HOT CORROSION OF LOW CARBON STEEL IN Na2SO4 AND NaC1

Abstract

The present study has been done to analyze the corrosion behaviour of low carbon steel (C-0.11%, Mn-0.42%, Si-0.01%, P-0.012%) used in manufacturing of tubes of the fire tube wet steam boilers for small industrial units, under the salt coating of Na2SO4, Na2SO4 and NaCl (1:1), NaC1 and synthetic ash [CaSO4-22.7; KZSO4-10.4; Na2SO4-4.3; ZnSO4-20.5; Si02-10.4; A1203-6.5; Fe203-22.3], in the temperature range 400-550°C. The study has been done for small duration of 8 hours in NaCl, NazSO4 + NaCl and Na2SO4 salt coatings, whereas 100 hours exposure has been given to samples embedded in synthetic ash. The corroded specimens were studied by weight loss measurement, SEM, XRD and EPMA. Thermogravimetric data showed minimum corrosion in Na2SO4 salt coatings and maximum in NaCl. The effect of temperature was more prominent in Na2SO4,°but the extent of corrosion at all temperatures was maximum for NaC1. X-ray diffraction study identified oxides of iron and sulfides of iron and manganese in NaZSO4, NaC1 + Na2SO4 and synthetic ash. While only oxides of iron were reported under NaCl coatings and no chloride was identified. The EPMA study indicated diffusion of Zn, Al and Si from synthetic ash to substrate. In case of NaCl pitting type corrosion is observed. This will make the use of these materials prohibitive in high pressure boilers. On the basis of above study it has been concluded that above grade of steel can not be used in boiler using coal containing high amounts of suipher, chlorides and ash. The steel should be modified by increasing the Cr and using additives viz. Si,