| Electroplating zinc is widely used in that it can effective improve the anticorrosion longevity of steel products. However, because it is prone to appear white rust in humid environment, electrogalvanized steel is often treated by traditional chromate conversion technology treatment in order to enhance its corrosion resistance. Due to the highly toxic carcinogenesis of hexavalent chromium, this technology has been restricted and prohibited in the world.Cerium-salt conversion coating has become research hotspot of alternative chromate conversion coating technology as a result of environmental protection and good corrosion resistance. But there are two big imperfections: the corrosion resistance is low when the coating thickness is small; the corrosion resistance is also low because thick coating is loose, cracking, falling off.Based on the previous research of cerium-salt conversion coating on galvanized steel by our group, the different corrosion inhibitors, thiourea, methenamine, sodium citrate, benzotriazole, potassium iodide, sodium sulfide, were studied in order to improve the corrosion resistance of cerium conversion coating on galvanized steel. The result showed that cerium-salt conversion coating modified by sodium sulfide had the best corrosion resistance on galvanized steel. Under the condition of sodium sulfide6g/L, 25℃ and 3min, salt spray time of cerium conversion coating increased from the24 h before modification to 96 h after modification.Scanning electron microscopy(SEM), energy dispersive spectrometer(EDS),Tafel and EIS test results showed that adhesive force of cerium-salt conversion coating on the surface of galvanized steel was improved due to the formation of zinc sulfide precipitation after the addition of sodium sulfide, which made cerium-salt conversioncoating not easy to crack and fall off when the coating thickness increased, the content of anti-corrosion element increase(mass fraction of O, P, Ce increased by 6.81%,7.21% and 7.27%). Corrosion potential was about 85 v more positive, and the corrosion current density decreased by about 10 times, and polarization resistance increased by about 9 times. Anti-corrosion performance of galvanized steel was greatly improved. |
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