Volume 4, Issue 1 (January 2007)
Investigation of Interaction Between Coolant Formulations and Flux Loading/Compositions in Controlled Atmosphere Brazed (CAB) Aluminum Surfaces in Heat Exchanger Applications
An investigation was made into the effects of flux formulation and after brazing residue on the pitting potential of the aluminum surface in various types of coolants during laboratory evaluations. Samples of CAB brazed radiator header alloy aluminum, with various flux loadings, were supplied by Behr and the Ford Motor Company. The flux is generally in the form of potassium fluoroaluminate, general formula K1–3AIF4–6.xH2O. The samples in the flux loading study were all passed through the brazing ovens with the following flux loadings: • No flux, • Half the regular amount of flux, • The regular amount of flux, • Double the regular amount of flux, • Regular amount of flux over half of the surface area. The individual coolants used in the flux loading study were 25 % volumes of the following, each with 100 ppm of chloride: • Conventional, • Hybrid (HOAT), • Organic Acid (OAT), • Modified Organic Acid (MOAT). In the flux composition study, different compositions of potassium fluoroaluminate from three different suppliers were evaluated with hybrid type coolant. All the aluminum samples were run according to a Ford Laboratory Test Method (FLTM) BL105-01 “A Rapid Method to Predict the Effectiveness of Inhibited Coolants in Aluminum Heat Exchangers” . The samples were heated and boiled in coolant for 1 h. The temperature was lowered to 70°C and the samples were electrochemically tested by either potentiodynamic polarization or modified Ford FLTM BL105-01. Each coolant at the end of the test was submitted for chemical analysis of fluoride content. The electrochemical results of the flux loading study showed that the higher the flux loading on the aluminum header material, the easier it is to initiate corrosion. Chemical analysis showed the presence of significant quantities of fluoride in the end of test fluids. As the only fluoride-containing component of the system is the flux residue, it was concluded that fluoride leached from the flux residue. The second study showed that not all potassium fluoroaluminate brazing flux is alike. Samples of three fluxes from three different suppliers of the same loading on aluminum were tested with hybrid coolant. When subjected to heating and potentiodynamic polarization, significantly different results were obtained from the three different fluxes.