D. R. Sigler, J. G. Schroth, GM Research and Development Center, Warren, MI

Summary: Failures of welding transformers were traced back to leaking brazed joints on the secondary winding within the transformer case. The leaks were caused by extensive corrosion of both the braze metal and the adjoining copper tube and/or casting that formed the secondary winding. The braze metals were Cu-rich, Ag-Cu-P alloys. The corrosion consisted of selective attack of copper within the braze metal as well as the adjoining copper tube and casting. The existence of extensive copper sulfide scale in the winding and particularly at the brazed joint along with the presence of high phosphorus levels within the corrosion products suggests that the corrosion mechanism was sulfide-induced corrosion that led to formation of phosphoric acid within the joint. This acid selectively attacked copper either within the joint or adjacent to it. This mechanism was reproduced in the laboratory using by exposing brazed joints to deionized water at elevated temperature containing high sulfide levels. A long term solution was identified, i.e., replacement of the Ag-Cu-P braze with a phosphorus-free Ag-Cu-Zn-Sn braze.