An insight into the stress corrosion cracking resistance of friction stir processed and micro arc oxidation coated ZE41 grade magnesium alloy

Abstract

The stress corrosion cracking behavior of friction stir processed, micro arc oxidation coated and a composite coated (friction stir processed-micro arc oxidation) ZE41 magnesium alloy were studied in modified simulated body fluid (m-SBF) solution at room temperature. Friction stir processing was carried out with a tool rotation speed of 650 rpm and a traverse speed of 50 mm/min. The results showed that fine equi-axed grain structure with an average grain size of ∼2.8 µm, fragmentation, and partial dissolution of T-phase and Zr-rich particles was achieved through friction stir processing. Specially designed two electrode coating method was adopted to produce samples with micro arc oxidation coating in silicate solution. The stress corrosion cracking susceptibility of base metal, friction stir processed, micro arc oxidation, and composite coated friction stir processed-micro arc oxidation samples were evaluated through constant load testing at five different stress levels. The results indicated higher stress corrosion cracking susceptibility for base metal compared to friction stir processed; micro arc oxidation and friction stir processed-micro arc oxidation samples. Grain refinement and microstructure homogenization enhanced stress corrosion cracking resistance of friction stir processed sample whereas the barrier effect of coating during initial stages of corrosion improved stress corrosion cracking resistance of micro arc oxidation sample compared to base metal. Among all the samples, friction stir processed-micro arc oxidation exhibited superior stress corrosion cracking resistance due to the self-repairing mechanism of friction stir processing, which in turn improved the corrosion resistance of micro arc oxidation layer. The fractographic features revealed different stress corrosion cracking mechanisms for base metal, friction stir processed, micro arc oxidation, and friction stir processed-micro arc oxidation samples. T-phase and Zr-rich particles played major role in the initiation of corrosion pits and resulted in stress corrosion cracks.