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Abstract
A vanadium-based conversion coating is proposed as an effective pretreatment for electroless Ni–P deposition on AZ91D magnesium alloy. The magnesium alloy substrate was immersed in a NaVO3 solution with various parameters such as the vanadium concentration, immersion time and bath temperature being varied. The results reveal the quality and corrosion protection performance of the coating increases with increasing treatment time up to a limit. However, excess treatment time induces the formation of cracks in the coating layer, leading to reduced corrosion resistance of the conversion coating. Increasing bath temperature decreases the crack density and increases the thickness of the conversion layer. Electrochemical polarisation results showed that the vanadium treatment in a bath containing NaVO3 of concentration 30 g L−1 at 80°C for 15–20 min gives good corrosion resistance. This optimum vanadium treatment was used as a pretreatment for the subsequent electroless Ni–P deposit and it had good passive parameters, showing better corrosion protection on AZ91D substrate than a chromium based treatment.
This work was supported by National Nature Science Foundation (grant no. 50871046), the Foundation of National Key Basic Research and Development Program (no. 2010CB631001) and the Program for Changjiang Scholars and Innovative Research Team in University. One of our authors (J. Sudagar) acknowledges the India–China cultural exchange scholarship program by the Ministry of Human Resource Department (India) and Ministry of Education (China).