Effect of sulfur on the stability of cementite

Abstract

Cementite FesC is an unstable carbide, which should decompose to iron and graphite. It was possible to grow cementite on iron samples in flowing CO-H 2-H20-H2S mixtures at temperatures between 400-700 "C, The cementite is stabilized by adsorbed sulfur, preventing graphite deposition, which would initiate metal dusting. Thus, this corrosion phenomenon, occurring in CO-H 2-H20 and other strongly carburizing atmospheres can be suppressed by the presence of some H 2S. The range of H 2S/H 2 ratios and temperatures in which iron is inert against metal dusting, corresponds to the range where a monolayer of sulfur is adsorbed in iron. It may be supposed that the iron carbide process, i.e. the production of FesC by reduction of ores in methane, is possible only by the presence of some sulfur. EinfluB vonSchwefel auf die Zementitstabilitat. Zementit FesC ist ein instabiles Carbid, das in Eisen und Graphit zerfallen sollte. In str6menden CO-H 2-H 20-H 2S-Gemischen konnte Wachstum von FesC-Schichten auf Eisenproben erzielt werden. Der Zementit ist durch adsorbierten Schwefel stabilisiert, dieser verhindert die Graphitablagerung, durch die 'metal dusting' eingeleitet wi.irde. Somit wird dieses in CO-H 2-H20-und anderen stark aufkohlenden Atmospharen von wenig H 2S unterdri.ickt. Der Bereich des H 2S/H2-Verhaltnisses und der Temperaturen, in denen Eisen inert gegen metal dusting ist, entspricht dern Bereich, in dem eine Monolage Schwefel auf Eisen adsorbiert ist. Man kann vermuten, daB die Eisen-Carbidproduktion, d.h. die Erzeugung von FesC durch Reduktion von Eisenerzen mit Methan, nur durch die Anwesenheit von etwas Schwefel m6glich ist, der das Produkt stabilisiert.

Effect of Sulphur on Iron Carbide Formation and Stability.

“…8) Sulphur adsorption on metallic iron is very strong, which effectively retards the carburisation process. Grabke et al 1,2) reported that the iron carburisation reaction rate began to decrease at sulphur activities a s Ͼ10 Ϫ4 , or at P H 2 S /P H 2 Ͼ10…”

“…The fraction of iron surface covered by sulphur can be calculated using data reported by Grabke et al 1) Calculated results are shown in Fig. 4.…”

“…Thus, the effect of surface-active sulphur on the catalytic activity of iron and the rate of topochemical reactions is well recognised. It was reported [1][2][3][4][5][6] that trace amount of sulphur significantly retarded cementite formation, free carbon deposition and cementite decomposition. In this paper, influence of sulphur on the iron carbide process was first investigated in the process of iron ore reduction by hydrogen.…”

Influences of Oxide Additions on Formation Reaction of Iron Carbide at 1023 K

“…However, since iron carbide is metastable at all temperatures compared with graphite and its saturated solution in iron, it is easy to decompose to solid carbon and metallic iron at elevated temperature. [4][5][6][7][8] The first commercial scale process for producing iron carbides at the Trinidad island did not succeed. This process needs long residence times and a few residual iron oxides leaves in the iron carbide depending on ore type.…”

“…The yield and the stability of iron carbide have been paid more attention. In 1995, Grabke and Müller-Lorenz 9) found that sulfur has the effect on the stability of iron carbide. In 1997, Hayashi and Iguchi 4,5) reported the synthesis of iron carbide by reaction of iron ores with H 2 -CO gas mixtures bearing traces of sulfur.…”