Effect of Fluoride on Corrosion of Cooling System Metals in Ethylene Glycol-Based Antifreeze/Coolants

Abstract Automotive and light-duty vehicle heat exchangers such as radiators are now predominantly made of aluminum alloys. For over 20 years, a brazing technique known as “controlled atmosphere brazing (CAB) with noncorrosive fluxes” has become the preferred process for producing aluminum automotive heat exchangers for original equipment manufacturers and aftermarket suppliers. Fluoride-based fluxes are typically used in the CAB processes to dissolve the aluminum oxide layer on the surface and inhibit reoxidation during the brazing cycle to allow the clad alloy to flow properly. The flux is generally a mixture of potassium fluoroaluminate, (K3AlF6, K2AlF5, and KAlF4). The flux residue left on the internal surface of heat exchangers after brazing is thought to be insoluble and noncorrosive, hence no removal is necessary. Although external corrosion of brazed aluminum heat exchangers has been studied using salt spray tests, a study addressing the effect of interaction between different antifreeze and flux residues on internal corrosion has not been reported. To the authors' knowledge, few studies on the effect of fluoride in engine coolants on metal corrosion are available. In this paper, laboratory test data are provided to show that flux residue is soluble in commercially available coolants and can generate fluoride ions that enhance cooling system corrosion. Systematic study on the effect of fluoride on metal corrosion in cooling systems and its remedies are also presented and discussed.