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The role of discontinuous precipitation in metal dusting
KTH, Superseded Departments, Materials Science and Engineering.
2004 (English)In: Materials Science Forum, ISSN 0255-5476, Vol. 461-464, 561-570 p.Article in journal (Refereed) Published
Abstract [en]

A high-performance commercial alumina-forming Ni-base alloy was studied after a two years field exposure at 540degreesC in a methanol plant. Comparative laboratory studies were performed at 650degreesC using a different synthesis gas mixture with the composition; 50%CO + 3%H2O + 47H(2). Post exposure metallographic examinations with LOM, FEG-SEM, SEMEDX analysis as well as thermodynamic calculations were made in order to identify and describe the metal dusting mechanisms. An estimate of the metal dusting corrosion rates was per-formed based on metallography and these were correlated to a theoretical model based on diffusion processes.

Place, publisher, year, edition, pages
2004. Vol. 461-464, 561-570 p.
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:kth:diva-24206ISI: 000223339700068OAI: oai:DiVA.org:kth-24206DiVA: diva2:345403
Note
QC 20100825Available from: 2010-08-25 Created: 2010-08-25 Last updated: 2010-08-25Bibliographically approved
In thesis
1. Mechanisms of Metal Dusting
Open this publication in new window or tab >>Mechanisms of Metal Dusting
2004 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The primary intention with this Doctoral thesis is to fillin the knowledge gaps and raise the level of understandingregarding the different metal dusting mechanisms in general andexplain the process in detail for high alloyed materials.Considerable effort is put into identifying the driving forcesand elucidating the diffusional processes in metal dusting.

The results are based on a series of long-term laboratory exposures of stainless steels and high-performance commercial alumina-forming Fe- and Ni-base alloys in synthesis gasmixtures, plus a separate shorter study on ultra pure iron. ANi-base alloy was also investigated after a two years field exposure in a methanol plant.

Post exposure metallographic examinations and analysis aswell as thermodynamic calculations were made in order toidentify and describe the operating metal dusting mechanisms.Two main mechanisms were previously used to explain metaldusting, one on the basis of decomposition of metastable carbides (Type I) the other on graphite formation (TypeII).

A new metal dusting mechanism has been identified in this Thesis which appears on high alloyed steels and Ni-base alloys,an active corrosion induced by carbon and oxygen, denoted TypeIII. Both the mechanisms and the type of corrosion products were consistent with the thermodynamic conditions of the material under the influence of a carbon and oxygen gradient.It was shown that this mechanism not only accelerates the metaldusting process, in fact, it determines the overall metaldusting kinetics on stainless steels and Ni-base alloys.

Another feature, which may occur at temperatures where metalbulk and even static grain boundary diffusion is too slow forexplaining the metal dusting corrosion process, was identifiedon a Ni-base alloy. It involves a fast growing cellular structure with discontinuous precipitated carbides whichprovides fast metal dusting kinetics by the Type IIImechanism.

A Type IV metal dusting mechanism, continued fragmentationby graphitization until nano-sized particles are formed andcatalyse carbon nanotube formation is also described. Thesteady state process and the driving force for metal dusting onpure iron was identified and described.

With these additional processes it is now possible to extendour understanding of the metal dusting processes to a widerange of engineering alloys.

 

Place, publisher, year, edition, pages
Stockholm: KTH, 2004. 81 p.
Keyword
metal dusting, mechanism, driving force, kinetics, high temperature corrosion, carburization, stainlesssteel, Ni-base alloys, iron, alumina formers, FeCrAl, oxidation, carbon nanotubes, discontinuous precipitation
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-3722 (URN)91-7283-713-6 (ISBN)
Public defence
2004-04-02, 00:00
Note
QC 20100825Available from: 2004-03-23 Created: 2004-03-23 Last updated: 2010-08-25Bibliographically approved

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