Study On The Corrosion Resistance And The Preparation Of Rare Earth Conversion Coatings Of Several Mg Alloys

The kind and content of elements added in Mg could produce different corrosion resistance of Mg alloys. Based on this issue, the corrosion resistance of pure Mg, AZ31 and LAZ532 alloys were investigated comparatively. The microstructures were observed by the optical microscope. The results revealed that the Mg17Al12phaze distributed along the grain boundary are discontinuous in the as-cast AZ31, and the AlLi phaze in LAZ532 have similar condition as Mg17Al12 in AZ31. This suggested that those phazes mainly acted as micro-galvanic cathode, which could accelerate the corrosion of a matrix. The corrosion resistance of samples was analyzed with hydrogen evolution experiment, weight loss experiment, polarization curves and electrochemical impedance spectroscopy (EIS) measurements. The polarization curves showed that corrosion current density of AZ31 is higher than that of Mg, but lower than that of LAZ532. This indicates that the corrosion resistance of the studied alloys decreases in the following order:Mg>AZ31>LAZ532.In order to improve the corrosion resistance of the two alloys (i.e. AZ31 and LAZ532), rare earth films were prepared on their surface by chemical conversion coating method under the same pretreatment and coating preparation condition. Two kinds of conversion solutions i.e. the mixtures of Ce(NO3)3 and KMnO4, Ce(NO3)3, La(NO3)3 and KMnO4 were uesd. Corrosion resistance of the coated alloys prepared at a temperature of 80℃was discussed. It was measured with weight loss and salt spray corrosion tests. And corrsion-rate of the alloys was analysed within 24h with hydrogen evolution experiment. Analyses by SEM indicated that rare earth conversion coatings were netty crackle-like morphology. Cross section test showed that some deep cracks could be observed only on the double rare earth coatings of LAZ532 specimens, which led to the decrease of corrosion resistance after 14 h corrosion test. The general results reported here indicated that whether single or double rare earth coatings could provide good protection for alloys, whilst generally speaking corrosion resistance of double rare earth coating was better than that of single rare earth coating.In addition, the process parameters of rare earth conversion coatings including solution concentration, treating temperature and treating time were optimized. The processing parameters of single rare earth conversion coatings on AZ31 were confirmed as follows:20℃, 10 min,1g/L KMnO4,2g/L Ce(NO3)3, and that of double rare earth conversion coatings on AZ31 are:20℃,10 min,1 g/L KMnO4,2 g/L Ce(NO3)3,5 g/L La(NO3)3. The processing parameters of single rare earth conversion coatings on LAZ532 were confirmed as follows: 20℃,10 min,5g/L KMnO4 and 2 g/L Ce(NO3)3, and that of double rare earth conversion coatings on LAZ532 are:20℃,10 min,5 g/L KMnO4,2 g/L Ce(NO3)3 and 5 g/L La(NO3)3. For LAZ532, the single rare earth conversion coating provided a better protection than double rare earth conversion coating.