Short‐term oxidation and hot corrosion resistance of a gradient CrN/Cr1−xAlxN coating

A CrN/Cr_1?xAl_xN coating comprised of an inner layer of CrN and an outer layer of Cr_1?xAl_xN with a gradient distribution of Al was deposited on two different alloys by a reactive sputtering method. Oxidation and hot‐corrosion tests of the gradient CrN/Cr_1?xAl_xN coating were performed at different temperatures. The phase compositions and morphologies of the as‐deposited coating and the corrosion products were investigated by using XRD and SEM/EDS. The results showed that the gradient CrN/Cr_1?xAl_xN coating exhibited good oxidation resistance at temperatures above 1000 °C owing to the formation of an α‐Al₂O₃‐rich oxide scale. The coating possessed good hot‐corrosion resistance in molten sulfate because the inner CrN layer could supply enough Cr to form a relatively protective Cr₂O₃ after the Al₂O₃‐enriched scale failed due to its dissolution in the molten sulfate.     

High temperature corrosion behavior of a multilayer CrAlN coating prepared by magnetron sputtering method on a K38G alloy

A multilayer CrAlN coating of Cr_0.58Al_0.42N/Cr_0.84Al_0.16N/Cr_0.51Al_0.49N has been fabricated by a reactive magnetron sputtering method. It consists of a bonding layer, a Cr-rich intermediate layer and an Al-rich outer layer. The multilayer structure provides the coating with good protection against different types of high temperature corrosion, i.e., high temperature oxidation and hot corrosion. The outer Al-rich layer gives the coating good oxidation resistance at 1000 and 1100 °C due to the formation of a continuous alumina scale. The parabolic rate constants of the coated samples decrease by about 2 orders of magnitude compared with that of the bare alloy samples. The intermediate Cr-rich layer can form a Cr₂O₃ scale to provide good protection under the hot corrosion conditions in the Na₂SO₄ salt fluxing at 900, 950 and 1000 °C. The incubation period of the hot corrosion extends several times longer when the alloy was coated by the multilayer coating at the three selected temperatures.     

Y2O3 对DZ125合金表面Cr-Al-Y渗层的组织结构及抗氧化性能影响

为进一步提高DZ125合金高温服役性能,通过扩散渗方法在其表面制备了稀土元素Y改性的Cr-Al共渗层,研究了Y₂O₃含量对渗层组织结构及抗高温氧化性能的影响。结果表明：不同条件下制备的Cr-Al-Y渗层均具有三层结构,由外向内依次为：Cr+Ni₃Cr₂外层,Ni₃Cr₂+Al₁₃Co₄中间层,以及Ni₃Al内层。当渗剂中Y₂O₃含量为0%～2%(质量分数,下同)时,渗层的厚度与密度显著增加;当稀土Y₂O₃的添加量过高时(5%),渗层的密度及厚度反而下降。1100℃高温氧化实验表明,Cr-Al-Y渗层显著提高了DZ125合金的抗高温氧化性能。     

Oxidation of Nickel and Cobalt-Based Alloys in the Presence of Condensed Sodium Sulphate

The hot corrosion behaviour of a number of nickel and cobalt-based superalloys has been examined by exposing samples to a high temperature oxidizing environment supersaturated with sodium sulphate vapour. This test seems more able to reproduce typical service behaviour than other laboratory tests. Pure cobalt is unaffected by the presence of the condensed sulphate, whereas CoW and low-chromium, CoCrW alloys undergo acidic fluxing. However, the major change produced by the continuous supply of Na₂SO₄, as opposed to the limited amount of salt available in the coating test is in the behaviour of the high chromium alloys, when the protective Cr₂O₃ layers are removed due to the formation of a Na₂CrO₄ species. Thus, the normally resistant Co25Cr7.5W alloy suffers acidic fluxing. Similarly, the Cr₂O₃ layer on the binary Co25Cr alloys is rendered ineffective; considerable ingress of sulphur into the alloy occurs. Aluminium and manganese additions seem to reduce this effect slightly by stabilising the protective oxide layer. Both these alloying additions have a higher affinity for sulphur than chromium, and this could be important. Binary Ni20Cr seems less susceptible to accelerated attack than the Co25Cr alloys, presumably due to its greater ability to maintain a protective Cr₂O₃ layer. However, addition of 3 vol.-% Y₂O₃ virtually prevents any attack by the Na₂SO₄, preventing sulphur penetration into the alloy and promoting the formation of a protective Cr₂O₃ layer. Under non-condensing conditions, all of the alloys tested oxidize in an unaccelerated manner, supporting the view that condensation of sodium sulphate is necessary for hot corrosion.     

两种铸造镍基合金的高温氧化行为

利用热重分析法、XRD和SEM (EDS)对比研究了700℃超超临界发电机组用K317和K325铸造合金在900和1000℃大气环境下氧化行为。结果表明,K317的氧化性能要优于K325。在900℃氧化时,2种合金的氧化增重遵循抛物线规律,而在1000℃氧化时,氧化增重均分段遵循抛物线规律。K317的氧化膜分3层,外层是NiO、TiO_2和NiCr_2O_4,中间层是致密的Cr_2O_3,内层是内氧化产物Al_2O_3。而K325的氧化膜分2层,外层是NiO, NiCr_2O_4和Nb_2O_5,内层是致密的Cr_2O_3和嵌入的Nb_2O_5颗粒,没有内氧化现象发生。在1000℃氧化时,K325中的Mo严重被氧化形成挥发性MoO_3;同时氧化膜发生了局部剥落现象,氧化膜的附着性相对较差。     

Inconel617合金表面电子束熔覆NbMoCr显微组织和耐腐蚀性的探讨

为了提高Inconel617合金(简称617合金)材料的表面性能,利用电子束熔覆技术在617合金表面制备了NbMoCr熔覆层. 对它的显微组织、硬度和耐腐蚀性能进行了研究,并与617合金进行了对比. 结果表明,NbMoCr熔覆层的组织更均匀,晶粒更细小,气孔等缺陷更少,且生成了微量M₂₃C₆,Cr₇C₃,Cr₄Si₄Al₁₃,CoCx等硬质相,提高了熔覆层的表面硬度及耐腐蚀性. 经检测,熔覆层硬度相比617合金硬度高出86 HV10. 电化学腐蚀测试表明,在1 mol/L H₂SO₄溶液中,617合金自腐蚀电流密度是NbMoCr熔覆层的5.16倍;在3.5 %的NaCl溶液中,617合金自腐蚀电流密度是NbMoCr熔覆层的4.6倍;在1 mol/L NaOH 溶液中,617合金自腐蚀电流密度是NbMoCr熔覆层的3.12倍.     

Factors Affecting Chromium Carbide Precipitate Dissolution During Alloy Oxidation

Ferrous alloys containing significant volumefractions of chromium carbides were formulated so as tocontain an overall chromium level of 15% (by weight) buta nominal metal matrix chromium concentration of only 11%. Their oxidation at 850°C inpure oxygen led to either protectiveCr₂O₃ scale formation accompaniedby subsurface carbide dissolution or rapid growth ofiron-rich oxide scales associated with rapid alloy surface recession, which engulfedthe carbides before they could dissolve. Carbide sizewas important in austenitic alloys: an as-castFe-15Cr-0.5C alloy contained relatively coarse carbides and failed to form aCr₂O₃ scale, whereas the samealloy when hot-forged to produce very fine carbidesoxidized protectively. In ferritic alloys, however, evencoarse carbides dissolved sufficiently rapidly to provide the chromium flux necessary to formand maintain the growth of a Cr₂O₃scale, a result attributed to the high diffusivity ofthe ferrite phase. Small additions of silicon to theas-cast Fe-15Cr-0.5C alloy rendered it ferritic and led toprotective Cr₂O₃ growth. However,when the silicon-containing alloy was made austenitic(by the addition of nickel), it still formed aprotective Cr₂O₃ scale, showing that the principal function of silicon was inmodifying the scale-alloy interface.     

Carburization of W- and Re-rich Ni-based alloys in impure helium at 1000 °C

The surface and microstructure stability of experimental W- and Re-rich Ni-based alloys in an impure-helium environment containing only CO and CO₂ as impurities (ppm level) have been investigated at 1000 °C. All the alloys carburized during 50 h of exposure, and, depending on the alloy composition, different carbides of the type M₆C, M₇C₃ and M₂₃C₆ formed on the alloy surface, in grain interiors and at grain boundaries. Microprobe analysis and Calphad-based calculations indicated that the chromium carbides (particularly Cr₂₃C₆) were enriched by rhenium. Extended exposure (225 h) led to the disappearance of surface transient carbides and the growth of surface oxide Cr₂O₃ occurred.     

Structure of Al–Ni intermetallic composite layers produced on the Inconel 600 by the glow discharge enhanced-PACVD method

The Plasma Assisted Chemical Vapor Deposition (PACVD) treatment conducted under glow discharge conditions in an atmosphere of trimethylaluminum vapors applied to an Inconel 600 substrate yielded composite surface layers built of intermetallic phases of the Al–Ni system with the outer zone composed of aluminum oxides. Such layers have very advantageous performance properties, such as high hardness, good corrosion and frictional wear resistance and, good adherence to the substrate.The present study is dedicated to microstructure characterization of the layers. The layers were examined using a variety of methods. Based on the results of these examinations, the microstructure of the composite layers was described as a multizone one with an outer Al₂O₃ zone, an intermediate AlNi₃ + Al₂O₃ zone and a diffusion zone of type Ni(Al,Cr,Fe) + AlNi₃ + Cr₇C₃. The mechanism of layer formation as well as the correlation between the microstructure and the observed improvement of the surface properties of the Inconel 600 alloy are discussed.     

Investigations of the degradation of high-temperature alloys in a potentially oxidizing-chloridizing gas mixture

The degradation of high-temperature alloys in argon-5.5% oxygen-0.96% hydrogen chloride-0.86% sulfur dioxide at 900°C under isothermal and thermal cycling conditions has been investigated. All the alloys showed reasonable resistance under isothermal conditions, although the Al₂O₂ ***-forming material, alloy 214, gave the lowest amount of corrosion, consistent with Al₂O₃ being a more effective barrier than Cr₂O₃ to inward penetration of chlorine or sulfur-containing species from the environment. Significant internal corrosion was observed for some alloys. Degradation of all the alloys was much more severe under thermal cycling conditions because of the failure of the protective scales. In all cases, formation of volatile chlorine-containing compounds was observed. Degradation of the alloys resulted from the penetration of chlorine-containing species through the initially formed oxide scale and formation of chlorides or, possibly, oxychlorides at the alloy-scale interface or in the subjacent alloy. The sulfur dioxide did not play any obvious role in the process.     

The combined effect of refractory coatings containing reactive elements on high temperature oxidation behavior of chromia-forming alloys

The high temperature oxidation behaviors of chromia-forming alloys (F17Ti and Fe-30Cr alloys) have been studied at 1273 K under isothermal conditions and at 1223 K under cyclic conditions, in air under the atmospheric pressure. To extend the oxidation lifetime, coatings have been applied onto the alloy surfaces. Al₂O₃ and Cr₂O₃ films doped with Sm₂O₃ or Nd₂O₃ were prepared via the metal-organic chemical vapor deposition technique. Single Cr₂O₃, Al₂O₃, Nd₂O₃ and codeposited Cr₂O₃-Nd₂O₃, Al₂O₃-Nd₂O₃, Al₂O₃-Sm₂O₃ coatings drastically improved the chromia-forming alloy high temperature oxidation behavior, since they decreased the oxidation rate and enhanced the oxide scale adhesion. Results showed that a critical amount of reactive element (Nd or Sm) in chromia or alumina coatings (11-18 at.%) was needed to observe the most effective effect. The fast precipitation of NdCrO₃ or NdTi₂₁O₃₈ and the segregation of reactive elements at the chromia grain boundaries slowing down outward cation transport and consequently blocking the chromia grain growth, was supposed to be the main reasons of the beneficial effect ascribed to the reactive elements in chromia scales.     

Effect of Al2O3 content and swaging on microstructure and mechanical properties of Al2O3/W alloys

Al₂O₃-reinfored tungsten alloys were fabricated by powder metallurgy method and hot swaging technology. The investigation was made on the microstructure, relative density, nano-hardness and fracture toughness (K_IC) of the sintered and swaged Al₂O₃/W alloys. The swaging process and addition of Al₂O₃ are beneficial to comprehensive properties of the sintered and swaged alloys. After swaging, the Al₂O₃/W alloys can achieve the full density. According to the nano-indentation test and three-point bend test, the swaged W-0.25 wt% Al₂O₃ alloy possesses the highest hardness of 7.02 GPa, the greatest modulus of 435.09 GPa and the maximum fracture toughness of 21 MPa·m^1/2. The observation of fracture morphology shows that the recrystallization behavior and grain growth occur above 1400 °C in the swaged pure W alloy, which leads to recrystallization brittleness. At the same time, the microstructure of the swaged W-0.25 wt% Al₂O₃ alloy does not change apparently.     

High temperature corrosion of pure Fe, Cr and Fe-Cr binary alloys in O2 containing trace KCl vapour at 750 °C

Pure Fe, Cr and Fe-Cr binary alloys were corroded in O₂ containing 298 ppm KCl vapour at 750 °C. The corrosion kinetics were determined, and the microstructure and the composition of oxide scales were examined. During corrosion process, KCl vapour reacted with the formed oxide scales and generated Cl₂ gas. As Cl₂ gas introduced the active oxidation, a multilayer oxide scales consisted of an outmost Fe₂O₃ layer and an inner Cr₂O₃ layer formed on the Fe-Cr alloys with lower Cr concentration. In the case of Fe-60Cr or Fe-80Cr alloys, monolayer Cr₂O₃ formed as the healing oxidation process. However, multilayer Cr₂O₃ formed on pure Cr.     

Improvement of cyclic oxidation resistance of a NiAl-based alloy modified by Dy

The effect of the rare-earth-element dysprosium on the cyclic oxidation behavior of NiAl-31Cr-3Mo alloy at 1300 and 1400 K in static air atmosphere was investigated in this paper. The results revealed that doping 0.1.% atom fraction of Dy greatly improved the cyclic oxidation resistance at 1300 and 1400 K. The microstructure and composition of the oxidation layer were studied by means of X-ray diffractometer (XRD), scanning electron microscopy (SEM) with EDAX unit. The oxidation scale, which is located on the surface of NiAl-31Cr-3Mo alloy, mainly consisted of α-Al₂O₃ and a little Cr₂O₃. While the continuous and sole α-Al₂O₃ scale formed on the surface of the alloys doped with the dysprosium. The adherency of scale improved due to a Cr-rich phase decreased the internal stress by thermal cycling and transformation of Al₂O₃ or transverse growing induced for the testing alloy modified by Dy. The oxidation mechanism was discussed from phase constitution.     

Thermal fatigue behavior of K125L superalloy

The thermal fatigue behavior of K125 L superalloy at the peak temperature of 1,050 °C was investigated by optical microscope(OM), X-ray diffraction(XRD), and scanning electron microscope(SEM). The experimental results show that the crack initiation sites of tested alloys are at the V-notch tip and the V-notch tip propagates by way of continuous cracking along grain boundaries. The formation of high-temperature oxides and MC carbides accelerates the crack propagation, and no secondary carbides precipitate out. Oxides between cracks are mainly the Al2O3 as well as Cr_2O_3, and carbides are Ta-rich and Tirich MC carbides.     

Corrosion of a stainless steel and nickel-based alloys in high temperature supercritical carbon dioxide environment

Corrosion of four alloys has been studied in supercritical carbon dioxide at 650 °C and 20 MPa, specifically AL-6XN stainless steel and three nickel-based alloys, PE-16, Haynes 230, and Alloy 625. The tests were performed for exposure durations of up to 3000 h with samples being removed for analyses at 500 h intervals. The corrosion performance of the alloys was evaluated by weight change measurements, and the surface oxide layers were characterized by scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. Weight gain measurements showed that the Al-6XN stainless steel exhibited the least corrosion resistance while the weight gains were nearly similar for the other alloys. The oxide layer in AL-6XN stainless steel was composed of large equiaxed grained outer layer of Fe₃O₄ (magnetite) and an inner layer of FeCr₂O₄. Oxide spallation was observed in this stainless steel even after 500 h exposure. In all alloys, Cr-rich oxides phases of Cr₂O₃ and Cr_1.4Fe_0.7O₃ were identified as the protective layers. In alloy PE-16 a thin layer of aluminum oxide formed that promoted the corrosion resistance of the alloy. Cr₂O₃ was identified as the main protective oxide layer in nickel base alloys Haynes 230 and 625.     

Study of a Novel DS Superalloy DZ142

研究了一种新型定向高温合金DZ142的微观组织,测定了DZ142合金持久性能、抗氧化性能、抗热腐蚀性能并与DZ125L合金进行比较。结果表明:热处理后的DZ142合金组织主要由γ、γ′、(γ+γ′)共晶和MC型碳化物组成;经长期时效处理,合金时效组织中未见有害的TCP相,碳化物类型仍为MC型。DZ142合金的持久性能,高温抗氧化性能和抗热腐蚀性能都优于DZ125L合金。     

不同Ti含量DZ125L镍基合金的组织

利用光学显微镜、扫描电镜、电子探针及X射线研究了两种不同Ti含量的DZ125L合金铸态和热处理后的微观组织以及合金中主要元素的偏析行为.结果表明:两种合金铸态和热处理后的组织主要由γ,γ',γ+γ'共晶和MC碳化物组成;两种合金中Ti表现了强烈的正偏析行为,W表现为强烈的负偏析行为;随着Ti含量的降低可以减少合金的γ+γ'共晶量,显著降低合金的偏析程度.     

Corrosion of iron-, nickel-, and cobalt-base alloys in atmospheres containing carbon and oxygen

The corrosion of iron-, nickel-, and cobalt-base alloys has been studied in atmospheres containing carbon and oxygen in the temperature range 894–1366 K. It was observed that preformed Cr₂O₃ films are not effective barriers to carbon transport in atmospheres in which the oxide is not stable but that stable, growing Cr₂O₃ films are excellent barriers to carbon penetration. The presence of Fe-containing oxides on Fe-Ni-Cr and Fe-Cr alloys cause the scales to be permeable to carbon. This phenomenon was found to be sensitive to alloy surface preparation. Carbon transport through oxide scales may occur by two mechanisms: diffusion or molecular transport through physical defects. The present work has evidence of the latter but cannot rule out the former in cases where the carbon activity is sufficiently large. In gases containing CO and CO₂ in which Cr carbide is stable Cr₂O₃ was found to form at the carbide-alloy interface by oxygen transport through the carbide. In A-CH₄ Fe-Ni-Cr were found to undergo graphitization attack. The results were consistent with the formation and subsequent decomposition of metastable carbides, as proposed by Hochmann.     

Comparative study on dry sliding wear and oxidation performance of HVOF and laser re-melted Al0.2CrFeNi(Co,Cu) alloys

Al_0.2CrFeNiCo and Al_0.2CrFeNiCu high entropy alloys were deposited with high velocity oxygen fuel (HVOF) on 316L substrate. Later, a laser re-melting (LR) process was applied to enhancing the coating microstructure. LR process effects on dry sliding wear and oxidation behaviors were investigated. The mixture of powders with free elements led to the formation of inner oxides in HVOF coatings. The oxide and porosity were eliminated using LR. After LR, FCC was the dominant phase in both alloys, while BCC, sigma and Cr₂O₃ phases were observed in Al_0.2CrFeNiCo alloy. The hardnesses of the Al_0.2CrFeNiCo and Al_0.2CrFeNiCu coatings after HVOF were HV 591 and HV 361, respectively. After LR, the hardnesses decreased to HV 259 and HV 270, respectively. Although HVOF coatings were most affected by increased load, they showed the highest wear resistance compared to other samples. The lowest wear resistance could be seen in the substrate. After the oxidation tests, HVOF coating layer was completely oxidized and also, the coating layer was delaminated from the substrate after 50 h oxidation due to its porous structure. LR coatings exhibited better oxidation performance. Al_0.2CrFeNiCo was dominantly composed of Cr₂O₃, exhibiting a slower-growing tendency at the end of the oxidation tests, while Al_0.2CrFeNiCu was composed of spinel phases.