Mechanism of corrosion protection of hot-dip aluminium–silicon coatings on steel studied by electrochemical depth profiling

The electrochemical behaviour of hot dip aluminium–silicon coatings on steel is studied in relation to the in-depth structure and composition, determined by SEM/EDX and EBSD. Progressive GDOES sputtering was used for electrochemical analysis using a micro-capillary three-electrode microcell in craters positioned in the various sublayers. Additionally, cross-sections were analyzed with SKPFM. Results show that the outer free aluminium layer of the coating is able to cathodically protect the underlying layers and substrate. Conversely, the interdiffusion layers are nobler than the steel. The galvanic series implies that cathodic protection will depend on the nature of the defect and the corrosion environment.