The Investigation On Inhibitors For Magnesium And Its Alloy In Nacl Solution

Magnesium and its alloys have many advantages such as low density, high strength-to-weight ratio, high rigidity and elastic modulus, good recyclability and are considered as important light engineering materials. However, magnesium and its alloys are highly susceptible to corrosion, particularly in industrial and marine atmosphere without protection. Their poor corrosion resistances are limiting their practical application. Although the usage of corrosion inhibitors is the simplest method and most effective method for protection of metals in aqueous environment, its application for protection of magnesium alloys is not very popular mostly because of the relatively high rate of the corrosion process. In this paper, according to the inhibiting mechanism of corrosion inhibitors and the structural characteristic of magnesium atom (it is very difficult to react with adsorption type of inhibitors due to the high 3d orbit energy) as well as the active chemical properties of magnesium alloy, we design a series of effective inhibitors for magnesium and its alloys. Traditional electrochemical methods in combination with weight loss and surface analysis methods are used to investigate those inhibitors. The main research work and results are as follows:1. The inhibition performance of organic silicate and inorganic Zn salts on corrosion of magnesium-based rare-earth containing alloy Mg-10Gd-3Y-0.5Zr (GW103) was evaluated in ASTM D1384-87solution (165 g/L NaCl,148 g/L Na2SO4,138 g/L NaHCO3, pH=8.2) with organic silicate and inorganic Zn salts. The results showed that organic silicate can provide moderate protection, and the inhibition efficiency increase with the increase of the concentration of organic silicate; the pH of solution would decrease due to the hydrolysis reaction of inorganic Zn salt, so inorganic Zn salts would provide moderate protection only at low concentrations. If combined the organic silicate and inorganic Zn salts, particularly when the concentration of organic silicate is higher than the concentration of Zn salts, there is very good synergism effects between organic silicate and inorganic salt and the inhibition efficiency can reach 95%.2. The chlorophyll is an insoluble magnesium porphyrin compound. Environment-friendly 5,10,15,20-Tetraphenylporphyrin (TPP) was synthesized as a corrosion inhibitor for magnesium and its alloys, which could react with Mg2+ to form insoluble Mg porphyrin on magnesium alloy surface. The calculation of quantum chemistry result showed that the formation of TPP-Mg compound due to the reaction of TPP and Mg2+ is thermodynamically possible. The inhibition effect of TPP on pure Mg and AZ91D alloy in 0.05 wt.% NaCl solution was studied by means of electrochemical measurement and immersion corrosion test. Results showed that TPP could provide excellent protection for Mg and its alloy even at low concentration. Besides, TPP inhibitor can also effectively decrease the galvanic corrosion of AZ91D-X80 carbon steel and AZ91D-Cu couples. FT-IR, ultraviolet-visible absorption spectrum (UV), fluorescent spectrometry and XPS analyses suggest that TPP molecules can bond with Mg ions via N atoms to form a TPP-Mg complex, which can precipitate with the original Mg(OH)2 to form a compact film on AZ91D, and then retards the dissolution of the Mg alloy.3. The inhibition effect of 2-hydroxy-4-methoxy-acetophenone (paeonol) and 2-hydroxy-acetophenone on the corrosion of AZ91D magnesium alloy in 0.05 wt.% NaCl solution was investigated. Electrochemical measurement results showed that the both inhibitors had good inhibition for magnesium alloy; the inhibition efficiency increases firstly and then decreases with the increase of inhibitor concentration. The inhibition efficiency reaches a maximum at 50 ppm. However, paeonol had better protection than 2-hydroxy-acetophenone at the same concentration. Molecular simulation indicates that the molecular structure of paeonol-Mg complex is considerably stable. UV-Visible spectroscopy measurement was used to detect the inhibitors concentration change, it is demonstrated that about 5 ppm paeonol was consumed on 1 cm2 Mg surface. FT-IR showed that through the deprotonation of hydroxyl group of paeonol, the formed benzene oxygen anion chelates with the dissolved Mg2+ions to form protective paeonol-Mg complex, which precipitates with the original Mg(OH)2 on magnesium alloy surface to inhibit the corrosion of magnesium alloy.