Temperature dependence of hot corrosion of 2.25Cr-1 Mo steel and its weldments

The oxidation and corrosion resistance of 2.25Cr-1 Mo steel have been examined at 550 to 700°C in the presence of K2SO4-NaCl mixture. The application of the salt mixture on the surface resulted in an increase in the isothermal oxidation rate of coated specimens. The oxide cracked, blistered and spalled with time. With the mixed corrodents containing both sulphate and chloride, the attack was still more severe. The synergistic effect of sulphate and chloride was the most detrimental causing severe spalling of the scale. The microstructure and composition of the corrosion products have been examined by optical microscopy and x-ray diffraction (XRD) respectively. The same tests were done for fusion (manual metal arc welded, MMAW) and non-fusion or solid state (induction pressure welded, IPW) joints of 2.25Cr-1 Mo steel and the rate of oxidation was compared. The corrosion rate followed the order MMAW > Unwelded > IPW.