Author(s)

S. G. R. Brown & N. C. Barnard

Abstract

An extension to a previously-developed numerical model is presented that predicts the sites of localized anodic dissolution on exposed surfaces of ZnAl alloy coating; often employed in the protection of steel-strip substrate material. Long-established thermodynamic concepts and galvanic-coupling mechanisms are used to identify sites susceptible to local metallic dissolution. Estimation of metal loss over time is linked to the electrode potentials predicted at the exposed alloy surface. Both alloy composition and the concentration of multiple species in the NaCl solution are considered in determining these electrode potentials. The temporal evolution of the Zn-Al-Fe system – represented using a cellular automaton framework – is predicted via field-based calculations on steady-state voltage and non-steady state diffusion/migration fields. Concentration perturbations in the electrolyte are captured and resultant potential fields are generated using a straight-forward finite difference technique on an irregularstructured computational grid. The influence of the microstructural features in the ZnAl alloy coatings is assessed in terms of metal loss, current density fields and pitted-depth. Simulated results are validated at the meso-scale in comparisons made to experimental observations in accelerated testing of these alloys in concentrated aqueous NaCl solutions. Simulations have been performed that quantitatively assess the localized corrosion at the surface of ZnAl alloy coatings and cut-edge scenarios where Zn, Al and Fe are exposed. The model is extended to predict the effect of altering the processing conditions, i.e. cooling rate and coating thickness. Keywords: Zn-Al coatings, galvanic corrosion, de-alloying, finite difference.

Keywords

Zn-Al coatings, galvanic corrosion, de-alloying, finite difference