Corrosion and Corrosion Inhibition Studies on Mg-Al-Zn Alloy in Aqueous Ethylene Glycol (30% V/V) Medium

Abstract

Magnesium alloys find applications from automobile industries to medical implant materials because of its enhanced properties in comparison with other light weight alloys. Mg-Al-Zn alloy is one such magnesium alloy, which finds variety of applications in automobile industries. The occurrence of corrosion is beneficial in the medical field of application as it avoids one more surgery for the removal of implants, but is a limitation in automobile industry applications. Therefore, the study of corrosion and its controlling measures have vital role in practical applications. In this thesis, corrosion of Mg-Al-Zn alloy and its mitigation were studied using electrochemical techniques - potentiodynamic polarization and AC impedance spectroscopy. The surface morphology and surface composition of the corroded alloy surface were examined by SEM and EDX analyses. The aqueous ethylene glycol (30% v/v) solutions, containing chloride and sulfate ions, were chosen as the corrosive media. The corrosion studies were performed at different chloride ion and sulfate ion concentrations, pH and temperature of the medium. The obtained results concluded the trend of higher corrosion rate at higher concentration of chloride and sulfate ions, lower pH and higher temperature. The synergistic mixtures of sodium dodecyl benzene sulphonate (SDBS) with trisodium phosphate (TSP) and sodium benzoate (SB) were employed as corrosion inhibitors. Similarly morpholine and its two derivatives N-methyl morpholine and dimethyl morpholine were also studied as corrosion inhibitors in the corrosive media. The synergistic mixtures acted as anodic type inhibitors by suppressing anodic reaction. The morpholine and its derivaties acted as mixed type inhibitors. All the inhibitors were more effective at lower temperature and in higher concentration of the corrosive media. The activation and thermodynamic parameters have been calculated and documented. The synergistic mixtures adsorbed on the alloy surface predominantly through physisorption and obeyed Langmuir adsorption isotherm. While the adsorption of morpholine and dimethyl morpholine obeyed Langmuir adsorption isotherm, that of N-methyl morpholine obeyed Temkin adsorption isotherm. The synergistic mixtures of organic-inorganic inhibitors showed higher inhibition efficiency than mixtures of organic-organic inhibitors. The efficiencies of morpholine and its derivatives decrease in the following order: dimethyl morpholine > morpholine > N-methyl morpholine. The possible mechanism of inhibition has been proposed for all the inhibitors.