Pre-oxidized Alloy Research Articles

Effect of pre-oxidation at 850 °C under low oxygen pressure on the zinc liquid corrosion resistance of Fe-20Cr-5B–3Al alloy

In the process of hot-dip galvanizing, components working in liquid zinc for a long period of time naturally corrode and fail prematurely. Meanwhile, Cr and Al elements can be used to improve the corrosion resistance of Fe-B alloy in Zn liquid, unfortunately, the α-Fe matrix still gets preferentially corroded. In this work therefore, pre-oxidation is performed under a low oxygen pressure to form oxides of Cr2O3, Al2O3 and FeCr2O4 on the surface of the alloy, which effectively prevents the diffusion of zinc liquid. The oxidation behavior of Fe-5B–20Cr-3Al alloy at 850 °C is then analyzed by thermodynamic calculation and experimentally studied. The corrosion properties of the pre-oxidized alloy in Zn-55 wt% Al liquid and pure zinc liquid are subsequently investigated. Findings revealed that the oxide film forms under oxygen pressure of 10−20.5 atm. is dense, and the thickness reaches 8 µm. Moreover, FeCr2O4 and a small amount of B and Cr oxides are distributed in the upper part of the oxide film, coupled with internal oxidation of Al2O3 on the α-Fe phase. These ensure the continuity and compactness of the oxide film, thus exhibiting the best corrosion resistance to zinc liquid and zinc aluminum liquid. Consequently, pre-oxidation of Fe-5B–20Cr-3Al alloy under low oxygen pressure at 850 °C is proposed as an efficient method to ensure corrosion resistance of the alloy in a zinc-aluminum solution.

Corrosion behaviour of alumina-forming heat resistant alloy with Ti in high temperature steam

The corrosion behaviour of an alumina-forming heat resistant alloy with high Ti (Fe-35Ni-16Cr-4.5Al-3Ti, wt%) was evaluated in high temperature steam. The alloy developed interfacial α-Al2O3 layer during the pre-oxidation treatment in air at 750 °C. However, it formed external Cr2O3 and internal γ-Al2O3 oxides on the austenite matrix during the steam corrosion at 750 °C for 1000 h. This behaviour is attributed to the increased oxygen permeability into the austenite matrix from the synergistic effect of steam and Ti. Meanwhile, the pre-oxidized alloy with interfacial α-Al2O3 layer showed improved corrosion protection with reduced weight gain in steam environment.

Experimental investigations on time, temperature and atmosphere influence on microstructure and mechanical properties of borosilicate glass to Kovar-alloy seals

Glass-metal seals have widespread use in chemical industries and electronic engineering. The sealing of Kovar alloy (Fe-Ni-Co) to borosilicate glass was performed by the use of an intermediate oxide layer which matched well with the two materials at both of its sides. The oxide layer was obtained by pre-oxidation of Kovar alloy in a humid nitrogen atmosphere at 850 °C and 20 min and was mainly composed of Fe3O4. The sealing of the glass to the pre-oxidized metal was then carried out considering temperature, time, and atmosphere. The optimum sealing conditions consisted of a nitrogen atmosphere, a temperature of 950 °C, and a time of 28 min. Other requirements were comprised of an adequate wetting of the pre-oxidized alloy surface by the glass and an appropriate dissolution of the surficial metal oxide in the glass, without formation of detrimental crystal phases. These requirements were shown to depend on the values of temperature, time and atmosphere of sealing, being optimized with the values mentioned above. The microstructural evolution was studied by the SEM and EDS analyses. The results showed that at temperatures of 1000 °C and higher, the more is the sealing time, the more is the extent of nucleation and growth of the fayalite phase, excessive growth of which results in embrittlement and mechanical-property degradation. Strength studies showed that the temperature and the time of sealing have significant effects on the seal strength, the maximum of which is met with sealing at 1000 °C for 4 min. The value of shear strength thus obtained was 9.84 MPa.

Effects of metal dusting relevant exposures of alloy 601 surfaces on carbon formation and oxide development

Ni and Fe are excellent catalysts for carbon formation, and industrial alloys are therefore susceptible to metal dusting corrosion; a costly issue in e.g. synthesis gas manufacture. The objective of this work is to better understand the initial reaction phenomena leading to metal dusting, and thereby minimize the corrosion through optimum alloy selection and pretreatment. Pre-oxidized alloy 601 samples were subjected to carburizing gaseous environments at 750 °C, and carbon formation and surface oxide layer development were investigated by SEM, optical microscopy, AES and Raman spectroscopy. Thin (S)TEM/EDS cross-section lamellae were prepared by Focussed Ion Beam milling.Beyond the initial incubation period, less carbon is formed under 10% CO/Ar than under synthesis gas with finite low carbon activity. Cr2O3 evolves as a thin surface oxide layer with only CO reacting and more ordered carbon develops with increasing exposure time. In contrast, oxidation yields (Ni, Fe, Cr)3O4 spinel formation while the materializing carbon remains its disorder during prolonged exposure to synthesis gas. The metal dusting corrosion rate is hence lowered due to Cr2O3 stabilization, while the spinel represents an unstable redox state that continuously yields new carbon. A fine-grained alloy surface structure is also found beneficial to the Cr2O3 formation.

Corrosion resistance of alumina-forming alloys against molten chlorides for energy production. I: Pre-oxidation treatment and isothermal corrosion tests

Advanced components in next-generation concentrating solar power (CSP) applications will require advanced heat-transfer fluids and thermal-storage materials that work from about 550°C to at least 720°C, for integration with advanced power-conversion systems. To reach the cost target, less-expensive salts such as molten chlorides have been identified as high-temperature fluid candidates. High-strength alloys need to be identified and their mechanical and chemical degradation must be minimized to be used in CSP applications. Approaches for corrosion mitigation need to be investigated and optimized to drive down corrosion rates to acceptable levels—in the order of tens of micrometers per year—for achieving a long system lifetime of at least 30 years. Surface passivation is a good corrosion mitigation approach because the alloy could then be exposed to both the liquid and the vapor phases of the salt mixture. In this investigation, we pre-oxidized the alumina-forming alloys Inconel 702, Haynes 224, and Kanthal APMT at different temperatures, dwelling times, and atmospheres to produce the passivation by forming protective oxides at the surface. The pretreated alloys were later corroded in molten MgCl2 – 64.41wt% KCl at 700°C in a flowing Ar atmosphere. We performed electrochemical techniques such as open-circuit potential followed by a potentiodynamic polarization sweep and conventional long-term weight-change tests to down-select the best-performing alloy and pre-oxidation conditions. The best corrosion results were obtained for In702 pre-oxidized in zero air at 1050°C for 4h. Metallographic characterization of the pre-oxidized alloys and of the corroded surfaces showed that the formation of dense and uniform alumina scale during the pre-oxidation appears to protect the alloy from attack by molten chloride.

Corrosion of pre-oxidized nickel alloy X-750 in simulated BWR environment

Samples of pre-oxidized Alloy X-750 were exposed to a simulated boiling water reactor environment in an autoclave at a temperature of 286 °C and a pressure of 80 bar for four weeks. The effect of alloy iron content on corrosion was investigated by comparing samples with 5 and 8 wt% Fe, respectively. In addition, the effect of two different surface pre-treatments was investigated. The microstructure of the formed oxide scales was studied using mainly electron microscopy. The results showed positive effects of an increased Fe content and of removing the deformed surface layer by pickling. After four weeks of exposure the oxide scale consists of oxides formed in three different ways. The oxide formed during pre-oxidization at 700 °C, mainly consisting of chromia, is partly still present. There is also an outer oxide consisting of NiFe2O4 crystals, reaching a maximum size of 3 μm, which has formed by precipitation of dissolved metal ions. Finally, there is an inner nanocrystalline and porous oxide, with a metallic content reflecting the alloy composition, which has formed by corrosion.

Assessment of wear coefficients of nuclear zirconium claddings without and with pre-oxidation

In the cores of pressurized water nuclear reactors, water-flow induced vibration is known to cause claddings on the fuel rods to rub against their supporting grids. Such grid-to-rod-fretting (GTRF) may lead to fretting wear-through and the leakage of radioactive species. The surfaces of actual zirconium alloy claddings in a reactor are inevitably oxidized in the high-temperature pressurized water, and some claddings are even pre-oxidized. As a result, the wear process of the surface oxide film is expected to be quite different from the zirconium alloy substrate. This study attempts to measure the wear coefficients of zirconium claddings without and with pre-oxidation rubbing against grid samples using a bench-scale fretting tribometer. Results suggest that the volumetric wear coefficient of the pre-oxidized cladding is 50 to 200 times lower than that of the untreated cladding. In terms of the linear rate of wear depth, the pre-oxidized alloy wears about 15 times more slowly than the untreated cladding. Fitted with the experimentally-determined wear rates, a stage-wise GTRF engineering wear model demonstrates good agreement with in-reactor experience in predicting the trend of cladding lives.

The effect of pre-oxidation treatment on the corrosion behavior of amorphous Al1−xZrx solid-solution alloys

The electrochemical behavior of as-deposited and pre-oxidized amorphous Al1−xZrx solid-solution alloys (x=0.25, 0.32, 0.49, 0.65, 0.74) in 1M HCl was investigated by the micro-electrochemical technique. It was found that, for higher Zr alloying contents, x≥0.49, the as-deposited Al1−xZrx alloys exhibit passive corrosion behavior due to the formation of a protective Zr-rich passive film on the Zr-rich alloy surface. The native oxide layer on the as-deposited Al-rich Al1−xZrx alloys (x<0.49) is much less stable, which rapidly results in a total break-down of the protective character of the native Al-rich oxide layer upon anodization in 1M HCl. The pre-oxidized am-Al1−xZrx alloys all exhibited superior corrosion behavior as compared to their as-deposited pendants, attaining passive current densities as low as 3.5×10−2μA/cm2 at 1000mV. The superior corrosion resistance of the pre-oxidized alloys is attributed to a combination of characteristics of the thermally-grown amorphous (Al0.33Zr0.67)O1.83 overlayer: (i) a relatively high Zr fraction, (ii) a structurally and chemically uniform microstructure, (iii) a stable, minimal residual growth defect structure (in the surface region) established by a sufficiently large time of oxidation.

Effect of pre-oxidation on the oxidation resistance of spinel-coated Fe–Cr ferritic alloy for solid oxide fuel cell applications

Low-temperature-sintered MnCo2O4–MnO2 coatings have been prepared on pre-oxidized SUS430 ferritic alloy by slurry coating. The effect of pre-oxidation treatment before slurry coating is then investigated. Microstructural and electrical characterizations show that 25 h of pre-oxidation at 800 °C could significantly improve the oxidation resistance of the coated alloy and effectively inhibit the increase in area-specific electrical resistance during long-term oxidation. These effects can be explained by the interfacial reactions between the coating and the pre-oxidized alloy during sintering and oxidation tests. Furthermore, this study suggests that the dense reaction layer at the coating-alloy interface could be the key to improving the oxidation resistance of metallic interconnects with low-temperature-sintered spinel coatings.

Effect of pre-oxidization on the cyclic coking and carburizing resistance of HP40 alloy: With and without yttrium modification

The pre-oxidation results demonstrate that rare earth elements play an important role on the scale adherence of alloy after the oxidation at elevated temperature. The better scale adherence of the yttrium modified alloy permits a superior cyclic coking and carburizing resistance. This is because the integrity chromia layer, which prevents fast ion diffusion, can retard the outer catalytic coking and the inner carburizing of pre-oxidized alloy. As for the alloy under the cast condition, severe coking and carburization has been detected, which was also observed on the pre-oxidized samples where the scale suffered from severe spalling.

Effect of H2S on High Temperature Corrosion of Fe–Ni–Cr Alloys in Carburizing/Oxidizing Environments

This investigation involves the corrosion behavior of two Fe–Ni–Cr alloys containing different Si content at 1050 °C in carburizing-oxidizing environments (typical of ethylene pyrolysis) with varied concentration of H2S. High-Si containing alloy could form thinner but less uniform oxide scale than low-Si alloy after pre-oxidation due to the barrier effect of continuous SiO2 at interface of scale/substrate. Pre-oxidized alloy showed a better resistance to carburization/sulfidation attacks than the bare alloy in absence of pre-oxidation. It was found that carburization and sulfidation of the Fe–Ni–Cr alloys could be prevented in the environment with a ratio of \( P_{{{\text{H}}_{ 2} {\text{S}}}} /P_{{{\text{H}}_{ 2} }} \) at 1.7 × 10−5. When the sulfur partial pressure was lower than this value, oxides were found to be converted to porous and non-protective carbides. When the sulfur potentials were increased, manganese or chromium sulfide on outer layer and internal sulfide stringers mixed with silicon oxide in substrate could be formed. Under high sulfur partial pressures, spallation of outer sulfide or oxide scale was observed on high-Si alloy due to less stability of oxide layer formed at surface which was converted to sulfide faster than on low-Si alloy.

Oxidation of pre-oxidized GH128 alloy implanted with Ce + at 1 000 °C

The influence of Ce implantation into preformed scales with a dose of 1×10 17 ions/cm 2 on the subsequent oxidation behavior of GH128 alloy at 1 000 °C in air was investigated. The pre-oxidation was carried out at 1 000 °C in air for 1 h and 5 h respectively. Cr 2O 3, NiO and NiCr 2O 4 formed on the surface of all specimens. Ce implantation decreased the subsequent oxidation rate of both the alloy and the 1 h pre-oxidized alloy, however, had no effect on that of the 5 h pre-oxidized alloy. The beneficial effect was most obvious in the directly implanted alloy. During the cyclic oxidation for 600 h, Ce implantation for all specimens with or without preferential oxidation played a similar beneficial effect on the oxide spallation resistance. The results indicate that Ce incorporated into the oxide scales affects the diffusion of the reaction species to some extent, the wavy interface and small grain structure make a significant contribution to improving the spallation resistance of the oxide scales.

Effect of composition control of precursor and pre-oxidation of alloy sheath on Jc properties of Ag–Cu alloy sheathed Bi2223 tapes

When an Ag–Cu alloy is used as inner sheath material of Bi2223 tape, the formation of many large (Sr,Ca)14Cu24Ox (14:24) particles in the early stage of sintering degraded the Jc values. To suppress this formation reaction for the improvement of Jc, the effects of composition control of precursor and the pre-oxidation of Ag–Cu alloy sheath were investigated in this study. The Jc value of the tape controlling precursor composition, in which tape the amount of impurity particles was slightly decreased, was recovered up to 1.2×104A/cm2. On the other hand, the tape sheathed with pre-oxidized alloy could suppress the formation of the 14:24 particles in the early stage of sintering through the reduction of Cu potential in alloy sheath. As a result, the size and number of 14:24 particles were decreased in the final tape and improved the Jc value. Based on these results, the composition control of precursor and pre-oxidation of alloy sheath were applied simultaneously to fabricate the tape. This combination of both processes resulted further reduction of the number and size of the 14:24 particles in the final tape, and the best Jc value of 1.8×104A/cm2 was achieved. This proves that the high Jc can be achieved in the Ag–Cu alloy sheathed tape by the suppression of impurity particles.

Characterization of iron-based alloy interconnects for reduced temperature solid oxide fuel cells

The oxidation kinetics and electrical properties of oxide scales thermally grown on the surface of a commercial ferritic alloy have been investigated on the un-oxidized and pre-oxidized alloys as functions of temperature and time under oxidizing atmospheres with four different electrodes. Oxidation kinetic studies with the un-oxidized alloys show a nearly parabolic dependence on time of oxide-scale growth rate, but a significantly increased growth rate with a coating of LSCo (La 0.6Sr 0.4CoO 3− δ ) compared to those without and with the coatings of LSM (La 0.85Sr 0.15MnO 3)+LSGM (La 0.8Sr 0.2Ga 0.83Mg 0.17O 2.815) and platinum. Short-term resistance measurements in stagnant air as a function of temperature with pre-oxidized alloys indicate that the oxide scale has a semiconducting transport property. The overall activation energy includes a term from small-polaron hopping inside the oxide scale Δ H m and terms Δ H i and Δ H j from charge transfers at the electrode/oxide-scale and alloy/oxide-scale interfaces, respectively. For the LSCo electrode, long-term resistance measurements as a function of time with un-oxidized alloys reveal a secondary oxidation mechanism related to the formation of an insulating spinel phase in addition to a primary oxidation mechanism associated with the formation of Cr 2O 3. SEM observations show that oxidation of the un-oxidized alloy in the presence of an oxide electrode results in considerable interdiffusion of Cr and the electrode cations, especially Co, across the interfaces. Since the ASR values of the oxide scale measured with oxide electrodes quickly approach the permitted limit of a practical SOFC, highly recommended for prevention of a secondary electrochemical oxidation of iron-based alloy interconnects is (1) the use of an oxide coating having purely electronic conductivity and/or (2) prior-to-use conditioning of the alloys via pre-oxidation.

The effect of pre-oxidation of chromia and alumina forming alloys on erosion in laboratory simulated fluidized-bed conditions

The effects of pre-oxidation of alloys have been investigated in laboratory simulated fluidized bed erosion-corrosion environments. Two alloys, Incoloy 800H and Feeralloy, which formed chromia and alumina respectively, under steady state oxidation conditions, were tested. The research consisted of preoxidizing the alloys in a range of oxidation environments, including air, oxygen and H 2/H 2O. The scales were then examined and analysed to determine the composition and adhesion to the metal. The erosion-corrosion tests were carried out at 500°C, using 200 μm silica as erodent, at velocities of 1.9 m s −1, and for exposure times of up to 168 h. The specimens were analysed subsequently by analytical scanning electron microscopy and X-ray diffraction. The pre-oxidation results showed that, for both alloys, the scales formed in H 2/H 2O were more adherent and slower growing than those formed in air or oxygen. The erosion-corrosion tests on pre-oxidized specimens indicated that the scales formed in H 2/H 2O were the most resistant to particle impacts. The results also showed that pre-oxidized Incoloy 800H eroded more rapidly than pre-oxidized Feeralloy. As the erosion-corrosion rates of the two non pre-oxidized alloys were dissimilar to each other, it was not possible to establish whether there was a significant difference between the erosion resistance of the pre-formed scales, under these conditions. Possible reasons for the improved performance of the scales formed in low oxygen pressures are discussed. The differences in erosion-corrosion rates of the alloys are also addressed, with emphasis on alloy deformation characteristics and transient oxidation rates.