Oxidation behavior of Fe-20Cr-5Al rare earth alloys in air and synthetic exhaust gas

Fe-20Cr-5Al alloy foils are used in automotive catalytic converters. This work examines oxidation behavior of four production-processed alloy foils in both air and synthetic exhaust gas environments. Oxidation tests were performed between 750° C and 1150° C for times to 96 hrs. Weight gain results in both atmospheres were similar, an indication that the same mechanism controls oxidation in both environments. At high temperatures (>-950° C) both atmospheres produce weight gains consistent with -alumina growth. Activation energies of 323 kJ/gmole and 271 kJ/gmole were calculated for oxidation in air and synthetic exhaust gas, respectively. At lower temperatures (<-850° C), accelerated weight gains can occur from growth of transition alumina. Despite similar weight gain results, the two atmospheres produce different oxide morphologies: at 950° C and above, air produces a rounded, porous oxide while synthetic exhaust produces a more compact, angular oxide. Unexpectedly, oxide spalling occurred on foils oxidized in synthetic exhaust at 1050° C and above.     

Effect of state of sea on atmospheric corrosion in coastal zones

Abstract

Several studies in the literature have shown a considerable effect of wind on marine aerosol production and atmospheric corrosion when the wind speed exceeds some 3–5 m s^-1. However, the current work has shown that the wind effect is not so clear at the Tarragona testing station, on the Mediterranean coast of Spain, where marine winds are generally moderate. Apparently, factors other than marine wind speed at the coast exert a significant influence on the aerosol production. In addition to wind data, the study considers data on the state of the sea in the coast close to the Tarragona testing station obtained from records from an experimentation buoy. These latter data suggest a direct relationship between chloride aerosol deposition rate and the square of wave height. The effect of chloride deposition rate on atmospheric corrosion at the Tarragona testing station has also been studied. Using bimetallic ‘Climat’ type specimens a proportionality has been revealed between corrosion and atmospheric salinity.     

Sulfidation properties of an Fe-23.4Cr-18.6Al alloy at temperatures 1073 and 1173 K in H2S-H2 atmospheres of sulfur at pressures 104-10−5 Pa

An investigation is reported on the sulfidation properties of an Fe-23.4Cr-18.6Al(at.%) alloy at 1073 and 1173 K in H₂S-H₂ atmospheres, 10⁴ > P_{S 2} 10⁵Pa. The sulfidation kinetics exhibited an early transient period before onset of parabolic kinetics. Values of the parabolic sulfidation rate constants increased by as much as 10⁵ from their smallest values at low sulfur pressures, P_{S 2} 10^–4 Pa, to maximum values at sulfur pressures P_{S 2} 10² Pa. Multilayered scales were formed, the number and types of layers dependent on sulfur pressure. A fully developed scale at sulfur pressures P_{S 2} > 10^–3 Pa.     

Effect of preoxidation and Ti or Hf additions on the corrosion behavior of Fe-23Cr-5Al alloys in Ar-10SO2 at 1200 K

An Fe-23Cr-5Al alloy and those containing 0.21% Ti or 0.12% Hf were cyclically corroded in flowing Ar-10SO₂ gas under atmospheric pressure and in a temperature range varying from room temperature to 1200 K. The corrosion kinetics were assessed by gravimetry, while morphological examinations were carried out using x-ray diffractometry, scanning electron microscopy, and microanalysis. Similar corrosion tests and examinations of the corrosion products were performed for the specimens preoxidized at 1200 K for 20 ksec in pure oxygen under atmospheric pressure. The main corrosion product under all the experimental conditions was -alumina, growing mainly outward in the form of thin needles. Many voids formed beneath the adherent scales. The degree of outward growth decreased by the additives. Partial spallation and formation of scales repeated on the base alloy, resulting in continuous mass losses. The addition of Ti prevented scale spallation, resulting in gradual mass gains; whereas the addition of Hf resulted in poorly adherent scales, under which grooving at alloy grain boundaries and void formation inside the grains occurred. Preoxidation resulted in the formation of adherent scales on all the alloys; however, with increasing corrosion cycles, the outward growth of the oxide became significant. Penetration of sulfur to the substrate under the adherent scale took place.     

The Response of Alloys to Erosion-Corrosion at High Temperatures

Pure nickel, Ni-20Cr and Ni-30Cr alloys wereexposed to conditions of erosion and corrosionsimultaneously at 700°C and 800°C. The exposureswere made using normal impact of an air stream loadedwith 20-m alumina. The alumina particles flowed at therate of 400 mg per min and the velocities used were 75m/s and 125 m/s. The reaction kinetics were measureddiscontinuously by interrupting the exposure and measuring the weight loss. The specimens wereexamined using SEM-EDAX both on the surface and incross-section. Under simple oxidation, the alloyspecimens developed a thin protective layer of chromia. Under erosion-corrosion conditions, thisprotective scale was prevented from forming and thealloys were found to undergo aggressive attack at a ratethat was the same as that experienced for pure nickel, the surface oxides were identified asCr₂O₃ and NiO. It is proposedthat, under erosion corrosion, the erosive streamprevents the formation of a continuous layer of chromiaby removing the oxide faster than it can spread laterally. The specimen issaid to be in a state of erosion-maintained transientoxidation. This mechanism implies that it would bedifficult for protective scales to form in the presence of erosion and the oxidation behavior of analloy cannot be used as a guide to its resistance toerosion-corrosion.     

Al对Fe-20Cr-35Ni-0.6Nb合金显微组织和抗氧化性的影响

采用增量法研究了不同Al含量(0.5%、1.5%、2.5%,质量分数)的Fe-20Cr-35Ni-0.6Nb含Nb合金在1000 ℃空气条件下的抗氧化。采用SEM、EDS、TEM、拉曼光谱等手段研究了合金的显微组织和氧化膜特性。结果表明,3种含Nb合金组织为单相奥氏体,基体中存在少量弥散分布的NbC沉淀相,氧化前后沉淀相含量和晶粒大小保持不变。添加0.5%和1.5%的Al后,含Nb合金的表面形成多层结构的氧化膜,最外层和第三层为Cr₂O₃,次表层主要为NiCr₂O₄、NiFe₂O₄和Fe₂O₃,最内层为Al₂O₃内氧化层。基体中的NbC析出相和氧化膜中少量Nb的氧化物(Nb₂O₅)加剧了氧化膜的疏松。当Al含量增加到2.5%时,含Nb合金表面形成连续致密的Al₂O₃氧化膜,降低了Fe-20Cr-35Ni-0.6Nb合金的氧化速率,提高了抗氧化性。     

Influence of Zr or Hf additions to Fe-23Cr-5Al on the protectiveness of preformed Al2O3 scales against sulfidation

An Fe-23Cr-5Al alloy and those containing 0.17 w/o Zr or 0.12 w/o Hf were oxidized to form -Al₂O₃ scales in a flow of pure O₂ at 1300 K for specified periods up to 400 ks, and subsequently sulfidized at 1200 K in an H₂ –10% H₂S atmosphere without intermittent cooling. The protectiveness of the preformed scale was evaluated by the protection time after which a remarkable mass gain takes place owing to the rapid growth of sulfides. In general, the protection time increases as the scale thickens. Both additives increase the protection time to some degree by forming more structurally perfect scales. However, ZrO₂ particles on or near the outer surface of the scale on the Zr-containing alloy provide sites for sulfide formation. The scales formed on the grain boundaries of the Hf-containing alloy are ridged. The tops of the ridges are associated with cracks, which provide preferential sites for sulfide growth.     

The reactive-element effect in the high-temperature oxidation of Fe-23Cr-5Al commercial alloys

The effect of yttrium and hafnium additions on high-temperature oxidation resistance of modified Fe-23Cr-5Al commercial, heat-resistant alloys was studied. The oxidation resistance was evaluated by the use of accelerated-life tests and isothermal oxidation for wires and flat samples, respectively. A solgel method of modification of the surface properties of ready-made heating elements was examined. The effect of several sol-gel coatings on the alloy oxidation behavior in the accelerated-life test was studied.     

Effect of Hf Additions on the Isothermal-Oxidation Behavior of TiAl at High Temperatures

The isothermal-oxidation behavior of TiAlcoupons containing Hf of up to 5.2 mass % has beenstudied in the temperature range 1100-1400 K in a flowof purified oxygen under atmospheric pressure. Theaddition of 0.2% Hf is very effective to decrease theoxidation rate at 1200 and 1300 K. Metallographicexamination using conventional methods revealed that theinitially-formed Al₂O₃ scale ismaintained very sound by the addition. However, further additions ofHf result in a slight enhancement of oxidation at 1200K and a gradual decrease of the effect at 1300 K.Finally, there is almost no effect by the addition of 5.2% Hf at 1300 K. Excess amounts of Hf leadto the formation of oxide mounds on theAl₂O₃ scale. They grow in size andnumber during subsequent oxidation until the wholespecimen surface is covered with a thick scale. Such a scale is notprotective having a structure often reported in theliterature. The effect of the addition of 0.2% Hfbecomes small at 1350 K and at 1400 K it is inverted.Possible mechanisms for the improvement attained by thesuitable addition are discussed.     

热-机械训练对Fe-20Mn-5Si-5Cr-3Ni形状记忆合金性能的影响

研究了热-机械训练对Fe-20Mn-5Si-5Cr-3Ni合金形状记忆效应(SME)的影响,对合金在HCI、NaOH和NaC1溶液中的耐腐蚀性进行了实验研究.结果表明,热-机械训练能够有效提高合金的形状回复率,在600℃进行中间退火,形状回复率较好;热-机械训练次数为3次时,形状回复率达到最大值(98%).Fe-20Mn-5Si-5Cr-3Ni合金的耐腐蚀性在NaOH溶液中较好,在NaC1溶液中次之,在HC1溶液中较差.     

Oxidation properties of Fe-5Cr-4Al (wt.%) alloys in oxygen at temperatures 1000°C–1320°C

Oxidation kinetics of a parent Fe-5Cr-4Al alloy subjected to two types of anneals were investigated at temperatures ranging from 1000°C to 1320°C. The alloy annealed at 850°C exhibited a rapid transient oxidation stage associated with growth of nodules containing iron oxides and internal precipitation of -Al₂O₃ in the alloy beneath these nodules. The nodules nucleated and grew from sites located in the regions of the alloy grain boundaries during the period of rapid alloy grain growth. Nodular growth virtually ceased when a continuous -Al₂O₃ film formed at the nodule-alloy interface. The alloy subjected to anneal at 1000°C and at the reaction temperature to stabilize the alloy grain size tended upon oxidation to form a protective -Al₂O₃, layer by parabolic kinetics at temperatures to 1250°C. If this alloy was oxidized in stages at 1000°C, a protective -Al₂O₃ scale was formed up to 1320°C. The temperature coefficient of the parabolic oxidation kinetics was consistent with diffusion processes at boundaries of the -Al₂O₃ grains playing an essential role during growth of this protective oxide layer.     

热机械“训练”工艺对Fe-20Mn-5Si-8Cr-5Ni-0．05N合金组织及形状记忆效应的影响

利用透射电镜、扫描电镜等设备研究了Fe-20Mn-5Si-8Cr-5Ni-0．05N合金在不同“训练”工艺下的组织,得到结论如下:“训练”2次时,试验合金的形状记忆效应最佳,此时可恢复的ε马氏体含量最大,并呈单一切变模式生长:“训练”3次时试验合金的母相中产生位错缠结,抑制了ε马氏体的逆转变,合金的形状记忆效应开始下降。     

热机械"训练"工艺对Fe-20Mn-5Si-8Cr-5Ni-0.05N合金组织及形状记忆效应的影响

利用透射电镜、扫描电镜等设备研究了Fe-20Mn-5Si-8Cr-5Ni-0．05N合金在不同“训练”工艺下的组织，得到结论如下：“训练”2次时，试验合金的形状记忆效应最佳，此时可恢复的ε马氏体含量最大，并呈单一切变模式生长；“训练”3次时试验合金的母相中产生位错缠结，抑制了ε马氏体的逆转变，合金的形状记忆效应开始下降。     

STEM Analysis of the Transient Oxidation of a Ni-20Cr Alloy at High Temperature

High-spatial scanning transmission electronmicroscopy (STEM) has been used to study the developmentof transient scales on a commercially available Ni-20Cralloy. The samples were examined usingelectron-transparent cross sections of the metal and oxide foroxidation times between 0 and 30 min in a furnace at950°C in laboratory air. The samples were polishedto 1 m diamond before oxidation, producing a recrystallized grain structure within 100 nm ofthe surface. Upon oxidation, the initial scale consistedalmost exclusively of chromia. However, at themetal-oxide interface, thin layers of silica and alumina were detected. At longer oxidation times,(>5 min), localized thickening of the silica layerwas observed. With increased oxidation time, (>25min), these regions spread along the metal-oxideinterface until an almost continuous silica layer hadformed. The silica layer was present at much shorteroxidation times than reported by other workers, however,this may be because of the thin layer being undetectable using microprobe techniques. The scale formedwas found to be adherent, although the alloy containedsulfur and did not contain reactive elements. However,sulfur was not found to segregate to the metal-oxide interface possibly because of the presence ofthe amorphous silica layer.     

Al、Si及Y多元合金化对Cr-20Nb合金高温氧化行为的影响

针对软第二相Cr稍微降低Laves相NbCr2合金的1200℃抗氧化性,采用Al、Si及Y多元合金化来提高Cr-20Nb合金的高温抗氧化性能。结果表明,多元合金化的Cr-20Nb合金1100℃及1200℃抗氧化性均好于加入单一合金化的及纯Cr-20Nb合金,并随着Si合金元素含量增加,Cr-20Nb合金的氧化增重变小,抗氧化性变好;SEM结果表明,添加合金元素后,氧化膜与基体的粘附性得到了明显提高。     

The Effects of Reactive-Element,Ion-Implantation-Induced Amorphous Layers on the Oxidation of Co-12Cr and Ni-12Cr Alloys

Nickel-chromium (Ni-12Cr, wt.%) andcobalt-chromium (Co-12Cr, wt.%) alloys were ionimplanted with 150 keV yttrium to fluences that rangedbetween 2 × 10¹⁶ and 1 ×10¹⁷ ions/cm². The influence ofthe implantation on the microstructure of the alloy wasdetermined. The effect of the highest dose implantationon the alloys' oxidation response at 1000°C, 48 hrwas measured. Both alloys contained an amorphous surface phase as a result of this fluence and one ofthe effects of oxidation was to recrystallize theamorphized alloy in the first few minutes of oxidation.The lower doses of 2 × 10¹⁶ions/cm² were sufficient to cause amorphization of both the Ni-12Cr andthe Co-12Cr. The implantation reduced the isothermalmass gain by a factor of 25% for the Ni-12Cr, but had anegligible effect on the Co-12Cr alloy. Short-term oxidation experiments at 600°C showed viatransmission electron microscopy that, in the absence ofthe yttrium implant, the Ni-12Cr alloy forms NiO in thefirst minute of oxidation and the Co-12Cr alloy forms CoO and CoCr₂O₄.The implanted Ni-12Cr, on the other hand (1 ×10¹⁷Y⁺/cm²), formsrecrystallized Ni-Cr, Y₂O₃, andNiO in the near-surface region, while the implantedCo-12Cr alloy forms CoO, CoCr₂O₄, and a recrystallized intermetallic alloy fromthe amorphized region.     

Effect of zirconium concentration on nodule formation during cyclic oxidation of Fe-15Cr-4Al alloy at 1300°C

The effect of small concentrations of zirconium on the cyclic oxidation behavior of Fe-15Cr-4Al was investigated at 1300°C. The zirconium concentrations studied were 0.2, 0.55, and 1 wt.%. Each cycle consisted of a 1-hr isothermal period followed by 30-min cooling of the sample in air. Nodule formation was observed in higher zirconium-containing alloys, 0.55% and 1% zirconium, and the base alloy. The nodules formed at the corners and edges of the specimen and grew with increasing number of cycles. EDAX and line scans of cross section of oxidized samples demonstrated that the nodules were composed of iron oxides. Suggestions are given regarding the mode of nodule formation. Results show that additions of 0.2% zirconium and 1% yttrium to the base alloy suppress the nodule formation and provide the best resistance to cyclic oxidation.     

机械抛光后的时效工艺对0Cr25Al5电热合金拉拔性能的影响

对机械抛光后的0Cr25Al5电热合金进行了不同温度(150、250、350 ℃)和不同时间(1、2、3 h)的时效处理,并与抛光前后的力学性能、显微组织以及析出相进行了对比。结果表明:0Cr25Al5电热合金机械抛光后有析出相Cr₃C₂析出,集中于晶界处分布,其形状以棒状、针状为主,并伴有少量球状析出相;机械抛光后再经250 ℃×1 h时效处理时的硬度下降,抗拉强度略有上升、伸长率略有下降,残余应力明显降低,晶粒度大小不变,析出相基本为球形,尺寸较小,分布较为集中且多分布于晶界,为析出相Cr₃C₂溶解并以离位析出的方式重新形成的Cr₇C₃,同时由于时效温度较低,氧化膜无再生现象,符合后续拉拔工艺要求。与未时效相比,经250 ℃×1 h时效处理的0Cr25Al5电热合金在后续拉拔过程的模具损耗、断丝率均明显下降,成材率上升。     

Influence of Nickel on the Oxidation Resistance at High Temperatures of Thin Chromium Coatings

Oxidation of Metals - The oxidation behavior of four valve steels (X33CrNiMn23-8, X50CrMnNiNbN21-9, X53CrMnNiN20-8 and X55CrMnNiN20-8) covered with thin sputter-deposited coatings containing...     

Effect of Lanthanum on Oxidation Behavior of Ni-20Cr- 18W-1Mo Alloys at 1373 K for 100 h in Air

对有无La元素的Ni-20Cr-18W-1Mo基高温合金进行了1373 K/100 h的等温氧化试验,分别采用扫描电镜(SEM),X射线能谱(EDX)和X射线衍射(XRD)分析了氧化试样的氧化膜表面和截面形貌特征。结果表明,La元素的添加能够极大地提高基体和氧化膜之间的结合力,使得Ni-20Cr-18W-1Mo基高温合金在1373 K时的抗氧化性提高,相比于未添加La元素的Ni-20Cr-18W-1Mo基高温合金,添加La元素的高温合金具有更小的抛物线氧化速率常数κ"和较弱的氧化膜剥落,同时,形成的表面氧化膜更加致密纤细。