High temperature corrosion of low-alloyed and stainless steels: mechanistic study of chlorine-induced corrosion

The global demand on power generation is constantly increasing and so far, also its environmental impact. The environmental impact could primarily be directed to the power generation being based on fossil fuels, giving a net increase of CO2 to the atmosphere when combusted. The need of renewable fuels such as biomass and waste for power generation, leading to no net release of CO2, is therefore increasing. However, biomass and waste vary in composition and the considerably high amounts of alkali- and chlorine-containing species of these fuels result in a highly corrosive fireside environment for the metallic components of the boiler. Chlorine-induced corrosion is speculated to play an important role in the corrosion of these metallic components. However, the consequences of event leading to corrosion in the presence of chlorine is still not fully understood and the corrosion mechanism is under debate. Thus, this study aims at investigating mechanism of chlorine-induced corrosion. The study is divided into two parts; field exposures showing the extent and initiation of a chlorine-induced corrosion attack and laboratory exposures aiming at investigating the mechanism of the chlorine-induced corrosion attack.The field exposures were focused on the startup sequence of probe exposures. The results showed that the initiation of breakaway corrosion is very rapid in this environment. Thus, the primary protection, i.e. the Cr-rich oxide scale on stainless steels, was immediately destroyed and the oxides and metal chlorides formed set the boundary conditions for further corrosion, i.e. secondary protection. The results showed that the different startup sequences had only a minor effect on the initial corrosion attack. Based on the corrosion attack observed in the field-exposed samples, a set of laboratory exposures was designed. The objective was to investigate the mechanism behind chlorine diffusion through oxide scales at high temperatures. A series of pre-oxidations were performed in order to investigate the role of oxide composition, microstructure, and thickness on chlorine-induced corrosion.The investigation showed that the presence of either KCl(s) or HCl(g) accelerates the corrosion rate of all the investigated materials. Both thickness and microstructure of the Fe-rich oxide, i.e. secondary protection, influences the incubation time to breakaway corrosion. In addition, cracking and spallation of the Fe-rich oxide, as well as the presence of metal chlorides at the oxide/metal interface below a crack-free scale, were observed. Thus, the corrosion attack may be driven by both for crack formation and chlorine diffusion through the oxide scale. Mechanisms for both the influence of crack formation on the corrosion attack and alternative diffusion paths for chloride is proposed. DFT calculations showed that the diffusion of chloride ions through the oxide scale is energetically favoured to occur via oxygen vacancies

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

Reactions between Mullite based Refractories and Slag at Elevated Temperatures

In the iron ore pelletizing, the sintering of the pellets takes place in a rotary kiln which is lined with refractory bricks. During the process, deposited materials such as iron ore slag, additives, binders and fly ash stick on the surface of the lining, and with time, by infiltration and diffusion, a corroded layer is formed. Some of the reactions occurred in that layer are promoted by the presence of alkalis. Refractory/deposited materials reactions and infiltration of deposited materials were studied at laboratory scale. In this study, techniques such as differential scanning calorimetry (DSC), x-ray diffractometry (XRD) and scanning electron microscopy (SEM) were used. Reaction temperatures, phase transformations and infiltration depth of deposited materials were determinated. Additions of alkalis into the materials involved were used to enhance the reactions between them. Using powder mixtures the results of DSC show that in addition of K2CO3 the reaction temperature range is 850-950°C showing formation of new phases around 850°C with kalsilite and around 950°C with leucite; in addition of the mixture Na2CO3 and K2CO3, there is a broader temperature interval of 600-925°C with formation of kalsilite at lower temperatures and nepheline at higher temperatures. In the characterization of the infiltration of deposited materials into the brick it was observed that nepheline was formed mainly in the corroded brick layer but K feldespathoids (leucite, kalsilite, kaliophilite) were formed beneath the corroded brick layer.Validerat; 20110705 (anonymous)</p

The effect of startup procedure of probe exposures on deposit and corrosion formation in a waste fired CFB boiler.

This paper investigates the startup sequence influence on the initial deposit formation and consequently, the corrosion attack in a waste fired Circulating Fluidized Bed (CFB) boiler. Two probes were exposed simultaneously; one probe starting exposure from room temperature and one probe starting exposure after 100 \ub0C preheating. The samples consisted of Sanicro 28 highly alloyed stainless steel. Regardless of preheating or preoxidizing the sample probe the corrosion attack is still very fast leading to thick oxide scales and formation of metal chlorides resulting in poor scale adhesion. By preheating and preoxidation, the attack as well as the amount of metal chlorides was decreased slightly. Thus, the startup sequence seems to have only a minor effect on the initial stages of exposure

Correlation between field and laboratory exposures for boiler corrosion test – mechanistic study of chlorine induced corrosion.

For boilers manufacturers and operators, smart selection of materials is important. Thus, the\ua0understanding of corrosion mechanisms involved in the degradation of metallic components of the boiler\ua0is of great interest. In order to address corrosion problems in the boiler, probe exposures are performed\ua0in research and industrial projects. Valuable information is obtained from this test, such as material\ua0corrosion rates and deposit formation. However, due to the complex environment in the boiler, it is\ua0complicated to identified and study the corrosion mechanisms taking place. Therefore, laboratory\ua0exposures simplifying the conditions in the boiler can be used to study the corrosion mechanisms in\ua0more detail.In order to correlate corrosion behavior between field and laboratory exposure, samples of same\ua0material were exposed in both field and laboratory scale. Field exposures were performed in a waste\ua0fired CFB boiler with horizontal design. Air-cooled test probes were used in order to control the\ua0temperature of the 347H sample materials to 600 \ub0C. Pre-oxidized samples were exposed for 24 and\ua0144 hours and compared to laboratory studies. Special focus was given to the study of the mechanism\ua0of chlorine induced corrosion.After field exposure, optical examination of samples showed that the deposit material was easily\ua0detached form the samples surface. Chemical analysis of the deposit material shows that the deposit\ua0was rich in chlorine. Cross sectional views of all the samples show similar corrosion attack for both field\ua0and laboratory but more extensive attack was observed in the field samples. Presence of metal\ua0chlorides were detected at the oxide/metal interface of both types of exposures same as corrosion\ua0attack at the grain boundaries. The well-controlled laboratory exposures gave the opportunity to a more\ua0detailed characterization resulting in a better understanding of the propagation of the corrosion attack\ua0observed in the field exposures

Microstructural Study of the Influence of KCl and HCl on Preformed Corrosion Product Layers on Stainless Steel

Metallic construction materials in biomass- and waste-fired boilers are exposed to corrosive environments due to the considerable amounts of alkali chlorides and HCl(g) released in renewable fuels combustion. Alkali chlorides corrosivity toward stainless steels exposed at high temperature has been extensively studied. Nevertheless, the corrosion attack propagation is still not fully understood and it is expected that chlorine diffusivity through oxide layers plays a major role in accelerating the corrosion. In order to investigate the role of chlorine on the propagation step of the corrosion attack, tailor-made oxides were produced. The samples were subsequently exposed to chlorine-containing environments for short period of time. The reaction atmospheres were O2\ua0+\ua0H2O\ua0+\ua0KCl(s) and O2\ua0+\ua0H2O\ua0+\ua0HCl(g) at 600\ua0\ub0C. Since in this study chlorine diffusivity through the corrosion product layer is of great interest, samples were analyzed with XRD and SEM/EDX. High-quality BIB cross sections were performed. Summarizing, for the preformed oxide layers on the stainless steel in the presence of HCl(g), chlorine seemed to penetrate to the oxide/metal interfaces in the material. However, in the presence of KCl(s) there seems to be no effect of the salt on the corrosion rate

Oxidation Driven Permeation of Iron Oxide Scales by Chloride from Experiment Guided First-Principles Modeling

In this comprehensive investigation, it is demonstrated how chloride ions may permeate a crack-free oxide scale, thus providing hitherto missing mechanistic insight as to the impacts of KCl(s) or HCl(g) exposures on the high-temperature corrosion of biomass- and waste-fired boilers. Guided by dedicated experimental analyses, mechanistic understanding is gained by means of density functional theory. Chloride ions, being accommodated in oxygen anion vacancies that are generated at the receding magnetite/alloy interface, are driven through the oxide scale by the oxidation process. Intermediate stabilities of quasi-homogeneous transient iron oxychloride species are found, employing potassium ferrite and goethite as complementary cation sinks for the KCl(s) and HCl(g) reactants, respectively. Spontaneous disproportionation of the supersaturated oxychlorides into two different types of chloride decorated magnetite grain boundaries is demonstrated. These motifs are proposed to explain loss of scale integrity as well as accelerated oxidation by offering short-circuiting transient pathways for ion diffusion