The effects of potential and kinetic parameters on the formation of passivating noble metal rich surface layers during the selective dissolution of binary alloys

This article presents a mathematical model of the selective dissolution process that occurs below a critical potential, E(C), in a thin alloy surface layer which becomes protective as it enriches in the noble metal. A kinetic-transport product h (cm(-1)) was obtained which includes the effects of electrode potential, E, exchange current density, i(o), transfer coefficient, alpha, and interdiffusivity, D: h = i(0)/nFDC(A)(0)exp(alpha nF(E-E(rev))/RT) = k/D where k (cm s(-1)) combines the polarization, kinetic parameters and the concentration of the less noble element A in the bulk A-B alloy, C-A(O). A dimensionless product h root Dt = k root t/D, where t is the time, was found to affect the rates of selective dissolution and surface recession, C-A and C-B at the alloy surface and the thickness of, and the concentration profiles within, the (characteristically defective) interdiffusion layer at the alloy surface. For h root Dt greater than or equal to 5 this layer is seriously depleted of A and proportionately enriched in B, under which condition the solution degenerates to a parabolic rate law which controls the process. The concentration C-A at the alloy surface decreases to 50% of its initial value at h root Dt = 0.75 and to only 10% at h root Dt = 5.6. Surface enrichment in B depends on its mole fraction in the bulk alloy, being greater for lower mole fractions. The model predictions are compared to experimental results in the literature. Copyright (C) 1996 Elsevier Science Ltd.