Protection of magnesium alloys from corrosion using magnesium rich surfaces.

Mg alloys have great potential in engineering applications for saving energy consumption due to the high strength to weight ratio. Mg alloys are also biocompatible and biodegradable with biomedical applications such as orthopaedic and vascular implants. Controlling the corrosion of Mg alloy components is necessary to sustain their performance over the design lifespan. A low corrosion rate is also preferred for implants to mitigate negative effects such as hydrogen evolution during corrosion. Surface films can be used to control the corrosion of an Mg alloy effectively. In this work, Mg(OH)₂ coatings were deposited on Mg alloy substrates (AZ31 and ZM21) by hydrothermal (H.T.) steam treatment as a benchmark and subsequently by novel processing using electrochemical (E.C.) and additive treatment with an Mg²⁺ rich solution. The microstructures and compositions of the alloys are characterised both with and without coatings. Corrosion tests were conducted in various test solutions, including 3.5% NaCl, 0.9% NaCl and Hank's solutions. Electrochemical techniques and mass change measurement are used for the corrosion testing of initial exposure and longer-term immersion, respectively. The processing parameters of the electrochemical and additive methods were optimised based on the corrosion behaviour of the coated samples. This research shows that the Mg(OH)₂ based coating can enhance the corrosion protection of the Mg alloy substrates, with at least a 3 fold reduction in corrosion rates compared to uncoated substrates. Comparing hydrothermal coatings, the electrochemical and additive (EC+Additive) coatings not only show similar corrosion performance but also greater manufacturing flexibility and repairability with potential for further enhancement.