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

• The in-depth electrochemical behaviour of aluminized steel is studied.
• The outer free aluminium layer can cathodically protect the steel substrate.
• The inner interdiffusion layers cannot cathodically protect the steel substrate.
• Potential for cathodic protection depends on type of defect and environment.
• Galvanic series of the sublayers and Volta potential series show similar trend.

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.