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 Heating Rates on the Formable Oxide Scale on a C-Steel Surface

Oxide scale formation on a C-steel surface has been investigated using linear heating rates ranging from 0.1℃/min to 10℃/min at high temperatures. The studies on the oxide scale formation at high temperature (650℃) at slower heating rate (0.1℃/min) shows that the kinetic regime is linear. X-ray diffraction measurements revealed that the scale constituents are significantly influenced by the heating rate. The adherence of the scale was improved by using slower heating rate (0.1℃/min-≤650℃), while above such degree the scale was susceptible to cracking and flaking out of the alloy surface. In fact, the development of oxide growth stresses can cause considerable scale cracking. As well, variation of the crystallite sizes under the aforementioned conditions might affect the scale stacking to the alloy surface. The secondary electron detector images of the oxide scale shows that the scale was imperfectly smooth and there were a number of voids and defects in the scale skin, especially at fast heating rate. This observation could be attributed to defects of the as-received alloy. In general, slower heating rate reduced the defects of the scale and improved its adherence.

Influence of oxide scales on the corrosion behaviors of B510L hot-rolled steel strips

The influence of oxide scales on the corrosion behaviors of B510 L hot-rolled steel strips was investigated in this study. Focused ion beams and scanning electron microscopy were used to observe the morphologies of oxide scales on the surface and cross sections of the hot-rolled steel. Raman spectroscopy and X-ray diffraction were used for the phase analysis of the oxide scales and corrosion products. The corrosion potential and impedance were measured by anodic polarization and electrochemical impedance spectroscopy. According to the results, oxide scales on the hot-rolled strips mainly comprise iron and iron oxides. The correlation between mass gain and test time follows a power exponential rule in the damp-heat test. The corrosion products are found to be mainly composed of γ-Fe OOH, Fe3O4, ？-Fe OOH, and γ-Fe2O3. The contents of the corrosion products are different on the surfaces of the steels with and without oxide scales. The steel with oxide scales is found to show a higher corrosion resistance and lower corrosion rate.

Influence of Cr_2O_3-Al_2O_3 Composite Oxide Scale on Oxidation Resistance of ZG40Cr24

Test alloys ZG40Cr24 with alloying of 3 wt% aluminium were cast by intermediate frequency induction furnace. The oxidation resistance of test alloys at 1 000 ℃ for 500 hours was examined according to oxidation weight gain method. The scale morphology and composition were studied using scanning electron microscope （SEM） and X-ray diffraction （XRD） respectively. By energy dispersive spectroscopy （EDS） studies, a kind of composite oxide scale compounded highly by Cr2O3, Al2O3 and spinel MCr2O4 in molecule scale came into being at high temperature. With flat and compact structure, fine and even grains, such composite scale granted complete oxidation resistance to alloy ZG40Cr24. The oxidation resistance mechanism was studied deeply in electrochemistry corrosion. The P＋N semiconductor composite scale composed plenty of inner PN junctions, of which the unilateral conductive and the out-of-order arrangement endowed itself insulating in all directions. The positive and negative charges in scale could not move, and the mobile number and transferring rate of them both dropped enormously, as a result, the oxidation rate of the matrix metal was cut down greatly. So the composite scale presented excellent oxidation resistance.

DEFLECTION METHOD FROM TWO SIDES OXIDATION TO STUDY EFFECT OF Y ON INTERNAL STRESS OF OXIDE SCALES

Au investigation was carried ont of the effect of Y addition upon the internal stress of Al_2O_3 scale formed during oxidation of sputtering coating on Co-30Cr-6Al alloy and the growth stress of oxidized film on Fe-23Cr-5Al alloy heating up to 900℃ in air,using a specially designed deflection method from thin strip specimen with coat- ings,ion-implanted Y on one side and oxidized onto both sides.Results indicate that Y may decrease the internal stress of oxide scale on 2×10^(17) Y^+/cm^2 implanted Co- 30Cr-6Al coating,and increase one on 2×10^(16) Y^+/cm^2 implanted Fe-23Cr-5Al alloy. This seems due to whether or not Y promotes the plastic deformation of oxide scale.In addition,at initial oxidation stage,the change of growth stress of oxide scale formed on Y-implanted Co-30Cr-6Al coating may be related to the influence of Y on oxidation process of the coating.

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.

Study on direct cold rolling of hot rolled austenitic stainless steel SUS 304 with oxide scale

The austenitic stainless steel SUS 304 with oxide scale was directly cold rolled at different reductions of 10%, 20% and 30% respectively. It was proved that the surface quality （lower surface roughness） of the cold rolled products was achieved after subsequent annealing and pickling possesses, compared to the conventional hot rolled No. 1 product, whereas the grain size, mechanical and corrosion-resistant properties were comparable to those of the No. 1 product.

Oxide Scale Growth on High Carbon Steel at High Temperatures

The structure and formation process of oxidation on high carbon steel were investigated with the aid of X-ray diffraction （XRD）, scanning electron microscope （SEM） and Laser Raman spectroscopy （LRS）. The oxide scale formed comprised a three-layer structure, similar to that formed on pure iron and low-carbon steel. For the high carbon steel, however, the scale was essentially a two-layered because of the low proportion of hematite （Fe2O3） formed. The scale thickness increased with the temperature and time of oxidation. The rate of scale thickening rapidly increased above 900 ℃, at which the rate was particularly fast in the first 20 s of oxidation. The proportion of wüstite （Fe1-yO） increased with time and temperature of oxidation, while the magnetite （Fe3O4） remained constant at about 2 μm.

Reduction of Oxide Scale with Hydrogen

During hot rolling process metals will inevitably oxidize because of high temperature and air condition. In order to guarantee the surface quality, acid pickling is applied to remove the oxide scale while waste acid will do harm to the environment. Faced with the problem, by means of reduction process of hot-rolled plates, the oxide scale will be reduced to iron, so that acid pickling is unnecessary. One pass cold rolling procedure was applied. The compres- sion ratios of hot-rolled plates with oxide scale were 10%, 18%, 26% and 31%, respectively. After that, samples mentioned above including a sample without deformation were separately reduced under hydrogen atmosphere condi- tion （5 % H2 ＋95 % Ar in volume percent） at 600-1 000 ℃. The thermal gravimetric apparatus （TGA） was used to establish accurate experimental condition and obtain complete mass loss data. Field emission electron probe microa- nalysis （EPMA） was applied to analyze scale morphology change and composition distribution through the oxide scale. It was found that the sample with 26% compression ratio could be reduced completely at 900 ℃which was favorable to galvanization.

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.

Effects of Coiling Temperature and Cooling Condition on Transformation Behavior of Tertiary Oxide Scale

The influences of coiling temperature and cooling condition on structural transformation of the hot-rolled tertiary oxide scale formed under continuous cooling conditions were studied by thermal gravimetric analyzer. The fourth oxide scale formed under different conditions were classified and plotted. Because the oxide scale structure transformation is diffusion-controlled and the transformation law is similar to ＂C＂ curve, the eutectoid transformation nose temperature is 450 ℃. Under condition of low temperature and high cooling rate, ion diffusion behavior is restricted so that the eutectoid reaction is suppressed, resulting in that the fourth oxide scale is mainly made up of pre-eutectoid Fe304 and FeO without eu- tectoid products. From scale structure transition diagram, the eutectoid reaction process was affected by coiling temperature and cooling rate, leading to various scale structures.

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.

Hot-dip Galvanizing of Carbon Steel after Cold Rolling with Oxide Scale and Hydrogen Descaling

A new processing method for producing hot dip galvanized steel is designed and tested, in which pickling is skipped. Hot-rolled low carbon steel sheets are roiled with oxide scale in an experimental mill at room temperature, prior to annealing under a 20% hydrogen reducing atmosphere and galvanizing on a hot-dip galvanizing simulator. Micro-cracks formed in the oxide scale during cold rolling roughen the steel surface and enlarge the specific surface. Through-thickness cracks provide transport channels for hydrogen, and hence the reduction of oxide scale is en- hanced. When the sheet is dipped in the zinc bath, cracks are submerged by liquated zinc and the defects are not dis- tinct after hot-dip galvanizing. The overlay coating occludes with rough surface of the sheet, whereby a superior coat- ing adherence is realized.

Theoretical consideration on composite oxide scales and coatings

The present paper discussed some fundamental aspects on composite oxide scales and coatings for protection of alloys from high temperature oxidation, the related thermodynamic conditions, special mechanical characteristics and a sealing mechanism. It was proposed that the oxide scales and coatings with a composite structure should possess superior mechanical properties than that with a single phase oxide. It also showed that the A1203 scales or coatings doped with 3（203 and ZrO2 （or YSZ）-A1203 composite coatings possessed superior properties at high temperatures. In such composite oxide scales and coatings, the fracture resistance of the scales was increased by the toughening effect, the thermal stress was decreased owing to the increase of thermal-expansion coefficients, and A1203 phase could seal the alloy substrate well In addition, the kinetic equation of thermal growth oxide on alloy covered with composite oxide coatings was derived.

Growth Rate,Microstructure and Phase Composition of Oxide Scales for Three Typical Steels in Simulated Continuous Casting Process

C-rowth rate, microstructure and phase composition of scale layer formed during oxidation in 56 % H2O9%O2 N2 and following continuous cooling in ambient air were experimentally investigated by means of optical microscopy, scanning electron microscopy （SEM）, energy-dispersive spectroscopy （EDS） and X-ray diffraction （XRD） for 45, 20CrMnTi and TTS443M steels respectively, to examine the effects of strand surface temperature and steel composition on the scale formation in simulated continudus casting process. The growth rates were found to be ap proximately parabolic and the calculated activation energy of TTS443M steel is much higher than those of the two other steels. For 45 and 20CrMnTi steels, the scale layers were looser and a distinct gap formed at the scale-sub strate interface at higher strand surface temperature. The dominant phases within the scale were iron oxides except for FeO · Cr2O3 phase simuhaneously existing in the oxide scale of 20CrMnTi steel. On the other hand, the scale layer formed on TTS443M steel was compact and tightly attached to the steel surface. At both lower and higher strand surface temperature, iron oxide was main phase in external layer of the scale, while chromia was dominant in inner layer with an appreciable Cr enrichment.

HIGH TEMPERATURE OXIDATION AND STRUCTURE OF SCALE AND WHISKERS FOR Fe-Ni-Cr SEALING ALLOYS

The oxidation rate,the growth,morphology and structure of oxide scale and whiskers for Fe-Ni-Cr sealing alloys in H_2-H_2O atmosphere at high temperatures have been studied. The growth rate of scale is controlled by diffusion.The scale is composed of Cr_2O_3 and spinel(Fe,Mn)O·Cr_2O_3 and the oxide whisker,are spinel(Fe,Mn)O·Cr_2O_3.

Effects of Scale Composition on Oxidation Kinetics of Fe-based Superalloy

By oxidation weight gain method, four groups of Fe-based superalloys with different content of chromium, aluminium and silicon were tested at 1 200 ℃ for 500 hours. According to the oxidation weight gains, the oxidation kinetic curves were plotted, and the equations were regressed by least square method and non-linear curve fitting. The effects of different scale compositions on the morphology and oxidation kinetic law were studied further by analysis of X-ray diffraction （XRD） and scanning electron microscope （SEM）. It is found that the compounded scale is composed of Cr2O3, Al2O3, SiO2 and FeCr2O4, with compact structure and fine grains, possessing complete oxidation resistance at 1 200 ℃, and the oxidation kinetic curve follows the power function of y=axb （a0, 0b1）. When the compounded scale lacks Al2O3 or SiO2, it becomes weak in oxidation resistance, but the oxidation kinetic curve still follows the power function with bigger parameter b. When Cr2O3 is absent, the kinetic curve shows two parts： the slow adding of oxidation weight gains at the beginning and the ascending line in the end. Such scale loses oxidation resistance completely.

Kinetics of hydrogen reduction and the influence of gas composition on hot- rolled steel scale after temper rolling

For replacing the presently employed pickling method with a more environmentally friendly descaling method, hydrogen reduction of oxide scale formed during hot rolling was studied at 800℃ under a varied atmosphere. The hydrogen level and water vapor content in the reducing atmosphere were found to influence the reduction rate wherein increasing hydrogen level as well as decreasing water vapor content resulted in faster oxide reduction. The reduction reaction substantially obeyed a parabolic rate law. Oxide scale of a usual thickness （approximately 7 micrometer） could be reduced almost completely in an atmosphere of 20 vol. % hydrogen with a water vapor content corresponding to a -40℃ dew point at 800℃. When lowering the hydrogen level to 10% and increasing the water vapor content to a 10℃ dew point,quite a large extent of the oxide scale was retained,which might be attributable to the formation of an outermost dense layer of pure iron at the early stage of reduction.

HIGH-TEMPERATURE OXIDATION OF FGH96 P/M SUPERALLOY

High temperature oxidation behaviors of FGH96 P/M superaUoy have been studied in air at temperatures ranging from 600 to 1000℃. By means of isothermal oxidation testing, X-ray diffraction, SEM （scanning electron microscopy）, and EDS （energy dispersive X-ray spectroscopy） analyses, the oxidation kinetics as well as the composition and morphology of scales were investigated. Thermodynamic calculations were used to explain the oxidation mechanism. The results showed that as the oxidation temperature increased, the oxidation rate, the scale thickness, and scale spallation increased. FGH96 P/M superalloy exhibits good oxidation resistance at temperature below 800℃. The oxidation kinetics follows an approximately parabolic rate law, and the oxide layer was mainly composed of Cr2O3 TiO2 and a little amount of NiCr2O4. The oxidation is controlled by the transmission of chromium. titanium, and oxygen through the oxide scale.