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The traditional methods of protecting aluminum surfaces in automotive cooling systems are under scrutiny because of problems caused by high levels of certain inhibitors, particularly silicate, and the lack of protection against crevice corrosion. This paper discusses new monoacid/diacid inhibitor technology which improves protection of aluminum, increasingly used in automotive engines, and it also extends the useful life of the automotive engine coolant.

A new liquid cooled gasoline powered aircraft piston engine has been introduced to the General Aviation marketplace. To achieve additional benefits of liquid cooling, higher coolant operating temperatures are incorporated. Initial aircraft operating experience with the initially selected commercial ethylene glycol based coolant using traditional inhibitor packages resulted in excessive radiator core plugging. A program was initiated to determine the cause for the radiator plugging and identify solutions. Another commercially available ethylene glycol based coolant with a revised inhibitor package was selected as a promising solution. Evaluation of the coolant has been conducted resulting in significantly reduced deposit formation.

An inhibitor package which is silicate-, nitrate-, borate- and phosphate-free has been developed as the basis for a world-wide automotive coolant formulation. The formulation contains aliphatic mono- and dicarboxylic acids and tolyltriazole as the sole inhibitors. Formulations containing carboxylic acid inhibitors have been studied in ASTM bench tests and found to sufficiently protect all prevalent cooling system metals. In addition, fleet tests have shown that carboxylic acid inhibitors deplete much more slowly than conventional inhibitors, making possible a much longer life coolant. Results from laboratory tests which simulate extended usage indicated that carboxylic acid-containing coolants have a significantly longer life span for the protection of all cooling system metals. Finally, the carboxylic acid/tolyltriazole inhibitor package is completely adaptable to a propylene glycol base.

Field-test samples cut from radiator tubes in two 1990 Chevy Luminas (3.1L engine) after 100,000 miles were analyzed to determine corrosion layer differences. One car used a carboxylic acid-based inhibitor technology (C1). The other car used a conventional coolant (C2). X-ray photoelectron spectroscopy (XPS) analysis of the two samples was performed. Results indicate a significant difference between the two samples. The C1 sample had a thin (<60Å) organic coating bound to the aluminum alloy surface, while the C2 sample had a much thicker (>1000 Å) silicate-rich layer. This resulted in the C2 sample exhibiting “surface charging” behavior. These results relate directly to the metal/insulator (conductor/insulator) characteristics of the two samples, and imply that the heat transfer of the protective coating provided by the carboxylate technology (C1) is significantly better than that of traditional inhibitor technology (C2).