Metallurgical investigation of a heat-exchanger tube of 70/30 cupronickel failed by fretting corrosion

摘要

The present paper describes a failure investigation of a tube that had shown leakage in a shell and tube type heat-exchanger with 70/30 cupronickel tubes and mild steel baffles (13 nos.). Corrosion damage or thinning of the failed tube was found to have occurred from the outer surface at all the baffle locations. Further examination of the failed tube was done using an optical microscope, a scanning electron microscope equipped with energy dispersive spectroscopy, Raman spectroscopy and an instrumented microhardness indentation machine for hardness measurement. Based on the results, it was concluded that the failure and the loss in wall thickness of the tube at locations where it was in contact with the mild steel baffles were caused by fretting corrosion. The focus of this investigation is to bring out the metallurgical changes and the surface conditions (including changes in microstructure and microhardness, mode of corrosion attack and type of oxides/corrosion products) of the tube material undergoing fretting corrosion at micro level. Recommendations to avoid such a failure are also suggested.