The water wall tube surfaces in the lower furnace areas of kraft recovery boilers are exposed to a reduced sulfidizing atmosphere. Water wall surfaces are known to experience temperature excursions. High temperature excursions are relatively infrequent and most temperature spikes in this area are of small amplitude. Although it was known from the previous studies that continuous temperature cycling increases the corrosion rate of exposed metals, but the mechanism by which infrequent spikes increase the corrosion rate of carbon steel alloys in sulfidizing environment was not clear. The present study was aimed at establishing the effect of infrequent thermal excursions on overall high-temperature sulfidation of carbon steel used for water wall tubes. Results from the present study showed that even a couple of infrequent high-temperature thermal excursions, which can lead to scale damage, followed by long post spike exposure may lead to a higher corrosion rate than the equivalent isothermal tests conducted at the higher temperature of the thermal excursion.
Thermal excursions are known to affect the corrosion behavior of metals in high-temperature gaseous environments. Most of the previously published work, studying the effects of thermal excursion on gaseous corrosion, was done with continuous thermal cycling for a given test period 4) These studies clearly demonstrate that the corrosion attack is more severe due to the thermal cycling. A previous study (s)on SA-210 carbon steel alloy in a 1°/0H$ environment at 320°C for 20 days reported that regularly spaced continuous spikes to 400C increased the weight loss 19 times (compared with weight loss fi-om320C isothermal test.) The same result represented a 374% increase when compared with weight loss from 400C isothermal test. In another study, 1,25Cr-O.5Mo steel in an oxidizing environment of 0.2% S02 with thermal cycling between high temperature and room temperature showed that corrosion rates were strongly dependent on higher temperature; where the corrosion rates abruptly increased five folds around 700C.(2)