| The technology of phosphating treatment on AZ91D magnesium alloy is a simple and effective means of corrosion protection.Pretreatment process plays the vital role for corrosion resistance of manganese-based phosphate conversion coating on AZ91D magnesium alloy.The paper systematically studied the effect of mechanical pretreatment and chemical pretreatment on microstructure and corrosion resistance of manganese-based phosphate conversion coating on AZ91D magnesium alloy.The results show that microstructure and performance of manganese-based phosphate conversion coating after different pretreatment are different.The surface of AZ91D magnesium alloy can get the different surface roughness by pretreatment of sand blast,sanding and polishing processing.The difference of corrosion resistance and microstructure morphology of manganese-based phosphate conversion coating which under the corresponding surface roughness is very obvious.The corrosion resistance of the phosphate conversion coating was characterized by potentiodynamic polarization curves(POL),rate of hydrogen evolution test and the salt spray test.The results showed that the phosphate conversion coating with the best corrosion resistance can be prepared with the surface roughness of AZ91D magnesium alloy controlled at about 1.456μm or 0.29 μm.The phosphate conversion coating displayed a two-layer structure,including an flocculent inner membrane with a thickness between 4μm and 6μm,and an spherical outer membrane with a thickness between 10μm and 14μm,which was observed by scanning electron microscope(SEM).The results showed that the inner layer is composed of P,O,Mg and a small amount of Mn element,and the outer layer is mainly composed of Mn,P,O elements by energy disperse spectroscopy(EDS)and electron probe microanalysis(EPMA).According to the open circuit potential(OCP)evolution during the conversion process,it was reasonable to divide the coating formation process into three stages.Meanwhile,a series of SEM micro-morphologies illustrating the evolution of the conversion coating are given,which supports that the formation of phosphate conversion coating is preferred in the bulge of polishing scratches,and the ravine part of polishing scratches forms a small amount of coating at the beginning of the reaction.With the increase of immersion time,the ravine part also forms phosphate conversion coating.Given all this,the two formation mechanisms were proposed to explain the effect of the different mechanical pretreatment on the formation of manganese-based phosphate conversion coating on AZ91D magnesium alloy.Effect of Sulfuric acid,nitric acid,oxalic acid,phosphoric acid,sodium dihydrogen phosphate pre-treatment on microstructure and corrosion resistance of phosphate conversion coating on AZ91D magnesium alloys.The results showed that microstructure and corrosion resistance of phosphate conversion coating after different acid pre-treatment were different.Observed by scanning electron microscope(SEM),the β phase and eutectic phase on the surface of magnesium alloy matrix both bulged over the a phase,after acid pickling with sulfuric acid,nitric acid or oxalic acid.However,the matrix surface performed relatively flat and there were not β phase and eutectic phase bulging,after acid pickling with phosphoric acid and sodium dihydrogen phosphate.The surface topography of the formed phosphating film which was preliminary treated,remained unchanged and performed relatively flat,except the one acid pickling in sulfuric acid.Analyzed by potentiodynamic polarization curve(POL)and salt spray test,the corresponding manganese-based phosphate conversion coating whose corrosion resistance is the highest were formed,pretreated in the phosphoric acid whose pH is among 1.68 and 1.87,the sulfuric acid whose pH is among 1.06-1.31,nitric acid whose pH is among 1.16 and 1.39,oxalic acid whose pH is among 1.37 and 1.70 or sodium dihydrogen phosphate whose pH is among 4.48 and 4.70. |
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