Effect of temperature on corrosion behavior of alloy 690 in high temperature hydrogenated water

The influence of temperature on the corrosion behavior of Alloy 690 is evaluated using potentiodynamic polarization curves,electrochemical impedance spectra(EIS),scanning electron microscopy(SEM),Xray photoelectron spectroscopy(XPS) and transmission electron microscopy(TEM).The corrosion rate of Alloy 690 reaches a local maximum at 250?C.The kinetic control step of the growth of oxide film changes from the diffusion process of aqueous-phase ions to the growth of Cr-rich barrier layer in the temperature range of 200–300?C.A modified double-layer model is proposed to describe the effect of temperature on the structure and composition of the oxide film.     

Oxidation behaviour and microstructural evolution of FeCrAl ODS alloys at high temperature

Abstract

The oxidation behaviour of a commercial oxide dispersion strengthened (ODS) FeCrAl alloy (MA 956) and of a dispersion-free FeCrAl alloy (APM) were studied during isothermal exposures in air between 1,100°C and 1,300°C. After short heat treatments, the oxide film developed on MA 956 was more protective than that developed on APM. Longer isothermal exposures were conducted on MA 956 in order to examine the long-term behaviour of the oxide layer and the microstructural evolutions of the alloy. Chemical analyses of the substrate for increasing treatment duration revealed a continuous decrease of the matrix aluminium content due to oxide growth. After 4 months at 1,300°C, aluminium content of the alloy decreased to a critical value below which a continuous and protective oxide film could no longer be formed. Longer exposure times were carried out in order to relate the different stages of the catastrophic breakaway oxidation and identify the parameters involved in this phenomenon. Other substrate evolutions like cavity growth and nature and morphology changes of the Y₂O₃ particles were observed and are discussed in relation with the oxidation behaviour.     

Proton irradiation assisted localized corrosion and stress corrosion cracking in 304 nuclear grade stainless steel in simulated primary PWR water

Localized deformation and corrosion in irradiated 304 nuclear grade stainless steel in simulated primary water were investigated.The investigation was conducted by comparing the deformation structure,the oxide scale formed at the deformation structure,and their correlation with cracking.The results revealed that increasing the irradiation dose promoted localized corrosion at the slip step and grain boundary,which was primarily attributed to the strain concentration induced by enhanced localized deformation and depletion of Cr at grain boundary.Further,a synergic effect of the enhanced localized deformation and localized corrosion at the slip step and grain boundary caused a higher cracking susceptibility of the irradiated steel.     

容错事故燃料包壳用FeCrAl合金的研究进展EI北大核心CSCD

福岛事故后,人们迫切需要开发相应的燃料包壳材料以忍受严重事故发生时的极端工况,从而提高核电站的事故承受能力。尽管FeCrAl合金的宏观中子吸收截面要远远高于锆合金,但其在严重事故下良好的耐腐蚀性、优越的高温力学性能及抗辐照损伤能力,使其被列为事故容错燃料包壳的候选材料之一。然而,现有FeCrAl合金难以满足核电站用材料的要求,因此需对其进行优化,以获得更佳的性能。本文系统总结了近年来关于优化后FeCrAl合金的腐蚀行为、力学性能、辐照后的微观结构及力学性能变化、焊接性及加工性等方面的研究进展,分析了FeCrAl合金的高温腐蚀机理以及引起FeCrAl合金微观结构及力学性能变化的主要原因,提出了FeCrAl合金在高温腐蚀、焊接性以及加工性等过程中存在的主要问题以及未来的研究方向。     

Viscoplastic behavior of a FeCrAl alloy for high temperature steam electrolysis (HTSE) sealing applications between 700 °C and 900 °C

High temperature steam electrolysis (HTSE) is one of the most promising technologies for the industrial production of hydrogen. However one of the remaining problems lies in sealing at high temperature. The reference solution is based on glass seals which presents several drawbacks. That explains why metallic seals are under development. The expected seal will be submitted to creep under low stresses between 700 °C and 900 °C, possibly involving complex loading and thermal history. The candidate material investigated in this work is a FeCrAl (OC404, Sandvik) supplied as a 0.3 mm thick sheet. The ability of this material to develop a protective layer of alumina was studied first, as well as grain size growth during thermal ageing. Creep and tensile tests were performed between 700 °C and 900 °C to determine its mechanical properties. This database was used to propose and identify an elasto-viscoplastic behavior for the material. Creep was described by the Sellars-Tegart law. This law was then used to simulate and predict creep indentation tests performed in the same range of temperatures.     

Evaluation of temperature effect on the corrosion process of 304 stainless steel in high temperature water with electrochemical noise

The effect of temperature on corrosion process of 304 stainless steel (SS) in high temperature water was investigated by electrochemical noise (EN), scanning electron microscope (SEM), Raman spectrum and X-ray photoelectron spectroscopy (XPS). The experimental results showed that the corrosion process could be divided into two stages (passivity and active dissolution) with the increasing temperature. At 100 °C, the oxide film was a single layer mainly consisting of Cr₂O₃. However, at 250 °C, it became a double layer with an inner layer of Cr–Fe spinel compound and an out precipitated layer. The related growth mechanisms of the oxide film were also discussed.     

Stress corrosion crack initiation in Alloy 690 in high temperature water

Initiation of stress corrosion cracks in Alloy 690 in high temperature water is a rare occurrence and depends on the method by which the sample is loaded. Only in dynamic straining experiments is crack initiation consistently observed. Stress relaxation in constant deflection tests, and lack of a means of rupturing the oxide film in constant load tests are the principle reasons for the difficulty of initiating cracks in these tests. These observations, combined with those from the much more susceptible Alloy 600 form the basis for a mechanism stress corrosion crack (SCC) initiation of Alloy 690. SCC initiation is proposed to occur in three stages: an oxidation stage in which a protective film of Cr₂O₃ is formed on the surface over grain boundaries, an incubation stage in which successive cycles of oxide film rupture and repair depletes the grain boundary of chromium, and a nucleation stage in which the chromium depleted grain boundary is no longer able to support growth of a protective chromium oxide layer, resulting in formation and rupture of oxides down the grain boundary. The mechanism is supported by the available literature on oxidation and crack initiation of Alloy 690 in hydrogenated primary water conditions.     

Effect of acetic acids on the corrosion crack growth of 3.5NiCrMoV steels in high temperature water: synergistic interaction between stress corrosion and corrosion fatigue

Stress Corrosion Cracking (SCC) tests (pH: 3 ~ 5) and Corrosion Fatigue (CF) tests (R = 0.2, 0.1 Hz) were conducted to evaluate the effect of acetic acid on the corrosion crack growth behavior in high temperature water at 150°C. Acetic acid significantly influenced the corrosion fatigue cracking behavior of turbine disc steels in high temperature water. The CF crack growth rates of turbine disc steels increase until the organic acid concentration reaches a critical saturation value (between pH 4 and pH 3) because of the crack tip sharpening. Below the critical value of pH, the CF crack growth rates decreases because of the crack tip blunting. The corrosion fatigue crack growth rate is accelerated by the interaction of the fatigue and the stress corrosion in the test environment. The synergistic interaction should be accounted for in the realistic prediction of the corrosion fatigue life of turbine steel (3.5NiCrMoV steels) in high temperature water of acetic acid solution. With the high temperature corrosion fatigue data obtained in this study, it is possible to assess the life of turbine components in high temperature and high pressure.     

High temperature corrosion performance of Q235 steel in sulfur-bearing solution

The high temperature corrosion performance of Q235 steel in sulfur-bearing solutions as a function of temperature, test time, and sulfur content was investigated by weight loss measurements in this study. The results indicate that the corrosion behavior of Q235 steel in sulfur-bearing solution is directly related to the experimental temperature, immersion time and sulfur content. Higher temperature leads to a higher corrosion rate of the steel. The corrosion rate increases during the initial test time and then decreases with increasing the test time. As the sulfur content increases in the solution, the corrosion rate increases, and further increasing the sulfur content leads to a decreased corrosion rate.     

As-cast microstructures and behavior at high temperature of chromium-rich cobalt-based alloys containing hafnium carbides

Hafnium is often used to improve the high temperature oxidation resistance of superalloys but not to form carbides for strengthen them against creep. In this work hafnium was added in cobalt-based alloys for verifying that HfC can be obtained in cobalt-based alloys and for characterizing their behavior at a very temperature. Three Co–25Cr–0.25 and 0.50C alloys containing 3.7 and 7.4 Hf to promote HfC carbides, and four Co–25Cr– 0 to 1C alloys for comparison (all contents in wt.%), were cast and exposed at 1200 °C for 50 h in synthetic air. The HfC carbides formed instead chromium carbides during solidification, in eutectic with matrix and as dispersed compact particles. During the stage at 1200 °C the HfC carbides did not significantly evolve, even near the oxidation front despite oxidation early become very fast and generalized. At the same time the chromium carbides present in the Co–Cr–C alloys totally disappeared in the same conditions. Such HfC-alloys potentially bring efficient and sustainable mechanical strengthening at high temperature, but their hot oxidation resistance must be significantly improved.     

The influence of chromium on early high temperature corrosion of ferritic alloys under SO2 atmosphere

Comprehensive insights into the early stages of corrosion mechanisms provide fundamental knowledge to further understand and model long time material behaviour. The present work studies the early stages of combined oxidation and sulphidation of ferritic model alloys for time scales up to 250 h at 650°C to observe the influence of chromium during the corrosion under SO₂. Model alloys were used to focus on the reaction of the intended elements: Fe, Cr, S, and O. Pure iron simultaneously forms magnetite and iron-sulphide in an early stage of corrosion, covered by a pure oxide layer after 100 h. Iron with 13 wt-% Cr shows hematite and mixed Fe–Cr-oxides first, before sulphides nucleate in the inner corrosion zone. With increasing ageing time a magnetite layer is observed below the hematite layer. Quantitative phase fractions of all corrosion products observed were determined from cross section images. Characterisation of the Fe13Cr corrosion scale by FIB revealed a highly porous structure in the inner corrosion zone where Cr-rich (Fe, Cr)-sulphides are present, and caused the scale to spall easily.     

Stress corrosion and corrosion fatigue of metals in high temperature aqueous environments

The dissolved oxygen content of water influences the susceptibility of austenitic stainless steels and reactor pressure vessel steels to environment assisted cracking by raising the electrochemical potential. The relevance of this fact to the integrity of welded stainless steel constructions, deaerator vessels and reactor pressure vessels steels is discussed. In particulal, the use of slow strain rate test procedures involving applied electrochemical potential control is discussed as a means of assessing the susceptibility to cracking of materials in a range of aqueous environments.     

Comparisons of 30Cr2Ni4MoV rotor steel with different treatments on corrosion resistance in high temperature water

Three different treatments, including the heat treatment, deep cryogenic treatment and laser surface melting, were carried out on the 30Cr2Ni4MoV rotor steel. Electrochemical polarization curve and stress corrosion test at the high temperature autoclave were employed to evaluate the corrosion resistance of treated specimens in high temperature water. Results indicate that the conventional heat treatment will increase the value of K_IH for specimens with the lower yield strength, and hence reduce the susceptivity of stress corrosion cracking. However, for the deep cryogenic treated specimen, no apparent improvement was observed on the hardness and corrosion resistance due to the limited carbon precipitate and austenite transformation. In comparison, the best corrosion resistance of laser treated specimens was gained among the three-method-treated specimens according to results of the electrochemical polarization tests at temperature of 90 °C. Nevertheless, some micro-cracks produced on the tensioned surface during the fabrication of self-loaded U-bend specimen due to the large deformation, and thus lead to a decreased stress corrosion cracking resistance in the environment of high temperature water.     

Materials for high temperature corrosion resistance in combined cycle power generation

Combined cycle power generation, using pressurized fluidized bed combustion or coal gasification, is potentially a very efficient way of producing electricity. Unfortunately, high temperature corrosion, especially sulphidation, of alloys used in the construction of such plants is a problem which must be taken into account in design specification. This paper considers the principles of sulphidation and why it can be encountered in combined cycle power generation plant. The main requirements for metallic components in such plants, and alloys suitable for their manufacture, are identified.     

Influence of sigma phase on corrosion behavior of 316L stainless steel in high temperature and high pressure water

In order to elucidate the impact of σ phase on the oxidation film formation and stress corrosion cracking (SCC) resistance of 316L stainless steel, corrosion, SCC and three-point bending tests were conducted and the microstructures of the σ phase in 316L safe-end pipes were characterized via optical microscopy, environmental scanning electron microscopy and scanning Kelvin probe force microscopy. The results indicated that the σ phase was detrimental to the SCC resistance of 316L in high temperature and high pressure environments and the existence of inherently hard and brittle σ phase could change the cracking mode.     

Corrosion behavior of leaded-bronze alloys in sea water

The corrosion behavior of leaded-bronze alloys (Cu–5Sn–5Zn–5Pb, Cu–8Sn–8Zn–8Pb and Cu–10Sn–10Zn–10Pb) in sea water was investigated using weight loss method, open-circuit potential measurements (OCP), polarization techniques and electrochemical impedance spectroscopy (EIS). The nature and morphology of the corrosion products were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results showed that the corrosion resistance decreases with decreasing copper content. The XRD indicated that the composition of patina depends on the concentration of Cu, Sn, Zn and Pb in each alloy.     

The effect of gas composition and contaminants on scale growth rates of FeCrAl alloys

Abstract

Fe–20Cr–5Al alloys were exposed to several environments crucial for industrial applications. Experiments in a simulated fuel-rich exhaust gas, in combustion gas, in air with additions of SO₂ and HCl, and a N₂–NO mixture were compared to the oxidation kinetics in air.

While the addition of HCl and SO₂ enhances oxide growth via increased spallation, the N₂–NO mixture and the simulated exhaust gas seem to act as a shield gas due to the reduced oxygen partial pressure. As the local geometry can also change the gas composition during oxidation, e.g. inside crevices, results from an experiment with a deep bore are presented, simulating such a situation.     

In-bed corrosion of alloys in atmospheric fluidized bed combustors

A series of tests, each of 250 h duration, which is designed to examine the effect of changes in operational parameters on in-bed corrosion, is in progress using a 0.3 m-square atmospheric fluidized bed combustor (AFBC). A 2000-h test has been conducted on a 1.1 mdiameter AFBC, partly to determine the comparability between the results of the tests in the small unit and those in more realistically sized beds, and partly to assess the performance of candidate materials of construction in a longer-term test. The results indicatethat the smaller unit is indeed a satisfactory representation of large AFBCs. Variables such as coal type, coal sulphur content, acceptor type, excess combustion air and bed temperature appear to have relatively little effect on in-bed oxidation/ sulphidation corrosion, certainly within the limits likely in normal operation. Several materials which appear to be suitable for construction of a fluidized bed boiler are identified, although it is emphasized that extrapolation of short-term tests is particularly dangerous for this form of corrosion.     

High temperature corrosion of typical positions in SUS 30432 tubing bend tested in simulated environment

Abstract

The corrosion behaviour of typical positions in an SUS 30432 bend under simulated corrosive environments was investigated in this paper. A scanning electron microscope equipped with an energy dispersive spectrometer was used to characterise the surface and cross-section morphologies of specimens. Results show that the corrosion kinetics of all specimens were similar showing near parabolic law behaviour. The largest weight gain was located at the samples which were cut from the bottom of a bend. The corrosion scale consisted of an outer layer containing iron oxides and an inner layer containing Cr rich spinels and sulphides. The distances between grains in the samples from the bend were larger than that from the straight tube, which became smaller as the experimental time increased. It may be closely related to aggregation of dislocations near the grain boundaries after tube bending.     

Cr含量对CrMnFeNi系高熵合金腐蚀行为的影响EI北大核心CSCD

利用XRD,SEM/EDS,EBSD,电化学测试等表征手段研究Cr含量对Cr_(x)MnFeNi(x=0.8,1.0,1.2,1.5)高熵合金微观组织与耐蚀性能的影响。结果表明:Cr_(0.8)MnFeNi高熵合金为单相FCC结构,Cr_(x)MnFeNi(x=1.0,1.2,1.5)高熵合金为FCC+BCC双相结构,且BCC相比例随着Cr含量升高而增加。在0.5 mol/L H_(2)SO_(4)溶液中,高熵合金的耐蚀性能随着Cr含量降低而增强,其中,Cr_(0.8)MnFeNi单相高熵合金的耐蚀性能最好,这是因为Cr_(0.8)MnFeNi高熵合金的成分更为均匀。此外,Cr_(x)MnFeNi高熵合金在0.5 mol/L H_(2)SO_(4)溶液中均具有宽泛的钝化区域以及明显的伪钝化区域,表明合金在耐蚀性能上具有较大的研究价值和开发潜力。