Fabrication Of Hydrophobic Layer With Cerium On The Surface Of Magnesium Alloy And Corrosion Behavior

The preparation of hydrophobic coating on the surface of magnesium alloy is a kind of widely used protective method.In addition,many studies have shown that the rare earth cerium has obvious effect of corrosion inhibition on magnesium alloy.In this pape r,three kinds of hydrophobic coatings,including single Ca salt coating and single Ce salt coating as well as Ca-Ce composite coating,whose compositions contain both calcium myristate and cerium myristate,are fabricated by the method of cathodic deposition;and all of those coating have significantly improved the corrosion resistance of AZ31 magnesium alloy.The effect of fabrication process parameters,including deposition voltage and deposition time,on the morphology and corrosion behaviour of single calcium myristate coating and single cerium myristate coating were studied;and the influence of C a-Ce composite coating’s deposition time ratio and Ca-Ce molar ratio on the composite coating’s morphology and corrosion behaviour were researched as well.The study find that fabrication process parameters have an obvious effect on the hydrophobic coatings’ morphology and corrosion property,but no effect on their phase compositions.The phase composition of the Ca salt co ating is((CH₃（CH₂）₁₂COO)₂Ca,and the phase of the Ce salt coating consists of（CH3（CH2）12COO）4Ce and（CH3（CH2）12COO）3Ce.The research results show that both single Ca salt coating and single Ce salt coating can obtain the best corrosion resistance when the deposition voltage is 50V;that single Ca salt coating’s corrosion resistance gets better as the deposition time gets longer;however,that Ce salt soating gets the best corrosion resistance when the deposition time is 60 minutes.The Ca-Ce composite h ydrophobic coating with the best corrosion resistance is prepared when the deposition voltage is 50 V,and deposition time reaches 60 minutes（the Ca-Ce deposition time ratio is 2/1）and the Ca-Ce molar ratio is 99/1,thus called 99Ca-1Ce coating below.Th e pitting potential of 99Ca-1Ce coating is 0.94 V,which is 0.61 V higher than that of Ca salt coating;what’s more,the pitting etching of 99Ca-1Ce coating appears on the 11 t h day of immersion test;additionally,the adhesion strength between the 99Ca-1Ce coating and the substrate is the strongest one among those coatings.The addition of cerium in Ca salt coating is helpful to improve its thickness and hydrophobic property as well as the adhesion,thus such coating with cerium has a better corrosion resistance.But the addition amount of cerium has a limit,once the amount of cerium exceeds the limit,it will increase the coating defects and reduce the adhesion strength between coating and substrate,which will decrease the cerium’s inhibition efficiency,finally worsens the coating’s corrosion resistance.In the Ce-containing electrolyte,ionic migration has a speed order,which is present as Ce4 +> Ce3 +>Ca2 +,and Ca2 + always exists in the solution,therefore,the co-deposition of Ca salt and Ce salt happens at the latter deposition stage.During the immersion test,the phase of（CH 3（CH2）12COO）2Ca in the coating will turn to((CH₃（CH₂）₁₂COO)₂Ca·4H2O,while cerium myristate has no change.After the long-term immersion corrosion,the coating is dissolved and dissociated from the substrate,then the substrate suffers the corrosion,and its reaction product is Mg（OH）₂.