Microstructure and corrosion resistance of magnesium alloy ZE41 with laser surface cladding by Al-Si powder

The microstructure and corrosion behavior of commercial alloy ZE41 modified by surface laser cladding with Al-Si powder mixture was studied by SEM, TEM, X-ray diffraction and electrochemical methods. The coating is composed of an Al-Mg matrix and dendrite precipitates of Mg2Si. In function of the laser speed, the matrix is formed by a Mg solid solution in Al or by the intermetallic phase Mg17Al12. The presence of different matrixes is responsible for galvanic corrosion and decrease of corrosion resistance in interfacial area between coats. Isolated samples of the bulk coatings material showed similar corrosion potentials inspite of different matrix composition. This interpreted in terms of a mechanism involving two steps: (1) an initial dissolution of anodic Mg2Si particles followed by (2) pitting in the formed crevices. The proposed mechanism corresponds well with the experimental observations and the mechanisms of localized corrosion observed for aluminium alloys in the chloride media described in the literature. Improved corrosion resistance can be achieved by the microstructure homogenization through the optimization of laser parameters and/or following heat treatment.