Evaluating corrosion regimes of Fe-based alloys in high temperature corrosion
Typ
Examensarbete för masterexamen
Program
Materials chemistry (MPMCN), MSc
Publicerad
2020
Författare
Lindström, Jennie
Somsri, Walida
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
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.
Beskrivning
Ämne/nyckelord
Breakaway oxidation , Fe-based alloys , High temperature corrosion , Primary corrosion protection , Secondary corrosion protection , Spallation