The corrosion resistance of several candidate furnace-wall materials w
as evaluated in a laboratory retort, simulating the reducing-sulfidizi
ng combustion environment existing in PC-fired utility boilers burning
coal substiochiometrically. These materials, including three alloys a
nd nine coatings, were exposed to a mixed gas containing 400 ppm H2S,
100 ppm HCl, 900 ppm SO2, and other essential gaseous components at 80
0 degrees F (degrees C) for 1000 hours with and without the coverage o
f a chloride-bearing ash deposit. The nine coatings consisted of three
weld overlays, two diffusion coatings, and four thermal-spray coating
s. The test results indicate that sulfidation was the primary mode of
attack on unprotected low-Cr steels, such as SA213-T2. Furthermore, th
e role of 900 ppm SO2 in the corrosion mechanism varied significantly
with the Cr content in the alloys. A Cr concentration of approximately
9 wt% appeared to be a threshold, below which the scale surface favou
red the catalytic conversion of SO2 to H2S, thus increasing the local
H2S concentration and gas corrosivity. Above this threshold, the SO2 r
eacted with Cr in the alloy preferentially to form Cr2O3, resulting in
a reduced sulfidation attack. With relatively high Cr concentrations,
the weld overlays generally performed well in the simulated reducing-
sulfidizing combustion environment. Both diffusion coatings also exhib
ited satisfactory resistance to the mixed gas under the test condition
employed. However, the protectiveness of the thermal-spray coatings w
as limited by their porous coating microstructures, which allowed loca
l penetration of sulfur into the base metal. The HVOF process appeared
to produce a denser coating than the are spray and thus provided bett
er corrosion resistance. No effect was found from the addition of a sm
all amount of HCl to the mixed gas and chlorides to the ash on the fur
nace-wall corrosion. The probable corrosion mechanism is briefly discu
ssed.