Cr-rich Oxide Research Articles

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.

Oxidation behaviour of alloy S16 in superheated steam and supercritical water

ABSTRACTThe performance of S16, a CoCrWC alloy, was evaluated in superheated steam (SHS) at 625°C and 800°C (at 0.1 MPa), and in supercritical water (SCW) at 625°C and 26 MPa for 500 hours. S16 experienced weight loss under the SHS conditions (625°C and 800°C) and weight gain under the SCW condition. Weight loss was associated with scale spallation and pitting while oxide growth on the surface contributed to weight gain. Formation of CoCr2O4 and Cr2O3 was observed in the oxide layer of samples tested under the three conditions, however, the Cr-rich oxide on SCW tested samples was found to be continuous. A thin, continuous Co3W subscale was believed to have provided an oxidation barrier under the SCW test condition. Based on the limited weight change and surface oxide formation, S16 has the potential to be used as a coating material for ultra-supercritical power generation systems at temperatures up to 625°C.

Thermal stability of multi-layered AlCrN/Cr/AlCrN coatings at elevated temperatures for low thermal emissivity applications

Multi-layered AlCrN/Cr/AlCrN coatings were designed and deposited on Ni-based substrates by a cathodic arc ion plating system for low thermal emissivity applications. The multi-layered coatings annealed at different temperatures in air and vacuum were systematically investigated by GIXRD, EPMA, XPS, DSC, GDOES, FESEM, HR-TEM and scratch tester as well as the infrared properties evaluation. The as-deposited AlCrN layer was consisted of Cr2N nanocrystallites embedded in AlCrN amorphous matrix, inhibiting element diffusion and presenting higher infrared emissivity. The amorphous matrix began to crystallize as fcc-Cr(Al)N structure at 750 °C. The diffusion of Ni element from the substrate to the middle Cr layer was observed as well as the Cr-rich oxides were substituted by chrome‑aluminum oxides above 750 °C in air. The grain boundaries resulted from the crystallization of amorphous AlCrN layer could promote the penetration of O element from air and the diffusion of Ni element from the substrate. The AlCrN/Cr/AlCrN coatings vacuum-annealed above 750 °C performed lower infrared emissivity than the as-deposited one, while the coatings air-annealed above 750 °C showed higher infrared emissivity than vacuum-annealed ones due to the low extinction coefficient of oxides formed on the coating surface. The multi-layered AlCrN/Cr/AlCrN coating could be proposed for low infrared emissivity applications below 750 °C.

Effects of nano-dispersoids on synthesis and characterization of low Cr-containing CoNiFeMnCr high entropy alloy by mechanical alloying

Low Cr-containing CoNiFeMnCr high entropy alloys (HEAs) dispersed with different amounts of nano-Y2O3 particles were fabricated by mechanical alloying. The effects of nano-dispersoids on synthesis, microstructural evolution and mechanical properties of the model HEAs were investigated. The results demonstrate that a solid solution phase was formed after a long time of mechanical alloying. The addition of Y2O3 oxides in the model HEAs can promote the formation of the fine, uniform distribution of Cr-rich oxide particles. It suggested that the presence of Y2O3 could act as a nucleation site for precipitation of Cr-rich oxides. A high hardness and elastic modulus of the HEAs can therefore be achieved.

Stainless steel surface structure and initial oxidation at nanometric and atomic scales

The durability of passivable metals and alloys is often limited by the stability of the surface oxide film, the passive film, providing self-protection against corrosion in aggressive environments. Improving this stability requires to develop a deeper understanding of the surface structure and initial surface reactivity at the nanometric or atomic scale. In this work we applied scanning tunneling microscopy to unravel the surface structure of a model stainless steel surface in the metallic state and its local modifications induced by initial reaction in dioxygen gas. The results show a rich and complex structure of the oxide-free surface with reconstructed atomic lattice and self-organized lines of surface vacancies at equilibrium. New insight is brought into the mechanisms of initial oxidation at steps and vacancy injection on terraces leading to Cr-rich oxide nuclei and locally Cr-depleted terraces, impacting the subsequent mechanism of chromium enrichment essential to the stability of the surface oxide.

Oxidation behavior of the dilution zone of a dissimilar metal weld between low alloy steel and Alloy 82 in high temperature water

The investigation on oxidation behavior of the dilution zone of a dissimilar metal weld (DMW) between low alloy steel (LAS) A508-III and Alloy 82 weld metal (WM) in high temperature water was carried out. The results indicated that there were two types of oxide films in the dilution zone of the weld metal depending on the types of the neighboring fusion boundaries (FBs). One of the oxide films has a single crystal grain with spinel structure as internal layer which formed in the dilution zone of weld metal close to the martensite FB (area B). The other oxide film has a compact internal layer composed of nano-sized Cr-rich oxides found in the dilution zone of WM close to the sharp FB or other areas in the dilution zone (area A). Both the two types of oxide films exhibited double layer structure，regardless of the thickness of the oxide film. The single crystal grain of the internal layer of the oxide film in area B was attributed to the low Cr content of the substrate matrix, which resulted in a small number of nucleation of oxides at early stage of oxidation and the low-oxidation-rate was because of the stability of Fe-rich oxide and the protection of martensite due to galvanic corrosion.

Effect of δ-ferrite on the stress corrosion cracking behavior of 321 stainless steel

The stress corrosion cracking behavior of 321 stainless steel has been studied in high temperature water. Analytical electron microscopy was utilized to characterize the cracking process to better understand the δ-ferrite effect. Results showed that δ-ferrite inhibited the crack growth by changing the cracking direction along the γ/δ boundary and creating a highly branched crack path. δ-ferrite was also a source of Cr to facilitate the formation of protective Cr-rich oxides at the γ/δ boundaries. Ti(CN) particles in the γ/δ boundary tended to blunt the crack tip and inhibit further oxidation, and thus impeded the total crack growth.

High-temperature corrosion performance of HVAF-sprayed NiCr, NiAl, and NiCrAlY coatings with alkali sulfate/chloride exposed to ambient air

The high-temperature corrosion of high velocity air-fuel (HVAF) thermal spray Ni21Cr, Ni5Al, and Ni21Cr7AlY coatings was investigated at 600 °C for 168 h in ambient air under KCl and 50-50 mol% KCl–K2SO4 salts. Chlorination-oxidation cycle and breakdown of the corrosion products layer were the dominant corrosion mechanism in the chromia-forming Ni21Cr and Ni21Cr7AlY coatings exposed to KCl. KCl–K2SO4 was less corrosive to the chromia-forming coatings as K2SO4 was less reactive to the protective Cr-rich oxide. The alumina-forming NiAl exhibited a better corrosion performance under KCl, though it partially suffered from selective sulfidation when exposed to the mixed salt.

Cr diffusion coating to improve the corrosion resistance of an ODS steel in super-critical carbon dioxide environment

The surface of oxide dispersion strengthened (ODS) steel was deposited with Cr by physical vapor deposition, followed by inter-diffusion heat treatment (Cr-IDHTed) to form a thin outer Cr-rich carbide layer and inter-diffusion zone below it. When exposed to super-critical carbon dioxide (S-CO2) environment at 650 °C in 20 MPa for 500 h, a continuous and protective Cr2O3 layer was formed on Cr-IDHTed ODS steel, resulting in marked reduction in weight gain compared to the as-received one. On the other hand, thick Fe-rich oxides were formed on the as-received ODS steel, resulting in a significant weight gain. The as-received ODS steel showed near complete loss of tensile ductility after S-CO2 exposure due to extensive carburization in matrix. However, because of the presence of thin Cr-rich oxide layer on the surface, carburization was prevented and the change in tensile properties was minimized for Cr-IDHTed ODS steel.

Corrosion behaviors of four stainless steels with similar chromium content in supercritical carbon dioxide environment at 650 °C

The corrosion behavior of four stainless steels (SSs) with similar chromium content was investigated in supercritical carbon dioxide environment. Overall, weight gain was the smallest for 630 SS which formed thin and continuous Cr-rich oxide layer. Due to large Mn3O4 nodules, weight gain was larger for 430 SS. For 347H and 316LN SSs, thick duplex oxide layers were formed following the extensive oxide spallation, resulting in much greater weight gains than 430 and 630 SSs. The oxide spallation was explained by the formation of Nb-rich layer or amorphous matters at the matrix/oxide interface as well as the matrix structure.

Corrosion behavior of 410 stainless steel in flowing lead-bismuth eutectic alloy at 550 °C

Material compatibility and corrosion in lead-bismuth eutectic alloy (LBE) systems have been the critical issues to develop the accelerator driven systems (ADS) and the fast reactors (FR). Corrosion tests of 410 stainless steel were performed in LBE to study the corrosion behavior at 550 °C with different relative velocities. In the first part of the corrosion test, 410 stainless steel was exposed to flowing LBE with the relative velocities of 0 m/s, 1.70 m/s, 2.31 m/s, 2.98 m/s for 600 h. The second part is to immerse 410 stainless steel in the flowing LBE with the relative velocity of 2.98 m/s for 200 h, 400 h and 600 h. The results show that the oxide layer on the surface of corrosion samples is divided into an outer oxide layer, an inner oxide layer and an internal oxidation zone (IOZ). Specifically, the outer oxide layer is mainly composed of Fe3O4 and partially infiltrated Pb–Bi, the inner oxide layer is mainly made up of spinel (Fe,Cr)3O4, and the IOZ is consisting of Cr-rich oxides at the grain boundaries. Within 600 h, the corrosion degree is enhanced with the increase of corrosion time and relative velocity (0 m/s to 2.98 m/s). Moreover, the effect of prolonging corrosion time is similar to that of enhancing the flow velocity of LBE. Finally, the growth mechanism for the oxide layer of 410 stainless steel in oxygen-saturated LBE at 550 °C is proposed.

The effects of intergranular carbides on the grain boundary oxidation and cracking in a cold-worked Alloy 600

Intergranular oxidation occurred in a cold-worked Alloy 600 after exposure to pressurized water reactor primary water. Micromechanical testing was conducted to study the process of intergranular cracking under the external loading. High-resolution analytical electron microscopy was conducted to characterize the microstructure and chemistry of the oxidized grain boundaries before and after the micromechanical testing. The matrix around the intergranular carbides was preferentially oxidized and Cr in the intergranular carbides diffused out to facilitate the formation of Cr-rich oxide. Cracks preferentially propagated along the oxide-matrix interfaces during the micromechanical tests. Intergranular carbides are assumed to increase cracking resistance after cold-working.

Structure of the Oxide Films Formed on Hydrogen-Charged and Uncharged 316L Stainless Steels in Oxygenated High Temperature Water

Austenitic stainless steels (SSs) are commonly used in pressurized water reactors (PWRs) nuclear power systems as key structural materials. The corrosion resistance are closely related to the passive film formed on the surfaces of austenitic alloys in high-temperature aqueous environments. It has been reported that hydrogen can affect corrosion, localized corrosion as well as stress corrosion cracking of materials in aqueous solutions. It is necessary to investigate the influence of hydrogen on the corrosion behavior of stainless steels material in high temperature water environments. In the present study, the oxide films formed on 316L SS with uncharged (316L) and hydrogen-charged (316L-H) in oxygenated simulated PWR primary water at 300℃were investigated. Oxygen was added in high temperature water to simulated occlude region condition or faulted water chemistry. The aim of this work is to clarify the effect of hydrogen in stainless steel on the corrosion behavior. The surface morphologies of the oxide films formed on 316L (uncharged) and 316L-H (charged) specimens after exposure to oxygenated PWR primary water at 300 oC for168 h were observed. The regular and irregular oxides particles homogeneous distributed on the outer surface of 316L specimen, and compact filamentary oxides in the sub-surface. On 316L-H specimen, large oxides dispersed and inhomogeneous distributed on the outer surface, and compact minor polyhedral oxide particles packed on the sub-surface. The number of the large oxides on the outermost surface of 316L specimen was more than that on the 316L-H sample. The cross-section of the oxide films formed on 316L specimens after exposed to oxygenated PWR primary water was analyzed by FIB-TEM. The interface front between the inner oxide layer and the base metal was wavy-shaped. There were compact Cr-rich oxides in the inner layer, and filamentary (Fe, Ni)-rich oxides in the outer layer. There was slightly Ni-enrichment near the inner oxides/matrix interface and Fe-enrichment beneath the inner oxides/matrix interface from EDS line scan profile. Some discontinuous cellular oxides distributed on the inner oxides/matrix interface on 316L-H after the exposure test, which was Fe-rich oxides identified by EDS maps and line scan. Similar to the 316L specimen, the inner layer oxide was compact and Cr-edriched, showing a slightly Fe-enriched beneath the inner oxides/matrix interface, but no Ni-enrichment close to the interface of inner oxides/matrix was observed. Figure 1

Effect of microstructure evolution and phase precipitations on hot corrosion behavior of IN718 alloy subjected to multiple laser shock peening

High-temperature hot corrosion experiments of IN718 alloy subjected to multiple laser shock peening (LSP) were conducted by coating a salt mixture (75 wt% Na2SO4 + 25 wt% NaCl) on its surfaces at 700, 800 and 900 °C for 10 cycles. The effect of microstructure evolution and phase precipitations on high-temperature hot corrosion behavior of the specimens was investigated. Results showed that high-temperature hot corrosion behavior is associated with δ phase precipitations. Excessive δ phase precipitations significantly accelerate hot corrosion at 900 °C due to Na-containing oxides formation. According to corrosion kinetics, multiple LSP treated specimens exhibit lower mass loss compared with untreated specimens, indicating multiple LSP can relieve high-temperature hot corrosion. This is mainly attributed to crystal defects, which promote outward diffusion of Cr generating protective Cr-rich oxides.

Helium Tribology of Inconel 617 at Elevated Temperatures up to 950°C: Parametric Study

This study investigates the friction and wear behavior of Inconel 617, one of the primary candidate materials for high-temperature gas-cooled nuclear reactors. Using a custom-built, high-temperature tribometer, a helium (He)-cooled reactor environment was simulated up to 950°C. To obtain a comprehensive understanding of the Inconel 617 tribological response, the effects of contact load, temperature, air and He environments, sliding speed, and sliding distance were studied. From the conditions investigated, the coefficient of friction and wear values are the highest in a high-temperature He atmosphere. Scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction techniques were used to analyze the Inconel 617 oxide layer. Analysis of the samples tested in the He atmosphere showed the presence of Cr-rich oxide with a lower presence of Co-Ni-Mo compared to the samples tested in air. Characterization also revealed the existence of a very hard protective glaze layer in air while such layer was not observed in the He environment, which was associated with higher wear/friction values.

Role of M23C6 carbide on the corrosion characteristics of modified 9Cr-1Mo steel in N2-O2-CO2-SO2 atmosphere at 650 °C

In this study, the oxidation and sulfidation characteristics of modified 9Cr1Mo steels according to Cr-rich M23C6 distributed in grain boundaries in the N2-O2-CO2-SO2 atmosphere at 650 °C were analyzed. The oxidation kinetic was obeyed by the parabolic law. The oxide layer presented a three-layer structure consisting of hematite, magnetite, and FeCr spinel from the outside. In the initial oxidation stage, Cr-rich M23C6 exposed to the surface acted as nuclei of Cr-rich oxide. However, it is thought to hardly effect on the growth of the outer oxide layer. The Cr-rich M23C6 distributed in grain boundaries preferentially formed a Cr2O3 layer at the oxide/matrix interface, and internal oxidation characteristics were shown to be different depending on the presence or absence of SO2 in the atmosphere.

Characterization of oxide scales formed on heating equipment in supercritical water gasification process for producing hydrogen

The external- and internal walls of stainless steel heating equipment installed in supercritical water gasification plants for converting fossil fuels or renewable biomass to produce hydrogen-rich gases, respectively, are exposed to high-temperature air and reducing subcritical aqueous systems, all confronting severe corrosion issues. In these two harsh environments, the oxidation characteristics of typical stainless steel 304 were investigated by series of analytical methods. The oxide scale formed on stainless steel 304 in air at 550–650 °C exhibits a three-layer structure: an outmost layer consisting of Fe-rich corundum-type oxides, an inner layer comprising Cr-rich oxides and unoxidized metals, and an intermediate layer. The reducing subcritical aqueous experiments indicate that the relatively severe corrosion occurs at 350 °C rather than at 250 °C and 425 °C, which can be attributed to the reduced H+ concentration at 425 °C comparing to that at 350 °C and the increased Arrhenius rate constant of corrosion reaction with further increasing temperature from 250 °C to 350 °C. A proper pH range (6.5–10.5) for improving the corrosion resistance of Fe/Ni-based alloys in reducing subcritical aqueous environments was obtained, of which the upper limit decreases with increasing oxidicability of solutions.

Corrosion behavior of an Al added high-Cr ODS steel in supercritical water at 600 °C

Corrosion behavior in the supercritical water (SCW) of an Al added high-Cr oxide dispersion strengthened (ODS) steel was investigated in this study. The steel was exposed to SCW at 600 °C and 25 MPa for various exposure times up to 1000 h, and the resultant oxide layers were characterized by using weight change measurements, X-ray diffraction (XRD), X-ray photoelectron spectroscopies (XPS), scanning/transmission electron microscopy (SEM/TEM), and energy dispersive spectroscopy (EDS). The weight gain after exposure for 1000 h was 6.24 mg/dm2, indicating the superior corrosion resistance of 16Cr-3Al ODS steel to SCW. Chemical analysis of the surface oxides showed that the content of Cr-rich oxides increased with the increase of exposure time in SCW. Cross-sectional observation of the oxide scales revealed that an outer layer composed of (Cr, Fe)2O3 and an inner layer composed of Al2O3 formed on the sample surface. The growth of both outer and inner oxide layers followed the parabolic law with the increase of exposure times. The corrosion mechanism was discussed in detail based on the microstructure evolution process with exposure time in SCW.

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.