Influence of Pre-oxidation on Filamentary Carbon Deposition on 20Cr25Ni Stainless Steel

R Ding,Y L Chiu,M P Taylor,N Smith,C W Mowforth,H E Evans

doi:10.1007/s11085-019-09891-4

R Ding, Y L Chiu + Show 4 more

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https://doi.org/10.1007/s11085-019-09891-4

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Abstract

The results of a pre-oxidation heat treatment at 930 °C in Ar/H2/H2O environments on a Si-bearing Nb-stabilised 20Cr25Ni austenitic steel are presented. The heat treatment was conducted under low pO2, achieved by the introduction of controlled amounts of moisture into the gas. The atmosphere promoted the formation of a continuous, dense, adherent, protective surface scale composed of Cr2O3 and MnCr2O4 with a thin Si-rich oxide at the oxide alloy interface. Samples with different oxide layer thicknesses were produced and further exposed at 700 °C, to a gas of nominal composition CO2/1%CO/1000 vpm C2H4 for 4 h. This gas mixture has a carbon activity greater than unity and readily forms filamentary carbon on the non-pre-oxidised alloy. This is catalysed by nickel particles formed intrinsically from the alloy during the early stages of oxidation of the unprotected surface. The oxide layers produced, as a result of the pre-oxidation process, could suppress carbon deposition onto the alloy; a significant reduction in carbon deposit was noted with an oxide of 125 nm thickness, and no deposit was found on the sample with an oxide thickness of 380 nm. The depth of depletion of chromium from the alloy correlated with the thickness of the oxide formed during the pre-oxidation heat treatment, but the chromium concentration at the oxide/metal interface remained at ~ 15–16 wt% and considered to be sufficient to reform a protective layer in the event of mechanical damage to the original. No additional chromium depletion of the alloy occurred during the 4-h deposition stage.

Highlights

It has been recognised for many years that selective pre-oxidation of chromiumbearing alloys to form a chromia layer reduces or eliminates carbon deposition and carburisation in CO2-based environments [1, 2]
The aim of this paper is to present the results of a study designed to prevent the formation of the catalytic particles by the early development of a protective, continuous surface layer of chromia
The results presented here will find relevance in applications which may benefit from the formation of a highly protective oxide layer formed during a pre-oxidation heat treatment, e.g. solid state fuel cells and heat exchangers

Summary

Introduction

It has been recognised for many years that selective pre-oxidation of chromiumbearing alloys to form a chromia layer reduces or eliminates carbon deposition and carburisation in CO2-based environments [1, 2]. This benefit has been observed in the steam cracking process where the onset of coking was delayed by the formation of a chromia layer at the surface of alloys [3]. The purpose of the present study was to examine the effect of thin sub-micrometre chromia layers on carbon deposition on a Nb-stabilised 20Cr25Ni austenitic stainless steel. This process does not lead to carburisation of the underlying alloy as in “metal dusting” [15,16,17] but may be a precursor to this

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