Study Of The High Efficiency Corrosion Inhibitor On Magnesium Alloy

Magnesium alloys, as promising candidates to substitute aluminium alloys, have potential wide applications in automobiles, electronics and aerospace industries because of their optimal weight-to-strength ratios, high thermal conductivity, dimensional stability and damping characteristics, good electromagnetic shielding and machinability as well as recyclability. However, the dissolution rate of magnesium alloy is high due to the large potential difference between the components in the different phases. The poor corrosion resistance of magnesium alloys is a major impediment to their applications in many fields. Among many corrosion protection methods, the use of inhibitors is one of the most practical methods for protecting metals or alloys from corrosion.It is generally acknowledged that the heteroatoms such as N, S and O in organic compounds show an inhibition effect toward the corrosion of iron, copper and aluminium alloys. The main role of heteroatoms in the corrosion protection is the formation of insoluble deposits on intermetallic inclusions, thus prevents local increase of pH which is responsible for the acceleration of the intermetallics dealloying. In terms of forming the insoluble complex compounds with the components of a phase and intermetallic phase of AZ91D magnesium alloy, organic compounds 8-hydroxyquinoline (8HQ) could be potential candidates to provide strong inhibition effect on the corrosion of AZ91D magnesium alloy. The addition of the dodecylbenzenesulphonate anion can form an adsorption film on the surface of AZ91D magnesium alloy and make a subsequent protection on the film.The inhibition effects of sodium dodecylbenzenesulphonate (SDBS) and 8-hydroxyquinoline on the corrosion of AZ91D magnesium alloy in ASTM D1384-87 corrosive solution were investigated by the electrochemical impedance spectroscopy and potentiodynamic polarization tests. For SDBS, the inhibition effect was not significant. For 8HQ, the inhibition effect was significant, and the component of the film was Mg(8HQ)2, which was characterized by three spectra methods. Upon mixing 8HQ and SDBS inhibitors, the inhibition effect can reach 98%. In this paper, a synergistic inhibition behavior was observed, and a proper synergistic inhibition mechanism was proposed.Due to the current general attitude toward problems of environmental pollution, inhibitors consisting of non-poisonous substances, for example, sodium silicate and whose cost is sufficiently low to permit widespread application, are of special interest. Soluble sodium silicates, as an important natural component of ground and surface waters, are probably one of the oldest and most widely used industrial chemicals. Among a wide variety of applications in industries, the use of sodium silicate as a "green" inhibitor in cooling water systems has been investigated. Along the development of magnesium alloy, the sodium silicate application on magnesium alloy has much significance.The corrosion inhibition effect of sodium silicate on AZ91D magnesium alloy were studied in ASTM D1384-87 corrosive solution. Experiments were carried out at different initial silicate concentrations and pH conditions using a series of electrochemical methods. The corrosion resistances of the silicate coatings were evaluated using open circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation tests. The microstructure and composition of the silicate coatings were analyzed by scanning electron microscopy (SEM), optical microscopy and energy dispersive spectroscopy (EDS). From the results of the corrosion tests, it can conclude that sodium silicate could effectively improve the corrosion resistance of the alloy at the optimum concentration 10 mmol/L, while the pH value range from 10.5 to 12.5 is preferable. Generally, sodium silicate treatments improve the corrosion resistance due to the formation of a transparent silicate coating which act as a barrier to oxygen diffusion to the metal surface. The silicate coating formed in high silicate concentrations showed a superior corrosion resistance in both the short-term and long-term immersion, indicating the formation of a more compact and planar coating on the surface of alloy. According to the electrochemical results, the decrease of the pH has a negative effect on the corrosion resistance.