Effect of H₂S on metal dusting of iron

Abstract

A review is given on the effect of H 2 S on metal dusting of iron which has been studied by gas carburisation in CO-H 2 -H 2 O-H 2 S and CH 4 -H 2 -H 2 S mixtures at 500 and 700 8C.The presence of H 2 S in carburising gas atmospheres leads to sulphur adsorption on the iron surface, which retards carbon transfer. Segregation experiments and surface analyses have shown that sulphur segregates (and thus adsorbs) on cementite surfaces as well as on iron surfaces. The adsorbed sulphur also suppresses graphite nucleation and thus can stop the reaction sequence of metal dusting.Experiments by thermogravimetric analysis (TGA) have shown that the extent of retardation of metal dusting depends on temperature, carbon activity and H 2 S content. The higher the carbon activity, the higher is the H 2 S content required for suppression of metal dusting.At carbon activities a C > a C (Fe/Fe 3 C) the metastable iron carbide, cementite (Fe 3 C), occurs as an intermediate phase during metal dusting. Carburisation experiments in CO-H 2 -H 2 O-H 2 S mixtures at 500 8C and X-ray diffraction analysis (XRD) of carburised samples have revealed that at very high carbon activities a second iron carbide, Hägg carbide (Fe 5 C 2 ), forms on the cementite surface. Microstructural investigations have shown that both metastable carbides decompose during metal dusting.Metal dusting experiments on iron at 700 8C have been performed in CH 4 -H 2 -H 2 S gas mixtures. By adding 15 ppm H 2 S to the CH 4 -H 2 atmosphere the onset of metal dusting can be retarded for more than 350 hours. By means of Auger electron spectroscopy (AES), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) it was shown that coke contains graphite, cementite and iron particles with adsorbed sulphur.

Effects of Alloying on the Chemistry of CO and H2S on Fe Surfaces

“…Grabke and co-workers have observed that a small concentration of H 2 S in a CO/H 2 gas mixture can retard the metal dusting process. 74,75 This is likely to be due to the facile deposition of S atoms that block the surface sites for CO adsorption and/or dissociation (i.e., S poisons the surface). Our prediction of strong bonding between Fe and S (∼6 eV) is consistent with this idea.…”

Growth of massive cementite layers; thermodynamic parameters and kinetics

“…3 Since thermal decomposition of NH 3 as well as decomposition of (metastable) iron nitrides are slow, 4 transfer of nitrogen from the gas atmosphere into the specimen can be described by the following heterogeneous chemical equilibrium: of nitrogen dissolved in the solid at the surface (assumed to be in equilibrium with the gas atmosphere) obey: denotes the socalled nitriding potential, where p NH 3 and p H 2 are the partial pressures of ammonia and hydrogen in the gas atmosphere. Hence, according to Eq.…”

“…3 At atmospheric pressure and at the applied treatment temperatures, N 2 does not react with solid iron; it can be regarded as an inert gas. 4 This implies that the reaction NH 3 1 2 N 2 þ 3 2 H 2 is necessarily not in equilibrium in the gas phase, which is assured by a sufficiently large gas flux though the furnace (cf. Sect.…”

“…This dearth of fundamental knowledge on cementite is mainly due to the difficulty in preparing single-phase (single-or polycrystalline) cementite specimens. In the past, several works dealt with the production of bulk cementite specimens applying different methods [1][2][3][4][5]. Yet, only recently, it was reported for the first time that massive cementite layers can be grown on ferrite substrates by a dedicated gaseous nitrocarburising process [6].…”

Nitrogen diffusion through cementite layers

“…In the presence of cementite compound layers it turns out that the nitrogen (and carbon) diffusion into the substrate is strongly decelerated; cementite acts as a diffusion barrier for [15] and carbon [16]. Nonetheless, although nitrogen transport through cementite is decelerated, it is shown in this paper that a certain time-dependent amount of nitrogen is transported through the cementite into the ferrite substrate, leading to an increase of the nitrogen content in (and the hardness of) the ferrite substrate.…”