1. Introduction
As the lightest structural metal material, magnesium and its alloys have a lot of excellent performances, including high strength to weight ratio, low density, high dimensional stability, good processing capability and recycling ability [1] and [2]. However, magnesium alloys are highly vulnerable to the attack of corrosive media due to the low standard potential, which impedes and limits their extensive applications. Up to now, there are many reported methods to protect magnesium alloys from corrosion, such as surface modification, corrosion inhibitor, and composition modification. Surface modification is an effective route to improve the corrosion resistance of magnesium alloys. Sol–gel films, electroless plating, electroplating, chemical conversion coating and so on, are typical examples of surface modification [3], [4], [5] and [6].