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

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 \ub0C). 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

Beyond Breakaway Corrosion: Secondary Corrosion Protection of Iron-based Alloys

Metallic materials intended for high temperature applications must resist both mechanical and environmental degradation. The ability to withstand corrosion is an important aspect of high temperature materials and is of major concern in, for example, heat and power production. Nevertheless, corrosion is often a limiting factor in the lifetime of boiler components and it reduces the electrical efficiency and hinders the development of more economical and environmentally sustainable processes. The challenge of high temperature corrosion is often addressed by the use of high-alloyed steels, such as stainless steels and FeCrAl alloys. The corrosion resistance of stainless steels and FeCrAl alloys rely on the formation of a slow-growing, chromium- and/or aluminium-rich, corundum type oxide. However, in harsh corrosive environments these oxides are known to break down (i.e. \u27breakaway corrosion\u27) and a less protective, multi-layered, Fe-rich oxide is formed. One such example is in biomass- and waste-fired boilers, where the combustion process produces a corrosive environment, often resulting in breakaway corrosion in an early stage of operation. Thus, the corrosion propagation and lifetime of many key parts of the boilers, depend on the oxide scale formed after breakaway. This oxide scale is often considered non-protective and studies on the oxidation mechanisms controlling the corrosion propagation after breakaway are scarce. \ua0In order to address, and systematically investigate the corrosion behaviour after breakaway, this thesis introduces the concept of primary and secondary corrosion protection for the oxide scales formed before and after breakaway, respectively. The concept is considered to be important for the development and selection of materials to be used in applications where the breakaway event cannot be prevented, e.g. in biomass- and waste-fired boilers, as well as for the development of lifetime predictive modelling tools for corrosion. A systematic study of the secondary corrosion regime is performed by well-controlled breakdown of the primary corrosion protection of Fe-based model alloys. The resulting oxide scales are subjected to detailed microstructural investigation to study the general aspects of the secondary corrosion protection and how its properties and microstructure changes e.g. by altered alloy composition.\ua0 \ua0The results show that the oxide scales formed after breakaway exhibit similar microstructural features on all the exposed FeCr(Ni/Al) model alloys and that the growth of the secondary corrosion protection is mainly diffusion-controlled. Thus, lifetime predictive tools using diffusion-based simulations, such as DICTRA, could be developed to predict corrosion both before and after breakaway. However, it is also shown that corrosive species (e.g. KCl) may affect the mechanical integrity of the oxide scale, resulting in growth processes that requires other types of models. Furthermore, the results show that the growth rate in the secondary corrosion regime may be influenced by the alloy composition, for example by adding Ni or a combination of Al/Cr. This behaviour is not directly connected to how well the primary corrosion protection withstands the exposure environment (i.e. the incubation time to breakaway). Thus, these findings indicate that research on the secondary corrosion protection has a large potential to improve the selection and development of alloys for use in corrosive environments, such as biomass- and waste-fired boilers

Beyond breakaway corrosion – Influence of chromium, nickel and aluminum on corrosion of iron-based alloys at 600 \ub0C

Breakaway corrosion remains a challenge for many high temperature applications. The oxide formed after breakaway is commonly considered non-protective. This study investigates the protective properties after breakaway on a wide set of (Fe,Cr,Al/Ni)-model alloys by thermogravimertric analysis, ion/electron microscopy and X-ray spectroscopy. The results show that the oxide scales formed after breakaway exhibit similar microstructural features on all FeCr(Ni/Al)-alloys, and that the growth rate is greatly influenced by alloy composition for some alloys while is has little influence on others. This observation may be of great help in the selection and development of materials for use in harshly corrosive environments

The long-term corrosion behavior of FeCrAl(Si) alloys after breakaway oxidation at 600 \ub0C

The long-term corrosion behavior of three FeCrAl(Si) alloys has been investigated in two environments (K2CO3 and KCl+H2O) at 600 \ub0C. The FeCrAl alloy experienced breakaway oxidation in both environments but displayed a higher corrosion rate in KCl+H2O. The FeCrAlSi alloys retained the primary protection in the presence of K2CO3 but underwent breakaway oxidation in the presence of KCl+H2O. The FeCrAlSi alloys displayed considerably reduced corrosion rates, suggested to be an effect of the prevention of internal oxidation and the formation of non-continuous corundum-type oxide dispersed within the inward-growing scale

Secondary corrosion protection of FeCr(Al) model alloys at 600 \ub0C – The influence of Cr and Al after breakaway corrosion

The influence of Cr and Al content on the oxidation behaviour of FeCr(Al) model alloys after breakaway oxidation at 600 \ub0C and the underlying mechanisms were investigated in detail with thermogravimetrical analysis (TGA), thermodynamic calculations and advanced electron microscopy. The results showed that a Cr-content of ≥18 wt% drastically reduced the growth rate of the Fe-rich oxide scale, formed after breakaway oxidation, for FeCrAl alloys but not for FeCr alloys. This was attributed to the ability of the Fe(18-25)CrAl alloys to prevent internal oxidation, which enables the formation of a healing layer

The Influence of KCl and HCl on the High-Temperature Oxidation of a Fe-2.25Cr-1Mo Steel at 400\ua0\ub0C

The influence of alkali- and chlorine-containing compounds on the corrosion of superheater alloys has been studied extensively. The current paper instead investigates the corrosive effects of KCl and HCl under conditions relevant to waterwall conditions. A low-alloy (Fe-2.25Cr-1Mo) steel was exposed to KCl(s), 500\ua0vppm HCl(g) and (KCl + HCl) in the presence of 5%O2 and 20% H2O at 400\ua0\ub0C. The results indicate that alloy chlorination by KCl occurs by an electrochemical process, involving cathodic formation of chemisorbed KOH on the scale surface and anodic formation of solid FeCl2 at the bottom of the scale. The process is accompanied by extensive cracking and delamination of the iron oxide scale, resulting in a complex, convoluted scale morphology. Adding 500\ua0vppm HCl to the experimental environment (KCl + HCl) initially greatly accelerated the formation of FeCl2 at the scale/alloy interface. The accelerated alloy chlorination is attributed to HCl reacting with KOH at the scale surface, causing the cathodic process to be depolarized. A rapid slowing down of the rate of chlorination and corrosion in KCl + HCl environment was observed which was attributed to the electronically insulating nature of the FeCl2 layer which forms at the bottom of the scale, disconnecting the anodic and cathodic regions

Oxidation of Fe-2.25Cr-1Mo in presence of KCl(s) at 400 \ub0C – Crack formation and its influence on oxidation kinetics

Accelerated corrosion of boiler equipment remains a challenge for efficiently utilising biomass- and waste for power production. To overcome this challenge a better understanding of the influence of corrosive species present is required. This study focuses on the influence of KCl(s) on corrosion of Fe-2.25Cr-1Mo at 400 \ub0C. This is done by well-controlled laboratory exposures and detailed microstructural investigation with ion and electron microscopy (TEM, FIB, SEM, EDX, XRD, TKD). The scale microstructures are linked to oxidation kinetics. The results indicate that KCl(s) increases the ionic diffusion through the oxide scale as well as introduces cracks and delamination resulting in a rapid periodic growth process

The Influence of Oxide-Scale Microstructure on KCl(s)-Induced Corrosion of Low-Alloyed Steel at 400 \ub0C

The high-temperature corrosion of low-alloyed steels and stainless steels in the presence of KCl(s) has been studied extensively in the last decades by several authors. The effect of KCl(s) on the initial corrosion attack has retained extra focus. However, the mechanisms behind the long-term behavior, e.g., when an oxide scale has already formed, in the presence of KCl(s) are still unclear. The aim of this study was to investigate the effect of the microstructure of a pre-formed oxide scale on low-alloyed steel (Fe–2.25Cr–1Mo) when exposed to small amounts of KCl(s). The pre-oxidation exposures were performed at different temperatures and durations in order to create oxide scales with different microstructures but with similar thicknesses. After detailed characterization, the pre-oxidized samples were exposed to 5%O2\ua0+\ua020%H2O\ua0+\ua075%N2 (+KCl(s)) at 400\ua0\ub0C for 24, 48, and 168\ua0h and analyzed with scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and focused ion beam. The microstructural investigation indicated that Cl-induced corrosion is a combination of oxide thickness and microstructure, and the breakaway mechanism in the presence of KCl(s) is diffusion-controlled as porosity changes prior to breakaway oxidation were observed

The influence of silicon on the corrosion properties of FeCrAl model alloys in oxidizing environments at 600 \ub0C

The present study investigates the influence of Si on the high temperature corrosion behaviour of FeCrAl model alloys in O2, O2+H2O and O2+H2O + KCl at 600 \ub0C for up to 168 h. The investigation by SEM/EDX showed that all alloys displayed a protective behaviour in dry O2. In the more corrosive environments (O2+H2O and O2+H2O + KCl) the addition of Si affected the oxidation properties in two ways; Alloys containing Si resisted breakaway oxidation caused by Cr-evaporation (O2+H2O) and the thickness of the oxide formed after breakaway oxidation decreased with increasing amount of Si (O2+H2O + KCl)