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Hot corrosion of MCrAlY coatings in sulphate and SO2 environment at 900 °C: is SO2 necessarily bad?
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, The Institute of Technology.
Linköping University, Department of Management and Engineering, Engineering Materials. Linköping University, The Institute of Technology.
Siemens Industrial Turbomachinery AB, Finspång, Sweden.
Swerea KIMAB, Kista, Sweden.
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2015 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 261, 41-53 p.Article in journal (Refereed) Published
Abstract [en]

MCrAlY coatings can be corroded due to the basic fluxing (Type-I corrosion) of suppose-to-be protective alumina scale in a molten sulphate environment. In this study, two MCrAlY coatings, coating A (10 wt.% Al, 20 wt.% Cr) and coating B (7 wt.% Al, 28 wt.% Cr), were tested in a sodium-potassium sulphate environment with and without SO2 (500 ppm) in air with 100-900 °C thermal cycling up to about 500 hours. The aim was to test the effect of SO2 at the typical Type-I-corrosion temperature – 900 °C. The results showed that the corrosion behavior of the MCrAlY coatings depended not only on the coating composition but also on the corrosion environment. It was found that in coating A alumina scale was more resistant in the sulphate plus SO2 condition than that in the sulphate only condition. Such phenomenon indicated a beneficial effect of SO2. On Coating B, however, a mixed oxide layer, consisting of alumina and other oxides and sulphides, after a certain cycles in the sulphate environments with and without SO2 gas. In this coating, the addition of SO2 in the sulphate environment changed the corrosion mode from the basic fluxing of alumina to the sulfidation of Cr.

Place, publisher, year, edition, pages
Elsevier, 2015. Vol. 261, 41-53 p.
Keyword [en]
Hot corrosion; Basic fluxing; MCrAlY; SO2; Alumina
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:liu:diva-111118DOI: 10.1016/j.surfcoat.2014.11.065ISI: 000348255500007OAI: oai:DiVA.org:liu-111118DiVA: diva2:753480
Note

On the day of the defence date the status of the article was Manuscript.

Available from: 2014-10-08 Created: 2014-10-08 Last updated: 2017-12-05Bibliographically approved
In thesis
1. Oxidation and Corrosion of New MCrAlX Coatings: Modelling and Experiments
Open this publication in new window or tab >>Oxidation and Corrosion of New MCrAlX Coatings: Modelling and Experiments
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

MCrAlY coatings (“M” for Ni and/or Co) are widely used for the protection of superalloy components operated at high temperatures such as in the hot sections of gas turbines. The exposure to high temperature can cause coating degradation due to oxidation or hot corrosion at the coating surface. Microstructures in the coating and the coating life are affected also by the diffusion of alloying elements through the coating-superalloy interface. This PhD project, by applying thermodynamic modelling and experimental tests, investigates the oxidation and hot corrosion behavior of new MCrAlX coatings, in which X, referring to minor elements, is used to highlight the functions of such elements.

In order to understand and predict the coating degradation progress during thermal exposure, an oxidation-diffusion model has been established for MCrAlX coating-superalloy systems, which integrates the oxidation of aluminum at coating surface, diffusion of alloying elements, and the diffusion-blocking effect in the materials. The predicted chemical composition profile and microstructure agreed well with experimental results in a CoNiCrAlYSiTa-Inconel 792 system. The model was further applied in several coating-superalloy systems to study the influence of coating composition, superalloy composition and temperature on the evolution of microstructure in the coating and the coating life. The results have demonstrated the potential of the model in designing new durable MCrAlX coatings. In addition to the applications in coating-superalloy systems, the model was also adapted for studying the microstructural development in a superalloy in which internal oxidation and nitridation occurred in an oxidation process.

The oxidation behavior of some HVOF MCrAlX coatings was studied by thermal exposure at different temperatures (900, 1000, 1100 °C). Different spinels formed above the alumina scale, depending on the oxidation temperature. The minor alloying elements, Ru and Ir, had no direct influence on the oxidation behavior but may affect the phase stability in the coating.

MCrAlX coatings were also tested in 48-hour cycles at 900 °C in different hot corrosion environments containing sulphates and/or SO2. The results showed that the coating performance was dependent on coating quality, concentration of Al and Cr in the coating, and the hot corrosion condition. It was also found that the addition of SO2 in the environment may not necessarily be bad for hot corrosion resistance of some MCrAlY coatings.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2014. 46 p.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1619
National Category
Materials Engineering Mechanical Engineering
Identifiers
urn:nbn:se:liu:diva-111119 (URN)10.3384/diss.diva-111119 (DOI)978-91-7519-247-5 (ISBN)
Public defence
2014-10-30, ACAS, Hus A, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Projects
ELFORSK, AGORA MATERIA, Strategic Faculty Grant AFM
Available from: 2014-10-08 Created: 2014-10-08 Last updated: 2015-09-17Bibliographically approved

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