Corrosion of magnesium alloys in commercial engine coolants

Abstract

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 organic-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.

Recent Progress in Corrosion and Protection of Magnesium Alloys

“…It is clearly shown that AM-SC1 was more severely corroded by the MBL coolant than in the LLC-T coolant (Long life coolant (Red), Genuine Toyota part, a modern organic acid-based long life coolant) +1 wt% KF while the other materials were still intact in both coolants. [134] These results suggest the acceptability of LLC-T+1 wt% KF for use in an AM-SC1 engine block.…”

“…For example, Mobil coolant 33 plus (MBL) is a traditional inorganic coolant. It is more corrosive to AM-SC1 at room temperature than at the high temperature of 95 C. [134] The average general corrosion rate of AM-SC1 (a new alloy with rare earth elements, zinc and zirconium as the main alloying elements, which was developed as an engine block material [119,120] ) immersed in MBL decreases as temperature increased. When the temperature was lower than 85 C, the general corrosion rate became unacceptable.…”

“…[134] When the concentration of KF was greater than 1 wt%, the corrosion rate becomes lower than the corrosion rate threshold of 0.67 mg/cm 2 /week. Similar results can be obtained in other coolants with and without KF as an inhibitor.…”

“…It indicates that KF has an inhibitive effect in all of the studied coolants. [134] The likely mechanism of the inhibitive effect of KF on AM-SC1 is probably similar to that for pure magnesium in fluoride containing ethylene glycol solutions, ie. fluorides react with the magnesium in the matrix of AM-SC1, forming a low solubility magnesium fluoride product which deposits on the alloy surface and prevents further corrosion attack to the alloy.…”

“…[134] The results are listed in Table 4. The corrosion rates of AM-SC1, ingot AZ91D and diecast AZ91D all decreased after KF was added to the LLC-T coolant.…”