Electrochemical Investigation of the Corrosion Behavior and Galvanic Compatibility Between Microchannel Tubes and Fins of an Aluminum Automotive Condenser

The condenser of air-conditioning unit for automotive application is an aluminum heat exchanger with three main components: the headers, microchannel tubes and fins. It is normally installed in the neighborhood of the front grill, so it is subjected to external corrosion. Of main concern is the possibility of tube perforation, leading to coolant leaking. One way to improve the corrosion resistance of the condenser is to protect the outside of the microchannel tubes with a sprayed zinc coating. The standard test for corrosion of automotive heat exchangers is saltwater acetic acid test (SWAAT) which is intended to reproduce lifetime performance. With the aim of satisfying recent demands in the industry for higher SWAAT performance, a condenser prototype was developed in this study using flat tubes made with a 1000 series alloy spray-coated with zinc and  fins made with 3003 series alloy with a double clad of 4343 alloy. The condenser prototype was brazed in a controlled atmosphere industrial furnace, using the Nocolok® flux brazing process. External corrosion was simulated by immersion, in SWAAT solution, of small samples cut from the tube/fin panel, taking care to seal the ends of the tubes with epoxy resin. Results of potentiodynamic polarization and electrochemical impedance spectroscopy were compared to those for individual components (uncouple tube and fin) and indicated that the electrochemical behavior is dominated by the fins. The pitting corrosion resistance of the condenser is afforded by the zinc diffusion layer on the tubes and this effect was investigated with cross-sectional corrosion potential profiles and potentiodynamic determination of the pitting potential. Galvanic compatibility between the tube and fins was evaluated with zero resistance ammetry (ZRA) and showed that there is an evolution in galvanic corrosion as zinc in the diffusion layer gets depleted. Furthermore, the localized corrosion behavior at the tube-fin joint was investigated with scanning electrochemical microscopy (SECM) and scanning vibrating electrode (SVET).