Did some research on this the other day after getting an '84 KTM with Mag alloy cases. I found a Paper written by a Uni Prof. from South Oz on the use of magnesium alloy in engine castings. It was as dry as a .............. to read but the guy new his stuff. I can not find it again
unfortunately. The magnesium sits much lower on the scale than any other metal common in cooling systems so basically it sacrifices itself as a cathode in a galvanic circuit. I found that the top level Motul Factory line coolant contains the correct chemical mix to at least limit damage to the Mag alloy. There are others. Below is the critical extract from that paper. Yes I know I need to get a life if I end up reading Uni papers on corrosion of metals. Just remember the mag alloy does not rust.
A number of magnesium alloys show promise as engine block materials. However, a critical issue for the automotive industry is corrosion of the engine block by the coolant and this could limit the use of magnesium engine blocks. This work assesses the corrosion performance of conventional magnesium alloy AZ91D and a recently developed engine block magnesium alloy AM-SC1 in several commercial coolants. Immersion testing, hydrogen evolution measurement, galvanic current monitoring and the standard ASTM D1384 test were employed to reveal the corrosion performance of the magnesium alloys subjected to the coolants. The results show that the tested commercial coolants are corrosive to the magnesium alloys in terms of general and galvanic corrosion. The two magnesium alloys exhibited slightly different corrosion resistance to the coolants with AZ91D being more corrosion resistant than AM-SC1. The corrosivity varied from coolant to coolant. Generally speaking. an oraganic-acid based long life coolant was less corrosive to the magnesium alloys than a traditional coolant. Among the studied commercial coolants. Toyota long, life coolant appeared to be the most promising one. In addition. it was found that potassium fluoride effectively inhibited corrosion of the magnesium alloys in the studied commercial coolants. Both general and galvanic corrosion rates were significantly decreased by addition of KF, 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 Toyota long life coolant with 1%wt KF addition is a promising coolant for magnesium engine blocks.
