The Effect of steel surface characteristics on coating behaviour in hot dip galvanizing

ABSTRACT Hot dip galvanizing takes an important place on the protection of steel products against atmospheric corrosion. The thickness and appearance of the coating achieved after galvanizing process are controlled through the progress of the reactions between Fe and Zn in zinc. Especially the development of the reactions depends on the composition, shape and surface characteristics of the steel substrate, the composition and temperature of the zinc bath and immersion time. In this study, the effect of the surface characteristics purposely produced by chemical and mechanical methods on the galvanizing behaviour has been examined. For these purposes, the samples with predetermined silicon contents that cause significant changes in coating characteristics have been used. The methods to change the surface characteristics are pickling, grinding and polishing. In the examinations performed under optical microscope, different coating characteristics have been observed for pickled, ground and polished samples. Additionally, the change of the thickness of the alloy layers against immersion time has been defined by measuring the their thickness under optical microscope. After experiments, it has been determined that the polished surfaces of the steel samples containing low silicon such as 0.010% increase reaction rate. Ground samples give the thinnest coatings among the others. Pickled samples lead to the coatings as in the Sandelin curve. As a result, it is determined that surface geometry is the main factor which results in various coating characteristics between the surfaces having the different surface roughness and deformation energy. In the Sandelin steels, the surfaces having sharp edges prevented the over formation of Ç alloy layer formed due to silicon content. VI