Abstract
A stannate conversion coating treatment was performed on die-cast AZ91D magnesium plates after acid pickling in hydrofluoric, nitric, and hydrochloric acid. The types of anions in acid solutions influenced the etching behavior of the AZ91D plate as well as the composition and microstructure of the resulting passive film, specifically the presence of fluoride anions favored corrosion arrack on the β phase over the primary α grain. The passive film formed in hydrofluoric acid comprised Mg hydroxide and fluoride and was more compact than that formed in nitric acid. A denser passive film containing more Mg fluoride promoted the formation of thicker stannate conversion coating with fewer uncoated spots and crevices. As a result, the corrosion resistance of the stannate conversion treated AZ91D plate was improved, as measured by electrochemical polarization and salt spray tests.
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