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Surface pretreatment is generally applied before application of protective coatings on Mg alloys, which influences surface microstructure and electrochemical activity of the substrate and has an effect on the coating properties. The effect of various pretreatment processes (sand-blasting, grinding and polishing) on the microstructure and corrosion protection performance of phosphate conversion coating (PCC) on AZ91D Mg alloy was investigated in the present study. Sand-blasting cleaning significantly increases the surface roughness and electrochemical activity of the substrate, leading to formation of a porous PCC with inferior corrosion protection performance. In the case of ground/polished Mg alloy, the uniformity and corrosion resistance of the resultant conversion coating are mainly related to the surface roughness. Relatively low surface roughness of the substrate facilitates formation of a corrosion protective PCC.

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This paper researched on metallographic image analysis of primary phase morphology in semi-solid alloys. The image processing and quantitative analysis system is based on labVIEW and IMAQ Vision. The system uses image processing and analysis technology, such as manual threshold, removing small particles and filling holes processing, edge detection and quantitative analysis of the volume fraction, size and shape of the primary solid. According to the testing results, this system has characteristics of less time-consuming, quick inspection speed and high measurement accuracy, which completely satisfies the requirements.