Recent studies have shown that gaseous environment in the mid furnace of kraft recovery
boilers fluctuate between oxidizing and sulfidizing under normal operating conditions. It
has also been shown that these fluctuations have significant effect on corrosion by
enhancing the spallation of protective oxide scales. As one of the possible methods to
control mid furnace corrosion, chromized and aluminized diffusion coatings were
developed for carbon steels by pack cementation process using NH4Cl halide activator
and elemental Cr or Al powder. Pack cementation was done for different times and at
temperatures to get optimum coating process parameters. Chromizing of SA210 carbon
steel resulted in formation of chromium carbides irrespective of coating temperature and
time. Aluminizing showed formation of Fe2Al5 at 800oC and FeAl at 950oC as major
phases irrespective of pack compositions. Variation in Cr/Al coating thicknesses ranging
from 20µm to 150µm was observed by changing process time from 2 to 10 hours. When
tested in cyclic gaseous environments at 300oC, both coatings showed excellent behavior
(mass change < 1mg/cm2) compared to the uncoated SA210 carbon steel samples (mass
change ~8mg/cm2). Uncoated samples showed formation of layered and porous sulfideoxide
scale, whereas chromized and aluminized steels showed formation of protective
Cr2O3 and Al2O3 scale respectively.