Cr-rich Oxide Scale Research Articles

Corrosivity of KOH(g) towards superheater materials in a simulated air reactor environment for chemical looping combustion of biomass

Chemical looping combustion (CLC) of biomass has the potential to improve the electrical efficiency in power plants that utilize carbon capture and storage (CCS). This is attributed to the mild corrosive environment anticipated in the air reactor (AR). However, previous studies have measured alkali emissions in the AR, likely in the form of KOH(g), suggesting that alkali may transfer from the fuel reactor (FR) to the AR. To investigate the corrosive effect of KOH(g) release in the AR, a novel experimental set-up was developed to simulate two scenarios for the AR 1) Clean scenario: no release of KOH(g) in the AR (5 % O2 + 3 % H2O + N2 bal.) and 2) Alkali slip scenario: continuous release of KOH(g) in the AR (5 % O2 + 3 % H2O + N2 bal. + 16 ppm KOH(g)). The exposure was carried out at 700 °C and six alloys, relevant as superheater material, ranging from stainless steels to nickel-base (Ni-base) alloys as well as a FeCrAl alloy were investigated. The samples were exposed for a total of 168 h and the morphology of the corrosion products was investigated using SEM-EDX and XRD. The presented results suggest that KOH(g) significantly accelerates the corrosion of the stainless steels and Ni-base alloys investigated, by rapidly destroying the protective Cr-rich oxide scale, resulting in the formation of a multilayer oxide scale with inferior protective properties. On the contrary, the FeCrAl alloy retained a protective Al-rich oxide scale irrespective of the presence of KOH(g). The findings in this study highlight that the release of KOH(g) in the AR during combustion of biomass in CLC could introduce corrosion challenges for installed superheaters that can be significantly mitigated by utilizing FeCrAl alloys.

Oxidation behavior of harmonic structured high-entropy cantor alloy

Present study deals with the oxidation properties of the harmonic structured high-entropy Cantor (CoCrFeMnNi) alloy at 600 °C, 700 °C, and 800 °C in zero-grade air. The microstructure of the Cantor alloy is modified to a harmonic distribution with grain sizes at shell of ∼2 μm and core of ∼20 μm. The oxidation behavior of the harmonic one is compared with its non-harmonic counterpart and as-received 304L stainless steel. Both the high-entropy Cantor alloys show inferior oxidation resistance than the 304L stainless steel in this temperature range. Studies show that an Mn-rich oxide scale forms on the surface of the Cantor alloys, whereas a Cr-rich oxide scale forms on the 304L stainless steel surface. The high Mn content in the Cantor alloys deteriorates their oxidation resistance. However, the harmonic structure improves the oxidation resistance in the Cantor alloy due to the preferential oxidation of the shell region and the formation of a more compact oxide scale than its non-harmonic counterpart.

Breakdown of Protective Cr-Rich Oxide Scale Formed on Heat-Resistant Steels for Superheater Tubes in a Waste Power Generation Boiler

Breakdown of Protective Cr-Rich Oxide Scale Formed on Heat-Resistant Steels for Superheater Tubes in a Waste Power Generation Boiler

Irradiation accelerated corrosion of alumina-forming austenitic steels in supercritical CO2: The oxide scale formed within an individual grain or affected by grain boundary

The irradiation accelerated corrosion within monolayer grains of a new AFA steel exposed to sCO2 are studied. The presence of irradiation induced dislocation loops accelerate element diffusion and reduces corrosion resistance by ∼44%. Many oxide scale features are firstly found, including a two-phase inner layer consisting of Ni-rich austenite and Cr-rich spinel, and Al2O3/SiO2 oxide precipitates. Irradiation leads to an additional Cr-rich oxide layer at the bottom of inner layer and a narrower Al2O3 band in the internal oxidation layer. Irradiation also induces Cr depletion at grain boundaries, thereby impeding the formation of a protective Cr-rich oxide scale.

High-Temperature Corrosion of Ni-Cr-Mo Cladding Layers with Different Si Contents in NaCl-KCl-Na2SO4-K2SO4 Mixed Salt Medium.

In this work, Ni-Cr-Mo cladding layers with different Si contents were prepared on Q235 steel using laser-cladding technology, and their corrosion characteristics were investigated in NaCl-KCl-Na2SO4-K2SO4 mixed salt at 550 °C. The corrosion resistance of each cladding layer was tested by weight loss method, and the phase compositions and microstructures of the cladding layers and corrosion products were determined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that Si contributed to the formation of a dense chromium oxide film on the surface, and the addition of Si can significantly improve the corrosion resistance of the cladding layer at high temperature. At 550 °C, the corrosion rate of the cladding layer with 5 wt.% Si was only 38.2% of that of the cladding layer without Si. After 168 h of high-temperature corrosion, no Cr-rich oxide scale was found in the outermost layer of the Ni-Cr-Mo cladding layer without Si. When Si content was 3 wt.% and 5 wt.%, the Cr-rich oxide scale of the cladding layer was denser than that of the coating with 1 wt.% Si content.

Effect of surface finishing on the oxidation characteristics of a Fe-21Cr-32Ni alloy in supercritical carbon dioxide

The effect of surface finishing on the oxidation behavior of a Fe-21Cr-32Ni alloy in supercritical carbon dioxide (650 ºC for 1000 h) was studied. After exposure, the cross-section of the specimen showed a typical inner-outer oxide layers structure with subsurface carburization. Reduced oxidation and carburization rates were identified for the ground surface, where a much thinner Cr-rich oxide scale and a shallower carburization depth were observed. Grinding introduced an ultra-fine grained layer and a highly deformed layer on the metal surface, where Cr diffusion rate was significantly enhanced. The high Cr diffusion led to the transition from internal to external oxidation by favoring the formation of a thin/protective Cr-rich oxide layer. This continuous Cr-rich oxide layer not only protected the substrate matrix from further oxidation, but also reduced the permeability of carbon into the oxides, thus improving the oxidation and carburization resistance of the ground specimen.

High-Temperature Oxidation of Steels in Direct-Fired CO2 Power Cycle Environments

Future direct-fired supercritical CO2 power cycles require steels resistant to oxidation/corrosion in high-temperature CO2 environments containing various impurities. Herein we studied the oxidation behavior of 14 candidate steels in a simulated direct-fired CO2 power cycle environment consisting of 95% CO2, 4% H2O, 1% O2 with/without 0.1% SO2 at 1 atm and 550 °C, 600 °C, 650 °C for up to 2500 h. Steels with ≥ 11.5 wt% Cr exhibited at least partial coverage by Cr-rich oxide scales leading to a significant decrease in the oxidation rates in both gases. While SO2 had little effect on low-Cr steels that formed Fe-rich oxides, it generally worsened performance of high-Cr (> 11.5 wt%) steels by hindering the establishment of a protective Cr-rich oxide. This effect was most pronounced at the lowest temperature of 550 °C, which was attributed to strong preferential adsorption of sulfur-containing species within the oxide at relatively low temperatures.

Electroless Ni–P coatings on low-Cr steels: A cost-efficient solution for solar thermal applications

The corrosion behavior of an electroless Ni–P coating on low-Cr X20CrMoV12-1 steel as a cost-efficient substrate-coating combination is investigated in molten NaNO 3 –KNO 3 nitrates at 600 °C. Results indicated that the as-deposited Ni–P coating was transformed to a thermally treated diffusion coating upon exposure to the molten salt without additional heat treatment prior to the exposure. Detailed evolution of the Ni–P coating and the oxidation behavior during the 1000-h immersion test is discussed. Results showed competitive corrosion resistance of the coating system compared to that of the sophisticated Ni alloy 230, with the advantage of an inherent barrier against Cr dissolution form the substrate. The significant corrosion resistance of the electroless plated Ni–P coating is attributed in part to the protective Ni--rich oxide scale and in other part to the in-situ formed Ni 3 P barrier at the oxide-substrate interphase boundary. - Electroless Ni–P coating was developed on a low-cost X20CrMoV12-1 substrate - Interdiffusion occurred during the immersion test at 600 °C - The as-coated substrate required no additional heat treatment prior to the molten salt exposure - An outer Ni–Cr rich oxide scale and an inner Ni 3 P layer formed during the immersion test - The Ni–Cr scale and Ni 3 P layer protected the substrate against molten salt corrosion and Cr-dissolution

Oxidation Behavior of Welded Fe-Based and Ni-Based Alloys in Supercritical CO2

Next-generation supercritical CO2 (sCO2) power cycles will require different classes of alloy throughout the operational temperatures to optimize tradeoff of creep strength, oxidation performance and cost. This will necessitate joining methods such as welding, which might pose compatibility concerns at the joined interfaces. In this study, similar and dissimilar metal welds were generated from a variety of candidate alloys for sCO2 systems including ferritic/martensitic steels, austenitic steels, and Ni-based superalloys. Samples were extracted from different regions of the welds and exposed to sCO2 at 550 °C and 20 MPa for 2500 h and then characterized to understand their behavior in this environment. Unsurprisingly, the local oxidation behavior was largely dictated by the Cr content in the underlying metal. High-Cr austenitic steels and Ni alloys formed slow-growing Cr-rich oxide scales with minimal carburization of the underlying metal, while low-Cr ferritic/martensitic steels formed fast-growing Fe-rich oxide scales with significant carburization. Most welds did not show any unusual oxidation behavior at the interfaces, considering the local Cr content. The one exception was the 347H similar metal weld, where a larger grain size and complex grain structure in the fusion zone led to a significantly higher rate of Fe-rich oxide nodule formation compared to the base metal. This suggests that microstructural changes at joined interfaces can play an important role on the oxidation-limited lifetimes in future sCO2 systems. The composition changes across the interfaces enabled study of the effect of Fe on the growth rate of Cr-rich oxides and of the origins of the subsurface recrystallization zone that forms beneath them.

High-Temperature Compatibility of Structural Alloys with Supercritical and Subcritical CO2

In searching for new working fluids for power generation, supercritical CO2 (sCO2) offers some attractive features for efficient cycles. However, compatibility with structural alloys is a concern. NiCr-based alloys have excellent compatibility at 600°-800°C at 20-30 MPa sCO2. However, conventional steels have restrictions in temperature because of carburization and accelerated oxidation in sCO2, similar to observations in CO2. To assess the impact of carburization on steel mechanical properties, small (25mm long) dogbone tensile bars are being exposed and tested after exposure at 450°-650°C. Only highly alloyed advanced austenitic steels are resistant to carburization at 650°-750°C, suggesting that Crrich oxide scales are good barriers to C ingress. Above 800°C, it is only possible to conduct subcritical evaluations at this time, but initial results suggest most conventional high-temperature Fe- and Ni-based alloys are more rapidly degraded by CO2 at higher temperatures. Coatings are a potential solution that require more study.

Electron beam-powder bed fusion of Alloy 718: Effect of hot isostatic pressing and thermal spraying on microstructural characteristics and oxidation performance

Alloy 718 manufactured via electron beam-powder bed fusion (EB-PBF) was coated with a thermally- sprayed NiCoCrAlY coating for enhanced oxidation protection. A high-velocity air fuel technique was used to deposit the coating. The specimens were then subjected to hot isostatic pressing (HIP). Oxidation of the specimens was undertaken in an ambient air environment at 650 and 800 °C for 168 h. The oxidation performance of EB-PBF-built Alloy 718 was improved after the deposition of the coating, particularly at 800 °C. In this temperature, a thick Cr-rich oxide scale was found on the uncoated Alloy 718 specimen, whereas a thin and stable Al-rich oxide scale was formed on the surface of the coated specimen. HIPing enhanced the oxidation resistance of uncoated Alloy 718; however, the oxidation behavior of coated Alloy 718 was negatively affected by HIPing.

High-temperature corrosion of an Fe–Ni-based alloy HR6W under various conditions at 750 °C and 810 °C: Effect of the temperature, water vapor, simulated ash and SO2

The influences of temperature, SO2, water vapor and simulated ash on the high-temperature corrosion of an Fe–Ni-based alloy HR6W at 750 °C and 810 °C for 1000 h was systematically investigated. The weight gains and characterization of the corrosion products formed on the HR6W alloy in various environments were used to obtain the high-temperature corrosion behaviors of the alloy. The results show that weight gains of alloy HR6W under various gas conditions at 750 °C and 810 °C was much lower than that of the samples coated with salt, indicating that alloy HR6W exhibited excellent corrosion resistance due to a Cr2O3-rich scale. The SO2 promoted crackings of the Cr-rich oxide scales at 750 °C and 810 °C. After exposure to air containing water vapor and SO2, the corrosion products were mainly composed of an outer layer of Fe2O3, a small amount of NiO and an inner layer of Cr2O3. Sulfidation of alloy HR6W underneath the Cr-rich oxide scale was clearly observed on the samples exposed to SO2 and on all samples coated with salt. Additional NiO was found on the samples coated with salt exposed to air containing SO2. Alloy HR6W coated with salt exposed to air containing SO2 at 750 °C and 810 °C experienced the most severe corrosion among all samples herein due to the high causticity of the alkali-iron-tri-sulfates. The key elements affecting the high-temperature corrosion of alloy HR6W in the various gas environments were temperature > SO2>water vapor, in the order. Under the gas-molten salt conditions, the SO2 played the most important role in determining the corrosion rate. The key elements were SO2> temperature > water vapor, in that order.

Subsurface grain refinement in electron beam-powder bed fusion of Alloy 718: Surface texture and oxidation performance

Subsurface grains of Alloy 718 additively manufactured via electron beam-powder bed fusion technique were refined using shot peening to improve the surface texture and oxidation performance. Oxidation of the specimens was performed at 650 and 800 °C in ambient air. Due to plastic deformation upon shot peening, compressive residual stress and high microstrain were generated in the subsurface region within a depth of approximately 50 μm. The shot-peened specimen exhibited lower surface roughness, finer subsurface grains, and higher hardness compared to the as-built specimen. Shot peening, coupled with hot isostatic pressing and heat treatment (HIP-HT), yielded superior oxidation performance with substantially low oxidation kinetics at 800 °C. The smooth surface, as well as dense and refined subsurface microstructure resulting from shot peening, facilitated the formation of a continuous, protective, and adherent Cr-rich oxide scale.

Temperature-Dependence of Corrosion of Ni-Based Superalloys in Hot CO2-Rich Gases Containing SO2 Impurities

Future power plants will require Ni-based superalloys resistant to high-temperature corrosion in CO2-rich environments containing impurities. In this work, several commercially available Ni-based alloys (617, 230, 625, 263, 740H) were exposed at 600°C, 650°C, 750°C and 1 atm to 95% CO2, 4% H2O, 1% O2 without/with 0.1% SO2 to simulate compositions expected in a direct-fired supercritical CO2 power cycle. The results indicate no effect of SO2 at 750°C, a small negative effect at 650°C, and a large negative effect at 600°C. Alloys exposed at the higher temperatures (650–750°C) formed thin Cr-rich oxide scales, whereas the lowest temperature (600°C) resulted in thicker scales consisting of non-protective oxides and sulfates. Thermodynamic analysis indicates this increased corrosion is associated with a transition in the stable compounds in contact with the gas. Understanding the factors that affect this transition will aid in the selection or design of alloys for future CO2-based power systems.

Corrosion Performance of AISI 304 Stainless Steel in CO2-Saturated Brine Solution

Corrosion behavior of 304 stainless steel exposed to a NaCl (3.5 wt %) solution saturated with CO2 has been analyzed using electrochemical techniques including, potentiodynamic polarization, polarization resistance, and electrochemical impedance measurements. The stainless steel samples were evaluated having different surface and pre-oxidation treatments. The oxide scales formed on 304 stainless steel oxidized in different pO2 at 1100°C have also been studied and compared. Different morphologies and chemical composition of the oxide scales were observed after oxidation at low and high oxygen partial pressures. Oxide layers with high chromium content were formed on the ground sample pre-oxidized in Ar while iron-rich oxides were mainly formed under air atmosphere. The electrochemical corrosion results indicate that non-oxidized 304 SS exhibits the best corrosion performance followed by the ground sample heat-treated in argon. For the oxidized stainless steels, the differences in the electrochemical responses are associated to the morphological characteristics and composition of the oxide layer. Homogeneous and dense Cr-rich oxide scale provides protection to 304 SS during exposure to CO2-saturated solutions while the formation of Fe-oxides with porous morphology increases the corrosion rate of 304 stainless steel.

Effect of Surface Finish on High-Temperature Oxidation of Steels in CO2, Supercritical CO2, and Air

Current and future power systems require steels resistant to high-temperature oxidation in CO2-rich environments. The introduction of structural defects by various surface treatments can profoundly affect the oxidation/corrosion behavior of steels in many environments. This effect is largely unexplored for steels exposed to high-temperature CO2, which is the focus of this work. We prepared Grade 22, Grade 91, 347H, and 310S steels with three different surface finishes, ranging from little to substantial surface damage, and exposed the steels to 1 bar CO2, 200 bar supercritical CO2, and laboratory air at 550 °C for up to 1500 h. Surface finish had little impact on the oxidation behavior of low-Cr (2 wt%) Grade 22 and high-Cr (25 wt%) 310S steels. In contrast, intermediate-Cr steels Grade 91 (8 wt%) and 347H (17 wt%) generally showed improved oxidation and carburization resistance with increasing extent of surface damage, which was attributed to the delay or prevention of the onset of Fe-rich oxide nodule growth. Comparison between exposure environments suggests that this effect is more complex for CO2 compared to air and that it is additionally affected by CO2 pressure. The results suggest that surface treatments should be considered as one approach to achieve improved corrosion resistance in high-temperature CO2, particularly for steels containing Cr levels near the transition that is required to form and maintain a protective Cr-rich oxide scale.

Investigation of fireside corrosion of austenitic heat-resistant steel 10Cr18Ni9Cu3NbN in ultra-supercritical power plants

Fireside corrosion behavior of austenitic heat-resistant steel 10Cr18Ni9Cu3NbN was investigated which was firstly applied in a 660 MW power plant. XRD, SEM and EDS were utilized to characterize the corrosion products and to reveal the corrosion mechanism. Results show that corrosion products formed on experimental tubes were composed of a molten salt layer (MSL), a metallic oxidation layer (MOL) and an internal sulfuration zone (ISZ). A thick MSL containing a mixture of spherical AlSi oxides/ CaSO4 and Fe2O3 is formed above Fe- and Cr-rich oxide scales. Fe and Cr sulfides are found underneath the MOL, which are developed at high pressure of sulfur. The corrosion rate at fireside facing the flue gas is much greater than that on the reverse side. Strict control of sulfur content in coal was therefore proposed relieving fireside corrosion of heat-resistant steels. The corrosion mechanism is put forward. Lastly, the exfoliation of MSL is caused by aggregate CaSO4 particles.

Steam Oxidation of Austenitic Heat-Resistant Steels TP347H and TP347HFG at 650⁻800 °C.

Steam oxidation of austenitic heat-resistant steels TP347H and TP347HFG at 650–800 °C was investigated. Comprehensive micro-characterization technologies containing Scanning Electron Microscope (SEM), Energy Dispersive X-ray Spectroscopy (EDS), X-ray Diffraction (XRD), and X-ray Photoelectron Spectroscopy (XPS) were employed to observe and analyze the oxidation products. Results show that breakaway oxidation behaviors were observed on TP347H at 700 °C and 800 °C. The oxidation kinetics of TP347HFG at 650–800 °C followed a parabolic law. The oxide scales formed on TP347HFG were composed of MnCr2O4 and Cr2O3. A thin and protective Cr-rich oxide scale was replaced by Fe2O3 nodules due to the insufficient outward migration of metallic ions, including Cr and Mn at the subsurface of coarse-grain TP347H. Smaller grain of TP347HFG promoted the formation of the compact Cr-rich oxide scales. At higher temperatures, the incubation period for breakaway oxidation of the Cr-rich oxide scale was much shorter because of quick evaporation of the Cr2O3 oxide scale and the slower outward diffusion of metallic ions via the grain boundaries.

Evaluation of electroplated Co-Cu metallic coatings for intermediate-temperature solid oxide fuel cells

Co-Cu alloys have been co-deposited onto 430 ferritic stainless steels via electroplating with a citrate solution. At the initial oxidation stage, a three-layer scale composed of a thin CuO outer layer, a thick (Cu,Fe,Cr)-doped Co3O4 middle layer and a (Cu,Fe)-doped (Co,Cr)3O4 inner layer was formed on the coated steel. With extended oxidation, the (Co,Cr)3O4 inner layer has been transformed into a Cr-rich oxide inner layer. An obvious outward diffusion of Fe appeared, leading to the formation of an (Cu,Cr,Mn)-doped (Co,Fe)3O4 interaction zone between the Co3O4-based spinel and the chromia oxides. The Co-Cu coating effectively blocked the outward migration of Cr from the substrate. No Cr element could be found in the coupled La0·8Sr0·2MnO3 (LSM) plate of the coated sample after oxidized at 800 °C in air for 500 h. The highly conductive coating with a structure of CuO/Co-based spinels significantly decreased the growth of the Cr-rich oxide scale, and thus a much lower scale area specific resistance (ASR). The electrical properties and the oxidation mechanism of the coated substrates were discussed.

Degradation of the oxide film formed on Alloy 690TT in a high-temperature chloride solution

Comparative analyses are adopted to examine the influences of Cl− on the degradation of the film on Alloy 690TT in high-temperature water. According to the results of surface analyses and TEM, the composition and structure of the oxide film formed in a chloride solution are determined. Unlike the duplex layer oxide film formed on the sample in pure water, a thinner and monolayer Cr-rich oxide scale with several little corrosion pits on the surface is formed, which is closely related to the adsorption of Cl− ions due to their affinity to material surface and the selective dissolution of Ni-rich oxides.