How to screen high temperature steels for optimal use in biofuel-heated burners for residential boilers: Role of alkali

Hot parts of burners for residential boilers are subject to irregular heating cycles up to 800 °C and higher, simultaneously exposed to alkali salts, variable oxygen partial pressure - and thermal shock. Fundamental hot corrosion studies under well-controlled conditions, in combination with simple ranking tests simulating the effect of ash deposited on the alloy surface, have been performed. The investigation included four types of high-alloyed steel and two types of ash from wood and bark pellets. The work was complemented with field studies of selected damaged parts. Two mechanisms involving interaction of potassium as well as carbon under intermittent oxygen-deficient conditions with chromium from the alloys have been corroborated. A simple ranking test procedure has also been designed. It proved relevant as a screening method only after incorporating a mild thermal shock in the heat treatment sequence. A significant difference in corrosive effect, particularly on a lower chromium-alloy like 304 L, has been observed for the ash derived from the wood and bark pellets, respectively.