The influence of carbonyl sulphide on the inhibition of filamentary carbon deposition on stainless steel

Abstract

This paper examines the role that COS plays in the inhibition of carbon deposition onto a Si-free 20Cr-25Ni-Nb-stabilized austenitic stainless steel. We have shown that the addition of just 240 vppb of COS to a standard gas deposit mixture of high carbon activity (1000 vppm C 2 H 4 , 1% CO, balance CO 2 ) yields a dramatic fall in the amount of filamentous carbon deposited. The inhibitive effect persists when treatment is continued after COS has been subsequently removed from the treatment gas. Clearly, in these experiments the phenomenon is a result of substrate surface modification by the COS rather than changes in gas phase kinetics. Deposition onto surfaces where filamentous carbon has already been laid down is similarly inhibited.

The Effect of Temperature and Carbonyl Sulphide on Carbon Deposition on 20Cr25Ni Stainless Steel

“…The results at 550°C confirmed the findings of the previous study [15] in that no filamentary carbon was deposited on the surface of the alloy with gases containing 234 vppb COS, Fig. 4a.…”

“…An extension of this approach involved minor additions of silicon to the alloy [8]: known to promote early formation of a protective, defect-free chromia on austenitic steels [15,16]. An alternative approach to suppression of carbon deposition involved changing the chemistry of the particles.…”

“…An alternative approach to suppression of carbon deposition involved changing the chemistry of the particles. This was achieved by increasing the oxygen potential of the depositing gas [9] or by the introduction of sulphur species into the depositing gas [15,17]. Both of these routes are thermodynamically plausible; the former produces an environment in which nickel oxidises thus preventing metallic particle formation, the later does not prevent the formation of the particles but modifies the catalytic nature.…”

“…Determination as to which, if either, of these possibilities occurred in this system was not confirmed in the earlier work due to the difficulty in isolating sufficient numbers of particles and the difficulty in resolving extremely thin sulphide layers or poisoned surfaces. It should be noted that no nickel sulphides were detected during the course of the investigations and thus it was postulated that surface poisoning was the most likely route to the suppression of filamentary carbon formation [15].…”

“…Much of the earlier work was conducted at 550°C [8,9,15,16]. The nominal composition of the pre-mixed gas used in that work was 1000 vppm C 2 H 4 in 1%CO balance CO 2 , and the alloy was a 20Cr 25Ni Nb-stabilised austenitic stainless steel.…”