The Research Of Chemical Conversion Coating With High Corrosion Resistance And Low Electrical Contact Resistance On AZ91D Magnesium Alloy Resisting Hot And Humid Environment

AZ91D magnesium alloy is widely used in 3C electronic products because of its excellent physical,mechanical and other properties.However,Magnesium alloy is of the biggest drawback,easy to be corroded,which could results in failure of the structure or mechanical properties.On the other hand,corrosion caused by the formation of corrosion products would further result in electrical contact failure which is a great concern to electronics industry.Therefore,it is necessary to develop a protective chemical conversion coating for magnesium alloy without sacrificing the surface conductivity.Compared with other protective chemical conversion coating treatments,chemical conversion treatment has the advantages of low cost,easy operation and easy realization.Therefore,the 3C electronics industry urgently needs a magnesium alloy chemical conversion coating with both high corrosion resistance and electrical conductivity.In response to this problem,we prepared a chemical conversion coating with high corrosion resistance and low electrical contact resistance in an acid solution containing various strong oxidizing agents.The experimental results show that corrosion resistance of the coating increased by about 5 times,and the coating surface contact resistance is only 114m?.The corrosion resistance of the magnesium alloy is due to the fact that a coating with a thickness of 0.8 to 1.2?m is formed on?phase.In addition,?phase is about 400 nm higher than?phase on the coating.When an electrical contact occurs,the?phase as a"conductive spot"preferentially contacts other components,giving the coating good conductivity.Hot and humid environment is the typical service environment for electronic equipment.By researching the service behavior of the chemical conversion coating with high corrosion resistance and low electrical contact resistance on AZ91D magnesium alloy in high temperature?85°C?and high humidity?85%?environment,the conductive performance of magnesium alloy electroconductivity coating quickly failed to meet the application requirements in 3C field.It is found the corrosion of?phase produce an amount of corrosion products,Mg?OH?₂,on the surface of which the volume expansion makes the?phase to be higher than the?phase.As the?phase loses the function of"conductive spots",the conductive performance of the coating loses too.In order to improve the conductivity of the chemical conversion coating with high corrosion resistance and low electrical contact resistance on AZ91D magnesium alloy in hot and humid environment and ensure the corrosion resistance of the coating,AZ91D magnesium alloy was pretreated with acidic solution.After the pickling treatment,the?phase in the chemical converse chemical conversion coating is about 10?m higher than?phase.After 960 h of hot and humid test,the?phase is still higher than?phase and the coating keep good conductivity.Hydrogen evolution experiments and salt spray experiments show that pretreated with acidic solution had no obvious effect on the corrosion resistance of the coating.Finally,the effects of casting shrinkage defects of AZ91D magnesium alloy body on the corrosion resistance and electrical conductivity of the chemical conversion coating were explored.Comparing to the coating without shrinkage,coating with shrinkage was seriously corroded at the defective site and the further corrosion propagation resulted in corrosion products covering over the?phase?conductive spots?which led to the decease of conductivity of chemical conversion coating on magnesium alloy.