CORROSION OF SHAPE 'WELDED STEELS IN AQUEOUS AND SALT ENVIRONMENTS

Abstract

The present study has been done to analyze the corrosion behaviour of low carbon steel (C-0.1%, Mn-1.5%, Si-0.3%, S-0.024°h, P-00.04%,-Cu-0.3%), processed by the new method of Shape Welding. This technique is being developed to manufacture the components like reactor pressure vessels etc,: by welding method alone. The corrosion behaviour, was studied under the salt coating of pure Na2SO4, NaCl and Na2SO4 mixture (1:19, 3.:17, 1:1), and aqueous environment, in the temperature range of 700-900°C.The study has been done for small duration of 8 hours in NaCl + Na2SO4 and pureNa2SO4 salt coatings, whereas 40 hours of exposure has been given to sample kept in aqueous corrosion chamber. The corroded specimens were studied by weight loss measurements and corrosion product identified by SEM, XRD an EPMA. It was found that the Shape Welded steel is most resistant to 50% NaCl + 50% Na2SO4 mixture at 700°C and is least resistant to pure Na2so4 at 900°C. At 700°C and 800°C thin and compact scale was formed which became massive at 900°C. In 5% NaCl + 95% Na2SO4 mixture and in 15% NaCl + 85% Na2'SO4mixture cracks were found in scale at 900°C. Nodular growth was observed in 15% NaCl + 85% Na2SO4 mixture. at 700 & 800°C. In 50% NaCl + 50% Na2SO4 mixture stingers were observed at 800°C indicating partial melting. EPMA analysis indicated the presence of duplex layer with pure Na2SO4 at 8000C, with 5% NaCl + 95% Na2SO4 mixture of 7000Cand with 15% NaCl + 85% Na2SO4 at 800°C, which consisted mainly of Fe and 02. In aqueous media the weight loss was very low indicating that the material is more resistive to aqueous corrosion as compared to hot corrosion in Na2SO4 and NaCl mixtures. On the basis of this study it can be concluded that Shape Welded Steel is not suitable for use at 900°C in Na2SO4 environment but at low temperatures it performs well, specially at 700°C being least susceptible to 50% NaC1 + 50% Na2SO4 mixture at that temperature. (