Characterization of plasma electrolytic oxidation coatings formed on Mg–Li alloy in an alkaline silicate electrolyte containing silica sol

We investigate the influence of silica sol addition on the ceramic coatings of Mg–Li alloy by plasma electrolytic oxidation (PEO) in an alkaline silicate electrolyte. Scanning electron microscopy, X‐ray diffraction, X‐ray photoelectron spectroscopy, and energy dispersive spectroscopy are employed to characterize the microstructure and composition of the ceramic coatings. The anti‐corrosion behavior of the ceramic coatings is evaluated by potentiodynamic polarization measurements in conjunction with electrochemical impedance analysis. The ceramic coating formed in the electrolyte containing silica sol contains SiO₂ and Mg₂SiO₄ phase and has more uniform morphology and higher corrosion resistance than that formed in the electrolyte without addition of silica sol.     

The Effects of Molybdenum on the High-Temperature Oxidation Resistance of Thin Foils of Fe–20Cr–5Al at Very High Temperatures

Thin foils of Fe–20Cr–5Al alloys are susceptible to breakawayoxidation once the aluminum content of the substrate has fallen below somecritical value. The combined addition of 0.1 wt.% lanthanum and 0, 1, or 2wt% molybdenum has a beneficial effect on the high-temperature oxidation ofsuch foils. Lanthanum has the well-known reactive-element effect on adhesionof the protective alumina scale, thereby increasing the time to onset ofbreakaway oxidation, while, for alloys containing molybdenum, breakawayoxide spreads relatively slowly over the specimen in comparison to alloysthat contain no molybdenum. In particular, molybdenum-containing alloys areable to develop a protective Cr₂O₃ layer at the breakawayoxide–substrate interface. Conversely, molybdenum-free alloys form aninternal-oxide zone in the substrate adjacent to this interface, rather thana Cr₂O₃ layer, so breakaway oxide spreads rapidly. A martensitic phase isobserved in the substrate adjacent to the breakaway oxide formed on Fe–20Cr–5Al–La specimens, which means that the-phase has transferred to the -phase at the temperature ofthe oxidation test (1150°C). Conversely, -phase is retained inthe molybdenum-containing alloy, even after breakaway takes place, sincemolybdenum, which is a strong ferrite former, is enriched in the alloyadjacent to areas of breakaway oxide. The diffusion rate of chromium isslower in the than in the -phase so a continuouschromium-rich oxide layer, which is effective in inhibiting breakawayoxide from spreading, cannot be established at the breakawayoxide–substrate interface for the molybdenum-free alloys.     

High-Temperature Oxidation Behavior of Iron–Chromium–Aluminum Alloys

The detailed oxidation behavior of Fe–10Cr alloys, containing aluminumin the range of 2–8% by weight, was studied in pure oxygen at 1 atmpressure. The investigations were performed over the temperature range950–1050°C under cyclic conditions (3-hr cycles) in each case. Thecyclic-oxidation resistance, as measured by the specific weight-gain values,was observed to progressively improve with increasing aluminum content inthe alloy. For a particular aluminum content, however, the oxidationresistance decreased with increasing temperature. Following the initialtransient-oxidation period, a healing layer of chromia was established onthe four alloys. The lower-aluminum alloys (2–4% Al) were observedto end up with Fe-rich oxide scales under the experimental conditions atall temperatures, whereas those containing aluminum in the range of6–8% formed -Al₂O₃ scales.     

原位合成TiC弥散强化304不锈钢的高温氧化行为

研究了TiC颗粒增强304不锈钢在950℃的高温氧化性能。结果表明:TiC的加入降低了304不锈钢氧化速率,且随着TiC的加入量越多,氧化速率降低的越明显。进入稳定氧化期后,304钢的氧化遵守线性规律,含2%TiC的304不锈钢氧化遵守线性-抛物线规律,而含6%TiC的304不锈钢的氧化遵守抛物线规律。TiC的加入显著的改善了304不锈钢氧化膜的抗剥落性。     

A study on the oxidation characteristics of cast irons containing aluminum

Isothermal-oxidation characteristics of cast irons containing aluminum (5–15% Al) from 700 to 1000°C in air have been studied. In addition to massgain measurements, the morphology and composition of the oxide scales have been examined by SEM-EDX system and XRD analysis. A normal Fe–5Al–C alloy does not develop protective, adherent scales. Even the addition of misch metal and calcium silicide to such an alloy does not improve its oxidation resistance. But aluminum cast iron develops considerable oxidation resistance only when a sufficient quantity of silicon is also present in the alloy. Treatment of the alloy with misch, metal and calcium silicide together assists in protective scale formation. Among the alloys investigated Fe–15Al–Si–C treated with misch metal and calcium silicide shows minimum oxidation at 1000°C.     

High-temperature oxidation of Fe-Cr-Al-Si alloys extruded into honeycomb structures

The oxidation behavior of Fe–20Cr–5Al–(0.5–5)Si and Fe–(12–20)Cr–(5–7)Al–(1–2)Si alloys extruded into honeycomb structures has been investigated at 1150°C in air for up to 500 hr. The oxidation weight gains decrease with increasing Si and Cr contents in the 5-Al alloys. Si additions are more efficient than Cr additions to reduce the weight gain. Increasing Si content in the 5-Al alloys suppresses the formation of an iron-chromium complex oxide, forming mullite and vitreous silica in the scale, although the location is not clearly indicated. The 5-Si alloy shows anisotropy in elongation of the honeycomb specimen during oxidation in the Fe–20Cr–5Al–xSi alloys, whereas alloying with Si and Cr does not improve the oxidation resistance of the 7-Al alloys significantly. These results are explained by Wagner's theory of a secondary getter. However, we point out additionally that the difference between Si and Cr in the Pilling-Bedworth ratio and the solubility of their oxides in the Al₂O₃ scale may contribute to the significant effect of Si additions. Finally, this paper demonstrates that the selected Fe–Cr–Al–Si honeycombs having walls 200 m thick show excellent oxidation resistance over 500 hr at 1150°C in air. The time to catastrophic oxidation is roughly proportional to the wall thickness in extruded honeycombs.     

Au/Al_2O_3层状复合纳米涂层的制备及其抗氧化性能(英文)

采用磁控溅射法成功制备Al2O3/Au层状复合纳米涂层,所制备的涂层结构致密且由Al2O3层和Au层交替组成。采用高温循环氧化实验对复合涂层在不锈钢基体上的高温抗氧化性能进行分析评价。结果表明:Al2O3/Au层状复合纳米涂层极大地改善不锈钢基体的抗氧化和抗剥落性能。其抗氧化机理与涂层能够有效地抑制氧向合金基体的扩散并促进不锈钢基体中Cr元素的选择性氧化有关;抗剥落机理可归因于复合涂层中的Au层和纳米结构的Al2O3层能够有效地松弛高温热循环过程中产生的热应力,从而提高涂层的抗剥落性能。     

Effect of La and Hf Additions on the High-Temperature Oxidation Resistance of High-Purity Fe–20Cr–5Al Alloy Foils

The oxidation behavior of 30- or 50-m thick high-purityFe–20 w/o-Cr–5 w/o Al alloy foil and similar alloy foilscontaining La and La–Hf was examined in cyclic-oxidation tests at1373 and 1473 K in air. The oxidation process proceeded in three stages. Inthe first stage, an Al₂O₃ scale grew until all the Alin the foil had been removed. In the second stage, a Cr₂O₃layer grew between the Al₂O₃ layer and the substrateon the alloys containing La or La–Hf, while a (Cr, Al)₂O₃layer formed on the alloy without La and La–Hf. In the third stage,breakaway oxidation occurred. The addition of La decreased the oxidationrate in both the first and the second stages. The addition of La–Hfdecreased the rate further. The growth rate of alloys containing La orLa–Hf in the second stage was found to be proportional to thediffusion rate of oxygen in the Al₂O₃ scale. Therefore,it is inferred that the inward oxygen diffusion rate in the Al₂O₃scale on the alloy containing La–Hf was reduced compared with that onthe alloy containing La, resulting in a decrease in the oxidation rate inthe first stage.     

Influence of Yttrium-Alloying Addition on the Oxidation of Alumina Formers at 1173 K

The oxidation behavior of three commercial Fe–Cr–Al alloys, Kanthal APM, Kanthal A1, and Kanthal AF (containing alloying additions of yttrium), has been investigated during isothermal exposures in air at 1173 K. After an initial transient stage, a diffusional process appears to predominantly control the oxidation kinetics of both alloys. During the transient stage, relatively important mass gains have been registered and the presence of yttrium does not seem to have a significant effect on the oxidation rate. On the contrary, the reactive element markedly influences the parabolic oxidation rate and the composition of the oxide scale. In situ X-ray diffraction (XRD) shows that yttrium promotes the transformation of transition alumina into -Al₂O₃, leading to the formation of a more protective oxide scale.     

Corrosion characterization of microarc oxidation coatings formed on Mg–7Li alloy

Ceramic coatings with thickness of 27 µm were fabricated on Mg–7Li alloy in Na₂SiO₃–C₆H₁₈O₂₄P₆ solution by microarc oxidation (MAO). The morphology and phase composition of MAO coatings were characterized by scanning electron microscopy (SEM) and X‐ray diffraction (XRD). The corrosion behavior of the bare and MAO coated Mg–7Li alloy was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Results showed that the MAO coatings were composed of MgO, Li₂O, and Mg₂SiO₄, and there existed some micropores on the coating surface with a diameter of 3–20 µm. The corrosion potential (E_corr) and corrosion current density (I_corr) of the MAO coated alloy were about ?1.4761 V and 7.204 × 10^?7 A/cm², respectively. The E_corr of the MAO coated alloy increased by 109.6 mV and its I_corr decreased by three orders compared with that of the bare Mg–7Li alloy. The EIS plots indicated that the impedance of the MAO coated alloy was 15 times higher than that of the bare alloy. The fitting parameters showed that the resistance of the MAO coatings was far greater than that of the bare alloy. The dense intermediate layer and the transition layer of the MAO coatings acted as a barrier to hinder the proceeding of solution permeation, remarkably improving the corrosion resistance of the Mg–7Li alloy.     

High-temperature oxidation of a 20/25 stainless steel in high-pressure carbon dioxide

Kinetics of high-temperature oxidation, and of oxide spallation, are reported for a 20Cr-25Ni Nb-stabilized stainless steel, exposed in carbon dioxide at 40 bar pressure in the temperature range 1115–1240 K. Gross weight-gain kinetics were parabolic with respect to time, with an activation energy of 370±16 kJ/mol. Oxidation rates are shown to be consistent with diffusion control in the inner chromia layer of a two-layer oxide. Intergranular oxidation of silicon was observed beneath the oxide scale. Oxide spallation occurred at all temperatures and increased linearly with gross weight gain. At 1240 K 50% of the oxide which formed spalled at the first weighing. Thereafter, the fraction spalling decreased to about 10%. The difference is attributed to a change in the oxide composition from an M₂O₃ structure to M₃O₄, which requires greater compressive stresses to induce spallation during cooling.     

The Application of a Wedge-Shaped Sample Technique for the Study of Breakaway Oxidation in Fe–20Cr–5Al Base Alloys

The evolution of the breakdown of protective alumina scales on Fe–20Cr–5Alalloys has been studied by the use of taper-sectioned samples. The oxidationin air of two model alloys is compared with that of two commercial alloys,Kanthal APM and MA 956 at 1350°C. It is shown that the length of thebreakaway-oxidation region along the taper may be used to rank theperformance of the alloys and that the wedge geometry allows a detailedstudy to be made of the chemical processes involved in the degradation ofthe oxide scale just prior to breakaway oxidation.     

Yttrium sol–gel coating effects on the cyclic oxidation behaviour of 304 stainless steel

The present results reveal the interest of sol–gel coating technique to improve 304 steel high temperature oxidation resistance. An yttrium sol–gel coating appears to enhance the oxidation resistance during isothermal oxidation test, to decrease widely the oxide weight gain and to reduce the initial transient oxidation stage generally observed in the case of blank steels. Moreover, the experimental results confirm that yttrium sol–gel coating also plays a significant role on the cyclic oxidation behaviour of the 304 steel. In fact, the yttrium addition promotes remarkably the prolongation of the period during which the oxide scale still remains adherent to the substrate.     

Phase stability,oxidation, and interdiffusion of a novel Ni−Cr−Al−Ti−Si bond-coating alloy between 900 and 1100°C

A novel, low-expansion experimental Ni–Cr–Al–Ti–Si bond-coating alloy was investigated in the as-cast state concerning its phase stability, oxidation resistance in air, and interdiffusion with single-crystal IN-100 at 900, 1000, and 1100°C. Isothermal oxidative thermogravimetry was employed up to 500 hr. Interdiffusion was compared to a commercial Ni–Co–Cr–Al–Y alloy on IN-100. Oxidized Ni–Cr–Al–Ti–Si specimens and diffusion couples were characterized by metallography, SEM, EDX, XRD, and XRF. The Ni–Cr–Al–Ti–Si alloy provides good oxidation resistance in air at least up to 1000°C. The alloy is an alumina former. Due to its coarse microstructure, other oxides (e.g., rutile) may form and considerably dominate the oxidation behavior. The kinetics of oxidation were correlated with temperature, formation of phases, and morphology of oxides. Interdiffusion fluxes between Ni–Cr–Al–Ti–Si and IN-100 were mainly directed to the superalloy. They were faster than in Ni–Co–Cr–Al–Y/IN-100 diffusion couples.     

Oxygen solubility and a criterion for the transition from internal to external oxidation of ternary alloys

Smith's model is expanded in order to derive expressions to quantitatively describe the oxygen-solubility behavior in ternary alloys as a function of alloy composition. Multicomponent-diffusion theory is used to establish a criterion for the onset of internal oxidation beneath the external scale when oxidizing conditions favor formation of the oxide of the least-noble metal in a ternary alloy. The oxygen-solubility model and the criterion are applied to the oxidation of Ni–Cr–Al alloys in 76 torr of oxygen at 1100 and 1200°C, predicting the minimum Al concentrations required to form a protective Al₂O₃ scale. It shows that sufficient Cr additions would significantly reduce the oxygen solubility and also alter the oxygen distribution in the ternary alloys, avoiding the oxygen supersaturation necessary for the onset of internal oxidation. These two factors make it easier to establish the protective Al₂O₃ scale.     

The Zn-rich corner of the 450 °C isothermal section of the Zn–Bi–Fe–Ni quaternary system

Following up on recent studies of the isothermal section of the Zn–Fe–Ni, Zn–Fe–Bi and Zn–Bi–Ni ternary systems at 450 °C, the Zn-rich corner of the 450 °C isothermal section of the Zn–Bi–Fe–Ni quaternary system with the Zn being fixed at 93 at.% was determined experimentally using the equilibrated alloys approach. The specimens were investigated by means of scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). It was found there exist 4 two-phase regions, 5 three-phase regions and 2 four-phase regions. Two liquid L (Zn) and L (Bi) can coexist with T, ζ and δ-Ni in this isothermal section, no new phase was found in this study.     

Cyclic-Oxidation Behavior of Fe–20Cr–4Al Alloys with Small Amounts of Sulfur at High Temperatures

The cyclic-oxidation behavior of Fe–20Cr–4Al alloys with 3, 35, 53, 104, and 171 ppm sulfur was studied in oxygen at 1273, 1373, 1473, 1573, and 1673 K by mass-change measurements, X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron-probe microanalysis (EPMA). The cyclic oxidation consisted of an 18.0 ksec exposure that was repeated up to five times. Amounts of spalled oxides of the alloys with 35 and 53 ppm sulfur increased after one and two oxidation cycles, and then decreased after three or more oxidation cycles at 1473 and 1573 K. On the other hand, spalling of oxide scales on the alloys with 3 and 171 ppm sulfur was scarcely recognized after all cycles used in this study. SEM observations of both sides of the spalled oxides indicated that spallation was related to the morphology of the surface oxides and to the morphology and volume of cavities at the oxide–alloy interface.     

The influence of thermal cycling on the oxidation and oxide spallation of a 1%Cr–0.5%Mo low-carbon steel

The oxidation and oxide spallation of 1%Cr–0.5%Mo low carbon steel disks in dry oxygen was studied isothermally at 800°C (1073 K) and in thermal cycling between 800 and 600°C (1073 and 873K) followed by cooling at rates from 3 to 100°C/min. Mostly parabolic oxidation kinetics were observed. Thin scales (10 ) were more prone to spalling than thicker scales (20 ). The thickness and growth imperfections of an inner scale layer enriched in chromium, molybdenum, and silicon strongly influenced the probability of cohesive failure exceeding that of adhesive failure of the scale. Cohesive failures in the bulk scale during thermal cycling were probably nucleated at voids and microcracks produced in the initial isothermal period of scale growth. The number of segmented scale layers that became detached during cycling was governed by the number of parallel rows of voids in the scale and not necessarily by the number of cycles.