Beyond Breakaway Corrosion: Investigating the Secondary corrosion protection of Iron-based alloys

Doctoral thesis, 2023

High temperature corrosion of metallic materials remains a major challenge for many in-dustrial applications. The challenges of high temperature corrosion are often addressed by using highly alloyed materials such as stainless steels or FeCrAl alloys. The corrosion protection of these alloys rely on the formation of a protective Cr- and/or Al-rich corundum-type oxide. However, under corrosive conditions, these oxides tend to break down, resulting in the formation of a less protective, multi-layered Fe-rich oxide scale, a process known as breakaway corrosion.

The oxide scales formed after breakaway are often considered to be non-protective. There-fore, previous studies on breakaway corrosion have mainly focused on how to delay, or prevent, the breakdown of the Cr/Al-rich oxide. Nevertheless, in many industrial appli-cations the breakaway event occurs in an early stage of operation and may be difficult to prevent. Thus, the corrosion propagation and lifetime of metallic components are often determined by the protection of the Fe-rich oxide scale formed after breakaway.

This thesis systematically investigates the protective properties of the Fe-rich oxides formed after breakaway at intermediate temperatures (400-600 °C). This is done through well-controlled breakdown of the Cr/Al-rich oxide, on a broad range of Fe-based model alloys that contain varying amounts of Cr, Ni, Al, and Si. The formed multi-layered Fe-rich oxide scales are subjected to detailed microstructural investigations, to elucidate how the properties and microstructures of the multi-layered Fe-rich oxide change as a result of altered alloy composition, and/or the presence of certain corrosive species.

The results clearly demonstrate the possibilities to improve the protection of the Fe-rich oxide by an altered alloy composition. The influences that alloying elements exhibit on the Fe-rich oxide scales are different from the previously demonstrated effects of the slow-growing Cr/Al-rich corundum-type oxides. Thus, the positive effects of certain alloy-ing elements in Fe-based alloys are not necessarily the same for the corrosion protection exhibited before and after breakaway. Therefore, this thesis introduces the concept of primary and secondary corrosion protection for the oxide scales formed before (Cr/Al-rich corundum-type oxides) and after (multi-layered Fe-rich oxide scales) breakaway. The terminology is considered to be important for future material research and development, as well as for the selection of materials to be used in applications in which breakaway corrosion cannot be prevented.