Effect Of Micro-arc Oxidation Surface Treatment On The Stress Corrosion Property Of AZ31 Magnesium Alloy

Based on the specific strength,specific stiffness and low vibration damping of Magnesium alloys as lightweight metal material,it has potential applications in aerospace,military industry,traffic,biomedical,electronic products and so on.However,the poor corrosion resistance highly restricts its applications,it can react corrosion medium in atmospheric environment easily such as Oxygen,water and chlorides,which can leads to corrosion damage.In practical application environment,the component bear the load inevitably.So,Magnesium alloy is easy to happen stress corrosion(SC)under the interaction of load and corrosion.And SC usually bring about local corrosion under tensile stress.The local corrosion can accelerate the corrosion and corrosion cracking,which is stress corrosion crack(SCC).It often causes serious accident.Comparing with other metal materials,the chemical property of Magnesium alloys is active,so the corrosion resistance is the key factor to affect the stress corrosion of Magnesium alloys.This work was to generate different thickness of ceramic coatings such as 5 μm,10 μm,20μm,30μm and 40 μm,using micro-arc oxidation(MAO)technique.The SC test was conducted with constant load and slow rate continuous loading three-point bending test,and the effect of ceramic coating on the SC property of AZ31 magnesium alloy was discussed combining with the thickness,compactness,surface morphology,roughness and electrochemical corrosion behavior.The result showed:In the constant load SC test in 3.5 wt%NaCl solution,the SC life of MAO sample is greater than AZ31 Mg alloy.The surface corrosion rating of AZ31 Mg alloy was lower than 5 when break under the maximum load(260.4 MPa).The critical SCC load of MAO sample was 163.9 MPa,182.1 MPa,168.9 MPa and 181.0 MPa for MAO-5,MAO-10,MAO-20 and MAO-30,respectively.With increasing the load,the SCC life of AZ31 Mg alloy and MAO samples decreased,and the effect of MAO ceramic coating on the SCC life was less and less.When the load was 187.5 MPa the SCC life of MAO samnle increased by 0.5-80.1 times relative to AZ31 Mg alloy,while 260.4 was 1～2.9 times.So the MAO ceramic coating can improve the SC and SCC property of AZ31 Mg alloy.(2)In the slow rate continuous loading SC test,the bending strength of MAO samples was all greater than AZ31 Mg alloy in air,deionized water,3.5 wt%NaCl and 3.5 wt%NaCl+K2Cr2O7 solution.And the bending strength increased by 2.8%,14.8%,22.7%and 29.2%relative to AZ31 Mg alloy in the above corrosion medium.So the effect of corrosion medium on corrosion resistance was more,the improvement of MAO ceramic coating on SCC property was more significant.(3)The reason that MAO ceramic coating can improve the SC property is that on the one hand,the ceramic coating improved the absorbed-in-fracture energy and bending strength,they increased by 7.3%～44.3%and 5.6%～14.9%respectively relative to AZ31 Mg alloy.On the other hand,the ceramic coating improved the corrosion resistance of AZ31 Mg alloy,which in the immersion corrosion test in 3.5 wt%NaCl solution,the surface corrosion rating of AZ31 Mg alloy was lower than 5 after 444 h,while MAO samples were up to 1302 h rating at 5 when break.The corrosion current of MAO samples decreased more than one order of magnitude.So the great corrosion resistance of ceramic coating can delay initiating crack,and then it improved the SCC life of AZ31 Mg alloy.