Corrosion of magnesium (Mg) alloys and metallurgical influence

An overview is provided of the following aspects which determine the form and rate of corrosion of typical two-phase magnesium (Mg) alloys: (i) measurement details; (ii) concentration of the impurity elements iron, nickel, copper and Co; (iii) volume fraction, size, distribution and electrochemistry of second phases; (iv) heat treatment; (v) propensity of the environment to cause surface film formation and breakdown; and (vi) the composition of the a-Mg matrix. Our understanding of the Mg corrosion mechanism is based on research using chloride solutions, which are appropriate for applications such as auto service. These chloride solutions are quite aggressive. The chloride ions tend to break down the partly protective film on the Mg alloy surface. The corrosion rate increases with exposure time until steady state is reached, which may take several weeks. This understanding can elucidate corrosion for the other application of growing importance to Mg alloys as biodegradable medical implants. Solutions that elucidate medical applications tend to form surface films and the corrosion rate decreases with immersion time. The surface films increase the resistance between micro-galvanic elements in the microstructure, so that there is less acceleration of the corrosion rate by second phase particles. Surface condition is an additional influence on the corrosion of Mg components. This is of significantly greater importance for auto applications than for medical applications because cost effectiveness is much more important for auto applications