The Chemistry of Oxalic Acid Cleaning of Engine Cooling Systems

Abstract

Engine coolant candidates undergo a progression of tests to evaluate various physical properties and performance characteristics. Key to this test progression is the evaluation of the coolant in vehicles. It serves as the final check point, tieing together the results of the other various tests. In the fleet test, the corrosion protection and performance characteristics of an engine coolant are evaluated under actual use conditions. The methodology used in testing a coolant in a passenger car or a light duty truck is critical. ASTM Practice for Testing In Car and Light Truck Service (D 2847) serves as an industry accepted standard for evaluating coolants in vehicles. This standard provides a full discussion of testing protocol. Sections 9.3 and 9.4 of ASTM D 2847 outline the cooling system cleaning and conditioning procedures. These sections stipulate the use of oxalic acid as a chemical cleaner followed by a borate buffer conditioning fluid. The purpose of these two steps are (1) oxalic acid cleaning removes inhibitors, rust and other deposition from the cooling system, and (2) borate conditioning fluid removes excess oxalate to leave a conditioned surface in the cooling system which is neither detrimental nor beneficial to the performance of the coolant to be evaluated. The ASTM Committee D-15 on Engine Coolants is reconsidering the use of oxalic acid cleaner in the automotive cooling system. Concerns have been raised regarding potential carryover effects oxalic acid could have on the performance of a coolant. This paper reviews the chemistry of the oxalic acid cleaner as well as discusses the effects that oxalic acid cleaning has on the cooling system surfaces and its potential carryover effects to the test coolant. An understanding of the chemistry of oxalic acid is crucial in judging its efficacy as a cleaner to bring cooling system surfaces of a test vehicle to a reproducible base condition before performance testing.