A 309 stainless steel wedge divider in a thermal oxidizer failed due to catastrophic metal loss after a short time in service. Failure caused shutdown of the oxidizer to replace the dividers.

A 309SS wedge divider was submitted for laboratory analysis due to significant metal loss in a high temperature exhaust gas environment. The wedge divider acted to contain catalyst and was exposed to hot exhaust gases from a rotary dryer that dried wood shavings and sawdust. The offgas from the wood contained combustion products of methanol, formaldehyde, acrolyn, acetaldehydes, and wood dust. The wood and resin reportedly contained sodium and potassium chloride salts and calcium sulfates. It was reported that the greatest metal loss occurred where the exhaust temperatures were the lowest. Metallographic examination of the wedge divider in the area of greatest metal loss (lowest temperature) revealed that the material was almost completely consumed and contained massive, multi-layered corrosion products. Metallographic analysis of the wedge divider at the top (highest temperature) end showed that the microstructure contained massive nitride needles, suggesting of exposure to an disassociated ammonia or high-temperature nitrogen gas environment. Observation of sigma phase in the top portion microstructure suggested exposure to temperatures between 1000F and 1600F. All evidence suggested that the wedge divider experienced metal loss due to exposure to molten fuel ash containing low-melting point vanadium, potassion and sodium salts.