Evaluating corrosion regimes of Fe-based alloys in high temperature corrosion

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Examensarbete för masterexamen
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2020
Författare
Lindström, Jennie
Somsri, Walida
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Sammanfattning
In high temperature applications, breakaway corrosion is a major issue where corrosion limits the useful life of alloy components. The focus for many previous studies have therefore been on preventing this event. However, in harsh and more corrosive environments, where the protective oxide scale generally cannot be retained, the protective properties must rely on the oxide formed after breakaway. Thus, Persdotter et al. [1] introduced the concepts primary and secondary corrosion protection. The primary corrosion protection is defined as a thin, well adherent slow-growing corundum type oxide, while the secondary corrosion protection is a iron-rich multi-layered fast-growing oxide. However, by altering the alloys composition, the secondary corrosion protective may be improved. The aim of this master thesis work is to collect data from previous studies to evaluate the applicability of the concept of primary and secondary corrosion regimes in a broader temperature range. The focus is however on the secondary corrosion protection for FeCr(Ni) alloys in various temperatures and environments. The literature review suggest that the concept can be applied in a broader temperature range but is however more complex with more factors to consider. It was observed that increasing the temperature improved the secondary corrosion protection for Fe-20Cr when isothermally exposed. In contrast, the effect of cyclic oxidation is more severe at higher temperatures for Fe-Cr alloys. In addition, under cyclic oxidation, adding Ni was shown to increase the risk of spallation since thermal stresses occur between the metal and the formed oxide. Thermal stresses more severe in alloys with high thermal expansion coefficients. Materials with an austenitic structure have a higher thermal expansion than ferritic materials and therefore a possible explanation for higher spallation risks. The result from the experimental part indicate that an alloy with poor secondary corrosion protection also formed a similar oxide scales after spallation and stabilized around the same oxide thickness, regardless how much of the scale that was removed. Moreover, when part of the oxide scales was removed from an alloy with a good secondary protection, a thicker oxide scale was observed when re-exposed. On the contrary, when the entire oxide was removed, a similar oxide scales as a good secondary corrosion protection, was formed. This suggests that a good secondary corrosion protection may be sensitive towards cyclic oxidation and spallation in general while this have minor effect on a poor secondary protection.
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Breakaway oxidation, Fe-based alloys, High temperature corrosion, Primary corrosion protection, Secondary corrosion protection, Spallation
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