Study on Corrosion Behavior and Corrosion Inhibition of Magnesium Alloy Ze41

Abstract

The alloys of magnesium are in the spot-light lately. With applications that run the gamut from automobile parts to medical implants, this class of alloys truly deserve all the adulation coming their way. ZE41 is one such cast alloy, which is lighter than aluminium, can be cast or machined into variety of desired shapes and has good damping and shock absorption abilities. A very low resistance to corrosion puts the otherwise remarkable efficacy of ZE41 in jeopardy. Hence understanding the corrosion of ZE41 and developing the measures to combat the same are indispensable. In the present thesis the corrosion of ZE41 and its mitigation were analyzed using techniques like potentiodynamic polarization and AC impedance, along with SEM and EDX analyses. Sodium sulfate and its mixture with sodium chloride were chosen as two corrosive media. The blank corrosion studies were performed at various medium concentrations, pH and temperatures. The results revealed a trend of higher corrosion rate associated with higher medium concentration, lower pH and higher temperature. Five different long chain alkyl monocarboxylates namely stearate, palmitate, myristate, laurate and caprylate were synthesized and tested as corrosion inhibitors for ZE41. The calculated activation and thermodynamic parameters have been documented in the thesis. The carboxylates were predominantly physisorbed and adsorption was in accordance with Langmuir adsorption isotherm. The studied carboxylates were found to function as mixed-type inhibitors which predominantly suppressed anodic reaction. The carboxylates were more efficient at lower temperatures and in combined medium. At an optimum concentration the efficiencies of the carboxylates decreased in the order: stearate > palmitate > myristate > laurate > caprylate and this has been accredited to the reduction in aliphatic chain length. The proposed mechanism attributed the cathodic inhibition to the blockage of the reaction spots by chemisorbed carboxylates. The anodic inhibition resulted from the compaction of porous film by precipitated magnesium carboxylate salts.