Investigation of the macroscopic and microscopic electrochemical corrosion behaviour of PVD-coated magnesium die cast alloy AZ91

The PVD plasma anodisation method developed by the authors allows the anodisation and subsequent PVD deposition of magnesium alloys in a process step using a conventional PVD sputtering unit. In the standard salt spray test, these surfaces show corrosion behaviour superior to conventional hard coatings, and they also show good wear characteristics [H. Hoche, H. Scheerer, E. Broszeit, C. Berger, Mater. Wiss. Werkst. Tech. 30 (2002) 395 [1]; H. Hoche, H. Scheerer, D. Probst, E. Broszeit, C. Berger, Surf Coat. Technol. 173 (2003) 1018 [2]]. To determine the corrosion behaviour, different test methods were applied to plasma-anodised and Al2O3-coated specimens of the magnesium die cast alloy AZ91D and CrN hard coatings with optimised deposition parameters for magnesium alloys. The investigation of the macroscopic electrochemical corrosion behaviour with current density-potential curves (10 mm testing diameters) does not allow any satisfactory description of the quality of the different surface treatments. Measurements with a microprobe (0.1 mm testing diameter) supply a suitable description of the electrochemical corrosion behaviour of PVD-coated magnesium alloys by comparing intact coating areas with specific coating defects produced by Vickers indentations. The results of the three testing methods are compared and brought into agreement with each other. It is shown that conventional macroscopic current density-potential measurements are not suitable to describe the corrosion behaviour of PVD-coated magnesium alloys. (c) 2004 Elsevier B.V. All rights reserved.