Passivation and Potential Fluctuation of AZ31B Alloy in Alkaline Environments

Magnesium (Mg) alloys are used in many industries because of their distinctive properties, but their high chemical reactivity and poor oxide film protection make them inferior. This project focused on investigating the effects of secondary phases and intermetallic particles (IMPs) on the corrosion behavior of AZ31B Mg alloy. The test solution was chloride free NaOH with varying pH levels between 10-14, as well as testing under deaerated conditions. The open circuit potential (OCP) measurements using AZ31B in static NaOH solutions showed potential fluctuation between approximately -1.6 VSCE (active state) and -0.4 VSCE (passive state) during the 24-h immersion. After long-term immersion (i.e., 120 h) in deaerated 1M NaOH, it was found that the OCP values stabilized at approximately -1.4 VSCE.At pH 10 and 11 it was found that the Al-Mn IMPs were the cathodes and an oxide film developed over them. Under pH 13 and 14, the Al-Mn IMPs partially dissolved and later were protected by an oxide film. The formation of oxide nodules under static conditions at the interface of the α-Mg matrix and the secondary phases revealed severe cracking due to increased volume within the oxide. This cracking may be the cause of the potential fluctuation that AZ31B undergoes