Technical And Performance Study Of Ti-W Composite Chemical Conversion Coating On Aluminum Alloy

Aluminum alloy is one of the most widely used metallic materials at present. It has widely used in defense industry, electronic industry, instrument industry, commodity manufacturing industry and so on. However, its corrosion resistance is very poor,Commonly, we use chemical conversing technology to improve the anticorrosive resistance of aluminum alloys,and chromates are the most widely used currently. With the implementation of ROHS, ELV and WEEE regulations in EU, the use of chromates will be limited strictly. In this context, the researchers have focused on the development of chrome-free chemical conversion technology on aluminum alloys presently.The chrome-free chemical conversion coatings on aluminum alloy surface were prepared by SnSO₄,CoCl₂,TiOSO₄,Na₂WO₄ as the main salt respectively, the influence of different factors on the corrosion resistance were studied. The optimal conditions of SnSO₄ as the main salt were SnSO₄ 0.15g/L,KMnO₄ 1.5g/L,NaF 2g/L,reaction temperature 50℃,reaction time 1.5min; The optimal conditions of CoCl₂ as the main salt were CoCl₂ 2g/L,KMnO₄ 2g/L,NaF 2 g/L,reaction temperature 30℃,reaction time 1 min;The optimal conditions of TiOSO₄ as the main salt were TiOSO₄ 0.3g/L,KMnO₄ 1g/L,NaF 1.5g/L,reaction temperature 40℃,reaction time 1min;The optimal conditions of Na₂WO₄ as the main salt were Na₂WO₄ 0.3g/L,KMnO₄ 0.5g/L,NaF 1g/L,reaction temperature 40℃,reaction time 2min。TiOSO₄ and Na₂WO₄ were chosen as the main salts for preparing composite chemical conversion coating on aluminum alloy by orthogonal experiment, the optimal conditions were the concentration ratio of TiOSO₄ and Na₂WO₄ was 0.3 g/L︰0.3 g/L,KMnO₄ 1.0 g/L,NaF 1.0 g/L,reaction temperature 40℃,reaction time 2min. The morphology of the coating was observed by scanning electron microscopy (SEM). Coating composition and the microcosmic phase structure were characterized using energy dispersive X-ray (EDX) analysis and X-ray diffraction (XRD) respectively. Electrochemical method was used to study the coatings corrosion resistance. The results indicated that the composite chemical conversion coating is a crystal structure material that composites with Al,Mn,W,Ti et al., the surface appears as a accumulation of fibroid spherical particles, the crystallinity of conversion coating is better than uncoated sample obviously. The corrosion potential of the coating is improved from-1.440V to -0.440V, the corrosive speed dropped from 0.168 g (/m²Â·h)to 0.048 g /( m2 ? h), they were both prove the corrosion resistance has improved.The results of surface structure and electrochemical test on chemical conversion coating show that the coating was formed is on the cathode,and the hydrogen evolution reaction on the cathode is control step; The mechanism of corrosion resistance on aluminum alloy chemical conversion coating is inhibition on cathode mainly, and both with anodic inhibition.