Copper-Silver-Phosphorus braze joint corrosion in production welding transformers

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Copper-Silver-Phosphorus braze joint corrosion in production welding transformers

Sigler, D.R. / Schroth, J.G.

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 Cu-Ag-P braze metal and the adjoining copper tube and/or casting that formed the secondary winding. 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 by exposing brazed joints to sulfide-contaminated deionized water at elevated temperature. A long term solution to the corrosion problem was identified: replacement of the Cu-Ag-P braze with a phosphorus-free Ag-Cu-Zn-Sn braze.