Effect of chromate on the electrochemical behavior of sintered Zn–Al coating in seawater

To improve the anticorrosion resistance of chromium-free Zn–Al sintered coating, characteristics of chromate on zinc–aluminum–chromium were studied by electrochemical methods. Depth profiles of elements were obtained by glow discharge spectrum (GDS) and X-ray photoelectron spectroscopy (XPS) was used to analyse the chemical states of main elements in Zn–Al coatings. Corrosion behaviors of steel samples coated with chromium-free Zn–Al and zinc–aluminum–chromium exposed to seawater were investigated by using electrochemical impedance spectroscopy (EIS) measurement. Both in open circuit potential and EIS monitoring self-healing phenomena of chromate were obviously observed. The self-healing function endows zinc–aluminum–chromium coating with better anticorrosion performance. Electrochemical analysis show that self-healing action of coatings is missing when replace chromate with molybdate and silane.