HOT_ CORROSION STUDIES ON STEEL WELDMENTS

Abstract

In the following study, an attempt has been made to investigate the behaviour of mild steel weldments made by submerged arc welding at three amperages (450, 550, 650 A) in salt environments of pure NaCl, pure Na2SO4 and NaCl-+ Na2SO4 mixture (50% each by weight) in the temperature range of 500 C to 900°C. The specimen coated with the given salt were exposed for eight hours at the required temperature in the silicon carbide furnace. The weight change per unit area was' measured after the hot corrosion run. The welding current has been found to affect the weight change in these salt environments both in the base metal and the welded specimen. At all temperatures and for three, different welding currents, NaCl attack is more severe as compared Na2S(34 and the mixture of NaCl + Na2SO4. The Na2SO4 is comparatively least corrosive but at 900°C it gives much higher weight change as compared to NaCL and NaCl + Na2SO4 mixture. The NaCL + N 004 mixture gives maximum attack at 700°C in case of both the welded and base metal specimen. At lower temperature of exposure the base metal shows more reactivity towards these salts as compared to the welded specimen whereas at 900°C the weight change in the welded specimen is more as compared to the 'base metal. The EPMA analysis shows that the scale/substrate interphase is irregular in shape and it penetrates differentially in the weld metal, the HA2 and the base-metal. The scanning electron micrographs showed some difference in morphology of scale in the RAZ the weld metal and the base metal and showed presence of multi-layered scale iii which were mainly oxides of iron (Fe203, Fe304 and Fe01 and iron sulphide in case of Na504. Mild steel can be used for boilers and other heat appliances operating at tower temperatures i.e. around 550°C (6) and salt corrosion on fire side of the tube can occur. The higher temperature of 700°C and 9000C has been taken to accelerate the corrosion reaction because the duration of the test was only eight hours. It can be inferred from this study that mild steel may be used at temperatures upto 500°C in applications like boiler tubes and heat transfer appliances. At higher temperatures, hot corrosion becomes very severe. As mentioned earlier, NaCL is many times more aggressive than Na2SO4 and NaCl Na2SO4 salt mixture. So in coastal areas for sea going vessels where the chloride content of the environment may be much higher, the mild steel submerged arc welded steel will not be suitable.