Influence of Low-Temperature Pre-oxidation by Ozone on Corrosion Resistance of T91 Steel in Steam at 600 °C

The influence of the low-temperature pre-oxidation by ozone on the corrosion resistance of T91 steel in a steam environment at 600 °C was investigated. XRD, SEM, EDS and XPS were employed to characterize the corrosion products. Results show that the corrosion kinetics of the studied materials obeyed the parabolic oxidation law. Thin (Fe, Cr)₂O₃ oxides were developed on the surface of T91 after pre-oxidation by the ozone at 150 °C. The low-temperature ozone pre-oxidation improved the oxidation resistance of T91 steel in steam, which can possibly be applied to industrial application.     

The Effect of Pre-oxidation Treatment on Corrosion Behavior of Ni–Cu–Fe–Al Anode in Molten CaCl<Subscript>2</Subscript> Salt

This article presents Ni–Cu–Fe–Al alloy as a novel inert anode used in FFC process (the Fray Farthing Chen) in molten calcium chloride salts for producing titanium. The alloy was prepared by vacuum induction melting; then utilized as anode material in molten CaCl₂ for 16 h at 900 °C. Morphology and the corrosion behavior of the samples were analyzed using scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The product on the cathode was analyzed using X-ray diffraction (XRD). After 16 h electrolysis of anodes, EDS and SEM analysis of the samples showed that the corrosion depth of the non-oxidized sample was shorter. Corrosion attacks more severe for the pre-oxidized sample than the non-oxidized sample, which indicated that the corrosion resistance of outer layer is higher on the non-oxidized sample. The XRD results show that the TiO₂ pellets were successfully reduced to the lower oxides using the Ni–Cu–Fe–Al inert anode.     

Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>-Deposit-Induced Corrosion of Mo-Containing Alloys

Disk alloys used in advanced gas turbine engines often contain significant amounts of Mo (2 wt% or greater), which is known to cause corrosion under Type I hot corrosion conditions (at temperatures around 900 °C) due to alloy-induced acidic fluxing. The corrosion resistance of several model and commercial Ni-based disk alloys with different amounts of Mo with and without Na₂SO₄ deposit was examined at 700 °C in air and in SO₂-containing atmospheres. When coated with Na₂SO₄ those alloys with 2 wt% or more Mo showed degradation products similar to those observed previously in Mo-containing alloys, which undergo alloy-induced acidic fluxing Type I hot corrosion even though the temperatures used in the present study were in the Type II hot corrosion range. Extensive degradation was observed even after exposure in air. The reason for the observed degradation is the formation of sodium molybdate. Transient molybdenum oxide reacts with the sodium sulfate deposit to form sodium molybdate which is molten at the temperature of study, i.e., 700 °C, and results in a highly acidic melt at the salt alloy interface. This provides a negative solubility gradient for the oxides of the alloying elements, which results in continuous fluxing of otherwise protective oxides.     

Hot Corrosion Behavior of Superalloy IN718 at 550 and 650 °C

This investigation was undertaken to evaluate oxidation and hot corrosion behavior of the Fe-Ni-based superalloy IN718, at 550 and 650 °C, to explore its performance as turbine engine components under marine environment. Uncoated and different salt-coated samples (100 wt.% NaCl, 75 wt.% Na₂SO₄ + 25 wt.% NaCl, and 90 wt.% Na₂SO₄ + 5 wt.% NaCl + 5 wt.% V₂O₅) were exposed in air at 550 and 650 °C under cyclic heating and cooling for 100 h. Weight gain was studied for both uncoated and salt-coated samples. X-ray diffraction, scanning electron microscopy, and electron dispersive spectroscopy were used to characterize the oxidation and corrosion products. A possible mechanism of corrosion, based on the corrosion compounds, is discussed. The variation in weight gain with time showed a parabolic growth of oxides. Coating with NaCl was found to be detrimental both at 550 °C as well as 650 °C. On the other hand, the salt mixture of NaCl and Na₂SO₄ had no effect at 550 °C; however, it was detrimental at higher temperature of 650 °C. Coatings of salt mixture of Na₂SO₄, NaCl, and V₂O₅ caused very slow oxidation at both the temperatures. Increase in thickness of salt coating was observed to enhance the rate of hot corrosion. Among the three types of salt coatings, the coating of NaCl was found to be most damaging both at 550 and 650 °C.     

Thermogravimetric Study of Oxidation-Resistant Alloys for High-Temperature Solar Receivers

Three special alloys likely to be suitable for high-temperature solar receivers were studied for their resistance to oxidation up to a temperature of 1050°C in dry atmospheres of CO₂ and air. The alloys were Haynes HR160, Hastelloy X, and Haynes 230, all nickel-based alloys with greater than 20% chromium content. The oxidation rate of specimens cut from sample master alloys was followed by thermogravimetry by continuously monitoring the weight change with a microbalance for a test duration of 10 h. The corrosion resistance was deduced from the total weight increase of the specimens and the morphology of the oxide scale. The surface oxide layer formed (scale) was characterized by scanning electron microscopy and energy dispersive x-ray spectroscopy and in all cases was found to be chromia. Oxidation was analyzed by means of parabolic rate law, albeit in some instances linear breakaway corrosion was also observed. For the temperature range investigated, all alloys corroded more in CO₂ than in air due to the formation of a stronger and more protective oxide scale in the presence of air. At 1000°C, the most resistant alloy to corrosion in CO₂ was Haynes 230. Alloy Haynes HR160 was the most oxidized alloy at 1000°C in both CO₂ and air. Hastelloy X oxidized to a similar extent in CO₂ at both 900°C and 1000°C, but in air, it resisted oxidation better at 1000°C than either at 900°C or 1000°C.     

Cyclic Oxidation Behavior of IN 718 Superalloy in Air at High Temperatures

Ni-base superalloy IN 718 was cyclically oxidized in laboratory air at temperatures ranging from 750 to 950 °C for up to 12 cycles (14 h/cycle). The kinetic behaviour as well as the surface morphology, and the oxide phases of the scales were characterized by means of weight gain measurements, cyclic oxidation kinetics, scanning electron microscopy equipped with energy dispersive spectroscopy (SEM-EDS), and X-ray diffraction (XRD) analysis techniques. The results showed that as the oxidation temperature increased, the oxidation rate, the external scale thickness, and internal oxidation zone increased. It was suggested that the oxidation rate was controlled by the diffusion of substrate elements in the alloy and the inward diffusion of oxygen through the oxide scale. The oxidation kinetics followed a sub-parabolic rate law and, the activation energy of oxidation was 249 ± 20 kJ mol^?1. The scaling process was controlled mainly by the diffusion of chromium, titanium, manganese, and oxygen ions through the chromia scale. IN 718 showed low weight gain and very slow reaction rates of substrate elements at 750 °C. At 850 °C, a continuous and very thin oxide scale was formed. At 950 °C, XRD and EDS-elemental mapping analysis revealed that a complex oxide scale had formed. It consisted of an outermost layer of TiO₂?CMnCr₂O₄ spinels, inner layer of Cr₂O₃, and the inner most layer composed of Ni₃Nb enriched with Nb, Ti and Al oxides underneath the chromia layer. The oxide scale at this temperature seemed to be thicker layer, significant spallation and volatilization had apparently occurred, and greater internal corrosion was identified. The doping effect of titanium was observed, where it was found to be diffused through the chromia scale to form TiO₂ at the oxide-gas interface as well as internally and at the oxide alloy interface. The amount of rutile (TiO₂) at the oxide surface increased with temperature. In view of Mn contents in the alloy, the manganese?Cchromium spinel oxide was inferred to have played an important role in cyclic oxidation behaviour of IN 718, where the change in oxidation kinetic was noted. The Al contents would cause internal Al-rich oxide formation at grain boundaries.     

The role of Ce ions in electrochemical corrosion of 304 SS in Ce(NO<Subscript>3</Subscript>)<Subscript>3</Subscript>.6H<Subscript>2</Subscript>O

Effects of temperature and potential on the electrochemical corrosion behavior of alloy AISI 304 (UNS S30400) Stainless steel were investigated in 3 wt.% cerium nitrate (Ce[NO₃]₃.6H₂O) solution. With an increase in electrolyte temperature from ambient temperature to 90°C, the corrosion potential of the alloy shifted towards the noble direction, and the resistance to polarization increased due to the formation of Ce-oxide on the electrode surface. The oxide films formed at the open circuit potential (OCP) and a passive potential of 0.4 V_SCE were examined by x-ray photoelectron spectroscopy (XPS). The oxide film formed at 50°C and a passive potentialof 0.4 V_SCE consists of mixed oxides of Ce and Cr, whereas that at OCP consists of only Cr oxide. The formation of Cr oxides on the electrode surface was primarily due to the nitrate (NO₃ ⁻) ions in Ce(NO₃)₃.6H₂O electrolyte.     

Performance of NiCrAlY Coatings Deposited by Oxyfuel Thermal Spraying in High Temperature Chlorine Environment

A microcrystalline Ni-22Cr-10Al-1Y (wt.%) coating was deposited on AISI 304 stainless steel by the oxyfuel thermal spray technique. The deposited coating was subjected to heat treatment to improve the microstructure characteristics and its corresponding high-temperature properties. The isothermal high-temperature corrosion behavior at 650 and 700 °C in synthetic air and in the presence of 1% Cl₂ was investigated using thermogravimetric analysis, x-ray diffraction, and scanning electron microscopy with energy-dispersive x-ray spectroscopy. The results indicated that the deposited NiCrAlY coating possessed acceptable oxidation-corrosion resistance at 650 °C owing to the formation of extensive amounts of the protective oxide of Cr₂O₃; NiO and a lesser amount of a Cr_1.12 Ni_2,88 metallic phase are also formed. At 700 °C, the coating lost its protective characteristic because of the excessive consumption of thermodynamically stable phases by oxidation-chlorination process. In this case, the steel base and the coating were attacked by chlorine during the exposure time; the mass gain of the NiCrAlY coating was slightly higher and provided only a limited protection up to 11 h; thereafter, breakdown of the layer of oxides occurred and this is attributed to the formation of non-protective oxides mainly β-Fe₂O₃ and Fe_21.33O₃₂ and the depletion of chromium.     

In-Situ Electrochemical Corrosion Behavior of Nickel-Base 718 Alloy Under Various CO<Subscript>2</Subscript> Partial Pressures at 150 and 205 °C in NaCl Solution

The electrochemical corrosion behavior of nickel-base alloy 718 was investigated using electrochemical impedance spectroscopy and potentiodynamic polarization techniques at various partial pressures of CO₂ (\(P_{{{\text{CO}}_{2} }}\)s) in a 25 wt% NaCl solution at 150 and 205 °C. The passive films composed of FeCO₃ exhibit good corrosion resistance with a feature of Warburg impedance, Tafel plots show a complete passivation and the anodic reactions was dominated by a diffusion process at low \(P_{{{\text{CO}}_{2} }}\)s (1.8–9.8 MPa) at 150 °C. While numerous dented corrosion areas appeared on the sample surface for the \(P_{{{\text{CO}}_{2} }}\) of 11.6 MPa at 205 °C, the Tafel plot with three anodic peaks and the Nyquist diagram with an atrophied impedance arc were present. This dented corrosion attribute to the synergistic effects of stress, temperature, \(P_{{{\text{CO}}_{2} }}\) and Cl^?, the temperature and stress could play crucial roles on the corrosion of the alloy 718.     

Effect of Pre-Oxidation at 800?��C on the Pitting Corrosion Resistance of the AISI 316L Stainless Steel

In situ X-ray diffraction was used to identify the oxides formed on the AISI 316L (containing 2% Mo) stainless steel during isothermal oxidation at 800 °C, in air. Good oxidation behavior was observed on this steel when considering kinetics, structural characteristics and scale adherence. It was demonstrated that molybdenum plays a protective role in that it hinders the outward iron diffusion and leads to the lower growth rate and the better scale adherence. The oxide scale was then composed of Cr₂O₃ with a small amount of Mn_1,5Cr_1,5O₄ at the external interface. Pre-oxidation of the AISI 316L also improved its aqueous corrosion resistance. No pitting corrosion occured during the corrosion test. Aqueous corrosion testing also showed that the oxide scale formed at 800 °C is crack-free and still adherent after cooling to room temperature.     

Hot Corrosion Behavior of Low-Pressure Cold-Sprayed CoNiCrAlY Coatings

CoNiCrAlY coatings were deposited by low-pressure cold spraying and pre-oxidized in a vacuum environment, and its hot corrosion behavior in pure Na₂SO₄ and 75 wt.% Na₂SO₄ + 25 wt.% NaCl salts was investigated. The pre-oxidation treatment resulted in the formation of a dense and continuous α-Al₂O₃ scale on the coating surface. After being corroded for 150 h at 900 °C, the pre-oxidized coating exhibited better corrosion resistance to both salts than the as-sprayed coating. The presence of preformed Al₂O₃ scale reduced the consumption rate of aluminum, by delaying the formation of internal oxides and sulfides and promoting the formation of a denser and more adherent Al₂O₃ scale. Moreover, we investigated the corrosion mechanism of cold-sprayed CoNiCrAlY coatings in the two salts and discussed the effect of the pre-oxidation treatment.     

Microstructure and corrosion resistance of vanadium films deposited at different target-substrate distance by HPPMS

High power pulsed magnetron sputtering（HPPMS）, a novel physical vapor deposition technology, was applied to prepare vanadium films on aluminum alloy substrate in this paper. The influence of target–substrate distance（Dt–s）（ranging from 8 to 20 cm） on phase structure, surface morphology, deposition rate, and corrosion resistance of vanadium films was investigated. The results show that the vanadium films are textured with a preferential orientation in the（111） direction except for that fabricated at 20 cm. With Dt–sincreasing, the intensity of（111） diffraction peak of the films decreases and there exists a proper distance leading to the minimum surface roughness of 0.65 nm. The deposition rate decreases with Dt–sincreasing. All the V-coated aluminum samples possess better corrosion resistance than the control sample. The sample fabricated at Dt–sof 12 cm demonstrates the best corrosion resistance with the corrosion potential increasing by 0.19 V and the corrosion current decreasing by an order of magnitude compared with that of the substrate. The samples gain further improvement in corrosion resistance after annealing, and if compared with that of annealed aluminum alloy, then the corrosion potential of the sample fabricated at 20 cm increases by 0.415 V and the corrosion current decreases by two orders of magnitude after annealed at 200 °C. If the annealing temperature further rises to 300 °C, then the corrosion resistance of samples increases less obviously than that of the control sample.     

High-Temperature Corrosion Behaviour of Aluminized-Coated and Uncoated Alloy 718 Under Cyclic Oxidation and Corrosion in NaCl Vapour at 750 °C

To evaluate the suitability of HR3C and 22Cr–25Ni–2.5Al AFA steels as the heat-resistant alloys, the oxidation behavior of them was investigated in air at 700, 800, 900 and 1000 °C. The evolution of oxide layer on the surface and subsurface was investigated using a combination of compositional/elemental (SEM, EDS) and structural (XRD, GDOES) techniques. A dense and continuous Cr₂O₃ healing layer on the HR3C was formed at the temperature of 700 or 800 °C, but the Cr₂O₃ oxide film on HR3C was unstable and partly converted into a less protective MnCr₂O₄ with the increase in temperature to 900 or 1000 °C. The composition and structure of oxide film of 22Cr–25Ni–2.5Al AFA steels are significantly different to the HR3C alloys. The outer layer oxides transformed from Cr₂O₃ to Al-containing oxides, leading to a better oxidation resistance at 700 or 800 °C compared to HR3C. Further, the oxide films consist of internal Al₂O₃ and AlN underneath the outer loose layer after 22Cr–25Ni–2.5Al AFA oxidized at 900 or 1000 °C. It can be proved that the internal oxidation and nitrogen would make 22Cr–25Ni–2.5Al AFA steels have worse oxidation resistance than HR3C alloys at 900 or 1000 °C.     

Hot Corrosion Behavior of Inconel 625 Superalloy in Eutectic Molten Nitrate Salts

Corrosion resistance of Inconel 625 Ni-based superalloy was studied in a molten nitrate salt consisting of 40 KNO₃–60 NaNO₃ (wt%) at 500 and 600 °C. Open-circuit potential, potentiodynamic polarization, electrochemical impedance spectroscopy and gravimetric tests were used to evaluate the degradation mechanism and corrosion behavior of the alloy. Surface morphology and chemical analysis of corrosion products were characterized by means of scanning electron microscopy and energy-dispersive X-ray spectrometry. The weight-loss curves showed that with the increase in temperature, the oxidation rate and mass gain increased; the relationship between the mass gain and time was close to the parabolic oxidation law. The electrochemical corrosion results confirmed that during the exposure of Inconel 625 alloy to the molten salts, nickel dissolves as a result of non-protective NiO layer formed. The formation of a non-protective oxide layer with low barrier property was responsible for observing the weak corrosion resistance of the alloy at high temperatures (500 and 600 °C). Cyclic polarization tests showed a positive hysteresis confirming the nucleation and growth of stable pits on the surface of Inconel 625 at high anodic overpotentials. Sodium nitrite acts as an efficient pitting inhibitor for this case. In this way, the sodium nitrite with the concentration of 0.1 molal was found to have an optimum inhibition effect on pit nucleation at 600 °C.     

Optical spectroscopic and electrochemical characterization of oxide films on a ferritic stainless steel

Colored oxide films that form on ferritic stainless steel in a high-temperature, oxidizing environment and correspond to different chemical compositions can cause a deterioration of pitting resistance and corrosion performance. Herein, optical spectroscopic and electrochemical techniques have been used to reveal the relationship between color, chemical composition, and corrosion resistance of oxide films formed in the temperature range from 400°C to 800°C for 30 min and at 800°C for 10, 20, 30, and 60 min. The substrate with a thin and dense passivation film leads to a low pitting potential but high corrosion resistance. Oxide films of yellowish or brownish color formed below 600°C are mainly iron oxides, which correspond to low corrosion resistance. No passivation characteristics can be observed for polarization curves of oxide films formed at 500°C and 600°C. The color of oxide films varies from blue to dark gray with the increase of oxidation time at 800°C. Corrosion resistance changes with different proportions of Fe₃O₄, Cr₂O₃, and FeCr₂O₄. The gray oxide films formed at 800°C for 30 min exhibit the lowest pitting susceptibility and the highest corrosion resistance.     

Effect of Temperature on the Corrosion Behavior of API X120 Pipeline Steel in H<Subscript>2</Subscript>S Environment

The corrosion behavior of newly developed API X120 C-steel that is commenced to be used for oil pipelines was studied in a H₂S saturated 3.5 wt.% NaCl solution between 20 and 60 °C using potentiodynamic polarization and electrochemical impedance spectroscopy techniques. The corrosion products formed on the surface of the alloy were characterized using x-ray diffraction and scanning electron microscopy. It has been noticed that the formation of corrosion product layer takes place at both lower and higher temperatures which is mainly comprised of iron oxides and sulfides. The electrochemical results confirmed that the corrosion rate decreases with increasing temperature up to 60 °C. This decrease in corrosion rate with increasing temperature can be attributed to the formation of a protective layer of mackinawite layer. However, cracking in the formed mackinawite layer may not be responsible for the increase in the corrosion rate. More specifically, developed pourbaix diagrams at different temperatures showed that the formed protective layer belongs to mackinawite (FeS), a group of classified polymorphous iron sulfide, which is in good agreement with the experimental results. It is also noticed that the thickness of corrosion products layer increases significantly with decrease in the corrosion rate of API X120 steel exposed to H₂S environment. These findings indicate that API X120 C-steel is susceptible to sour corrosion under the above stated experimental conditions.     

Effects of Platinum on the Hot Corrosion Behavior of Hf-Modified �á�-Ni3Al?+?��-Ni-Based Alloys

The establishment of a protective ??-Al₂O₃ scale is critical for providing high temperature protection from oxidation and hot corrosion, thereby improving lifetimes of advanced gas turbine engine components. Recent work by our group has shown that a wide range of Pt + Hf-modified ?á?-Ni₃Al + ??-Ni alloy compositions form a very adherent and slow-growing Al₂O₃ scale and exhibit excellent oxidation resistance. The main thrust of the present study was to understand the effects of Pt addition on the Type I (900 °C) and Type II (705 °C) hot corrosion (HC) behavior of model Hf-modified ?á? + ?? alloy compositions. The salt used to bring about hot corrosion was Na₂SO₄. It was found that the Type I HC resistance of ?á? + ?? alloys improved with up to about 10 at.% Pt addition, but then decreased significantly with increasing Pt content up to 30 at.% (the maximum level studied); however, under Type II HC conditions the resistance of ?á? + ?? alloys progressively improved with increasing Pt content up to 30 at.%. The effect of pre-oxidation on hot corrosion resistance was also examined, and the results indicated that pre-oxidation generally improved Type II HC resistance for the test duration studied.     

纯Ti和Ti-Pd合金在有、无添加H2O2的磷酸盐缓冲溶液中的耐腐蚀性能（英文）

采用动电位和电化学阻抗谱技术研究了纯Ti(2级)和Ti-Pd合金(7级)的耐腐蚀性能。实验温度为36.6℃,实验溶液包括模拟的健康人体条件的pH7.4的PBS溶液和添加了H2O2(0.015mol/L)的pH5.2的炎症状态的PBS溶液。Ti-Pd合金(7级Ti),在含H2O2的PBS溶液中,其耐腐蚀性能比纯Ti的好(较低的腐蚀电流密度),表明其是一种很好的骨科植入材料。     

Effect of heat treatment on corrosion properties of CuAlNi shape memory alloy

The effect of heat treatment on corrosion properties of CuAlNi shape memory alloy was investigated in 0.9% NaCl solution at pH 7.4 and 37 °C by open circuit potential measurements, polarisation techniques, and electrochemical impedance spectroscopy. Investigations were performed on CuAlNi alloy samples in as-cast state and after heat treatment procedure containing annealing at 850, 885 and 920 °C followed by water quenching. Electrochemical impedance measurement results indicate that heat treatment of CuAlNi alloy leads to the increase in charge transfer resistance and surface layer resistance and the decrease in values of capacitance of the double and surface layers, indicating higher corrosion resistance compared with the as-cast CuAlNi alloy. The increase in polarisation resistance and the decrease in corrosion current density of heat-treated CuAlNi alloy also suggest beneficial influence of heat treatment on corrosion resistance of CuAlNi alloy. Optical microscopy, SEM/EDX and XRD analysis of samples surface after polarisation measurements show the occurrence of pitting corrosion on the electrode surfaces, with the existence of CuCl₂, AlCl₃ and Cu₂Cl(OH)₃ compounds as the surface corrosion products.     

Salzschmelzkorrosion gerichtet erstarrter eutektischer CO-Cr-C-Basis Superlegierungen bei hohen Temperaturen

Corrosion of directionally solidified eutectic Co-Cr-C-Superalloys by molton salts at high temperatures The corrosion behaviour of various directionally solidified 73C-class eutectic alloys (Co-Cr₇C₃) and the conventionally cast nickel base alloy IN 738* were investigated using a eutectic sulphate melt (sodium, calcium, and magnesium with 2% sodium chloride). As these materials are designed for high temperature applications, tests were carried out at 900°, 1000°, and 1100°. The additions to 73C were nickel, aluminium, and manganese. Corrosion surface attack for 73C and IN 738 was found to be similar. The grain boundary formation of sulphides and oxides in IN 738 is shown up as a disadvantage when compared with 73C as 73C has no grain boundaries perpendicular to the surface. This could possibly be compensated by directionally solidifying IN 738. A 10% nickel addition to 73C was found to increase the corrosion resistance, a 2% aluminium addition showed a minor improvement, and a 4.7 or 10% manganese addition to 73C to influence the corrosion resistance considerably.