Abstract
The Cr content of the surface regions of stainless steel tubes that are used as superheaters and reheaters in boilers can be enriched to more than 70 wt-% during chromisation. Such layers are susceptible to the formation of sigma (σ) phase while its prevention through the use of chromised Ni plated layers is offset by the occurrence of Kirkendall voids. This paper describes how the microstructure of chromised surface protective Ni-Fe layers can be controlled through the optimisation of the conditions under which the chromising and post-solution heat treatments are carried out. The Ni plating and chromising conditions required for the production of layers in which neither σ phase nor Kirkendall void formation takes place are described. The improved chromised surfaces are shown to be highly resistant to hot corrosion, under both coal fired and oil fired environments, as well as to steam oxidation.
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