Investigate Of Chromate Conversion Coatings On Aluminum Alloy

The traditional hexvalent-chrome coating have been largely used in surface treatment for its low cost,simple process and corrosion resistance obviously excelled all other chemical conversion coatings. However, in the situation of severe environmental requirements,it would be replaced by low or non-toxic chemical conversion coating for the highly toxic hexavalent chrome, the trivalent-chrome coating is considered the best alternative technology, because its toxicity is only 1% of hexvalent-chrome coating. The rare earth elements are widely applied in the metal surface treatment, their special electronic structure and strong chemical activity can refine crystal grains and improve the protective properties of coating, so they are used as good accelerators.In this paper, the formation mechanism,morphology and corrosion resistance of hexvalent-chrome coatings were investigated by SEM, the open circuit potential-time curve, current-time curve, potentiodynamic polarization curves and EIS,The effect of different rare earth chloride (0-60mg/l CeCl₃,LaCl₃,YCl₃) on hexvalent-chrome coating were also studied. On this basis, the experimental system of trivalent-chrome coating was optimized, and its preparation and performance were researched and discussed by SEM and the electrochemical test system.According to the results of orthogonal test, the best formula and process parameters of trivalent-chrome coating were CrO₃ 3.5 g/L,NaF 1.2 g/L,K₂Cr₂O₇ 1.2 g/L,pH 2.0,processing temperature: 40℃, processing time: 5 min.The formation of hexvalent-chrome coating is an electrochemical reaction, and the time of prepartion process is very short about 200s. There are some cracks on the hexvalent-chrome coatings that its major chemical composition are hydrated aluminum oxide and chromium trioxide, it is that the solid insulating layer improves the protective properties of aluminum alloy. Part of Cr⁶⁺ may be trapped in the film, when the coating has small damage, it would play the role of self-healing.Compared the effect of different content of (0-60mg/l) rare earth additives (CeCl₃,LaCl₃,YCl₃) on hexvalent-chrome coating, the results indicated that the rare earth elements had little impact on the formation process, but they could make the film more uniform and compact, so that the deposited coatings on AA 6061 could dramatically reduce the corrosion current density (icorr) and shift the corrosion potential (Ecorr) towards the more positive region, and 40 mg/l CeCl₃ had the excellent effects on the microstructure and protective properties of hexvalent-chrome coating.The optimized experimental system of trivalent-chrome coating were KCr(SO4)2 20g/l, H3PO4 3ml/L, NaF 1.2g/L, CeCl₃ 40mg/L, a small amount of surfactant, pH 1.5-2.0, processing temperature: 35℃, processing time:5 min. At this conditions, the protective properties of trivalent-chrome coating was equivalent to that of the traditional hexvalent-chrome coating.