A novel approach to heal the sol–gel coating system on magnesium alloy for corrosion protection

Sol–gel-based coatings exhibit high potentiality to be as an alternative to toxic chromate coatings for surface pre-treatment of metals and alloys. However, as soon as even small defects appear in the coating, the coating cannot stop the development of corrosion process. Present work demonstrates the possibility to use zinc nitrate as healing agent to repair the organic silane coatings in NaCl solution. The zinc nitrate was added to the 0.005 M NaCl solution where AZ91D magnesium alloy coated with organic silane coating was immersed. The healing process and the healing mechanism were investigated by electrochemical measurements and scanning electron microcopy coupled with energy dispersive spectroscopy. The results demonstrated the introduction of zinc nitrate to the electrolyte could stop the development of corrosion process of the coating system and a remarkable recovery on corrosion resistance could be obtained. This effect may be attributed to the formation of zinc oxide/hydroxide on the defective areas, hindering the corrosion activities.