Research On Structure And Corrosion Resistance Of Phytic Acid Conversion Coating For Magnesium Alloys

Chemical conversion coating is one of the most efficient mothds to protect magnesium and its alloys from corrosion. But the solution of traditional chromate conversion technology contains toxic hexavalent chromium(Cr6+), which is harm to the human and pollutes the environment. In this paper, the nontoxic phytic acid convertion coating was prepared, the structure and corrosion resistance of the coatings were studied formed in different conditions and repeated convertion technic, and the formation mechanism of the coating was discussed. In the same time, the phytic acid convertion coatings for pure magnesium and AZ91 magnesium alloy were prepared, and the influence of the matrix materials was studied for the phytic acid convertion coating. In the experiment, the morphology of the convertion coatings were observed by SEM, the composition and structure were analysed by EDS,XPS,FTIR, and the corrosion resistence of the coatings was estimated by the polarization and EIS.When preparing phytic acid convertion coating for AZ31 magnesium alloy, we found that extending the processing time, increasing temperature, reducing the pH value of the solution, or increasing phytic acid concentration can make the thickness of the coating increase, but the compactness became worse, and even appeared dried mud morphology. In proper conditons, the coating which was thick and compact could be obtained.By EDS and XPS, the main components of the convertion coating were C,O,Mg,P, when extending the processing time, increasing temperature, reducing the pH value of the solution, or increasing phytic acid concentration, Al may appear in the coating.The formation mechanism of the coating was discussed. The result showed that when the convertion coating begun to form, the coating mainly contained compounds of Mg, which consisted of magnesium salt of phytic acid, MgO and Mg(OH)₂; increasing the processing time, the concent of the metal salt of phytic acid became bigger, and Al begun to appear in the coating, which exised in the form of Al-O.The phytic acid conversion coating for AZ31 magnesium alloy formed in optimized conditions showed obvious passivation in 0.9wt% NaCl solution, pitting corrosion potential and corrosion potential separated obviously. The pitting potential decreased about 290 mV, corrosion potential rised about 50 mV, and the corrosion current density decreased two orders, impedance values were obviously higher than that formed in other conditions. its corrosion resistance has been highly improved.For AZ31 magnesium alloy, repeated conversion technic could make the coating thicker and the corrosion resistance was improved than a single conversion for 10 min. But its corrosion resistance became worse than that done in the coequal time.For pure and AZ91 magnesium alloy, we found that in the experiment, the phytic acid conversion coating for pure magnesium had worse compactness;βphase has higher potential of AZ91 magnesium alloys, and it was more difficult to form conversion coating for the second phase of AZ91 magnesium alloy.