A Metallurgical Approach to Metal Dusting of Nickel-Base Alloys

Abstract

Preliminary laboratory corrosion tests of two conventional Ni-base alloys were conducted at 650 C for 200 h in various CO-H 2 -CO 2 -H 2 O gas mixtures. For gas mixtures of high a c and CO content, alloy 600 (75 mass %Ni-15%Cr) specimens lost its mass due to metal dusting and deposition of coke was heavy on the surface of the specimen. Meanwhile, any pit formation was not occurred on the alloy 690 (60%Ni-30%Cr) specimens. Microscopic observation indicated that for the corroded Ni-base alloy specimens, inward diffusion of carbon presumably at cracks and flaws in the oxide scale was prominent. Carbon in the diffusion zone reacted with chromium to precipitate carbides in the matrix, followed by the ''direct'' formation of graphite platelets with a lamellar structure at the revealed metal surface. This direct precipitation of graphite platelets can be considered by a eutectoid reaction, which is similar to the pearlite transformation observed in Fe-C system. In the lamellar structure, the matrix thinned with the growth of graphite plates, and small metal particles of Ni and Fe which may catalyze the coke deposition reaction were crumbled and detached from the lamella.

Metal Dusting Corrosion of Nickel-Based Alloys

“…The calculated diagram in Figure 19 shows that graphite precipitation is favored when the carbon content in the alloy exceeds 1.0 wt.% for Fe-60Ni-6Cr (in wt.%). A similar conclusion has been made by Nishiyama, Kudo and Otsuka [24].…”

A Model for High-Temperature Pitting Corrosion in Nickel-Based Alloys Involving Internal Precipitation of Carbides, Oxides, and Graphite

“…In particular, the graphite that forms as a result of the carbide oxidation is seen to have a strong effect on the volume expansion and will, thus, contribute significantly to the surface disintegration accompanying the corrosion process. In the past, several investigators have noted that internal graphite formation may be responsible for the pitting corrosion attacks observed in Ni-based alloys, [29,80,81] but to the present authors' knowledge, its origin has not yet been associated with the internal oxidation of carbides. Nevertheless, the calculated volume changes related to the Cr 2 O 3 , Al 2 O 3 and graphite formation are so large that they must be the main factor contributing to the observed disintegration of the alloys 602 and 693 during high-temperature pitting corrosion, leading to the development of the layered pit microstructures observed previously in Figures 7(a) and 8(a).…”

Analysis of Cu segregation to oxide–metal interface of Ni‐based alloy in a metal‐dusting environment

“…As a result, it is said that the metal is missing from alloys, and a pitting-type corrosion proceeds. [3,4] The material becomes thin because of the repetition of this process.…”