Accelerated Corrosion Test Methods for Evaluating External Corrosion Resistance of Vacuum Brazed Aluminum Heat Exchangers

A program to develop an aluminum tube stock alloy for vacuum-brazed heat exchangers with improved external corrosion resistance necessitated the use of accelerated laboratory corrosion tests to rank trial materials and ultimately qualify an alloy for production and sale. Brazed radiator sections and sheet samples were exposed to a variety of ASTM standard corrosion test environments. Metallographic examination of samples thus exposed demonstrated that the SWAAT test (ASTM G85:A3) closely replicated the air-side corrosion morphology found in aluminum radiators from the field. Samples exposed in test environments other than SWAAT did not generally exhibit the mixed intergranular/pitting morphology found in field retrievals. In neutral salt spray (ASTM B117), for instance, smooth-sided hemispherical pitting was the corrosion mode observed.

Test techniques were developed for ranking SWAAT corrosion performance of brazed sheet samples and heat exchangers based on time to perforation - the criterion of particular interest in this application - rather than subjective visual appearance ratings or unreliable pit depth measurements. These techniques were applied in the development of a new perforation-resistant tube alloy, “K319”.