Electrochemical characterization of three corrosion-resistant alloys after processing for heating-element sheating.

The development in the production of heating-element sheathing to be used for domestic boilers is addressed to replace traditional copper with corrosion-resistant Fe-Cr-Ni based alloys. Here, an electrochemical study was carried out by recording the anodic polarization curves of three corrosion-resistant alloys (Incoloy 800, Incoloy 825 and Sanicro 28) in three tap waters having different chloride concentrations. The process used to produce heating-element sheathing consists of different steps (mechanical forming, welding and heat treatment) which lead to modifications to the material. Hence, tests were performed on samples taken after each processing step. Some tests were performed on samples submitted, after the end of the working process, to laboratory treatments (removal of the surface oxide and passivation). Pitting and protection potential were obtained from cyclic voltammetry. The corrosion potential was measured before the polarization experiments. From these potentials, the perfect and the imperfect passivity regions were obtained. The results showed that Incoloy 825, less expensive than Sanicro 28, has about the same corrosion resistance and so it appears to be the best material for this application even in aggressive tap water.