Effect of Navo₃ concentration and hcl pre-treatment on corrosion behavior of anodized coated AZ91D magnesium alloys

Abstract

Magnesium and its alloys have some excellent properties, such as the lowest density of industrial metals, a high strength-to-weight ratio, vibration absorbency and electromagnetic wave shield characteristics. Because of these properties, magnesium and its alloys can be applied in the manufacture of components for motor vehicles, in housing for electronics, and in the manufacture of sporting goods. In addition, magnesium and its alloys are more easily recycled than synthetic resins, which make these materials eco-friendly. The poor resistance of magnesium and its alloys to corrosion has limited the application of these materials in corrosive environments. Anodizing coating is one of the most cost effective and simple techniques for introducing a metallic coating to a substrate. This study deals with the preparation of oxide films on AZ91D magnesium alloy by anodizing coating in a solution containing La(NO3)3 and/or Mg(NO3)2, with additive NaVO3 concentration in the solution. The anodizing coating on the AZ91D magnesium alloy has been carried out and the effect of NaVO3 concentrations and pre-treatment was investigated. A chemical compound of AlO, MgO and VO was detected from the diffraction peaks of XRD pattern confirming the formation of an oxide layer on the surface of AZ91D magnesium alloy. The corrosion resistance of AZ91D magnesium alloy increases remarkably with corrosion rate of 0.2 mm/y was obtained. A self-healing morphology could be observed from SEM images indicated the recovery of the NaVO3 rich oxide film from the severe environment of NaCl solution. This result is remained constant for further improvement by HCl pre-treatment. A rough surface was obtained after immersing in HCl solution for 20 s and followed by anodizing, has yielded the corrosion rate up to 0.19 mm/y. Two peaks assigned for VO and MgO was detected from XRD patterns. The corrosion protection of AZ91D magnesium alloy has been increased by the following ways; the formation of selfhealing morphology to recover from the severe environment by the addition of NaVO3, and formation of a passive oxide layer on the surface of AZ91D magnesium alloy by immersion in HCl solution. Based on the electrochemical experiments of potentiodynamics polarization and immersion test result, coating with NaVO3 and HCl pre-treatment successfully increased the corrosion resistance of AZ91D magnesium alloy. The result demonstrated here will strongly contribute to the broad application of magnesium alloy in the field of the automotive industry.