Corrosion inhibition of magnesium alloys in coolants

A number of magnesium alloys show promise as engine block materials. However, corrosion of magnesium alloy engine components by the engine coolant is an important issue in the automotive industry. This paper shows that the corrosion rate of magnesium was increased by dilution and contamination of ethylene glycol. Fortunately, the corrosion of magnesium in ethylene glycol can be effectively inhibited by addition of fluorides. This finding was further verified by assessing the corrosion performance of AZ91D and a recently developed engine block magnesium alloy AM-SC1 in several commercial coolants. Generally speaking, the tested commercial coolants were corrosive to the magnesium alloys in terms of general and galvanic corrosion, but an organic-acid based long-life coolant appeared to be less corrosive than a traditional coolant. It was found that both general and galvanic corrosion rates were significantly decreased by addition of a properly selected inhibitor, and there were no evident side effects on the other engine block materials, such as copper, solder, brass, steel and aluminium alloys, in terms of their corrosion performance. The ASTM D 1384 test further confirmed these results and suggested that a commercial long service life coolant with the inhibitor addition is a promising coolant for magnesium engine blocks.