Formation Of Protective Scale Research Articles

Protective silicide coating fabricated on ductile VNbTa refractory complex concentrated alloy: Microstructure, high-temperature oxidation and wear behaviors

VNbTa refractory complex concentrated alloy (RCCA) possesses broad application prospects in a wide temperature window owing to its remarkable strength-ductility synergy and super rolling formability, whereas poor oxidation resistance will give rise to excessive wear above 800 ℃. In this contribution, we aim to fabricate a protective silicide coating on VNbTa RCCA for the purposes of enhancing high-temperature oxidation as well as wear resistance. Microstructural characterizations revealed that a uniform and intact coating was obtained under the optimized processing condition of pack cementation. The coating is mainly composed of a single-phase (VNbTa)Si2, along with a thin M5Si3-type interdiffusion zone. Compared with bare alloy, the coated specimens exhibit remarkable oxidation resistance at 1000 ℃ due to the preferential formation of protective SiO2 scale. Meanwhile, the coated specimens maintain desirable wear resistance from 800 ℃ to 1000 ℃. The results of this study would offer a valuable reference to expedite the future engineering applications of RCCAs.

Oxidation resistance of Cr-modified MoSi2 composites at high temperature

Alloying is an important measure to improve the oxidation resistance of MoSi2. Cr-modified MoSi2 composites were prepared by hot pressing sintering in this study, and their oxidation behaviors at different temperatures were examined by XRD, SEM and EDS. The results show that the MoSi2 matrix forms CrSi2 compound with the increase of Cr content, which is conductive to the formation of surface oxide scale. Besides, the microstructure study shows that the modified element Cr promotes the formation of SiO2 protective scale at 500 °C–900 °C. When oxidized at 1300 °C, Cr2O3 in the form of Cr oxidation dissolved in MoSi2 is dispersed in the SiO2 oxide scale. The high temperature oxidation resistance of MoSi2 composites is improved by adding Cr modified element.

Laser shock peening of AlCoCrCuFeNi high-entropy alloy fabricated by laser powder bed fusion: An enhanced oxidation-resistance mechanism at high-temperature

The high-temperature oxidation behaviors of laser powder bed fusion (LPBF)-fabricated AlCoCrCuFeNi high-entropy alloy (HEA) with and without laser shock peening (LSP) were investigated over different exposure durations and temperatures. Results revealed that LSP converted the surface stress from tensile to compressive, while also inducing grain refinement and high-density dislocations. Under high temperatures, the coupled effects of grain refinement and compressive residual stress resulted in the formation of dense protective oxidation scales, which inhibited the inward diffusion of oxygen. LSP improved the high-temperature oxidation resistance of LPBF-fabricated AlCoCrCuFeNi HEA, and the enhanced oxidation-resistance mechanisms were discussed.

Influence of the W and Ta content on the High-Temperature Oxidation Resistance of Multinary Co/Ni-Based Superalloys at 1050 °C and 1150 °C

Outstanding inherent environmental resistance is a precondition for the use of superalloys in high-temperature applications. Besides high Al and Cr levels, also refractory metal concentrations (W and Ta) are reported to affect protective scale formation, as these elements are expected to affect the chemical activity and also the transport of protective scale formers within the alloy. In this study, we elucidate the high-temperature oxidation behavior of 3 Co-based (Co/Ni ratio: 1.4) and 3 Ni-based (Co/Ni ratio: 0.7) superalloys differing in W and Ta levels. Time-resolved thermogravimetric analysis (TGA) in synthetic air at 1050 °C and 1150 °C for 100 h, scanning electron microscopy analysis (SEM), thermodynamic calculations using the CALPHAD software Thermo-Calc, and diffusion couple experiments were conducted to assess the impact of the Co/Ni ratio and the refractory metal content on the oxidation performance. The results indicate that a low W content (3 vs. 5 at.%) and a high Ta content (2.1 vs. 1.5 at.%) beneficially affect the oxidation resistance, as alumina scale formation is facilitated.

Evaluating Wagner Oxidation Criteria for Protective Al2O3 Scale Formation in Ni-Based Superalloys

An assessment is made of the Wagner transition criteria for predicting the formation of a continuous Al2O3 scale in Ni-based superalloys. Predictions are compared with data from an experimental Ni-based superalloy as well as commercial superalloys for which published data are available. The methodology was generally successful in predicting the transition temperature of the commercial superalloys but underpredicted the transition temperature of the experimental superalloy by approximately 50–100 °C. The difference in the transition temperature of the experimental superalloy to form a continuous Al2O3 scale is primarily attributed to a complex oxide subscale that increased the internal volume fraction of oxide and led to reduced oxygen ingress. The sensitivity and limitations of the methodology are discussed, and recommendations are made to refine the methodology to facilitate the interpretation of oxidation behaviour in polycrystalline Ni-based superalloys.

Challenges in computationally designing high temperature Fe-based austenitic alloys: Addressing the role of Ni additions

Alumina-forming austenitic (AFA) alloys are relatively inexpensive high performance materials which combine the creep resistance of low-cost austenitic alloys and the oxidation resistance of expensive alumina forming alloys. However, a fundamental understanding of the role of key alloying elements such as Ni, Cr, Al, Nb, Ti, V, B and C in the experimentally observed oxidation behavior of these alloys is still lacking. The present work is a first in a series of studies aiming to quantitatively describe the role of Ni in promoting or disrupting protective Al2O3 scale formation on AFA alloys. Ternary Fe-Al-xNi model alloys with three different Ni contents were isothermally exposed in an atmosphere with a low partial pressure of oxygen between 800–1000 °C for 24 h to evaluate the role of Ni in the observed internal oxidation behavior. Increasing Ni contents had no impact on the internal oxidation behavior of the alloys. The experimental and theoretical analyses in the present work suggested a negligible effect of the internal oxide precipitates on the inward diffusion of oxygen, typically expected in these systems, while simultaneously highlighting the barriers in the development of reliable models for computation-assisted design of these alloys.

Effect of silicon on the formation of protective scale on Fe-10Ni-10Cr-4Al alloy pre-oxidized at low oxygen pressure

Effect of silicon on the formation of protective scale on Fe-10Ni-10Cr-4Al alloy pre-oxidized at low oxygen pressure

Thickness Effects on Oxidation Behavior and Consequent γ’ Degradation of a High-Al Ni-Based Single Crystal Superalloy

High temperature oxidation is considered to play an essential role in the thickness debit effect on the creep rupture life of Ni-based single crystal (SC) superalloys. In order to clarify thickness effects, thin-walled specimens of different thickness (t = 0.1, 0.3, 1.0 mm) were prepared. Cyclic and isothermal oxidation tests of a high-Al Ni-based SC superalloy IC21 were carried out at 900 °C and 1100 °C in order to study the thickness effects on the oxidation behavior and consequent microstructural degradation. Thin-walled specimens of S01-N exhibited a good oxidation resistance when the protective scale-forming elements were enough in the matrix. Specimen thickness tuned the oxidation kinetics by changing the spallation behavior but had few influences on the thermodynamics. The easier stress relief via creep deformation in thin specimens is the main reason behind this phenomenon. Moreover, the obvious temperature effects on the multilayer oxide scales caused the different thickness-related spallation behaviors. Weaker microstructural degradation appeared and was further mitigated by reduced specimen thickness. Sufficient Al content is considered to be indispensable for the formation of protective α-Al2O3 scale with less degradation of matrix during the high temperature oxidation of thin-walled Ni-based SC superalloy castings.

Role of Nb in promoting protective scale formation on Al2O3-forming austenitic alloys

The high-temperature oxidation behavior of Fe–25Ni–17Cr–7.7Al–0.03Zr alloys with and without 0.5 or 1 at% Nb addition was investigated at 800 °C in air. Nb addition was found to promote the formation of an external Al2O3 scale. The Nb-free alloy formed a scale consisting of a thick Fe/Ni-rich oxide layer and a Cr2O3 layer, beneath which Al was internally oxidized. However, a continuous protective Al2O3 scale was developed below a thin Cr2O3 layer on the 1 Nb alloy, which significantly decreased the oxidation rate. Examination of the initial oxidation process during heating to 800 °C revealed that all of the alloys formed a Cr2O3 layer and an amorphous Al2O3 layer upon heating to 650 °C, but these layers became discontinuous upon further heating to 800 °C for the Nb-free alloy. The beneficial role of Nb was ascribed to its ability to maintain the initially formed thin Cr2O3 layer by the formation of a Nb-containing Cr-rich oxide layer at the interface between the Cr2O3 and amorphous Al2O3 layers. This Nb-containing Cr-rich layer prevented the breakdown of the transient amorphous Al2O3 layer until its transition to a protective crystalline Al2O3 scale.

Research Progress of Alumina-Forming Austenitic Stainless Steels: A Review

The development of Alumina-Forming Austenitic (AFA) stainless steel is reviewed in this paper. As a new type of heat-resistant steel, AFA steel forms an alumina protective scale instead of chromia in a corrosive environment. This work summarizes the types of developed AFA steels and introduces the methods of composition design. Various precipitates appear in the microstructure that directly determine the performance at high temperatures. It was found that alloy elements and the heat treatment process have an important influence on precipitates. In addition, the corrosion resistance of AFA steel in different corrosive environments is systematically analyzed, and the beneficial or harmful effects of different elements on the formation of alumina protective scale are discussed. In this paper, the short-term mechanical properties, creep properties and influencing factors of AFA steel are also analyzed. This work aims to summarize the research status on this subject, analyze the current research results, and explore future research directions.

Thermodynamic Probability Analysis of the Effects of Rb on the Corrosion Susceptibility of Cr-Containing Steels for Nuclear Materials Canisters

Rubidium (Rb) generated from the β-decay of Kr-85 has been theorized to be corrosive toward steel, specifically in the storage of Kr-85 nuclear waste streams. In the present study, the phase equilibria of RbxCryOz with Rb in dry oxygen and water are investigated to understand a possible pathway to unusual deterioration of the corrosion resistance of canister steels in the presence of Rb. It was found that, in dry oxygen environments, the accumulation of Rb (more than 0.01 mol) can completely consume the Cr in 1 mol of AISI 4130 steel by forming α-Rb2CrO4 and Rb3CrO4 and prevent the formation of protective Cr2O3 scale. In aqueous environments, RbxCryOz are metastable species. In order to investigate their role, the probability of forming various oxides is invoked in order to avoid the all-or-nothing approach to oxide formation typical of E-pH diagram, which only predicts the most stable species dissolved, ionized, or solid ionized. Thus, the probability of forming RbxCryOz was considered and reported herein. It was found RbxCryOz can possess a larger than 7% probability of forming over Cr2O3 in the Rb-rich case and 15% in the Cr-rich case, indicating that it is expected to find a small amount of RbxCryOz in the thermodynamically formed reaction products. Even though Cr2O3 is more stable than RbxCryOz, the protective Cr2O3 scale is likely to have some vulnerability to Rb, leading to one possible route for the decline in the corrosion resistance of steel canisters in aqueous environments. Therefore, from a thermodynamic perspective, the current study supports the hypothesis that Rb can thermodynamically react with Cr in steels and can lead to the formation of RbxCryOz at certain potentials and pH levels, showing the Rb influence of steel corrosion cannot be discounted. The paper considers experimental mixed potential and pH levels observed and their relationship to thermodynamic probability. From this relative corrosion resistance can be assessed in a preliminary way in aqueous environments.

Rare Earth Elements Enhanced the Oxidation Resistance of Mo-Si-Based Alloys for High Temperature Application: A Review

Traditional refractory materials such as nickel-based superalloys have been gradually unable to meet the performance requirements of advanced materials. The Mo-Si-based alloy, as a new type of high temperature structural material, has entered the vision of researchers due to its charming high temperature performance characteristics. However, its easy oxidation and even “pesting oxidation” at medium temperatures limit its further applications. In order to solve this problem, researchers have conducted large numbers of experiments and made breakthrough achievements. Based on these research results, the effects of rare earth elements like La, Hf, Ce and Y on the microstructure and oxidation behavior of Mo-Si-based alloys were systematically reviewed in the current work. Meanwhile, this paper also provided an analysis about the strengthening mechanism of rare earth elements on the oxidation behavior for Mo-Si-based alloys after discussing the oxidation process. It is shown that adding rare earth elements, on the one hand, can optimize the microstructure of the alloy, thus promoting the rapid formation of protective SiO2 scale. On the other hand, it can act as a diffusion barrier by producing stable rare earth oxides or additional protective films, which significantly enhances the oxidation resistance of the alloy. Furthermore, the research focus about the oxidation protection of Mo-Si-based alloys in the future was prospected to expand the application field.

Galvanic Interactions Between Fe Electrodes in CO2-Saturated Solutions with Different pH

Localized CO2 corrosion is a very common problem in the oil and gas industry. Severe damage of the surface is attributed to the formation, and breakdown, of protective iron carbonate (FeCO3) scales. When the corrosion layer is compromised, the difference between the open-circuit potentials of the FeCO3-covered and noncovered regions present on the same metallic surface acts as the driving force for a galvanic interaction. Depending on the area ratio of the anodic and cathodic areas, the surface could suffer severe localized damage. The present study was focused on the galvanic interactions between iron samples in solutions with different pH. CO2-saturated 1% NaCl solutions with bulk pH of between 6 and 8 and temperature ranging from 20°C and 80°C were studied. A split cell allowed for customization of different environments in each of the half cells, along with simultaneous monitoring of the galvanic current and driving force as indicated by the difference in open-circuit potential. Corrosion product layers were characterized using scanning electron microscopy, energy-dispersive x-ray spectroscopy, and x-ray photoelectron spectroscopy. The results indicated that the pH of the bulk solution plays a major role in the formation of protective FeCO3 scales. Fe exhibited passive-like behavior when immersed in a solution at 80°C with pH adjusted to 8. After reaching a passive-like behavior, Fe samples were cathodic when coupled to samples immersed in a solution with lower pH. The magnitude of the galvanic current increased with increasing pH gradient.

Promoting Protective Scale Formation at Lower Temperatures via Surface Finishing: Effects on the Establishment, Structure, and Chemistry in Haynes 214 High-Temperature Oxidation-Resistant Nickel Alloy

Promoting Protective Scale Formation at Lower Temperatures via Surface Finishing: Effects on the Establishment, Structure, and Chemistry in Haynes 214 High-Temperature Oxidation-Resistant Nickel Alloy - Volume 27 Issue S1

Oxidation behaviors of compositionally complex MAX phases in air

The oxidation behaviours of a newly synthesized MAX phase composite mainly containing a multi-component 413 MAX phase with Ti, Nb and Ta equally and evenly distributed at M site were investigated at 1000–1400 °C in air. Results indicated that the multi-component MAX phase exhibited superior oxidation resistance compared with traditional monolithic 413 MAX phases such as Nb4AlC3 and Ta4AlC3. Dense and passivating Al2O3 layers that formed at the interfaces between the substrate and the oxidation scale is the origin of the high oxidation resistance. The presence of Cr–Al alloy phases is essential for the formation of protective Al2O3 scale.

Improved oxidation resistance through an in-situ formed diffusion barrier: Oxidation behavior of amorphous multi-component FeCrAlMoSiY-coated Zr in high-temperature steam

Long-term reliability of some systems for high-temperature applications has been hindered by intensive coating-substrate interdiffusion. Herein, we report a novel column-free multi-component FeCrAlMoSiY coating sputtered on zircaloy. During the 1200 °C steam oxidation, the Si atoms preferentially migrated to the coating-substrate interface to form a continuous Zr2Si-rich diffusion barrier to inhibit both inward diffusion of the coating and outward of substrate elements. Therefore, the formation of protective α-Al2O3 scale has been promoted, which effectively prevented the underlying coating and substrate being oxidized. Formation of the in-situ diffusion barrier and oxidation protection mechanisms of this coating were then discussed.

Aluminide coating on Mar-M246 nickel superalloy by halide activated pack cementation (HAPC)

Nickel based superalloys are widely applied in high temperatures systems because they have excellent mechanical properties. However their use over long period can lead to excessive degradation oxidation and corrosion. Protective aluminum-based coatings increase the oxidation resistance of these alloys which may be achieved using the Halide Activated Pack Cementation (HAPC) technique. In this study, the Mar-M246 nickel superalloy was coated with aluminum rich intermetallic layers, obtained by the HAPC process at 700 °C, 800 °C, 900 °C and 1000 °C, for treatment duration of 9 h. Thermodynamic calculations have been carried out previously to determine process parameters, such as the choice of the activator and the suitable temperature. The microstructural characterization of the coatings was performed by SEM and EDS analysis. The coatings obtained were uniform and homogeneous in terms composition, and free of failures and cracks containing Ni2Al3 and/or NiAl3 phases. The efficiency of the coatings to enhance the oxidation resistance of the substrate was proved through the oxidation tests carried out at 1000 °C for about 240 h in which there was the formation of protective Al2O3 scale.

High-Temperature Corrosion Behaviour of CNT-reinforced Zirconium Yttrium Coatings on Boiler Tube Steel in Coal-Fired Boiler of Thermal Power Plant

Corrosion behaviour of carbon nanotubes (CNT)-reinforced zirconium yttrium coatings at high temperature under cyclic thermal loading has been investigated in actual coal-fired boiler environment of thermal power plant. The amount of CNT was varied from 0.5 to 6 wt.%, and the coating was developed by the plasma spray technique. The comparative effects of variation in CNT content on hot corrosion behaviour were studied by weight change measurements. The corroded products were then analysed with XRD, FE-SEM with EDAX and X-sectional analysis techniques. The results revealed that the variation in CNT percentile enhances the hot corrosion resistance of boiler steel in actual boiler environment at elevated temperature. The reinforced coatings showed the lower weight gain measurements with the formation of protective scale of oxides during the experiment. Corrosion rate was observed to be reduced with the increased content of CNT in the coating composite.

The Effect of Nickel and Aluminum Concentration on Cyclic Oxidation Resistance of Mild Steel (AISI 1020)

Experimental studies have been made to determine the oxidation be heavier of several coatings system on commercial mild steel alloy (AISI 1020). The oxidation kinetics of a single stage coating such as Aluminized and Nickel have been under )1000°C( using a thermal cyclic oxidation.The structures were studied using optical microscope to identify a phases. Most coating use to enhance the formation of protective oxid scale, in this research, the results for Nickel coating showed some reduction in resistance of )AISI 1020( alloy and It is found they follow linear rate law, but Aluminized coating using a pack – cementation technique, exhibited the greatest resistance to oxidation, followed parabolic law (protective oxidation behavior). because it has a good ductility and its capability to form a protective oxid Layer )Al2O3(, such Alumina scale is known as thermally and electrically insulator and characterized with slow gowning rate.

High-temperature oxidation behavior of a Ti5Si3-incorporated MoSiBTiC alloy

The oxidation behavior of a Ti5Si3-incorporated MoSiBTiC alloy (38Mo–30Ti–17Si–10C–5B, at.%) was systematically studied in the temperature range of 700–1000 °C. Temperature dependence of oxidation kinetics and oxide scale formation was revealed for this alloy. The oxidation kinetics at 700 °C featured an initial transient stage of weight gain and then switched to weight loss. At 800 °C, the oxidation kinetics featured an initial transient stage with fast weight loss, a steady stage with small weight loss, and an acceleration stage leading to catastrophic failure. At 900 and 1000 °C, the alloy exhibited a rapid transient weight loss as followed by a decreased weight loss fitting to parabolic kinetics. The scales that formed at 700–1000 °C mainly consisted of rutile TiO2 and amorphous SiO2. The high viscosity of SiO2 at 700 and 800 °C caused the formation of porous and microcracked oxide scales while the occurrence of viscous flow during oxidation at 900–1000 °C promoted the formation of dense and protective scales.