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08081 Corrosion of Copper Braze Alloys in Sulfide Containing Water

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Product Number: 51300-08081-SG
ISBN: 08081 2008 CP
Author: Yar-Ming Wang, David Sigler, and Dusanka Radovic
Publication Date: 2008
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The corrosion behavior of copper and copper braze alloy junctions found in resistance welding transformers was studied using synthetic cooling water with and without sulfide contamination. Electrochemical methods including measurements of corrosion potential, potentiodynamic polarization, linear polarization, and galvanic current, were used to determine the galvanic corrosion tendency and corrosion rates. The results showed that the presence of sulfide greatly increases the corrosion rates of both copper and copper braze alloys. The copper-phosphorus braze alloys showed higher corrosion rates than copper and, thus, form copper sulfide faster than copper, which has been observed in transformer joint analyses as the first stage in joint degradation. For copper-phosphorus braze alloys, the corrosion potentials are close to that of pure copper in sulfide-containing solutions, so there is no cathodic protection of the brazed joint from nearby copper components. The corrosion potential of the phosphorus-free alloy (Ag-Cu-Zn-Sn), in sulfide-containing solution is more noble than that of pure copper, thus providing cathodic protection for the joint. In sulfide-containing solutions with up to 5 ppm sulfide, an addition of 20 ppm azole can still retard copper and braze alloy corrosion and sulfide formation.
The corrosion behavior of copper and copper braze alloy junctions found in resistance welding transformers was studied using synthetic cooling water with and without sulfide contamination. Electrochemical methods including measurements of corrosion potential, potentiodynamic polarization, linear polarization, and galvanic current, were used to determine the galvanic corrosion tendency and corrosion rates. The results showed that the presence of sulfide greatly increases the corrosion rates of both copper and copper braze alloys. The copper-phosphorus braze alloys showed higher corrosion rates than copper and, thus, form copper sulfide faster than copper, which has been observed in transformer joint analyses as the first stage in joint degradation. For copper-phosphorus braze alloys, the corrosion potentials are close to that of pure copper in sulfide-containing solutions, so there is no cathodic protection of the brazed joint from nearby copper components. The corrosion potential of the phosphorus-free alloy (Ag-Cu-Zn-Sn), in sulfide-containing solution is more noble than that of pure copper, thus providing cathodic protection for the joint. In sulfide-containing solutions with up to 5 ppm sulfide, an addition of 20 ppm azole can still retard copper and braze alloy corrosion and sulfide formation.
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