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. 1. Introduction 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. PCCs provide uniform surface texture and increased surface area, and, when used as a base for paint, they promote good adhesion, increase the resistance of the paint to humidity and water soaking, and eventually increase the corrosion resistance of the painted system [1]. Most galvanized steel used in manufacturing industries (car, household appliances, etc.) is phosphate coated and painted. However, some authors [2] have reported the harmful effects of phosphating, mainly those compositions that contain nickel. 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]. Although chromate is an excellent corrosion inhibitor, it is highly toxic; it has carcinogenic effects and must be handled and disposed of with extreme care. Therefore, there are severe restrictions on its use. Much effort has been devoted to replace chromate chemicals with safe, nontoxic alternatives that are environmentally benign, and many environmental friendly coating systems are under development [4–7]. However, preparation and corrosion behavior of these
References
[1]
X. G. Zhang, Corrosion and Electrochemistry of Zinc, Plenum Press, New York, NY, USA, 1996.
[2]
J. F. McIntyre and R. J. Brent, “Tendencies of the pretreatment technology in next millennium,” Pinturas y Acabados Industriales, vol. 42, no. 261, pp. 36–44, 2000.
[3]
R. G. Buchheit, S. B. Mamidipally, P. Schmutz, and H. Guan, “Active corrosion protection in Ce-modified hydrotalcite conversion coatings,” Corrosion, vol. 58, no. 1, pp. 3–14, 2002.
[4]
S. A. Furman, “Novel coating technology replaces chromate in zinc galvanizing process,” Materials Performance, vol. 49, no. 1, pp. 10–12, 2010.
[5]
J. W. Bibber, “Chromium-free conversion coatings for zinc and its alloys,” Journal of Applied Surface Finishing, vol. 2, no. 4, pp. 273–275, 2009.
[6]
T. Peng and R. Man, “Rare earth and silane as chromate replacers for corrosion protection on galvanized steel,” Journal of Rare Earths, vol. 27, no. 1, pp. 159–163, 2009.
[7]
A. D. King and J. R. Scully, “Sacrificial anode-based galvanic and barrier corrosion protection of 2024-T351 by a MG-rich primer and development of test methods for remaining life assessment,” Corrosion, vol. 67, no. 5, Article ID 055004, 22 pages, 2010.
[8]
M. G. Fontana, Corrosion Engineering, McGraw-Hill, Singapore, 3rd edition, 2008.
[9]
M. Toga, P. Moyo, and D. J. Simbi, “Corrosion of galvanized steel in simulated mine water environments,” in Proceedings of the 15th International Corrosion Congress, paper 231, 9 pages, Granada, Spain, 2002.
[10]
M. Sagiyama, A. Hiraya, and T. Watanabe, “Electrochemical behavior of electrodeposited zinc-iron alloys in 5%NaCl solution,” Journal of the Iron and Steel Institute of Japan, vol. 77, no. 2, pp. 244–250, 1991.
[11]
R. L. Zeller and R. F. Savinell, “Interpretation of A.C. impedance response of chromated electrogalvanized steel,” Corrosion Science, vol. 26, no. 5, pp. 389–399, 1986.
[12]
A. A. O. Magalhaes, I. C. P. Margarit, and O. R. Mattos, “Electrochemical characterization of chromate coatings on galvanized stell,” in Proceedings of the International Symposium on Electrochemical Impedance Spectroscopy (EIS '98), pp. 239–241, Rio de Janeiro, Brazil, 1998.
[13]
F. Deflorian, V. B. Miskovic-Stankovic, P. L. Bonora, and L. Fedrizzi, “Degradation of epoxy coatings on phosphatized zinc-electroplated steel,” Corrosion, vol. 50, no. 6, pp. 438–446, 1994.
[14]
C. Lin, T. Nguyen, and M. E. McKnight, “Relation between AC impedance data and degradation of coated steel. 1. Effects of surface roughness and contamination on the corrosion behavior of epoxy-coated steel,” Progress in Organic Coatings, vol. 20, no. 2, pp. 169–186, 1992.
[15]
L. M. Callow and J. D. Scantlebury, “Electrochemical impedance on coated metal electrodes. Part1: Polarization effects,” JOCCA, vol. 64, no. 2, pp. 83–87, 1981.
[16]
R. G. Buchheit and A. E. Hughes, “Chromate and chromate-free conversion coatings,” in Corrosion: Fundamentals, Testing and Protection, vol. 13A of ASM Handbook, pp. 720–35, ASM International, Materials Park, Ohio, USA, 2003.
[17]
I. Suzuki, Corrosion-Resistant Coatings Technology, Marcel Dekker, New York, NY, USA, 1989.
[18]
W. Zhang and R. G. Buchheit, “Effect of ambient aging on inhibition of oxygen reduction by chromate conversion coatings,” Corrosion, vol. 59, no. 4, pp. 356–362, 2003.
[19]
W. Feitknecht, “Studies on the influence of chemical factors on the corrosion of metals,” Chemistry and Industry, vol. 36, pp. 1101–1109, 1959.
[20]
V. Kucera and E. Mattsson, “Atmospheric corrosion,” in Corrosion Mechanisms, F. Mansfeld, Ed., pp. 211–284, Marcel Dekker, New York, NY, USA, 1987.
[21]
X. G. Zhang, Corrosion and Electrochemistry of Zinc, Plenum Press, New York, NY, USA, 1996.
[22]
C. M. Rangel and L. F. Cruz, “Zinc dissolution in lisbon tap water,” Corrosion Science, vol. 33, no. 9, pp. 1479–1493, 1992.
[23]
V. Barranco, S. Feliu Jr., and S. Feliu, “EIS study of the corrosion behaviour of zinc-based coatings on steel in quiescent 3% NaCl solution. Part 1: directly exposed coatings,” Corrosion Science, vol. 46, no. 9, pp. 2203–2220, 2004.
