12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

Zinc-coated steels are utilized for home appliances, construction materials and automobiles. On zinc-coating layer, chemical conversion coating is formed. Chromate-free chemical conversion coatings generally contain phosphate compounds to prevent zinc corrosion mainly in the flat areas. On the other hand, when zinc-coating layer is damaged during the manufacturing process, the underlying steel can be exposed to the atmosphere and subjected to corrosion factors such as water, oxygen and salt. In this work, with the purpose of establishing guidelines for improvement of anticorrosion property, we investigated the effect of phosphoric acid on the corrosion protection in areas of exposed steel substrate. We showed that, in the salt spray test, phosphoric acid added to a chemical conversion coating enhances corrosion resistance even in scribed area that expose the steel substrate, preventing zinc corrosion from the scribed areas. We think compounds consisting of Zn2+ and PO43− covered the steel substrate and acted as a barrier against the corrosion factors. In addition, the amount of PO43− eluted from the chemical conversion coating during the test correlated with the corrosion rate. We therefore demonstrated that, to effectively inhibit the zinc corrosion in scribed areas, PO43− is necessary to be continuously eluted from the chemical conversion coating.