STUDY OF SURFACE MODIFICATION FOR RARE-EARTH MAGNESIUM ALLOYS BY HYDROTHERMAL METHOD

STUDY OF SURFACE MODIFICATION FOR RARE-EARTH MAGNESIUM ALLOYS BY HYDROTHERMAL METHOD
　　Magnesium alloys have extensive application potential in automotive, aerospace,electronic communication and national defense by its outstanding properties,such as low density,high strength-to-weight ratio and so on.And rare-earth magnesium alloys have excellent mechanical property, antioxidation property and high creep resistance.However,chemical properties of magnesium alloys are very active,which restrict the development of magnesium alloys by the bad Neodymium Magnets corrosion resistance.Therefore, improving anti-corrosion capacity on magnesium alloys by surface
　　modification is more significant.There are many surface modification methods,such as chemical conversion,plating or electroless plating, anodizing,laser modification and so on.All those methods possess itsadvantages and disadvantages.Chemical conversion has extensive application in magnesium alloys surface modification by its easy operation, low cost and other advantages.But chemical conversion is done at low temperature and atmospheric pressure.The conversion coatings are thin and bad adhesive.There is little report on rare-earth magnesium alloys surface modification in hydrothermal conditions at relative high temperature and high pressure.
　　Hydrothermal surface modification on rare earth magnesium alloys,a kind of new non-chromate chemical conversion technique,was studied in this paper.The contents include surface modification on Mg-3Nd-Zn-Zr and Mg-10Gd-3Y-Zr alloys in silicate and molybdate solutions by hydrothermal method;discussed the effect of bath composition and temperature to the anti-corrosion capacity of the coatings;the coatings formed by hydrothermal method followed by heat treatment and sealing,and discussed the reasons that leading to corrosion resistance improving.
　　This article studied the micro-morphologies,compositions and corrosion resistance of the coatings by FE-SEM,EDX,XRD and electrochemical testing methods.The results showed that the compositions of the conversion coatings on NZ30K alloy were mainly consist of round particles and flakes of Mg(OH)2 phase.A few of structurally amorphous compounds containing Si element also were found in the coatings.Moderate CH3COONa can promote round particles and flakes Mg(OH)2 agglomerating without affecting the coatings’growth rate,which shrinking the gap of flakes Mg(OH)2,so the porosity of the conversion coatings was decreased.The thickness of the coatings which have great adhesion with the substrate can reach about 25μm, and the corrosion resistance was improved obviously.
　　The coatings on GW103K alloy formed by hydrothermal method were mainly consisting of assemble cloud-like and flakes of Mg(OH)2 phase.The treated GW103K alloy testing in 3.5 wt.%aqueous solutions showed that the open circuit potential positively moved by 100mV compared with the untreated GW103K alloy.The corrosion current intensity was lowed by one order of magnitude,and the obvious passivation appeared at anodic polarization process.The pitting corrosion resistance was improved and reduced the cathodic hydrogen evolution rate.Charge transfer resistance was increased which improved the GW103K alloy’s anti-corrosion capacity. Moderate heating temperature is benefit for Mg diffusion,promote Mg(OH)2 coatings forming,and the best heating temperature is 180℃.
　　The uniform and compact MgO coatings were prepared on GW103K alloy surface by hydrothermal method followed by heat treatment in air. Testing in 3.5wt.%aqueous solutions,we found the corrosion potential increased by 200mV compare to no heat treatment alloy,the corrosion current intensity is lowed by one order of magnitude,the polarization resistance increased from 2844.7Ωto 6407.3Ω.After sealing treatment,the GW103K alloy by hydrothermal surface modification,testing in 3.5wt.% aqueous solutions.We found the corrosion potential increased by http://www.everbeenmagnet.com/en/products/110-sintered-neodymium-magnets 400mV,the corrosion current intensity is lowed by one order of magnitude,the polarization resistance increased from 2844.7Ωto 5072.0Ω.The corrosion resistance improved effectively.
　　Chemical conversion on magnesium alloys by hydrothermal method at relative high temperature and high pressure,which increased the coatings thickness and binding capacity compare to traditional chemical conversion coatings.This paper expands the new magnesium alloys surface modification technology.And it has great enlightenment for magnesium alloys surface modification technology’s further development.