Differences in corrosion behaviour of cunifer welds

Abstract

Copper-nickel alloys are used for pipe systems that come into contact with seawater. These alloys are known under the group name cunifer alloys. In this research cunifer alloy CuNi10Fe (with 10 percent nickel) was investigated. The weld filler was a cunifer alloy with 30 percent nickel (CuNi30Fe). While generally there should be little corrosion problems, some will occur. In this research more information was acquired about the corrosion properties of a copper-nickel alloy used by Damen Schelde Naval Shipbuilding in the case of welding. The corrosion properties were investigated in different zones over the weld, i.e. the weld zone, heat affected zone, and base material. The differences in terms of corrosion performance between welding with backing gas and without backing gas were studied. The alloy can be protected against corrosion by a self-assembling monolayer of sodium diethyldithiocarbamate. The influence on the corrosion properties of this layer was also investigated. Potentiodynamic measurements, surface analysis and Kelvin probe measurements were used. The unprotected weld zone of a weld welded without backing gas resulted in a significantly higher corrosion rate in the sulphide containing solution. Welding without backing gas caused a porous surface to form on the inside of the pipes. This is not beneficial for the corrosion properties on the long run. The protection with sodium diethyldithiocarbamate decreased the corrosion rate to levels also obtained in the solution without sulphides for the samples welded without backing gas.