Electroless Ni–P deposition with vanadium based coating as pretreatment on AZ91D magnesium alloy J. Sudagar 1,2 , J. S. Lian* 1 , Y. Q. Liang 1 and J. A. Liu 1 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 NaVO 3 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 NaVO 3 of concentration 30 g L 21 at 80uC 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. Keywords: Vanadium based coating, Electroless, Corrosion, Magnesium alloy Introduction Magnesium alloys have low density and this property makes them promising for several industrial applications. They also have the advantages of high specific strength modulus and excellent antishock resistance. Thus, they are prime candidates for aerospace and automotive industries and also in manufacturing electrical equipment such as cellular phones, television sets, and for sporting equipment. 1 Their heat conductivity and electromagnetic shielding effectiveness are attractive features for the information technology industry and for communication satellites. 2 However, magnesium alloys have poor atmo- spheric corrosion resistance and are very reactive in air. It is also difficult to apply corrosion protective electro- chemical treatment to them because of their high chemical affinity to aqueous solutions. Thus magnesium alloys are not suitable without a protective layer, especially in a humid environment, and require an appropriate surface treatment correctly applied onto the surface to improve the adhesion, wear 3 and corrosion resistance. 4 Various surface treatment techniques have been report- ed such as conversion treatment, electro-and electroless plating, and anodic treatment. Conversion coatings are produced by chemical or electrochemical treatment of a metal surface to make a superficial layer of substrate metal oxides, chromates, phosphates, or other compounds that are chemically bonded to the surface. 5,6 There are different types of conversion coatings such as chromate, phosphate/ permanganate, 7,8 stannate based, 9 cerium based, lantha- num and praseodymium conversion coatings, 10–12 tannic based treatments 13 and organic based (silane) coatings. 14 The above mentioned conversion coatings are unable to afford good mechanical properties, hence subsequent metallic coatings are essential. Electro- or electroless plating are among the most cost effective and simple techniques for depositing a metallic coating to the magnesium alloy substrate. In both cases, a metal salt in the solution is reduced to its metallic form on the surface of the workpiece. Electroless deposition is a more convenient method to deposit metal alloys than electroplating, involving the deposition with uniform coverage and thickness, crack free and good adhesion onto magnesium alloy, without applying an external electrical circuit. 15 The most difficult part of the process of plating magnesium is to develop a suitable pretreatment; once a suitable pretreatment is in place, many desired metals can be deposited. Conventionally, the magnesium alloy has been etched in a solution of Cr(VI) ions plus nitric acid and soaked in HF solution to form a conversion film (MgF 2 ), before electroless plating. But, the hexavalent chromium compounds are legislatively restricted (carcinogenic sub- stances) and HF also exhibits a strong corrosive nature. 1 College of Materials Science and Engineering, Jilin University, Changchun 130025, China 2 School of Civil Engineering, The Queen’s University of Belfast, Belfast BT7 INN, UK *Corresponding author, email lianjs@jlu.edu.cn ß 2012 Institute of Metal Finishing Published by Maney on behalf of the Institute Received 01 August 2011; accepted 12 March 2012 DOI 10.1179/0020296712Z.00000000022 Transactions of the Institute of Metal Finishing 2012 VOL 90 NO 3 129