Molybdate Pro-teatment for Rear Earth Conversion film on LF21 Aluminum Alloy to Improve Corrosion Resistance

Cerium conversion film was prepared on LF21 aluminum alloy by chemical deposition,and the cracking behavior and improvement of corrosion resistance of the film by molybdate pro-treatment were studied with methods of SEM,EDX,polarization testing and salt fog testing.The results show that deposition time had significant influence on cracking of the film.With the deposition time prolonged,cracking tendency of the film increased and the cracks were wider and deeper.After the film was treated in 10 g/L molybdate solution at 50 ℃ for 30 minutes,the cracking degree was alleviated and corrosion resistance of the film was increased.The passive current density decreased by almost an order of magnitude and the property to resist salt fog testing was also improved.The mechanisms for molybdate treatment to improve corrosion resistance may include the deposition of molybdate film at cracking sites,the inhibitive effect of molybdate and the “self sealing” effect of cerium conversion film.