Conversion layers substituted to hexavalent chrome in the anti-corrosion protection of galvanized steel

Metals/metal alloys are among the most used materials in the industrial environment. Among these, ferrous alloys have been widely applied in different segments of the industry such as: automotive, chemical, oil and gas, etc. Due to the different environmental conditions in which this material is applied, there are situations that can cause steel degradation, in a phenomenon known as corrosion. The degradation of the metal by the effect of corrosion generates environmental and economic impacts, which cause a decrease in the cost-benefit ratio. In order to reduce these losses, several protection methods are used such as corrosion inhibitors, passivates and paints. The galvanization of steel is widely used in the metallurgical industry as a metallic coating for protection, where it presents a considerable increase in the useful life of the material and because it is economically viable. In turn, chromating is a procedure performed on metallic coatings in order to prolong the useful life of these materials. However, this method uses chromate ions as a source of passivation, which are known to have high toxicity and carcinogenic effects. The search for new passivators that replace hexavalent chromium and present less toxicity is necessary. In this work, galvanized steel samples were passivated with two possible substitutes for hexavalent chromium: Molybdate and Permanganate. The corrosion resistance of the conversion layers was verified by electrochemical tests of polarization curve, open circuit potential, chronoamperometry and impedance. For comparison purposes, the tests were also performed in dichromate solution. Through the comparative evaluation of the corrosion resistance of galvanized steels with or without conversion coatings, it was verified that both the permanganate and the molybdate coatings presented a good potential for replacing Hexavalent Chromium. However, under the experimental conditions tested, the conversion to permanganate showed more significant results than to molybdate.