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An electrochemical impedance spectroscopy study on the effect of condensate on oxides formed on a 25Cr/20Ni cast stainless steel in exhaust environments
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Mechanical Metallurgy.ORCID iD: 0000-0001-5212-2227
KTH, School of Chemical Science and Engineering (CHE), Chemistry, Surface and Corrosion Science.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

 Condensation of diesel exhaust gases inside an exhaust- or turbo manifold may occur during cold-start and cooling of an engine, resulting in acidic liquid covering the oxide surfaces inside the manifolds. In the present study, the interaction between a chloride-containing exhaust-gas condensate of pH 2.4 and oxide scales formed on a 25Cr/20Ni cast stainless steel in air and in two different exhaust environments, 10%H2O-5%O2-85%N2 and 10%H2O-5%CO2-85%N2, at 900°C has been examined by means of electrochemical impedance spectroscopy, EIS. Interpretation of impedance spectra was coupled to oxide scale structures, revealed by SEM, EDX, XRD and GDOES, and to metal release studies using ICP. It was observed that the acidic condensate caused oxide spallation, followed by corrosion of the underlying metal surface for all test samples. The metal release rate of the oxide scale and underlying alloy was found to decrease at longer immersion times (>1h), most likely due to precipitation of corrosion products on the surfaces. EIS combined with equivalent circuit fitting showed to be a useful technique in describing the electric properties of the oxide scales, suggesting oxidation in H2O/O2 to result in formation of oxide scales being more resistive compared to the other environments. This was coupled to higher thickness and higher defect density, which correlated well with oxide scale analysis.

National Category
Metallurgy and Metallic Materials
URN: urn:nbn:se:kth:diva-122016OAI: diva2:620299

QS 2013

Available from: 2013-05-08 Created: 2013-05-08 Last updated: 2013-11-15Bibliographically approved
In thesis
1. Development of a ferritic ductile cast iron for improved life in exhaust applications
Open this publication in new window or tab >>Development of a ferritic ductile cast iron for improved life in exhaust applications
2013 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Due to coming emission legislations, the temperature is expected to increase in heavy-duty diesel engines, specifically in the hot-end of the exhaust system affecting components, such as exhaust- and turbo manifolds. Since the current material in the turbo manifold, a ductile cast iron named SiMo51, is operating close to its limits there is a need for material development in order to maintain a high durability of these components. When designing for increased life, many material properties need to be considered, for example, creep-, corrosion- and fatigue resistance. Among these, the present work focuses on the latter two up to 800°C improving the current material by additions of Cr, for corrosion resistance, and Ni, for mechanical properties. The results show improved high-temperature corrosion resistance in air from 0.5 and 1wt% Cr additions resulting in improved barrier layer at the oxide/metal interface. However, during oxidation in exhaust-gases, which is a much more demanding environment compared to air, such improvement could not be observed. Addition of 1wt% Ni was found to increase the fatigue life up to 250°C, resulting from solution strengthening of the ferritic matrix. However, Ni was also found to increase the oxidation rates, as no continuous SiO2-barrier layers were formed in the presence of Ni. Since none of the tested alloys showed improved material properties in exhaust gases at high temperature, it is suggested that the way of improving performance of exhaust manifolds is to move towards austenitic ductile cast irons or cast stainless steels. One alloy showing good high-temperature oxidation properties in exhaust atmospheres is an austenitic cast stainless steel named HK30. This alloy formed adherent oxide scales during oxidation at 900°C in gas mixtures of 5%O2-10%H2O-85%N2 and 5%CO2-10%H2O-85%N2 and in air. In the two latter atmospheres, compact scales of (Cr, Mn)-spinel and Cr2O3 were formed whereas in the atmosphere containing 5%O2 and 10%H2O, the scales were more porous due to increased Fe-oxide formation. Despite the formation of a protective, i.e. compact and adherent, oxide scale on HK30, exposure to exhaust-gas condensate showed a detrimental effect in form of oxide spallation and metal release. Thus, proving the importance of taking exhaust-gas condensation, which may occur during cold-start or upon cooling of the engine, into account when selecting a new material for exhaust manifolds. 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. 51 p.
Material development, SiMo51, HK30, exhaust manifolds, high-temperature corrosion, high-temperature low-cycle fatigue
National Category
Metallurgy and Metallic Materials Corrosion Engineering
urn:nbn:se:kth:diva-122006 (URN)978-91-7501-711-2 (ISBN)
2013-05-24, Marcus Wallenberghallen, Scania, Nyköpingsvägen 33, Södertälje, 13:00 (English)
Vinnova, 2009-01433

QC 20130508

Available from: 2013-05-08 Created: 2013-05-07 Last updated: 2013-11-15Bibliographically approved

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