Effect of Water Density/Pressure on the Corrosion Behavior of 304 and 310 Stainless Steels

Two stainless steels, AISI 304 and 310, are evaluated for their oxidation behavior in low-pressure steam (0.1 MPa), subcritical water (8 MPa) and supercritical water (29 MPa) at 625 °C for 1000 h. The water density is found to have a significant effect on both the weight change per unit surface area, oxide structure and thickness. Under low-pressure steam condition, very little weight change and limited oxide scale formation are observed on both steels while exposure to subcritical water results in excessive oxide formation and weight gain. With further increase in pressure to supercritical condition, a denser oxide layer near the 310 substrate is formed, decreasing the oxidation rate. However, this oxidation decreased is not significant on 304 tested under supercritical condition. No proportional relationship between the oxidation rate and water pressure is observed.     

Effect of Cobalt Content on the Oxidation and Corrosion Behavior of Ni–Fe-Based Superalloy for Ultra-Supercritical Boiler Applications

The oxidation and corrosion behavior of three model alloys with different cobalt contents (6–20 wt%) were investigated in static air and a simulated coal ash/gas environment at 750 °C. The model alloys follow a parabolic law approximately during the oxidation in static air. High cobalt level improves the oxidation resistance, however, without noticeable improvement in coal ash/gas corrosion resistance. The sample with the highest cobalt content grows the thinnest oxide layer and the fewest internal oxidation products in the oxidation test. Cobalt in the model alloys promotes the establishment of a protective chromium oxide scale during the oxidation test, but did not show much difference in restraining the inward diffusion of sulfur by increasing its content. The oxidation and corrosion products formed on the sample surface consist mainly of a protective chromium oxide film. Internal aluminum oxide particles have been found especially along the grain boundaries of the base alloy.     

Effect of SiC Addition on Oxidation Behavior of ZrB<Subscript>2</Subscript> at 1273 K and 1473 K

The oxidation behavior of ZrB₂–SiC composites with different contents of SiC addition was investigated at 1273 and 1473 K in air for 12 h in this study. The SiC addition contents ranged from 0 to 30 wt%. The results showed that when ZrB₂–SiC composites were oxidized at 1273 K in air, a two-oxide layer-structure forms: a continuous glassy layer and a ZrO₂ layer contained unoxidized SiC. When SiC content is 5 and 10 wt%, the glassy layer is mainly composed by B₂O₃. When SiC content is 20 and 30 wt%, a borosilicate glass could be formed on the top layer, which could improve the oxidation resistance of ZrB₂. When ZrB₂–SiC composites were oxidized at 1473 K in air, the oxide layer was composed of ZrO₂ and SiO₂ and unreacted SiC. Additionally, when SiC addition content was higher than 10 wt%, a continuous borosilicate glass layer could be formed on the top of the oxide layer at 1473 K. With the increase of SiC content in ZrB₂, the oxide layer thickness decreased at both 1273 and 1473 K.     

Effect of Dilute Tellurium and Selenium Additions on the High-Temperature Oxidation Resistance of Copper Alloys

The high-temperature oxidation resistance of Cu–Te–Se alloys and Cu–Se alloys at 300, 400, 500 and 600 °C was studied by measuring weight gain per unit area after fixed oxidation times. The morphologies of the oxide scales formed were observed using a scanning electron microscope, with the distribution of element detected by an energy-dispersive spectrometer, and the phases identified using X-ray diffraction. The focus of this study was to understand the effects of tellurium (Te) and selenium (Se) additions on the high-temperature oxidation resistance of copper alloys. At the dilute levels studied (≤0.5 wt% total), these elements underwent internal oxidation. Meanwhile, new phases formed, which made oxidation films more compact and increased the adherence between the oxide film and the alloy matrix, as well as prevented oxygen diffusing in the copper alloy matrix, so the oxidation resistance of copper alloys was improved.     

Degradation Behavior of Nanostructured Coatings Deposited by High-Velocity Arc Spraying Process in an Actual Environment of a Coal-Fired Boiler

FeCr-based nanostructured coatings were deposited on a 301S stainless steel substrate by the high-velocity arc spraying process in the current work. The oxidation behavior of the coatings exposed to elevated temperatures (700°C and 900°C) under laboratory conditions as well as in an actual industrial environment of a coal-fired boiler (at 700 ± 10°C) was investigated. X-ray diffraction, scanning electron microscopy/energy-dispersive analysis, and transmission electron microscopy techniques were used to characterize the coating as well as to analyze the corrosion products for elucidating the corrosion mechanisms. The microhardness of the coating was found to be 520–1100 HV. The (FeCr)-based nanostructured coating showed good adherence to the 310S substrate and excellent oxidation resistance during the exposures with no tendency for spallation of its oxide scales in both environments. The nanosized grain morphology of the coating facilitated the formation of protective scales, which is continuous, adherent, and nonporous due to the higher diffusivity of alloying elements in the coatings. It precludes high-temperature oxidation by acting as a diffusion barrier between the environment and the coating.     

Optimization of Cr-Content for High-Temperature Oxidation Behavior of Co–Re–Si-Base Alloys

The oxidation behavior of Co-17Re-xCr-2Si alloys containing 23, 25, 27 and 30 at.% chromium at 1,000 and 1,100 °C were investigated. Alloy Co–17Re–23Cr–2Si showed a poor oxidation resistance during exposure to laboratory air forming a two-layer external scale and a very thin discontinuous Cr₂O₃ layer at the oxide/substrate interface. The outer layer of the oxide scale consisted of CoO, whereas the inner layer was a porous mixture of CoCr₂O₄ spinel particles in a CoO matrix. The oxide scale was found to be non-protective in nature as the vaporization of Re-oxide took place during oxidation. An increase of chromium content from 23 at.% to 25 at.% improved significantly the alloy oxidation resistance; a compact protective Cr₂O₃-scale formed and prevented the rhenium oxide evaporation. The oxidation behavior of alloys containing 27 at.% and 30 at.% chromium were quite similar to that of Co–17Re–25Cr–2Si. The oxidation mechanism for Co–17Re–25Cr–2Si alloy was established and the subsurface microstructural changes were investigated by means of EBSD characterization.     

Dependence on the Chromium Content of the High-Temperature Oxidation Behavior of Ta-Rich Nickel-Based Cast Alloys

The high-temperature stability of primary tantalum carbides is a problem of importance for chromium-rich cast alloys, based on cobalt or nickel. The focus of this study was nickel-based alloys, as these alloys are particularly sensitive to a lack of TaC in the as-cast state and by dissolution due to high-temperature exposure. In this work, a possible way for promoting the formation of many TaC precipitates by changing from the usual 30 wt% chromium content was investigated. Five alloys with Cr content varying from 10 to 50 wt% were prepared and then subjected to microstructure characterization and to oxidation tests. In contrast with what was expected, decreasing the Cr content in comparison with the Ni–30Cr–0.4C–6Ta reference alloy did not succeed in obtaining more TaC precipitates, but instead had the opposite effect. Concerning the high-temperature oxidation behavior at 1127 and 1237 °C, loss of resistance was observed only for a Cr content at the lower level of 10 wt%. It was noticed that a subscale CrTaO₄ developed during oxidation and seemed to promote oxide spallation during cooling.     

Preparation of Self-Healing α-Al2O3 Films by Low Temperature Thermal Oxidation

This paper reports a new approach to lowering the temperature necessary for the preparation of α-Al₂O₃. Oxidation of Al–Cr alloys, with Cr contents of 18, 23 and 27 %, was performed at temperatures ranging from 620 to 720 °C in air for 100 h. The resulting oxide films were analyzed by SEM, EDS, XRD and XPS. The results showed that α-Al₂O₃ films were obtained following oxidation of the 18 and 23 wt% Cr alloy samples at 720 °C and that rough surfaces were conducive to the formation of α-Al₂O₃ such that peened surface samples showed significant α-Al₂O₃ growth while polished samples showed no oxide by XRD. A 23 wt% Cr sample with a roughened surface exhibited the formation of α-Al₂O₃ at a temperature of 670 °C. Conversely, only a very thin oxide film was observed on a 27 wt% Cr sample after oxidation at 720 °C.     

Long-Term Oxidation of Candidate Cast Iron and Stainless Steel Exhaust System Alloys from 650 to 800 °C in Air with Water Vapor

The oxidation behavior of candidate cast irons and cast stainless steels for diesel exhaust systems was studied for 5,000 h at 650–800 °C in air with 10 % H₂O. At 650 °C, Ni-resist D5S exhibited moderately better oxidation resistance than did the SiMo cast iron. However, the D5S suffered from oxide scale spallation at 700 °C, whereas the oxide scales formed on SiMo cast iron remained relatively adherent from 700 to 800 °C. The oxidation of the cast chromia-forming austenitics trended with the level of Cr and Ni additions, with small mass losses consistent with Cr oxy-hydroxide volatilization for the higher 25Cr/20–35Ni HK and HP type alloys, and transition to rapid Fe-base oxide formation and scale spallation in the lower 19Cr/12Ni CF8C plus alloy. In contrast, small positive mass changes consistent with protective alumina scale formation were observed for the cast AFA alloy under all conditions studied. Implications of these findings for exhaust system components are discussed.     

Oxidation Behavior of Fe–25Cr–20Ni–2.8Si During Isothermal Oxidation at 1,286 K; Life-time Prediction

The oxidation behavior of an austenitic steel, type 1.4841, with a Cr content of 25 wt% and a high-Si content of 2.8 wt% was studied during isothermal oxidation at 1,286 K in air. A thick, crystalline Cr₂O₃ layer, on top of a much thinner, amorphous SiO₂ layer, developed on the alloy substrate. After formation of a closed Cr₂O₃ scale, parabolic growth kinetics prevailed as long as the associated constant, steady-state Cr concentration in the alloy at the substrate/oxide interface of about 13 ± 1 wt% was maintained. Upon prolonged oxidation, successive cracking and spallation of the thickening oxide scale eventually led to breakaway oxidation, because the “bulk-”Cr concentration in the interior of the alloy dropped below the critical value required to ‘heal’ the protective oxide layer after oxide spallation. Application of a lifetime prediction model of the alloy substrate under isothermal oxidation conditions allowed determination of the breakaway-oxidation time as a function of alloy-sheet thickness, by employing the Cr volume-diffusion coefficient in the alloy and the parabolic growth-rate constant, both determined in the present study by fitting calculated to experimental Cr-depletion profiles for various oxidation times.     

High Pressure Steam Oxidation of Alloys for Advanced Ultra-Supercritical Conditions

A steam oxidation test was conducted at 267 ± 17 bar and 670 °C for 293 h. A comparison test was run at 1 bar. All of the alloys showed an increase in scale thickness and oxidation rate with pressure, and TP304H and IN625 had very large increases. Fine-grained TP304H at 267 bar behaved like a coarse grained alloy, indicative of high pressure increasing the critical Cr level needed to form and maintain a chromia scale. At 267 bar H230, H263, H282, IN617 and IN740 had k_p values a factor of one–to-two orders of magnitude higher than at 1 bar. IN625 had a four order of magnitude increase in k_p at 267 bar compared to 1 bar. Possible causes for increased oxidation rates with increased pressure were examined, including increased solid state diffusion within the oxide scale and increased critical Cr content to establish and maintain a chromia scale.     

Tantalum Oxidation in Steam Atmosphere

The oxidation behavior of tantalum in steam was investigated in the temperature range of 600–1100 °C. Thermogravimetric measurements were used to determine the oxidation behavior of the Ta metal at different steam partial pressures (10, 50 and 100 kPa). As a result of the oxidation tests, the samples show parabolic behavior for the initial period of oxidation, which is almost not visible at temperature higher than 700 °C. After the short parabolic kinetics, breakaway occurs and turns the kinetics from parabolic to linear. In this work kinetic parameters of the linear oxidation were determined. The influence of the samples´ shape was investigated using both plate and cylinder specimens. Experiments aimed at quantifying the hydrogen uptake of tantalum were conducted at temperatures between 600 and 1000 °C, and results differ from the expected ones applying the Sieverts’ law. Post-test optical microscopy was performed in order to analyze the post-oxidation appearance of the materials. The oxide scale appeared non-protective, especially at low temperature.     

Effects of Temperature and Straining on the Oxidation Behavior of Electrical Steels

The effect of Si content (in the range of 0.01–1.91 wt%) on scale formation of electrical steels in dry air at temperatures ranging from 850 to 1200 °C was investigated. The effect of applied tensile strain on oxidation behavior was also explored. A thermo-mechanical simulator (Gleeble machine) was employed to conduct the oxidation tests at different load conditions. The experimental results showed that at 1000 °C the oxidation rate decreased with increasing Si content in the steel. The formation of an inner scale, mainly consisting of amorphous silica, was responsible for the improved oxidation resistance. However, a substantial increase in oxidation rate due to the formation of molten eutectic fayalite (Fe₂SiO₄) was observed when the temperature was raised to 1200 °C. Under straining conditions at a very short oxidation time, the inner scale structure was slightly modified though the scale thickness remained almost unchanged for the steel containing 1.91 wt% Si.     

Cyclic Oxidation Behavior of the Ti–6Al–4V Alloy

The cyclic oxidation behavior of the Ti–6Al–4V alloy has been studied under heating and cooling conditions within a temperature range from 550 to 850 °C in air for up to 12 cycles. The mass changes, phase, surface morphologies, cross-sectional morphologies and element distribution of the oxide scales after cyclic oxidation were investigated using electronic microbalance, X-ray diffractometry, scanning electron microscopy and energy dispersive spectroscopy. The results show that the rate of oxidation was close to zero at 550 °C, obeyed parabolic and linear law at 650 and 850 °C, respectively, while at 750 °C, parabolic—linear law dominated. The double oxide scales formed on surface of the Ti–6Al–4V alloy consisted of an inner layer of TiO₂ and an outer layer of Al₂O₃, and the thickness of oxide scales increased with an increasing oxidation temperature. At 750 and 850 °C, the cyclic oxidation resistance deteriorated owing to the formation of voids, cracks and the spallation of the oxide scales.     

Microstructural Characteristics of Oxide Films Grown on Zr–0.7Sn–1Nb–0.03Fe–0.2Ge Alloy Corroded in Lithiated Water at 360 °C

The corrosion resistance of a Zr–0.7Sn–1Nb–0.03Fe–0.2Ge (wt%) alloy was investigated by autoclave test in lithiated water with 0.01 M LiOH at 360 °C under a pressure of 18.6 MPa. The microstructure of the oxide film which formed was examined by TEM and SEM. The results revealed that there were a few micro-cracks and more ZrO₂ columnar grains in the oxide film formed after exposure for 190 days. The oxidation of second-phase particles (SPPs) was slower than that of α-Zr matrix. The c-ZrO₂ was observed around the [Zr–Nb–Fe–Cr–Ge]O SPPs. The amorphous phase produced around the [Zr–Nb–Fe–Cr–Ge]O SPPs could relax the stress in the oxide film. The addition of Ge can reduce micro-pores and micro-cracks formed in oxide film, and delay the microstructural evolution from columnar grains to equiaxed grains. Therefore, the addition of Ge can improve the corrosion resistance of the Zr–0.7Sn–1Nb–0.03Fe alloy.     

Effect of Pressures on the Corrosion Behaviours of Materials at 625°C

The corrosion behaviors of austenitic stainless steels (SS) 310, 304 and Ni- and Fe-based A-286 exposed to 0.1 MPa, 8 MPa and 29 MPa at 625°C for 1000 h were investigated. These represent exposure to superheated steam, subcritical and supercritical water (SCW) at 625°C, respectively. As SS 310 showed the smallest weight change, the oxide cross-sections made from 310 samples were examined by transmission electron microscopy. The results revealed a single-layer oxide at 0.1 MPa and dual-layer oxides at 8 MPa and 29 MPa, followed by a Cr-depleted region into the austenite substrate. The compositions of the inner oxides at 8 MPa and 29 MPa are Cr-rich and largely similar to those of the single-layer oxides at 0.1 MPa exposure. These results suggest that corrosion testing in superheated steam may be a suitable surrogate for scoping tests of materials under SCW conditions at >650°C.     

Microstructure,hot oxidation resistance and damping capacity of Al- alloyed cast iron

Ductile cast iron was developed by adding Aluminium (up to16 wt%Al). Effects of aluminium on microstructure, hardness, oxidation resistance at high temperature and damping capacity of cast iron were studied. Adding Al up to 4.0 wt% raised degree of graphitisation. More than 4.0 wt% up to 13.8 wt%Al showed precipitation of hard carbides. Hardness was increased signi?cantly by increasing Al due to formation of intermetallic compounds. More than 5.8 wt%Al showed a higher oxidation resistance due to formation of complex Fe-Al oxide, acted as a protective and adherent scale. Damping capacity of 2.6 and 4.0 wt%Al was increased as the volume fraction of graphite increased as well as the change of graphite from spheroidal to compacted ones. Damping capacity of 16 wt%Al was enhanced due to high ferrite (86%) containing graphite. However, decreasing of ferrite and disappearing of graphite as well as presence of carbides highly deteriorated damping capacity of 5.8, 9, 12.8 and 13.8 wt%Al cast iron.     

Influence of Aluminum and Rhenium on the Isothermal Oxidation Behavior of CoNiCrAlY Alloys

The oxidation behavior of a Co32Ni21Cr8Al0.6Y (wt%) alloy with and without the addition of 3.5 wt% rhenium, 2 wt% aluminum or a combination of the two was investigated at 1000 °C. Results showed that increasing the Al content from 8 to 10 wt% led to an increase of the alloy ??-phase, but did not affect the oxidation behavior. Re addition induced (Cr,Re,Y)-rich phase to precipitate in the alloy, accelerated the ??- to ??-alumina transformation, reduced the oxidation rate and enhanced the rate of alloy Al diffusion. Adding both Al and Re further improved the oxidation behavior by promoting the development of the external alumina scale and suppressing the formation of Ni, Co containing spinel. This alloy also showed the largest reduction of oxidation rate and emerged to be the most beneficial. A continuous Cr?CRe rich layer was observed at the oxide/alloy interface of the Re, Al containing alloy after longer oxidation times, but this layer is not expected to affect the continued growth of the alumina scale.     

A Study on Effect of Reactive and Rare Earth Element Additions on the Oxidation Behavior of Mo–Si–B System

Mo–9Si–8B–1Ti, Mo–9Si–8B–1.8Ti, Mo–9Si–8B–0.2La and Mo–9Si–8B–0.4La₂O₃ (at.%) alloys were prepared using mechanical alloying followed by hot isostatic pressing and field assisted sintering. XRD, SEM and EBSD analysis confirmed the formation of Mo solid solution, A15 and T2 phases in the alloys. Isothermal oxidation behavior of the specimens was studied in the temperature range from 750 to 1,300 °C for up to 100 h. Both the Ti and La containing alloys showed superior oxidation behavior compared to unalloyed Mo–Si–B at 900 °C at the initial periods of oxidation. Ti-added alloys suffered higher rate of weight loss at higher temperatures (1,000–1,300 °C) due to the formation of non-protective low viscosity SiO₂-TiO₂-B₂O₃ scale. La-alloyed Mo–Si–B showed superior oxidation resistance at intermediate temperatures (900 °C) as well as at higher temperatures. Enrichment of La at the oxide/alloy interface was found to be the reason for improved oxidation behavior of La-alloyed Mo–Si–B. Amongst the four materials studied, the La₂O₃ containing alloy showed the best oxidation resistance at 900 °C.     

Roles of Mn in the High-Temperature Air Oxidation of 9Cr Ferritic–Martensitic Steel After Severe Plastic Deformation

Oxidation resistance of 9Cr ferritic–martensitic steel before and after cold-swaging process in air at 923 K was compared, and the oxide scales were characterized in detail by secondary ion mass spectrometry (SIMS). Both the cold-swaged sample and post-annealed samples showed considerably greater oxidation resistance than the initial as-tempered sample. Mn played a key role in scale formation of the cold-swaged sample and post-annealed sample; although the Mn content was only 0.5 wt% in the matrix, formation of Mn oxide (MnCr₂O₄ and Mn₂O₃) was favored. Roles of Mn in the oxidation behaviors of the cold-swaged sample and post-annealed sample were discussed.