Abstract
Conventional weight loss tests and both DC and AC electrochemical techniques were used to study if an organic inhibitor containing an alkanolamine salt of a polycarboxylic acid can substitute toxic coatings as chromating and certain phosphating procedures in the protection of galvanized steel. The electrolyte used was a 0.5 M aerated NaCl solution. All tests gave concordant results, indicating that the chromate-free organic inhibitor does protect galvanized steel in this environment, even though the provided protection was less than that of the chromate conversion coating. It was observed that, after a moderate initial attack, the corrosion rate diminishes due to the appearance and growth of passivating corrosion products layers, mainly constituted by zinc hydroxychloride and two varieties of zinc hydroxide, among other crystalline compounds.
Highlights
Conversion coatings are applied to galvanized steel to improve adhesion of additional protective coatings and for corrosion protection of the zinc coating
The purpose of this paper is to evaluate the corrosion protection provided by a chromatefree organic inhibitor that is suggested as a nontoxic alternative to chromate and phosphate conversion coatings on galvanized steel
It has been reported that, as far as the surface preserves its dark orange, almost brown aspect, Cr6+ ions have not been leached and the coating is still protecting the zinc substrate [18]. With regard to this white corrosion products layer, Feitknecht [19] previously reported the presence of zinc hydroxychloride and zinc hydroxide in immersion tests in a NaCl solution of the same concentration as that used in this study, and they appear in the reaction sequence for zinc exposed in natural marine atmospheres [20]: Zn −→ Zn(OH)2 −→ ZnCl2 · nZn(OH)2 (1)
Summary
Conversion coatings are applied to galvanized steel to improve adhesion of additional protective coatings and for corrosion protection of the zinc coating. Phosphate conversion coatings (PCCs) provide adhesion but do not provide substantial corrosion protection. Most galvanized steel used in manufacturing industries (car, household appliances, etc.) is phosphate coated and painted. Chromate conversion coatings (CCCs) for zinc have been the most widely used, as they enhance bare or painted corrosion resistance, improve the adhesion of paint or other organic finishes, and provide the metallic surface with a decorative finish. CCCs are distinguished by their easy application, their applicability to a wide range of alloys and, in many cases, their ability to improve the galvanized corrosion resistance by virtue of a built-in inhibitor reservoir [3].
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