Fireside Corrosion of Ni Base Self-Fluxing Alloy Coatings in WtE Plant: Effect of Flue-Gas and Metal Temperature

Abstract

Increase in the efficiency of energy recovery facilities is one of the challenges facing waste-to-energy (WtE) operators in the EU. To achieve this target, one option is optimization of the water/steam cycle to increase electrical efficiency. Nevertheless, increase in steam temperature in heat exchanger tubes results in increased fireside corrosion risks, particularly in superheater tubes, where severe corrosion loss of materials, frequent shutdowns for repairs and high operational costs occur. In this study, two heat-treated self-fluxing Cr–Mo–Si–B–Ni base alloys containing, respectively, 15.6 and 16.4 wt.% Cr, were applied to low- and high-temperature pendant superheater tubes (LTSH and HTSH). Detailed analyses for flue-gas and metal temperatures in service at different locations of the pendant tubes (around 850 and 400 °C, respectively) and analyses of deposits collected on both tubes (Na, K, Ca, Si, Zn, Pb, Cl, S) were conducted. Both coatings exhibited a corrosion rate close to ones reported for Inconel 625 in similar conditions. Localized corrosion was found and associated with molten phase-assisted corrosion mechanism damaging Si-rich protective layer. Correlations between corrosion rate and %Cr in the coating, T flue-gas, T metal and amount of molten phase in deposit are presented.