Similarity and Difference in the Plating Pretreatment for Mg and Al Alloys

Abstract

Mg and Al are light-weight less-noble metals and are expected as a key material for potential applications in the fields of transportation to save energy. Especially for Mg alloys, the addition of corrosion resistance is essentially important for outside use. Plating has been used to provide various functionalities to metals such as good appearance, wear resistance, sliding performance, wettability or repellency, and corrosion resistance. Both Mg and Al have similar difficulty in the plating process because the surface of metals is always covered with the passive film (Al) or corrosion film (Mg) that prevents the plating process as the electron transfer and the metal-to-metal bonding. In the plating process, therefore, such surface films should be removed before or during the treatment process. The zincate pretreatment has been applied to these metals in which the surface was replaced with Zn layer that provides a good substrate for the subsequent plating process. In the zincate process, Al or Mg surface is dissolved, and Zn is deposited simultaneously by the substitution reaction in the zincate bath containing Zn(II) ions to cover the substrate. In the case of Al, this substitution reaction proceeds rapidly, and the substrate surface is almost completely covered with Zn layer. In the case of Mg, however, the coverage with Zn is more important than Al because Mg can be rapidly dissolved in the zincating bath. Excess dissolution of Mg substrate causes the undesired poor adhesion of the plating film to the substrate. Therefore, the zincating condition for Mg alloys should be optimized to take a balance between the Mg dissolution and the substitutional deposition of Zn. On the other hand, a complicated structure composed of the area where dense pillars are formed and a bank surrounding this area is sometimes found on AZ31 alloy after the zincate treatment, as shown in Fig. 1. Such a structure is formed by the combination of the continuous dissolution of Mg substrate and the local deposition of Zn that protects Mg substrate from the dissolution. A bank is formed on the grain boundary of AZ31 surface, where initial Zn deposition preferentially proceeded. Such micro-structure can be compared with a well-known ordered porous alumina film formed on Al by the dynamic process of the continuous growth of anodic oxide film and mass transportation of ions in the growing film. Figure 1