Effects of composite oxide scale on oxidation resistance of superalloy K273

A Si-containing K273 superalloy was made using intermediate frequency induction furnace in the study. In the testing of oxidation resistance,the oxidation process of the alloy specimens during the testing at 900℃ for 500 h was examined by oxidation weight gain method.The morphology and composition of the oxide scales were determined using scanning electron microscope(SEM)and X-ray diffraction(XRD),respectively.The effects of the transferring of ions and electrons on the oxidation resistance were further analyzed microscopically by semiconductor oxide models.The results show that the composite oxide scales consist of Cr2O3,SiO2 and spinel- type oxide MCr2O4,with flat and compact structure,and fine grains in uniform distribution.All of these endow the superalloy K273 with strong oxidation resistance.The reason for the powerful oxidation resistance of the composite scale is that the formation process of P+N type semiconductor oxide enables to consume most of the surplus negative and positive ions in the oxide scales,which makes the number of the mobile ions and electrons dropped enormously,and the transfer rate of them falls heavily.So the oxidation rate of the metal phase in the alloy matrix is reduced significantly.

Effect of Hot-Rolled Oxide Scale on Dry/ Wet Cyclic Corrosion Behavior of Q370qNH Steel in Simulated Industrial Atmosphere Environment

The corrosion behavior of Q370qNH steel in the presence and absence of hot-rolled oxide scale in simulated industrial atmospheric environment was studied by dry/wet cycle accelerated corrosion experiments.The experimental results show that the corrosion type of bare steel is uneven overall corrosion and large size pitting corrosion in small areas;that of oxide scale sample is local dissolution corrosion and small size pitting corrosion in large areas,and corrosion rate is much smaller than that of bare steel.The corrosion products of both steels are composed ofα-FeOOH,γ-FeOOH,Fe_(2)O_(3),and Fe_(3)O_(4),but the formation mechanism is different.The bare steel generatesα-FeOOH andγ-FeOOH through“acid regeneration cycle mechanism”;the oxide scale sample generates hydroxides mainly through the gradual dissolution of the oxide film,and then through“the acid regeneration cycle mechanism”.With the extension of corrosion time,the electrochemical stability of the sample with oxide scale increases,but the change of tafel curve of bare steel sample is not obvious.In simulated industrial atmosphere,the existence of hot-rolled oxide scale can facilitate the formation of dense rust layer on the surface of Q370qNH steel,which is more protective than bare steel.

Influences of Alloying Elements on Oxidation Behavior of Steels and Microstructure of Oxide Scales

In order to figure out the oxidation behavior of steels during heating,five micro-alloyed steels were subjected to continuous and isothermal oxidation using the thermo gravimetric analyzer and the Gleeble-3500thermo-mechanical simulator.The microstructure of oxide scales,especially the thickness fractions of Fe2O3,Fe3O4 and FeO layers,was analyzed using the scanning electron microscope(SEM),electron probe microanalyzer(EPMA)and electron backscattered diffraction(EBSD)techniques.The micro-alloyed steels containing alloying elements(Si,Cr,Ni and Cu)show a higher oxidation resistance compared with the low carbon steel.It is found that alloying elements accumulated at scale/substrate interface during high temperature oxidation.Alloying elements function in two ways in the oxidation of steels:one is enhancing the scale/substrate interface and consequently suppressing the blister of scales;and the other is impeding the outward diffusion of iron cations from substrate to scales,resulting in the decrease of oxidation rate.As the diffusion of iron cations is impeded,the thickness fractions of Fe2O3 and Fe3O4of micro-alloyed steels are more than those of low carbon steels.

Dynamic evolution of oxide scale on the surfaces of feed stock particles from cracking and segmenting to peel-off while cold spraying copper powder having a high oxygen content

The oxide scale present on the feedstock particles is critical for inter-particle bond formation in the cold spray(CS)coating process,therefore,oxide scale break-up is a prerequisite for clean metallic contact which greatly improves the quality of inter-particle bonding within the deposited coating.In general,a spray powder which contains a thicker oxide scale on its surface(i.e.,powders having high oxygen content)requires a higher critical particle velocity for coating formation,which also lowers the deposition efficiency(DE)making the whole process a challenging task.In this work,it is reported for the first time that an artificially oxidized copper(Cu)powder containing a high oxygen content of 0.81 wt.%with a thick surface oxide scale of 0.71μm.,can help achieve an astonishing increment in DE.A transition of surficial oxide scale evolution starting with crack initiations followed by segmenting to peeling-off was observed during the high velocity particle impact of the particles,which helps in achieving an astounding increment in DE.Single-particle deposit observations revealed that the thick oxide scale peels off from most of the sprayed powder surfaces during the high-velocity impact,which leaves a clean metallic surface on the deposited particle.This makes the successive particles to bond easily and thus leads to a higher DE.Further,owning to the peeling-off of the oxide scale from the feedstock particles,very few discontinuous oxide scale segments are retained at inter-particle boundaries ensuring a high electrical conductivity within the resulting deposit.Dependency of the oxide scale threshold thickness for peeling-off during the high velocity particle impact was also investigated.

Effect of oxide scale on corrosion behavior of HP-13Cr stainless steel during well completion process

The well completion process in oil and gas industry,aiming to build effective exploitation,is divided into acidizing and formation water production process.Oxide scale(OS)formed on the inner wall of the HP-13Cr stainless steel tubes during the hot extrusion process changes the surface roughness.The effects of OS on the corrosion of HP-13Cr stainless steel during well completion process were studied by corrosion measurement,spectra analysis,microscopic observation and numerical simulation.The results indicate that the OS make no change of phase distribution and element composition of corrosion scale,while the increasing OS roughness is the dominant factor for accelerating corrosion rate during the well completion process.In acidizing process,the greater surface roughness OS of HP-13Cr stainless steel increases the corrosion rate obviously due to a larger interfacial area in contact with the aggressive environment.During subsequent formation water production process,the turbulence eddy,formed at locations characterized with greater surface roughness OS,can deteriorate the corrosion scale and accelerate the mass transfer of the corrosive species,resulting in more serious corrosion.

EFFECT OF MICROSTRUCTURES ON CYCLIC-OXIDATION BEHAVIOUR OF TiAl ALLOY

ＥＦＦＥＣＴＯＦＭＩＣＲＯＳＴＲＵＣＴＵＲＥＳＯＮＣＹＣＬＩＣ－ＯＸＩＤＡＴＩＯＮＢＥＨＡＶＩＯＵＲＯＦＴｉＡｌＡＬＬＯＹＺｈｏｕＫｅｃｈａｏ；ＨｕａｎｇＢａｉｙｕｎ；ＱｕＸｕａｎｈｕｉ；ＨｅＹｕｅｈｕｉ；ＫｏｎｇＧａｏｎｉｎｇ（ＰｏｗｄｅｒＭｅｔａ...

Microstructural analysis and hot corrosion behavior of HVOF-sprayed Ni–22Cr–10Al–1Y and Ni–22Cr–10Al–1Y–SiC(N) coatings on ASTM-SA213-T22 steel

The present paper deals with the investigation of microstructure and high-temperature hot corrosion behavior of high-velocity oxy fuel(HVOF)-produced coatings. Two powder coating compositions, namely, Ni22Cr10Al1Y alloy powder and Ni22Cr10Al1Y(80 wt%;microsized)–silicon carbide(SiC)(20 wt%;nano(N)) powder, were deposited on a T-22 boiler tube steel. The hot corrosion behavior of bare and coated steels was tested at 900°C for 50 cycles in Na2SO4–60 wt%V2O5 molten-salt environment. The kinetics of corrosion was established with weight change measurements after each cycle. The microporosity and microhardness of the as-coated samples have been reported. The X-ray diffraction,field emission-scanning electron microscopy/energy dispersive spectroscopy, and X-ray mapping characterization techniques have been utilized for structural analysis of the as-coated and hot-corroded samples. The results showed that both coatings were deposited with a porosity less than2%. Both coated samples revealed the development of harder surfaces than the substrate. During hot corrosion testing, the bare T22 steel showed an accelerated corrosion in comparison with its coated counterparts. The HVOF-sprayed coatings were befitted effectively by maintaining their adherence during testing. The Ni22Cr10Al1Y–20 wt%SiC(N) composite coating was more effective than the Ni–22Cr–10Al–1Y coating against corrosion in the high-temperature fluxing process.

Analytical modeling of oxide thickness variation of metals under high temperature solid-particle erosion

The paper presents a study of model development for predicting the oxide thickness on metals under high temperature solid-particle erosion.The model is created based on the theory of solid-particle erosion that characterizes the erosion damage as deformation wear and cutting wear,incorporating the effect of the oxide scale on the eroded surface under high temperature erosion.Then the instantaneous oxide thickness is the result of the synergetic effect of erosion and oxidation.The developed model is applied on a Ni-based Al-containing(Ni–Al)alloy to investigate the oxide thickness variation with erosion duration of the alloy at high temperatures.The results show that the thickness of the oxide scale on the alloy surface increases with the exposure time and temperature when the surface is not attacked by particles.However,when particles impact on the alloy surface,the oxide thickness is reduced,although oxidation is continuing.This indicates that oxidation does not benefit the erosion resistance of this alloy at high temperatures due to the low growth rate of the oxide.

Hot Corrosion Behavior of Hf-Doped NiAl Coating in the Mixed Salt of Na_(2)SO_(4)+K_(2)SO_(4)at 900℃

The hot corrosion behavior of the NiAl coating and the 5Hf-NiAl coating induced by mixed salt at 900℃was investigated.Comparing with the NiAl coating,the 5Hf-NiAl coating exhibited superior hot corrosion resistance because the addition of Hf promoted the formation of protective oxide scale and reduced the growth rate of oxide scale.Therefore,internal sulfides were not present in the 5Hf-NiAl coating after hot corrosion for 140 h.Cr exhibited different distribution in the two coatings since the addition of Hf changed the hot corrosion process of the coating.Hf and Ti in the 5Hf-NiAl coating trapped and captured sulfur,preventing the penetration of sulfur into the coating.The hot corrosion mechanism of the two coating and the effects of Hf on this process were discussed in this work.

高密度金属间化合物纳米颗粒强化型单晶高熵超合金的高温氧化行为

本文系统研究了单晶L_(12)强化富Co高熵超合金Co_(41)Ni_(35)Al_(11.5)-Ta_(2.5)Cr_(4)Ti_(6)(at%)在空气中800,900和1000℃下的氧化行为.结果表明,该合金在800至1000℃范围内均表现出优异的抗氧化性.其中,在800℃氧化后,合金表面形成了双层氧化膜,由TiO_(2)/Al_(2)O_(3)混合层和Cr_(2)O_(3)层组成.连续但不致密的Cr_(2)O_(3)层不能有效抑制内部氧化和内部氮化.在900和1000℃时形成复杂的分级结构氧化膜,主要成分为尖晶石、Cr_(2)O_(3)、TiTaO_(4)和Al_(2)O_(3).值得注意的是,在900至1000℃的温度范围内发现抗氧化性出现了反常提高.具体来说,在1000℃下形成的更复杂和连续的五层氧化层,特别是底部的连续致密的Al_(2)O_(3)层,赋予合金在1000℃下优异的抗氧化性.

Effect of Silicon and Aluminum Addition on Corrosion Behavior of ODS Iron-Based Alloys in Liquid Lead–Bismuth Eutectic

The long-term corrosion behaviors of four variants of oxide dispersion strengthened(ODS)iron-based alloys in the stagnant oxygen-saturated lead–bismuth eutectic(LBE)at 550℃ were studied herein.The effects of silicon and aluminum content on the thickness,morphology and composition of the oxide scale were explored with the aid of X-ray diffraction(XRD),scanning electron microscopy(SEM),electron probe micro-analyzer(EPMA)and X-ray photoelectron spectroscopy(XPS).The addition of 1.5 wt%silicon is not able to contribute to forming a protective external silicon oxide film on the surface of aluminum-free ODS iron-based alloy,while the addition of aluminum promotes the formation of a thin and continuous alumina oxide scale.In the meantime,an appropriate amount of silicon becomes the heterogeneous nucleation site for alumina during the initial stage of oxidation,giving rise to the rapid formation of a protective alumina scale.However,excessive silicon has a negative impact on the formation of continuous alumina scale,because it may compete with aluminum to absorb more oxygen.The result of oxidation kinetics in ODS iron-based alloy shows that the parabolic rate constant of the alumina oxide scale is 3–4 orders of magnitude lower than that of the scale mainly composed of iron and chromium oxide.

Oxidation behavior of ferritic/martensitic steels in flowing supercritical water

The oxidation behavior of two Ferritic/Martensitic(F/M)steels including novel SIMP steel and commercial P91 steel were investigated by exposure to flowing deaerated supercritical water(SCW)at 700℃for up to 1000 h.The kinetic weight gain curves follow parabolic and near-cubic rate equations for SIMP and P91 steels,respectively.X-Ray Diffraction analysis showed the presence of magnetite and a spinel phase in flowing SCW for both steels.The morphology and structure of the oxide scales formed on these two steels were analyzed.The relationship between the microstructure and oxidation behavior and the reason that SIMP steel showed better oxidation resistance than P91 steel were discussed.