Preparation At Room Temperature And Characterization Of Ce-mn Conversion Coating On Al Alloy

As one of the most popular surface treatment technologies, chemical conversion coating based on Cr⁶⁺ is widely used to improve the corrosion resistance of aluminium alloys. However, Cr⁶⁺ is toxic and its application has been prohibited in electrical and electronics industries by the European Union. There is an emergent need to develop an environmentally friendly chromate-free technology for the surface treatment of aluminium alloys. Rare-earth metals conversion coating is one of the most promising green alternatives.Currently most rare-earth metals conversion coating technologies are complicated, time consuming and elevated temperature is needed for the treatment. Its industrial application is thus limited. In this thesis, a high efficient, environmentally friendly conversion coating technology based on rare-earth metals is presented. The treatment is carried out at room temperature and the coating is of golden color. The detailed mechanism of coating formation and the reason for corrosion resistance enhancement are discussed.The room temperature treatment solution is based on Ce(NO₃)₃ and KMnO4. Four parameters (deposition time, pH value, the concentration of Ce(NO₃)₃ and KMnO4) were investigated for the influence on corrosion resistance of Ce-Mn conversion coating by orthogonal and single factor experiments, respectively. The solution composition and treatment parameters were optimized as follows: treatment time is 30 min, pH = 2, and the concentrations of Ce(NO₃)₃ and KMnO4 are 10g/L and 2g/L respectively. Scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and X-ray photo-electron spectroscopy (XPS) were used for the conversion coating characterization. Ce was found in the coating with the valence state of +3 and +4, while the valence state of Mn was +4.In order to improve the corrosion resistance and room temperature formation speed of Ce-Mn conversion coatings,H₃BO₃,Zr(SO₄)₂,NaF,HF,NaBF₄å’ŒNa₂ZrF₆ were selected as additives in to the above mentioned treatment solution. Both NaF and NaBF4 can effectively increase the coating formation speed and in dued golden conversion coating with exellent corrosion resistance. As indicated by both potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS), NaF is the best additive for the forming of the coating of high quality. The solution composition was further optimized as: 10 g/L Ce(NO₃)₃ + 2 g/LKMnO₄ + 0.6 g/L NaF, the coating process only 9 min.It is interesting to note that pre-treatment process is crucial to the formation speed and performance of conversion coating. A new pre-treatment was developed: 3 min soaking in home made solution (100mL/L HNO₃ + 10mL/L Na₃PO₄ + 10mL/L HF + 1.5g/L OP-10 + 0.5g/L thiourea) at room temperature, followed by 30 seconds soaking in 10 wt.% NaOH. It was found that after such pre-treatment, there was no dissolution and re-precipitation of iron, copper and zinc during conversion coating process. Cathodic area was small and distributed closely and uniformly on the aluminium alloy surface. The coating formation speed was raised and the resultant coating was compact and uniform. The coating corrosion resistance was enhanced.The formation mechanism of conversion coating was investigated. The critical pH value for the deposition of different elements was calculated. The effects of treatment conditions (time, pH value, concentration of Ce(NO₃)₂, KMnO₄ and NaF) on the growth of conversion coating was studied by electrochemical techniques. A model of conversion coating formation mechanism was put forward and justified by XPS.Thermal spray was carried out on aluminium alloy after Ce-Mn conversion coating treatment. The conversion coating technology developed in this study was shown to meet the Part 4 of National Standard GB 5237.4-2004 as required by powder coating profiles. A golden conversion coating with good corrosion resistance was achieved on 6063 aluminium alloy. Such conversion coating treatment on aluminium profiles is simple and environmentally friendly, and might be used as an alternative for the treatment that is based on toxic chromate.