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07209 Corrosion Behavior of Aluminized and Chromized Carbon Steel in Fluctuating Oxidizing-Sulfidizing Environments

Picture for 07209 Corrosion Behavior of Aluminized and Chromized Carbon Steel in Fluctuating Oxidizing-Sulfidizi
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Product Number: 51300-07209-SG
ISBN: 07209 2007 CP
Author: Vikas R. Behrani and Preet Mohinder Singh
Publication Date: 2007
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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.
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.
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