Modelling the localized corrosion effects experienced by electroplated zinc and zinc - 4.5 wt.% aluminium steel coatings.

A general modelling approach is described for the numerical simulation of localized corrosion phenomena. The model is demonstrated using several simple cases and compared both to analytical solutions and experimental measurements. The model is intended to operate at the microscopic-mesoscopic length scales and involves two- or three-dimensional field calculations performed in a finite difference computational framework. Limitations and possible extensions to the algorithm are discussed. Experimental work has been reported that demonstrates the effects of microstructural variations within Zn-Al Galfan type coatings on the corrosion behaviour of cut-edge material, i.e. those cases where both the underlying steel and the organic coated Galfan layer are simultaneously exposed to a corrosive environment. An attempt to model the localized corrosion effects in electroplated zinc and hot-dip Galfan coatings has been made. The model combines both diffusive and electrochemical phenomena and describes corrosion effects on micro scale coating layers in NaCl electrolyte. The model predicts the 3D form of electrical potential, localized current densities and concentrations and also the time-dependent degradation of the micro scale coating layer. Results of the prototype model are quantitatively compared with measured current densities obtained from Scanning Vibrating Electrode Technique (SVET) studies. Simulations have been performed to predict the microstructural influence on the corrosion of Galfan coatings cooled at different rates. The model is in good agreement with experimental findings with respect to the cut-edge behaviour of these coatings, although contradicts those SVET measurements made in respect of the surface corrosion performance