Metal dusting of high temperature alloys

Abstract

Metal dusting, i.e. disintegration into fine metal particles and carbon, was induced on a selection of chromia forming high temperature alloys in a flowing CO-H2-H20 atmosphere in exposures at 650°C, 600"C, 5004 and 450°C. The materials were pretreated by annealing in Hz at 1000 "C and electropolishing, this leads to large grain size and low surface deformation, both is disadvantageous for formation of a Crz03 scale. The resistance to metal dusting is only dependent on the ability to form a protective Crz03 scale, thus the high Cr ferritic steels proved to be very resistant, the femtic steels with 12-13% Cr were less resistant. Due to the lower Cr diffusivity in the austenitic steels, these were very susceptible, especially two alloys with about 30% Ni (Alloy 800, AC66). The appearance of metal dusting was somewhat different for Ni-base materials but they were also attacked under pitting. The metal dusting is preceded in all cases by internal carburization whereby the chromium is tied up, afterwards the remaining Fe or Fe-Ni matrix can react to the instable intermediate carbide M3C which decomposes to metal particles and carbon, in case of Ni-base materials a supersaturated solid solution of carbon is the intermediate. Metal dusting, d.h. ein Zerfall in feine Metallpartikel und Kohlenstoff, wurde auf einer Auswahl von chromoxidbildenden Hochtemperaturlegierungen in einer stromenden CO-Hz-HzO-Atmosphare ausgelost , bei Auslagerungen bei 650", 600°, 500" und 450 "C. Die Werkstoffe waren vorbehandelt worden durch Gluhen in Wasserstoff bei 1000°C und Elektropolieren, dies fuhrt zu grobkornigem Gefuge und geringer Oberflachenverformung, beides ist nachteilig fur die Ausbildung einer Cr203-Schicht. Der Widerstand gegen metal dusting hangt nur ab von der Fahigkeit, eine schutzende CrzO3-Schicht zu bilden, daher zeigen sich die hoch chromhaltigen ferritischen Stahle als sehr widerstandsfahig, die ferritischen Stahle mit 12-13% Cr waren weniger bestandig. Aufgrund des niedrigeren Chromdiffusionskoeffizienten in den austenitischen Stahlen waren diese sehr anfallig, insbesondere zwei Legierungen mit etwa 30% Ni (Alloy 800, AC 66). Fur Nickelbasiswerkstoffe war das Erscheinungsbild des metal dusting etwas andersartig, aber auch diese wurden unter Grubchenbildung angegriffen. In allen Fallen geht dem metal dusting eine innere Karbidbildung voran, durch die das Chrom gebunden wird, nachdem kann die verbleibende Feoder Fe-Ni-Matrix zu dem instabilen intermediaren Karbid M3C reagieren, das dann zerfallt zu Metallpartikeln und Kohlenstoff, im Falle der Nickelbasiswerkstoffe ist eine ubersattigte feste Losung von Kohlenstoff in dem Material das Zwischenprodukt.

A Metallurgical Approach to Metal Dusting of Nickel-Base Alloys

“…In addition to low-alloy steels reported widely so far, [1][2][3][4][5][6] experience with metal dusting of high-alloy steels, originally resistant to carburization, has been reported in actual plants as well. [7][8][9][10][11] Moreover, laboratory corrosion tests for development of eminent alloys required for severe metal dusting attack have been conducted.…”

Study of metal dusting phenomenon and development of materials resistant to metal dusting. Final report.

“…It has been reported that Ni-base alloys possess better resistance against metal dusting attack than the Fe-base alloys (Grabke et al 1993, Maimer and Norton 1999. The reason has not been clear thus far.…”

“…It was suggested that an oxide scale protects alloys, and metal dusting corrosion starts at defects in the oxide scale (Grabke 1998, Grabke et al 1993, 1998. Carbon penetrates through the defects and undermines the oxide scale.…”

“…Oxide scales are reported to play an important role in preventing alloys from metal dusting corrosion since carbon does not diffuse through the oxide layers (Grabke et al , 1993. Because oxide scales are important in metal dusting corrosion, it is necessary to study the effect of their composition and defects on metal dusting.…”

Analysis of Cu segregation to oxide–metal interface of Ni‐base alloy by HX‐PES

“…It is suggested that the important step in the metal dusting for carbon steels is the formation of an unstable cementite (Fe 3 C) as an intermediate followed by its decomposition. [2] On the other hand, the alloys which demonstrate high corrosion resistance in high-temperature oxidization atmosphere by containing high Cr. Although the protective oxide layer forms on these alloys in that environment, carbon, beyond the limit of solid solution, goes through the defective part of that oxide layer on the alloy surface as graphite while in use.…”