V/Ti Composite Conversion Coating On Al Alloy LY12

With good characteristics, Al alloys are widely used in light industry and construction materials, aviation and aerospace, electronic communications and many other fields. However, easily corroded in the processes of production and use, Al alloys are often treated by chromate treatment technology which is economic and effective in order to improve their anti-corrosion performance. Because of carcinogen, Cr(VI) was limited to use in the metal surface treatment by the European Union RoHS Directive(Restriction of certain hazardous substances in electrical and electronics equipment) as early as 2006, and China ?Heavy metal pollution comprehensive prevention and control 12 th five year plan?, the first 12 th five year plan, was put forward in 2011, giving strict control requirements to the five heavy metals including hexavalent Cr. Therefore, an alternative to chromate conversion coating is urgently needed.Al alloy LY12 is the mostly easily corroded among all Al alloys due to Cu-containing intermetallics. Because of environmentally friendly and good anti-corrosion performance, titanium-salt conversion coating was firstly studied in this thesis. A single factor experimental result indicated that titanium-salt conversion coating with 66 h salt spray time was obtained under the condition of NaF 0.3g/L, TiOSO4 0.5g/L, pH 4, 25? and 10 min.On the basis of the above research, the effect of assisting film-forming agents(potassium fluozirconate, sodium metavanadate and ammonium fluosilicate) on titanium-salt conversion coating was investigated by salt spray test. The results showed that sodium metavanadate could improve the corrosion resistance of the coating most effectively. The optimal preparation conditions were as follows:0.5g/L TiOSO4, 0.4g/L NaVO3, pH 4, 25?, 10 min. The results from scanning electron microscopy(SEM), energy dispersive spectrometer(EDS) and polarization curve analysis showed that the compact and uniform conversion coating was composed of elements O 4.79%?Ti 1.20%?V 1.63%?F 0.22%?Al 83.00%?Cu 4.78%. The formation of conversion coating consisted of anti-corrosion composition, vanadium and titanium oxides,especially, their higher content on Cu-containing intermetallics made the corrosion potential move 56 mV more positively than that of LY 12, the corrosion current density reduce by about 1/5, radius of capacitive reactance arc increases, impedance value of conversion coating rises, salt spray time increase to 91 h. The corrosion resistance of Al alloy LY12 was greatly enhanced.