Structure and formation mechanism of phosphate conversion coating on die-cast AZ91D magnesium alloy
Phosphate conversion coating, which is considered as an alternative to chromium conversion coating for improving the corrosion resistance of die-cast AZ91D magnesium alloy is studied. The structure and formation mechanism of the coating was investigated in details using ESEM/EDX, XRD, EPMA, ICP and...
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Veröffentlicht in: | Corrosion science 2008-02, Vol.50 (2), p.329-337 |
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Format: | Artikel |
Sprache: | eng |
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Zusammenfassung: | Phosphate conversion coating, which is considered as an alternative to chromium conversion coating for improving the corrosion resistance of die-cast AZ91D magnesium alloy is studied. The structure and formation mechanism of the coating was investigated in details using ESEM/EDX, XRD, EPMA, ICP and electrochemical method. It was found that the conversion coating was composed of complex phosphate containing magnesium and aluminum and showed amorphous structure. The thickness of the coating was about 10
μm. A possible formation mechanism for phosphate conversion coating was proposed. Local pH rise due to the evolution of hydrogen in the vicinity of micro-cathode attributed to the movement of hydrolysis reaction balance for manganese dihydro phosphate in treating solution and resulted in the phosphate coating precipitated on metal surface. Initial depositing position was related to the microstructure of substrate. Galvanic effect between α phase and β phase caused the flower-like phosphate nuclei preferential depositing on β phase. Ball-like phosphate nuclei deposited mainly in α phase interior due to the micro-galvanic effect within α phase. The primary α phase was removed from the metal matrix due to the corrosion dissolution, which suggested the anodic dissolution mechanism during phosphate conversion coating growth. The inner stress in dried film resulted in random distribution of the network micro-cracks on film surface, the crack site did not correspond to β phase. |
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ISSN: | 0010-938X 1879-0496 |
DOI: | 10.1016/j.corsci.2007.08.007 |