E. M. Müller-Lorenz

Metal dusting of nickel-base alloys

The carburization behavior of nine commercial nickel-based alloys and four iron-nickel-chromium alloys was investigated at 650°C in a carburizing carbon monoxide (CO)-H 2 hydrogen oxide (H 2 O) gas with a carbon activity (a c ) of a c > 1. The iron-nickel-chromium alloys suffered severe metal dusting after a very short test period. Nickel-based alloys were generally less susceptible to metal dusting than iron-based alloys. Nickel-based alloys with chromium concentrations of 25% and above, on the other hand, showed no significant evidence of metal dusting even after 10,000 h of exposure. It was found that these alloys were protected against metal dusting by the formation of a dense, self-healing chromia scale, which prevented the penetration of carbon into the base metal.

Influence of H2S on metal dusting

Presence of H 2 S in a carburizing atmosphere causes S-adsorption which retards carbon transfer and deposition and can suppress metal dusting of iron and steels. In the latter process cementite Fe 3 C is an intermediate, graphite deposition would initiate its decomposition but graphite nucleation is prevented by adsorbed sulfur. Thus continued Fe 3 C growth can be observed in the presence of H 2 S. Thermogravimetric studies in CO-H 2 -H 2 O-H 2 S mixtures have been conducted at 500 °C at various carbon activities a C and H 2 S/H 2 -ratios. With increasing a C higher H 2 S/H 2 -ratios are needed to suppress metal dusting, with increasing H 2 S/H 2 -ratio the kinetics of Fe 3 C growth change from diffusion controlled parabolic kinetics to linear carbon transfer controlled kinetics. At very high a C ≥ 1000 besides Fe 3 C also the Hagg carbide Fe 5 C 2 was observed as an outer layer on the cementite.

Coking by metal dusting of steels

In process industries coking is an annoying phenomenon, the carbon deposition causes decrease of heat transfer and hinders gas flow. Coking in a process may indicate metal dusting, i.e. the disintegration of metals and alloys in carbonaceous atmospheres under formation of graphite and fine metal particles. The metal particles act as catalysts for vast coke formation. The thermodynamics, mechanisms and kinetics of metal dusting have been studied on iron and steels in synthesis respectively reduction gas CO-H2- H2O, here the aspects are presented of coking due to metal dusting. From the interplay of the metal disintegration and carbon deposition rather complex coupled kinetics are resulting, even different in a low temperature range where the decomposition of the intermediate cementite is rate determining and in a higher temperature range where the carbon transfer from the atmosphere is rate controlling. Coking by metal dusting can be suppressed in the same way as metal dusting, by sulfur addition to the atmosphere and/or by a stable dense protective oxide layer. Coking durch Metal Dusting von Stahlen In der Prozesindustrie ist „coking“ ein argerliches Phanomen, die Kohlenstoffabscheidung bewirkt eine Abnahme der Warmeubertragung und behindert die Gasstromung. Das Auftreten von coking in einem Prozes kann anzeigen, das metal dusting stattfindet, d.h. der Zerfall von Metall oder Legierungen in kohlenstoffhaltigen Atmospharen unter Bildung von Graphit und feinen Metallpartikeln. Die Metallpartikel wirken als Katalysatoren fur eine starke coke-Bildung. Thermodynamik, Mechanismen und Kinetik des metal dusting sind auf Eisen und Stahlen in Synthese- bzw. Reduktionsgas CO-H2- H2O untersucht worden. Hier werden die Aspekte des coking aufgrund von metal dusting beschrieben. Aus dem Zusammenspiel des Metallzerfalls und der Kohlenstoffabscheidung resultieren recht komplexe gekoppelte Kinetiken, die sogar unterschiedlich sind in einem Bereich niedriger Temperaturen, wo der Zerfall des als Zwischenprodukt gebildeten Zementits geschwindigkeitsbestimmend ist, und in einem Bereich hoherer Temperaturen, wo die Kohlenstoffubertragung aus der Atmosphare geschwindigkeitsbestimmend ist. Coking durch metal dusting kann unterdruckt werden in derselben Weise wie das metal dusting, namlich durch Schwefelzusatz zur Atmosphare und/oder durch eine stabile, dichte, schutzende Oxidschicht.

Protection of high alloy steels against metal dusting by oxide scales

The metal dusting resistance of various high alloy steels was tested by exposures of specimens in different surface states at temperatures in the range 500-650°C. In addition surface analytical studies (AES) were conducted on the initial stages of oxide formation and the oxide layer after long term exposure under metal dusting conditions. The factors leading to a protective, Cr-rich oxide layer were elucidated and are summarized here.

Formation of chromium rich oxide scales for protection against metal dusting

Abstract Metal dusting is a disintegration of metals and alloys into graphite and metal particles, caused by strongly carburizing gas mixtures mainly in the temperature range 400–700°C. Protection of steels against metal dusting is possible through the formation of dense chromium rich oxide scales but it is not guaranteed that such scales are formed at low temperatures, even on high Cr-steels. Surface analytical studies have been conducted on the formation and composition of the oxide scales on 9–20%Cr steels. The growth of oxide films was followed by AES for 3 hours at 10–7 mbar O2 great differences were observed in dependence on surface finish. On ground samples, Mn and Si appeared early and Cr-rich oxide was formed, whereas on chemically etched samples Fe-rich oxides grew. After long term exposures (240 h) under metal dusting conditions, i.e. in CO–H2–H2O mixtures at 600°C, thin Cr-rich scales were observed on ground steels which were impermeable to carbon whereas on chemically etched steels thick Fe-rich scales had grown and carbon penetration was detectable. Accordingly, the oxide formation on Cr-steels at relatively low temperatures strongly depends on the surface treatment. Any surface working such as grinding and sand-blasting etc. introduces dislocations and causes a fine-grained microstructure near the surface, and the dislocations and grain boundaries act as rapid diffusion paths for supply of Cr to the surface in the first minutes of exposure, which leads to the formation of a protective oxide scale.