Mechanistic features of molten salt corrosion in coal fired boilers

摘要

AbstractIn general, the hottest outlet sections of both superheaters and reheaters in modern CEGB boilers are tubed in austenitic stainless steels. Experience over the last 10–15 years has indicated that molten salt fireside corrosion can significantly reduce design lives, corrosion rates being a function of(i)metal temperature(ii)gas temperature(iii)coal chlorine content(iv)the tube's aerodynamic positionThe characteristic visual features of this form of corrosion such as “classical” wastage flats, honeycomb pitting, and localised pitting at tube geometrical features such as weld beads, and lugs is described. Also discussed are some of the metallographic features of molten salt corrosion, including substrate carburization, internal oxidation/sulphidation and corrosion scale morphology.Whilst uncertainty remains in understanding the precise mechanism, the CEGB have adopted a pragmatic approach using new/alternative materials such as shields, coatings and co‐extruded tubes. However, such a policy may not be the most economic: for example, in some circumstances the alloy 50 Cr 50 Ni is required to give a guaranteed degree of protection. What is still required is the quantification of the role of Na, K, Cl etc. in fuel on deposition and corrosion rates, and the source and influence of carburization so that further operational control of this molten salt corrosion is av