A surface and corrosion characterisation of micro arc oxidation treated friction stir welded ZM21 and ZE41 magnesium alloy: a comparison study

Abstract

Magnesium alloys have gained attention as promising materials in industrial applications, for their high specific strength and low density. Magnesium alloys have desirable mechanical properties, but their poor corrosion resistance prevents their safe implementation. Alloys such as ZM21 and ZE41, possess unique properties that provide improved machinability and increased red-hot strength, respectively, while remaining prone to corrosion. To improve corrosion resistance, surface treatments and coating processes are employed. Comparing the corrosion characteristics of ZM21 and ZE41 is vital for aerospace and automotive applications, directly affecting component durability, reliability, and performance against corrosion. Magnesium alloys are frequently joined through friction stir welding (FSW), hence, similar importance is provided to studying the corrosion performance of welds, since FSW introduces microstructural changes that alter corrosion performance of welded joints. The paper discusses electrochemical corrosion mechanisms and analyzes the effect of Micro Arc Oxidation (MAO) coating on electrode potential, passivity, and electrical resistance of ZM21 and ZE41 plates welded through FSW. MAO treatments were performed on both base material and FSW joints. The corrosion performance of MAO-coated FSWed ZM21 and ZE41 alloys was compared through the Electrochemical Impedance Spectroscopy (EIS) and Potentio-Dynamic Polarisation (PDP) tests. The PDP test revealed that MAO treatment enhanced the corrosion resistance of both base and FSWed ZM21 and ZE41 magnesium alloys. There was an improvement in potential polarization (Rp) values from 565 Ω cm2 to 11245 Ω cm2 for ZM21 and from 1184.4 Ω cm2 to 11435.69 Ω cm2 for ZE41 alloys. While exhibiting improvements in corrosion resistance, MAO-treated ZE41 performed better than MAO-treated ZM21. PDP results were verified through confirmatory EIS results. Therefore, MAO treatments are effective methods to improve the corrosion performance of Mg alloys. Evaluation of MAO coating performance on various FSW Mg alloys and studying their corrosion performance is crucial for engineering material selection.