Corrosion of Alloys in a Simulated Wood-Waste Fluidizing Bed Power Boiler Environment

Rapid corrosion of Type 310H (UNS S31009) stainless steel fluidizing bed nozzles was observed shortly after start-up of a fluidizing bed power boiler designed to combust a mixture of salt-laden, wood-waste (hogged-fuel), and sludge. The primary cause of corrosion was previously identified as active oxidation, a high-temperature chloridation reaction occurring under thick, chloride-rich ash deposit. Replacing the nozzle metallurgy with a more corrosion-resistant material was one life extension strategy pursued by the mill. To assist the mill in this regard, an accelerated laboratory-based screening test was developed and used to identify materials from which to fabricate nozzles for field testing in the fluidizing bed power boiler. The accelerated screening test was validated by its consistency with field observations, namely, the corrosion mode and rate exhibited by Type 310H stainless steel. Of the various materials evaluated, the nickel-based Alloy 625 (UNS N06625) and Alloy HR160 (UNS N12160) were the most resistant to corrosion in the simulated environments. Alloy 625 was found to be at least seven to 10 times more corrosion resistant than Type 310H stainless steel when sulfur dioxide (SO₂) was present, and about 100 times more corrosion resistant in the absence of SO₂. Based on these tests, four alternative materials were selected for field testing.