Abstract
In the present work, strips of steel St-37 were coated with zinc (Zn) using wire flame spraying and with tin (Sn) using plasma spraying. Their corrosion behaviour was examined by exposure in a salt spray chamber. The exposure time ranged from 24 h up to 18 days. The as corroded samples were studied by optical microscopy, scanning electron microscopy and X-ray diffraction, and in the case of zinc coatings, the weight of the corrosion products was also measured. From this research, it was deduced that zinc coatings corrode uniformly and pits are also formed. A layer of different zinc compounds including zinc oxide and hydroxides, hydrated zinc chlorides and hydroxides covers the coating, which has an inhibiting effect on corrosion. However, hot dip galvanised coatings seem to be more resistant probably due to the presence of Fe–Zn phases. Regarding the tin coatings, Sn and Fe oxides and chlorides were detected on the coating surface along with the formation of large cavities. A possible mechanism of internal oxidation (intergranular corrosion) was used to explain these observations. Their performance seems also inferior to that of the hot dip galvanised coatings.
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