The microstructural impact on high-temperature corrosion in ferritic Fe-Cr-C model alloys in single and combined SO$_2$ + H$_2$O environments

Falk, Florian; Zander, Brita Daniela (Thesis advisor); Krupp, Ulrich (Thesis advisor)

Aachen : RWTH Aachen University (2023)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2023

Abstract

The conversion of heat- into electrical energy requires the combustion of matter, which is harmful to components of combustion power plants, automotive, air- and space industry because of the corrosive impact of combustion products i. e. alkali chlorides $NO_x$, $CO_2$, $O_2$, $H_2O$, sulphurous gases ($H_2S$, $SO_2$, $S_2$, $SO_3$) and ash. Essential structural alloys are austenitic and ferritic steels that can be protected from high-temperature corrosion by a slowly growing, dense but fine-grained chromium oxide ($Cr_2O_3$) layer. The $Cr$ content above a critical value or the addition of alloying additives i.e. $Al$ or reactive elements such as $Ce$, $Zr$, $La$, $Hf$ and $Y$, in principle promote protection by $Cr_2O_3$. However, diffusion and transport mechanisms at various interfaces of the alloy's microstructure and the formed scale (alloy/oxide; oxide/oxide; oxide/gas) are proceeding during the dynamic corrosion process. In austenitic stainless steels, the diffusivity of the protective layer forming elements is much slower than in ferritic stainless steels. The microstructural impact in atmospheres where oxygen is the only oxidising agent is well known. In contrast, the understanding of the microstructure impact (considering: grain boundaries, carbides, grain size and a duplex phase) of ferritic model alloys with high $Cr$ content ($>13$ % $Cr$ weight fraction) on corrosion in media containing $SO_2$ and $H_2O$ is missing to improve the materials corrosion resistance. In the present work, $Fe-16Cr-0.2C$ model alloys were used as a model system to investigate the microstructural impact on the high-temperature corrosion behaviour in environments where $SO_2$ and $H_2O$ were the harmful species. The alloys grain size, grain boundary misorientation angle and corresponding grain boundary line length, the carbides diameter and area fraction were extracted employing \ac{EBSD} to attribute the microstructural effect after exposure at $650$ °C. The focus of this work was the derivation of the microstructure-dependent corrosion mechanisms after short- and long-term exposure ($30$ min up to $1000$ h) considering thermodynamic calculation principles as well as qualitative phase analysis with electron and X-ray diffraction methods, mass spectrometry and electron microscopy. By the comparison of samples with different microstructural states, it was shown that phase boundaries carbide/alloy and \ac{HAGB} are the preferred diffusion paths for initial oxide and sulphide growth at the material interface and that the material's damage can be reduced by a targeted heat-treatment. The correlation between phase growth sequence and microstructural state was observed in real-time via in situ X-ray diffraction in the early stage of oxidation ($5$ sec. up to $4$ h) at $800$ °C in dry air. All samples show the initial growth of $Cr_2O_3$, and the time to breakaway oxidation was significantly reduced in the fully recrystallised state. The dissolution of sub-micrometre sized carbides of low area fraction is proposed to explain a contributing effect on $Cr_2O_3$ formation. The hypothesis that high fractions of short circuit diffusion paths contribute to protective oxide layer formation cannot be confirmed for $800$ °C in dry air because of fast proceeding recrystallisation. The improved understanding of the causes of internal sulphidation and higher oxidation rates for microstructure manipulated $Fe-16Cr-0.2C$ model alloys in hot $SO_2$ and $H_2O$ environments, the following implications for appreciating ferritic microstructures can be deduced to decrease the corrosive attack. A $16$ wt.% $Cr$ content in ferritic model alloys is high enough for $Cr_2O_3$ formation. Sulphidation, as well as high oxidation rates, can be reduced by a targeted heat-treatment to reduce the high angle grain boundary line length. Furthermore, the carbides diameter must be in the sub-micrometer scale and their area fraction low to effect the chromia maintainance at prolonged exposure times by decarburisation.

Institutions

• Division of Materials Science and Engineering [520000]
• Chair of Corrosion and Corrosion Protection [522710]