Effect of molybdate coating for white rusting resistance on galvanized steel

摘要

Purpose - Chromium based conversion coatings have been used widely to delay the formation of white rust on galvanized steel sheets. As chromium is carcinogenic, an alternative chromium-free conversion coating was developed for use on galvanized steel surfaces to delay the onset of white rusting. This paper seeks to address this issue. Design/methodology/approach - The molybdate coating was produced on galvanized steel surface by dipping in a phosphate solution bath using Na{sub}2MoO{sub}4 as a corrosion inhibitor. Findings - The cyclic polarization tests conducted on molybdates-treated samples showed a wider potential band in 3.5 per cent NaCl solution, indicating that the coating is passivating in nature. The polarization (R{sub}p) of both treated and untreated samples was evaluated in 3.5 per cent NaCl solution by electrochemical impedance spectroscopy (EIS) technique. It was observed that the polarization resistance (R{sub}p) of molybdate treated galvanized sample is 2.3 times higher compared with the untreated one. The improved corrosion resistance also was evident from the simulated salt spray test results where the treated sample showed a five times improvement over the untreated sample. Research limitations/implications - The corrosion resistance of inorganic-based chromium-free conversion coatings has not been equivalent to that of chromium based conversion coatings. A chromium-free conversion coating with enhanced corrosion performance by using water reducible polymer will be described in a future paper. Practical implications - The developed chromium-free passivation solution has been implemented for galvanized tube passivation. It is one of the best alternative chromium-free chemicals for the prevention of white rust formation on galvanized materials during transit and storage. Originality/value - This paper describes a new coating system for the galvanized material which will provide a significant improvement to white rusting resistance for the galvanizing industry.