Kinetics and mechanisms of nickel metal dusting I. Kinetics and morphology

Geers

et al. 2011

Materials & Corrosion

122

101

Recent experimental investigations have widened the understanding of metal dusting significantly. Microscopic observations have been used to dissect dusting mechanisms. Iron dusts by growing a cementite surface scale, which catalyses graphite nucleation and growth. The resulting volume expansion leads to cementite disintegration. Cementite formation on iron can be suppressed by alloying with germanium. Nonetheless, dusting occurs via the direct growth of graphite into the metal, producing nanoparticles of ferrite. This process is faster, because carbon diffusion is more rapid in a-Fe than in Fe 3 C. Austenitic materials cannot form cementite, and dust via formation of graphite at external surfaces and interior grain boundaries. The coke deposit consists of carbon nanotubes with austenite particles at their tips, or graphite particles encapsulating austenite. TEM studies demonstrate the inward growth of graphite within the metal interior. It is therefore concluded that the dusting mechanism of austenitic materials like high alloy Cr-Ni steels and Ni base materials is one of graphite nucleation and growth within the near surface metal. In all alloys examined, both ferritic and austenitic, the principal mass transfer process is inward diffusion of carbon. Alloying iron with nickel leads to a transformation from one mechanism with carbide formation to the other without. Copper alloying in nickel and high nickel content stainless steels strongly suppresses graphite nucleation, as does also an intermetallic Ni-Sn phase, thereby reducing greatly the overall dusting rate. A surface layer of intermetallic Ni-Sn Fe-base materials facilitates the formation of a Fe 3 SnC surface scale which also prevents coking and metal dusting. Current understanding of the roles of temperature, gas composition and surface oxides on dusting rates are summarised. Finally, protection against metal dusting by coatings is discussed in terms of their effects on catalysis of carbon deposition, and on protective oxide formation.

Section: Second-phase Effect On Cementite Disintegrationmentioning

confidence: 78%

Section: The Role Of Gas Compositionmentioning

confidence: 99%

mentioning

confidence: 99%

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Recent advances in understanding metal dusting: A review

Geers

et al. 2011

Materials & Corrosion

122

101

“…Metal dusting is a catastrophic form of high temperature corrosion in which carbon-supersaturated gas degrades iron-, nickel-and cobalt-base alloys into a dust of carbon and particulate metal or metal carbides [1][2][3][4][5][6][7][8][9][10][11][12][13]. Two mechanisms have been proposed according to whether or not metastable carbide forms.…”

Section: Introductionmentioning

confidence: 99%

Metal dusting behaviour of several nickel‐ and cobalt‐base alloys in COH₂H₂O atmosphere

2011

Materials & Corrosion

The metal dusting behaviour of total 11 nickel-and cobalt-base alloys at 680 8C in a gas of 68%CO-31%H 2 -1%H 2 O (a C ¼ 19.0, p O 2 ¼ 5.4 Â 10 À25 atm) was investigated. All samples were electropolished and reacted in a thermal cycling apparatus. On the basis of their reaction kinetics, these alloys can be classified into three groups: the first, with rapid carbon uptake and significant metal wastage, consists of alloys of relatively high iron content (AC 66, 800H and NS-163); the second, with intermediate rates, consists of some Co-base alloys (HAYNES 188, HAYNES 25 and ULTIMET) and the third, with very low reaction rates, consists of nickel-base alloys with high chromium levels (601, HAYNES HR 160, 230, G-35 and EN 105). An external chromia scale protected group 3 alloys from carburization and dusting. However, this protective scale was damaged and not rehealed for group 1 and group 2 alloys, allowing carbon attack. In all cases, coke deposited on the surface with two typical morphologies: filaments and graphite particle clusters. Subsurface spinel formation in high iron-content alloys led to rapid dusting due to the significant volume expansion. Alloy carbon permeability was calculated from a simple law of mixtures, and shown to correlate reasonably well with initial dusting rate except for one cobalt-base alloy in which iron spinel formation was significant.

“…Particular attention is directed to the role of cementite and the microprocesses of nanoparticle formation and underlying metal destruction (16)(17)(18)(19)(20)(21). This work was carried out using surface observation, metallographic cross-section analysis and electron microscopic examination of coke deposits at different stages of the reaction.…”

Section: Introductionmentioning

confidence: 99%

Understanding Metal Dusting Mechanisms

2009

ECS Trans.

Self Cite

Examination of iron dusting shows that cementite forms as a layer on the surface and as particles in the overlying coke deposit. On this basis, the mechanism of cementite disintegration is deduced to be one of internal graphite nucleation and growth within the scale. Cementite formation on iron is suppressed by alloying with germanium. Nonetheless, dusting occurs via the direct growth of graphite into the metal, producing nanoparticles of ferrite. This process is faster than dusting via cementite formation and disintegration.Austenitic materials cannot form cementite, and dust via formation of graphite at external surfaces and interior grain boundaries. The coke deposit consists of carbon nanotubes with austenite particles at their tips, or graphite particles encapsulating austenite. TEM studies demonstrate the inward growth of graphite within the metal interior. It is therefore concluded that the mechanism of nickel dusting is one of graphite nucleation and growth within the near surface metal. Alloying copper with nickel strongly represses graphite formation, thereby reducing greatly the overall dusting rate.