amorphism on precursor pitting sites in passive films. Variations in the degree of amorphism are treated by using different ϕ Δ G a - c values, where 0 < ϕ < 1 , and examining the resulting effects on pH-dependent oxide solubility and E – pH equilibria. Estimated surface energies of high-angle grain boundaries are used to show that grain boundaries in crystalline oxide films may be treated as amorphous regions.