Corrosion And Protection Of AZ40Magnesium Alloy In NaCl Solution

Magnesium alloys have been widely used in automotive, aerospace and electronicmanufacturing industry and other fields due to its excellent performances, such as lowdensity, high strength-to-weight ratio, excellent castability and cutting properties.However, magnesium alloys have a number of undesirable properties like activechemical reactivity and poor corrosion resistance that have hindered its widespread usein many applications. Therefore, it is very important to study the corrosion behaviorsand protection technique of magnesium alloy under specified conditions. The corrosionbehaviors and protection technique of AZ40magnesium alloy in NaCl solution havebeen investigated in this study. The obtained results would be important significance inimproving corrosion resistance of magnesium alloy and promoting its application.The electrochemical techniques such as electrochemical impedance spectroscopy(EIS) and Tafel polarization curve were used to investigate the influence of Cl-concentration in NaCl solution on AZ40magnesium alloy corrosion behaviors and theinhibitive effect of sodium silicate, sodium dodecylbenzenesulphonate, sodiumtungstate and lanthanum nitrate on AZ40magnesium alloy. Then the corrosionresistance of silane coating, sodium silicate conversion coating, lanthanum nitrateconversion coating and hydrophobic films which were prepared by modifyingconversion coatings using dodecafluoroheptyl-propyl-trtimethoxysilane (G502), wereestimated. The surface composition, morphology, and wetting property of themagnesium alloy samples were characterized by fourier transform infraredspectrophotometers (FTIR), scanning electron microscopy (SEM) and OCA20contactangle measuring instrument. The film-forming mechanism and the anticorrosionmechanism of protection films were discussed. The results were discussed as follows:AZ40magnesium alloy was easily corroded and more seriously with the increaseof Cl-concentration in NaCl solution. The corrosion products were attached to thesurface of the magnesium alloy so as to form new barrier layers that could inhibitmagnesium alloy corrosion in a certain degree.The inhibition efficiency of sodium silicate and sodium dodecylbenzenesulphonateon AZ40magnesium alloy increased with increasing the inhibitor concentration, whilethe inhibition efficiency of sodium tungstate and lanthanum nitrate had a thresholdvalue. Within the range of tested concentrations, the optimum additive concentration of sodium silicate, sodium dodecylbenzenesulphonate, sodium tungstate and lanthanumnitrate were0.001,0.001,0.0001and0.00005mol/L, respectively, and thecorresponding inhibition efficiency were83.53%,43.39%,50.03%and31.14%.The silane coating with successive, integrated and compact structure was formedon AZ40magnesium alloy by sol-gel method. The Si-O-Mg bonds was formed betweensilane coating and magnesium alloy substrate. The silane coating could effectivelyinhibit the corrosion of magnesium alloy for a long time and corrosion protectionefficiency value was up to as much as99.97%.The sodium silicate conversion coating and lanthanum nitrate conversion coatingwere formed separately on AZ40magnesium alloy by chemical conversion treatment.The surface of two kinds of chemical conversion coating exhibited a hydrophilicproperty with water contact angle of8°and13°respectively. After the chemicalconversion coating surafce were treated with G502, the magnesium alloy exhibits ahydrophobic property with water contact angle of27oand115orespectively. Comparedwith the chemical conversion coatings before hydrophobic treatment, the stability ofhydrophobic films were improved obviously, so that the effective protection ofhydrophobic films on magnesium alloy could last for a longer time.