Hexavalent Chromium-Free Pre-Treatment for Galvanized Steel

Abstract

The corrosion protection provided by an organic inhibitor containing an alkanolamine salt of a polycarboxilic acid that is suggested as non-toxic alternative to chromate and phosphate conversion coatings on galvanized steel was studied. Open circuit corrosion potential evolution, linear polarization, potentiodynamic polarization, electrochemical impedance spectroscopy, x-ray diffraction, salt spray (fog) tests and immersion - weight loss tests were used to evaluate the provided protection in a 0.5 M aerated NaCl solution. All tests gave concordant results, indicating that the chromate-free 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, and except for the phosphated samples, the corrosion rate diminishes due to the appearance and growth of passivating corrosion products layers, mainly con stituted by zinc hydroxychloride (Zn5(OH)8Cl2•H2O) and two varieties of zinc hydroxide.