11194 How to Screen High Temperature Steels for Optimal Use in Biofuel-Heated Burners for Residential Boilers: Role of Alkali

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Product Number: 51300-11194-SG

ISBN: 11194 2011 CP

Author: Robert Pompe, Carolina Pettersson, Jan-Erik Svensson and Lars-Gunnar Johansson

Publication Date: 2011

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Hot parts of burners for residential boilers are subject to irregular heating cycles up to 800 oC 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.

Key words: alkali corrosion, residential boilers, pellet ash, thermal shock, screening, high temperature corrosion-resistant alloys