Modelling of atmospheric corrosion of steel protected by aluminium based alloys, applied by hot dip processing (ATCORAS) : final report

The project deals with the development of modeling tools for the prediction of (atmospheric) corrosion behavior of Al-based alloys coated steel. In view of the project objectives, a series of AlZnMg coatings was produced by Rhesca simulator. These coatings were characterized using a vast set of surface analysis and electrochemical techniques in order to obtain the information about their composition, microstructure, local and global electrochemical behavior, distribution of pH and solid corrosion products over the corroding surface in time and space. The main experimental observations are: Addition of zinc and especially of magnesium, has a beneficial effect on the protective action of Al-based coatings; The corrosion activity of the coatings is highly non-uniform (at micrometer level) and has a stochastic nature. Active corrosion areas passivate and re-activate every 3-6 hours; Corrosion products formed on the cut-edge surface do not prevent corrosion either in immersed or in atmospheric conditions. Based on the experimental results, we developed local and global numerical modeling tools that are respectively based on balance equation models and the potential model. The local model predicts for all metallic coatings correctly the pH distribution and localization of the precipitated corrosion products over a corroding surface. Both models were validated against the results of accelerated corrosion tests and outdoor exposure. They can adequately reflect the protective action of the AlZnMg coatings when making abstraction of their non-uniform corrosion behavior. These results form a strong basis for further developments towards very thin film conditions (below 25-50 μm) for which still much quantitative information is missing.