Study On Corrosion Inhibitors For AZ31Magnesium In NaCl Solution

Having high specific strength, high specific stiffness, super plastic deformation energy absorption, good comprehensive performance and being rich in resources, magnesium and its alloys have a wide range of applications in automotive, aerospace, electronic communications, biomedical and other various departments. As the standard electrode potential of magnesium is-2.37V, the corrosion potential is much lower than the other commonly used light metals. For the poor corrosion resistance, which limits further application of magnesium and magnesium alloy, improving its corrosion resistance is of particularly importance. The usually method is to generate a layer of protective film on the surface of magnesium alloy through physical and chemical methods, which can lessen the corrosion. On the other hand, a certain amount of corrosion inhibitor can be added in its application system to enhance the corrosion inhibition, which is worth to further investigate due to its convenience, effectiveness and economy.The affection of the inorganic and organic corrosion inhibitors, such as sodium phosphate, sodium borate, sodium citrate, L-asparagine,2-amino benzimidazole and sodium alginate, were examined by means of electroche-mical impedance spectrum, polarization curve and the full-immersion weight-loss experiment. The influence of six kinds of corrosion inhibitors to electrochemical processes of AZ31magnesium alloy were analysed in immersion in the different medium, and the inhibitors with excellent inhibition efficiency for AZ31magnesium were get as follow,0.1%sodium phosphate-81.9%,0.05%2-amino benzimidazole-48.3%.According to the inhibition efficiency study of the inorganic-organic compounds with the already selected inhibitors, we get the best efficiency of every compounds with ratio as following:with0.1%sodium phosphate+0.15%sodium alginate, its inhibition efficiency is as high as91.6%.Combined with scanning electron microscopy (SEM) analysis of surface and cross section topography of AZ31magnesium alloy after immersion in3.5wt.%NaCl solution,we analyze the influence of corrosion process of magnesium alloys after adding sodium phosphate,2-amino benzimidazole, sodium alginate and its compounds in the solution. The results are given as follow:(1) In3.5wt.%NaCl solution, AZ31magnesium alloy was easily corroded and the pitting corrosion occured on magnesium alloy surface because of the existence of chloride ions in corrosive solution.(2) In the condition of using sodium phosphate as inhibitor separately, the surface of magnesium alloy was covered with the generated magnesium phosphate as film space, reducing the reaction of anodic polarization; Using2-amino benzimidazole separately, there exists imidazole structural, of which has central atom lone pair electrons to bond with magnesium ions as π bond. The π bond shows as planar structure and absorbs on the metal surface. Thus the corrosion rate increased significantly; Using sodium alginate separately, the hydrolysate of inhibitor in the medium are free mannose hyaluronic acid root and gulonic hyaluronic acid root. On one side of the two roots is negatively charged carboxylic acid, which can combine with positively charged magnesium ions and adsorption in the surface of magnesium alloy easily. For chloride ions is prevented from contacting with magnesium alloy, the corrosion is suppressed.(3) Sodium phosphate and2-amino benzimidazole almost show no synergy inhibition effect. When2-amino benzimidazole content increases, phosphate ions are preferential absorbed by the product of corrosion inhibitor after the protonation. Without enough PO43-ions reacting with magnesium passivation, the film layer on surface of magnesium alloy becomes thin and the inhibition effect declines.(4) Sodium phosphate and sodium alginate have good synergy inhibition effect. With the less content of sodium alginate, the role of corrosion is largely played by sodium phosphate, which can generate Mg3(PO4)2filled in the gap of Mg(OH)2. Thus the protective film layer is of great dense;When sodium alginate content increases, the negatively charged carboxylic acid ions reacted from hydrolysis is absorbed on the metal surface, Thereby the inhibitor restrains corrosion of magnesium alloys effectively.