engleski

Characterization of coatings on hot dip galvanized steel by optical microscopy

ne of the most effective methods for the corrosion protection of ferrous materials is zinc hot dip galvanizing. Although the benefits of such coatings are well studied and commercial technologies for hot dip galvanizing are already available, there is a need for more informations concerning the coating behaviour in contact with corrosive environment. This work presents the microstructural characteristics of galvanized coatings and their corrosion behaviour in the contact with chloride containing media (3.5% NaCl). Information of that kind is very useful for the prediction of the corrosion performance of galvanized constructions. Microstructure of the as-cast Zn coatings and their cross-sections were observed on pre- galvanized steel plates containing 0.16% C, 0.91% Mn, 0.33% Si and 0.017% P. The thickness measurement was also accomplished. Corrosion investigations have shown that on a freshly deposited Zn surface, oxidation in air leads to an uneven surface covered with crystals of different shape and, locally, dendritic crystalls of white rust are grown. However, after longer exposure to chloride containing solution, on the top of crystal planes many small randomly distributed globular nuclei were formed. White rust morphology was governed by the substrate microstructure as well as by the distribution of the active sites in the roughened Zn surface.

zinc coatings; corrosion stability; characterization methods; optical microscopy

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