Oxide Formation and Degradation during Erosion-Corrosion of Iron and Nickel Based Alloys

Erosion-corrosion is, in many technical applications, a serious material degradation process caused by dynamic particle/fluid mixtures. Pipes transporting particle containing liquids, jet engines, gas turbines, and fluidised bed combustion (FBC) power plants are examples of systems which are considered to be especially susceptible. The aim of this thesis is to investigate the mechanisms of erosion-corrosion in an environment that relates to FBC power plants. Five steels (Fe2.25Cr1Mo, Fe9Cr1Mo, AISI 304, Esshete 1250 and 353 MA) and one Ni based alloy (Inconel 625) were studied in this work. Two different types of erosion-corrosion test rigs were used for the exposures; one nozzle type test rig and one fluidised bed on laboratory scale designed to allow a wide variety of gas atmospheres, bed materials and fluidising conditions. The exposures were performed at low particle velocities and at temperatures ranging from room temperature up to 700 \ub0C. The exposure time varied from 24 h up to 3 weeks. Air was primarily used as exposing atmosphere, but in a minor study 50 ppm HCl or 50 ppm SO2 was added. The erosion-corrosion mechanisms at the different exposure conditions were determined from the surface chemistry and morphology. The degraded surfaces were mechanically characterised by stylus profilometry. Oxide thicknesses and compositions were measured by Auger electron spectroscopy (AES) depth profiling. Complementary information was received from SEM, X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD). At 350 \ub0C all the alloys investigated exhibit a minimum in wastage rate, due to the formation of a composite scale with a high erosion resistance. The scale is composed of fragments from the erodent material, metal oxides and bulk metal. The degradation mechanism is erosion of both the composite scale and the bulk metal. At 550 \ub0C an oxide scale is present, which grows at a higher rate than the corresponding scale formed during pure oxidation. It was shown that formation of cracks and other defects allowing molecular oxygen access to the oxide/metal interface was the major cause of the enhanced oxidation rate. The degradation mechanism is chipping of small oxide fragments. At 700 \ub0C less crack formation takes place, but it is still sufficiently comprehensive to enhance the oxidation rate markedly. The degradation mechanism is extensive oxidation at the bottom of the cracks resulting in flaking of the overlaying oxide. The presence of SO2 in the atmosphere results in a decreased wastage rate and altered degradation mechanism as reflected from the wastage patterns recorded on the exposed tubes

Oxide Formation and Degradation during Erosion-Corrosion of Iron and Nickel Based Alloys

Erosion-corrosion is, in many technical applications, a serious material degradation process caused by dynamic particle/fluid mixtures. Pipes transporting particle containing liquids, jet engines, gas turbines, and fluidised bed combustion (FBC) power plants are examples of systems which are considered to be especially susceptible. The aim of this thesis is to investigate the mechanisms of erosion-corrosion in an environment that relates to FBC power plants. Five steels (Fe2.25Cr1Mo, Fe9Cr1Mo, AISI 304, Esshete 1250 and 353 MA) and one Ni based alloy (Inconel 625) were studied in this work. Two different types of erosion-corrosion test rigs were used for the exposures; one nozzle type test rig and one fluidised bed on laboratory scale designed to allow a wide variety of gas atmospheres, bed materials and fluidising conditions. The exposures were performed at low particle velocities and at temperatures ranging from room temperature up to 700 \ub0C. The exposure time varied from 24 h up to 3 weeks. Air was primarily used as exposing atmosphere, but in a minor study 50 ppm HCl or 50 ppm SO2 was added. The erosion-corrosion mechanisms at the different exposure conditions were determined from the surface chemistry and morphology. The degraded surfaces were mechanically characterised by stylus profilometry. Oxide thicknesses and compositions were measured by Auger electron spectroscopy (AES) depth profiling. Complementary information was received from SEM, X-ray photoelectron spectroscopy (XPS), secondary ion mass spectroscopy (SIMS), energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD). At 350 \ub0C all the alloys investigated exhibit a minimum in wastage rate, due to the formation of a composite scale with a high erosion resistance. The scale is composed of fragments from the erodent material, metal oxides and bulk metal. The degradation mechanism is erosion of both the composite scale and the bulk metal. At 550 \ub0C an oxide scale is present, which grows at a higher rate than the corresponding scale formed during pure oxidation. It was shown that formation of cracks and other defects allowing molecular oxygen access to the oxide/metal interface was the major cause of the enhanced oxidation rate. The degradation mechanism is chipping of small oxide fragments. At 700 \ub0C less crack formation takes place, but it is still sufficiently comprehensive to enhance the oxidation rate markedly. The degradation mechanism is extensive oxidation at the bottom of the cracks resulting in flaking of the overlaying oxide. The presence of SO2 in the atmosphere results in a decreased wastage rate and altered degradation mechanism as reflected from the wastage patterns recorded on the exposed tubes

Effect of KCl on high-temperature corrosion of low-alloyed steel under low oxygen partial pressure

Low oxygen environments in biomass gasification and the presence of chlorine in feedstocks can influence the corrosion rate of steel by affecting the formation of protective oxide scales. The effect of KCl on the high-temperature corrosion of low-alloyed steel (13CrMo4-5) under low oxygen partial pressure is investigated by KCl salt spray (0.1 mg·cm−2) and exposure to 3 vol% H2 + 30 vol% H2O + Ar (balance) at 500°C for up to 168 h. Specimens without KCl salt are exposed for reference. Specimens are characterized after exposure by mass change, SEM/EDS, and XRD. KCl-deposited specimens exhibit about 30% lower mass gain after exposure compared to non-sprayed specimens. Their scale shows a porous innermost layer and a denser layer on top. No Fe or Cr chlorides are identified. The specimens without salt exhibit a similar two-layered scale, with a porous inner Fe-Cr oxide, followed by a denser and thicker Fe-oxide above. KCl could potentially protect the surface from further degradation by physically covering the specimen, altering the scale morphology, and forming a less permeable barrier, hindering the transport of species through the scale. Eric Börjesson is gratefully acknowledged for performing valuable laboratory work. The work was performed within the Swedish High Temperature Corrosion Centre (HTC) and it was funded by the Swedish Energy Agency. Alleima (formerly Sandvik), Cortus Energy, Kanthal, and Phoenix Biopower are acknowledged as valuable project partners. Alleima (formerly Sandvik) and Kanthal are acknowledged for providing the test materials.</p

Metal loss and corrosion attack of FeCrAl overlay welds on evaporator tube shields of a waste-fired power plant

Three FeCrAl alloys (APMT, EF100 and EF101) from Kanthal® and the reference Ni-Cr Alloy 625 were used as weld cladding materials on tube shields in the evaporator tube bank of a waste-fired combined heat and power plant. For each alloy type, the overlay welded tube shields were placed in both roof and floor positions within the evaporator for 6 months. The metal-loss rate, the microstructure and hardness of the overlay welds before and after exposure and the corrosion products were analysed. The results showed higher metal-loss rates in the welds placed in the roof position, confirming heterogeneities in the evaporator bank environment. Alloys were ranked from higher to lower erosion–corrosion resistance as follows: APMT ≈ Alloy 625 &gt; EF101 &gt; EF100. The analysis of the corrosion attacks showed a significant variation among the alloys, from a primarily homogeneous corrosion attack on APMT to intergranular corrosion in EF100 and pit formation in EF101. Alleima AB (formerly Sandvik Materials Technology AB), E.ON Värme Sverige AB and Kanthal AB are gratefully acknowledged for their participation and support in these research projects, and especially for providing the alloys, supporting the exposures in the boiler and valuable discussions. Eric Börjesson is acknowledged for performing valuable laboratory work at RISE and Kjell Hurtig is gratefully acknowledged for the preparation of the overlay welds at University West. Anna Jonasson is acknowledged for valuable planning and assistance regarding the exposure in the P14 plant at Händelö. This work was supported by the project, funded by the Stiftelsen för Kunskaps och Kompetensutveckling (KK‐stiftelsen) under Grant reference 20170316; and as part of the project KME‐802 , funded by the Swedish Energy Agency under Grant reference 46471‐1. HÖG‐FECRALCLAD Svetsbarhet och korrosionbeständinghet av nya FeCrAl‐legeringar Increased fuel flexibility and performance for boilers with challenging fuels</p

Influence of PbCl2 and KCl salt mixture on high temperature corrosion of alloy 625

Funding Information: This work was carried out within and funded by the High Temperature Corrosion Centre (HTC) at Chalmers University of Technology, with support from the Swedish Energy Agency. Publisher Copyright: © 2023 The AuthorsAggressive corrosion can occur when firing waste or bio-based fuels, due to the presence of high concentrations of heavy metals, alkali metals, and chlorides. These deleterious compounds deposit on furnace walls and can form mixtures that can rapidly accelerate corrosion. The effect of salts containing lead had not been studied extensively at temperatures lower than 400 °C in nickel-based materials. This study investigates the effect of the individual salts PbCl2 and KCl and their mixture on the high temperature corrosion of alloy 625 at 340 °C and 380 °C. Samples of alloy 625 were covered with individual salts or a salt mixture and exposed to high temperatures in an atmosphere of synthetic air, 20-vol% H2O, and 100 ppm HCl. The results show that the presence of individual salts does not induce observable corrosion attack on alloy 625 after 168 h at any tested temperature. The salt mixture did cause a severe corrosion attack at 380 °C, observed after 24 h of exposure. It is suggested that the salt mixture induces the formation of lead chromates that may prevent or disrupt the formation of a protective chromia scale. It is believed that a key part of the mechanism is the formation of eutectic melts by the interaction of the scale with the salt mixture. Thermodynamic equilibria calculations show that the first melting temperature of PbCl2 and KCl salt mixture after reaction with oxygen can be as low as about 382 °C, and even lower (357 °C) if chromates are present.Peer reviewe