Hydrogen in Chromium: Influence on the High-Temperature Oxidation Kinetics in H2O,Oxide-Growth Mechanisms,and Scale Adherence

Oxidation of 1-mm thick chromium samples with 7 to <1 wt. ppm hydrogenhas been studied at 900°C in a closed reaction chamber. The gases usedwere O₂, H₂O, and gas mixtures of O₂+H₂Oat a total pressure of about 20 mbar. By means of oxygen labeling intwo-stage oxidations, H₂ ¹⁶O followed by H₂ ¹⁸O,the position of oxide growth within the oxide scale was determined withsecondary-ion mass spectrometry (SIMS). The results are consistent withvisual observations and scanning electron microscopy (SEM), which showsflat oxides with good adherence when formed in H₂O. Whenreplacing O₂ with H₂O in the atmosphere, the oxidationrate increases by a factor of two with an increase of oxide growth at boththe inner and outer part of the oxide. An increased metal (cation) transportis observed when 5 wt. ppm hydrogen is present in the chromium metal priorto oxidation in both O₂ and H₂O. This is detrimentalfor the adherence of the oxide scale. The uptake of hydrogen in H₂Oexposure for 1600 min was measured to 1.5 wt. ppm. In exposures toO₂+H₂O mixtures, no H₂ is formed and no netwater is consumed as long as O₂ is present. A plausible reactionscheme based on an experiment with H₂ ¹⁸O is presentedto explain this observation.     

Effect of Ti and Zr elements with equal mass ratio on microstructure and corrosion resistance of Zn-11Al-3Mg alloy

In this study, the effect of Ti and Zr elements with equal mass ratio on microstructure and corrosion resistance of Zn-11Al-3Mg alloy was investigated. The microstructure was significantly refined and Al-rich phase transformed from dendrite to petal-like with the addition of Zr and Ti elements, due to the Al₃(Ti_xZr_{1 − x}) phase as the nucleation substrate. The corrosion resistance of Zn-11Al-3Mg-x(Ti,Zr) alloy was effectively improved. Moreover, the corrosion products of Zn-Al-Mg alloy were not changed by the addition of Ti and Zr, which are mainly composed of Zn₅(OH)₈Cl₂·H₂O and Zn₆Al₂(OH)₁₆CO₃·4H₂O.     

The oxidation behavior of Fe-20Cr alloy foils in a synthetic exhaust-gas atmosphere

Oxidation tests of rare-earth-modified and Ti-modified Fe–20Cr alloy foils, which are under consideration for catalytic converter supports, were performed in a synthetic exhaust-gas atmosphere (N₂+H₂O+CO₂) between 900°C and 650°C. Between 900°C and 750°C, the rare earths had no effect on oxide growth rates while Ti increased growth rates. Oxide growth rates for the rareearth alloys at 800°C and 750°C are much lower than those found in the literature for oxidation of Fe–Cr alloys or pure Cr in O₂-rich atmospheres. The slow growth rates for the rare-earth alloys agree with literature data for oxidation of stainless steels containing >20% Cr in wet atmospheres and are caused by growth of an oxide scale only one grain thick. At temperatures 700°C, Fe–20Cr alloys grow massive Fe oxides; however, this can be suppressed by adding rare earths or Ti. To ensure good oxide adherence, free sulfur must be eliminated in the alloy by tying it up with a reactive-element addition. Both Ti and the rare earths can be used to tie up S, but the rare earths are more effective. For converter applications, the optimum alloy composition may contain rare earths for good oxide adherence and a small amount of Ti to suppress growth of Fe-rich oxides.     

The effect of gas composition and contaminants on the lifetime of surface‐treated FeCrAlRE alloys

The degradation of a variety of alumina‐forming Fe20Cr5Al based alloys has been investigated at temperatures ranging from 1100°C to 1300°C for up to 4000 h (100 h/cycle) in different oxidising environments such as laboratory air, air + 10 vol% H₂O, air + 60 vol% H₂O and simulated automotive exhaust gas. Seven model alloys with controlled levels of impurities such as P, S and C and carefully controlled levels of additional elements (Y, Zr, Ti, Hf, Si, La, etc.) and two different commercial alloys (Aluchrom YHfAl and Kanthal APMT) were chosen for this study. The investigation showed that the model alloys containing La, Y + Si and Y with added C, and the commercial alloy APMT usually had the lowest initial oxidation growth rates, whereas model alloys containing Y plus Zr, and the commercial alloy YHfAl had the higher oxidation rates regardless of the different oxidising environments. Scale spallation was more prevalent in the case of alloys with low oxide growth rates but changing the levels of water vapour in the oxidising atmospheres had only a minor influence on the degree of spallation or the oxide morphology. The scales formed on the alloys containing La, Y + Si and high C spalled in an adhesive manner (at the scale/metal interface), whereas scales formed on alloys containing Y plus Hf, Zr or Ti cracked and spalled in a cohesive manner (within the scale). Inhomogeneities in the distribution of the alloying additions led to greater changes in the oxide morphologies than any difference in oxidising atmosphere, but the crystallographic textures of all the oxides were similar. This pilot study enabled us to rank the alloys according to their resistance to spallation and also to determine the influence of minor elements when added as tightly controlled single or multiple element additions. Some of these alloys were then used as mechanically weak PVD coatings on a strong (APMT) substrate, and further oxidation experiments confirmed that the coatings then oxidised in a similar way to the bulk alloys.     

Bildung von Aluminiumoxid-Deckschichten auf Eisenbasislegierungen

Formation of alumina layers on iron-base alloys The formation of Al₂O₃-layers has been studied for ferritic alloys Fe-6 Al-M and austenitic alloys Fe-27 Ni-4 Al-M where M – Ti, Zr, V, Nb, W, B, Si… (concentrations in wt%). One or more alloying elements M had been added and in some cases carbon. The oxidation was performed at 1000 °C in H₂O-H₂ mixtures at PO₂ = 10^?19 bar. After ½ h oxidation the oxide layers were investigated by X-ray structures analysis, scanning electron microscopy and Auger electron spectroscopy (AES). The alloy Fe-6 Al and most doped alloys form badly adherent layers, however, on alloys with additions of 0.1 to 1% Ti, Zr, V or Y the oxide layers are fine-grained and well-adherent. The Ti-doped ferritic alloys showed very protective layers, which is caused by the formation of a Ti(C, O)-layer beneath the α-Al₂O₃. The presence of the oxicarbide induces nucleation and improves the adherence of α-Al₂O₃, according to epitaxial relations between ferrite and oxicarbide and between oxicarbide and alumina. The favourable influence of Ti and Zr on the Al₂O₃ formation is also effective on the austenitic alloys.     

Effect of atmosphere composition on the oxidation behavior of MCrAlY coatings

In the present work the effect of atmosphere composition on the growth rate and adherence of the alumina scales was studied using free‐standing MCrAlY‐coatings and TBC‐specimens with MCrAlY‐bondcoats. The exposures comprised isothermal and cyclic exposures in laboratory air and Ar‐H₂‐H₂O at 1100 °C. It is shown that minor Zr‐addition to the bondcoat results in enhanced scale growth and internal oxidation. This effect is independent of the atmosphere composition. As a consequence of the rapid oxide formation the times to TBC failure on the Zr‐containing bondcoat in both atmospheres were much shorter compared to those with Zr‐free bondcoat. In the latter case the formation of a thin compact alumina TGO was slower in H₂/H₂O than in air resulting in significantly longer TBC‐lifetime in the former atmosphere.     

The effect of small additions of Zr, Cr, Mg, Al, and Si on the oxidation of 6∶4 brass

The oxidation behavior of 60%Cu-40%Zn brass having small amounts of Zr, Cr, Mg, Al, and Si was studied between 873 and 1043 K in air. The alloying element of Mg was harmful, while other alloying elements were beneficial to oxidation resistance. Particularly, the simultaneous addition of Al and Si decreased the oxidation rate drastically. During oxidation, Zr formed ZrO₂ Cr formed CuCr₂O₄, Mg formed MgO, Al formed Al₂CuO₄, and Si formed amorphous SiO₂. These oxides were incorporated in the oxide scale composed predominantly of ZnO. The oxide scales formed on all the tested alloys were prone to cracking, wrinkling, and spallation.     

Aluminum oxide adherence on Fe-Cr-Al alloys modified with group IIIB,IVB, VB,and VIB elements

A recent theory, explaining the effect of reactive elements on oxide adherence, states that sulfur adversely affects adherence and that reactive elements improve adherence by tying up sulfur as refractory sulfides. This theory is supported by work presented here, which correlates adherence behavior of Fe-Cr-Al alloys containing group IIIB through VIB elements with the stability of the sulfides that these elements form. Results show that poor adherence is produced by elements that form sulfides less stable than Al₂S₃ (VB and VIB elements), while good adherence is produced by elements which form sulfides more stable than Al₂S₃ (IIIB and IVB elements). In addition, the relative stability of sulfides, oxides, nitrides, and carbides must be considered. For example, group-IIIB elements are effective at much lower concentrations than group-IVB elements, because they react preferentially with S while group IVB elements react with C, N, and O before S.     

Improvements in oxidation resistance by dispersed oxide addition: Al2O3-forming alloys

The improvement in oxidation resistance produced by small additions of active elements to Al ₂O₃-forming CoCrAl alloys is primarily dependent on the formation of oxide pegs which grow into the alloy around the internal oxide particles of the active element; void formation at the alloy-scale interface is also suppressed. The distribution of these pegs is critical and this paper demonstrates that an internal oxidation pretreatment can be used to convert the active element to its oxide in a controlled manner, thereby optimizing the peg distribution. Al₂O₃-forming CoCrAl containing 1% Hf or Ce is internally oxidized in a sealed quartz capsule containing a 50/50 powder mixture of CoAl-Al₂O₃; it was not possible to oxidize internally Y-containing alloys. The isothermal and cyclic oxidation resistance of these alloys is superior to that of alloys not given a prior treatment. Detailed metallographic examination indicates that the prior internal oxidation treatment produces a finer, more uniform distribution of oxide pegs penetrating into the alloy which is more efficient in combatting scale spallation. Furthermore, the lower residual hafnium content in the alloy minimizes large HfO₂ precipitates and the formation of gross Al₂O₃ intrusions, which can initiate scale failure. Thus, by internally oxidizing the alloy first, the advantages of a high alloy Hf content (1%) in producing sufficient oxide pegs, but of the right size, coupled with minimal thickening of the surface scale, can be achieved.Formerly: Department of Metallurgy and Materials Science, University of Liverpool, Liverpool, England.     

几种Ni-Cr-Al墓低偏析高温合金的抗氧化性能研究

在１０００～１１００℃之间,对三种Ｃｒ、Ａｌ含量及微量元素Ｓ、Ｚｒ含量不同的Ｎｉ－Ｃｒ－Ａｌ基合金进行了恒温和循环氧化实验。结果表明,高Ｃｒ（１６Ｗｔ％）,低Ａｌ（３．５Ｗｔ％）、Ｓ（０．００５ｗｔ％）含量,且不含Ｚｒ的合金其氧化膜层主要由连续致密的Ｃｒ２Ｏ３外氧化层和树根状Ａｌ２Ｏ３内氧化层构成,氧化膜的粘附性较好。高Ａｌ（５ｗｔ％）,低Ｃｒ（９ｗｔ％）、Ｓ（０３００５ｗｔ％）含量,且不含Ｚｒ的合金其氧化膜主要由Ａｌ２Ｏ３,ＴｉＯ２,Ｃｒ２Ｏ３构成,氧化膜的粘附性较差,但在该合金中加入０．１Ｗｔ％Ｓ和０．１Ｗｔ％Ｚｒ,氧化膜的粘附性变好,而恒温氧化速率增加。文中对实验结果进行了综合分析。     

High-temperature corrosion of titanium-, niobium-, and manganese-rich Fe-25Cr alloys in H2-H2O-H2S gas mixtures

The simultaneous sulfidation and oxidation of Fe-25Cr, Fe-25Cr-4.3Ti, Fe-25Cr-7.5Nb, and Fe-25Cr-9.0 Mn alloys were studied at 1023, 1123, and 1223 K, respectively, in H₂-H₂O -H₂S gas mixtures. The influences of titanium, niobium, and manganese on the transition from protective oxide formation to the formation of sulfide-rich corrosion products of Fe-25Cr alloys have been investigated. It has been found that additions of titanium and niobium can improve the scaling resistance of Fe-25Cr alloys against sulfidation in H₂ -H₂O -H₂S gas mixtures at high temperatures. However, the addition of manganese does not increase the resistance to sulfidation of Fe-25Cr alloy. The oxide Cr₂Ti₂O₇, which can suppress sulfide formation, formed on the Fe-25Cr-4.3Ti alloy. The addition of manganese to Fe-25Cr does not form more stable and protective oxides than Cr₂O₃ which formed on Fe-25Cr. Thermodynamic stability diagrams are used to explain the experimental results.     

Microstructure,corrosion behavior,and surface mechanical properties of Fe oxide coatings on biomedical ZK60 Mg alloy

The Mg‐5.5Zn‐0.6Zr (in wt%, ZK60) alloy has been surface modified by dual Fe&O ion implantation and deposition (II&D) under different O₂ fluxes from 0 to 40 sccm. The microstructure is investigated by glancing angle X‐ray diffraction, atomic force microscope, and scanning electron microscopy. The results show that the modified layers, with a gradient microstructure of outer deposition region and inner implantation region, are composed of α‐Fe + Fe&Mg mixture, FeO/Fe‐rich oxide + Fe&Mg mixture, and α‐Fe₂O₃/Fe‐rich oxide + Fe&Mg mixture for 0, 10, and 40 sccm O₂ fluxes, respectively. The electrochemical and immersion tests in 37°C Hank's solution indicate an improvement in corrosion behavior under 0 and 10 sccm O₂ fluxes, but a deterioration in corrosion resistance under 40 sccm O₂ flux. In addition, the nanoindentation tests suggest that the dual Fe&O II&D simultaneously enhances the surface hardness and elastic modulus due to the formation of Fe and its oxide coatings.     

The influence of Y,Zr or Ti additions on the high temperature oxidation resistance of Fe-Ni-Cr-Al alloys of variable purity

The influence on oxidation behaviour of both minor elements, such as Si and Mn, and of deliberate small additions of Y, Zr or Ti, has been studied in an austenitic Fe-Ni-Cr-Al alloy. During oxidation, this material develops an inner layer of alumina and an outer layer of Cr₂O₃ and spinels. Y enhances oxidation resistance by promoting alumina formation, decreasing the weight gain under isothermal conditions and improving oxide adherence during temperature cycling. Zr, when added alone, has little effect but produces an additional amelioration in spalling resistance when combined with Y. To a minor extent, Si also appears to have a favourable effect, whereas Ti and Mn, which concentrate in the outer oxide layer, tend to accentuate spinel formation and enhance spalling under cyclic conditions. The stress level measured by deflection during isothermal oxidation is in general agreement with the behaviour under temperature cycling conditions. It is attempted to explain some of these observations on the basis of diffusion mechanisms within the oxide.     

Grain refinement of DHP copper by particle inoculation

Abstract

The viability of grain refining phosphorus deoxidised, high residual phosphorus Cu (DHP-Cu) by particle inoculation through the formation of in situ Zr, Ti, Nb, La, Ce, Gd and Nd oxide particles, additions of WC and NbB₂ particles as well as additions of Ti+B, Zr+B, Al+B and Ca+B with the purpose of forming in situ boride particles have been investigated under TP-1 casting conditions. Nominal additions of P in the range of 300–690?wt?ppm and of Ag of 150?wt?ppm have also been explored. In comparison, it appears that it is more difficult to grain refine DHP-Cu by particle inoculation than it is by the combined addition of P+Ag solute elements to a residual content of 396?ppm P and 143?ppm Ag (and having very low contents of Ti, Zr, Al, Ca and B microalloying elements) for the conditions studied.     

Radiotracer studies of carbon permeation through oxide scales on commercial high temperature alloys and model alloys

Chromia- and alumina-forming commercial high temperature alloys and model alloys were preoxidized at 900 or 1000 °C in H₂-H₂O at a low oxygen potential. The oxide layers were characterized by different methods. The carbon permeation through the oxide layers was studied by exposing the preoxidized specimens to an atmosphere CO-CO₂-H₂-H₂O, tagged with radiocarbon, for long time. The carbon was detected by stepwise polishing and measuring the radioactivity. A slow carbon ingress occurs through chromia layers, differences in the protection by the oxide scale could be tested by the radiotracer method for the different alloys. The alumina layer on Fe-6 Al is not protective, but no carbon ingress could be detected for an alloy Fe-6Al-0.5Ti. Autoradiography, AES and X-ray structure analysis showed the presence of Ti(O,C) beneath the outer Al₂O₃-layer. The oxicarbide improves the nucleation and adherence of the Al₂O₃ and prohibits the carbon penetration. The results were confirmed by gravimetric experiments, after preoxidation samples were exposed to CO-CO₂-H₂-H₂O at high carbon activity (a_c = 1.02), carburization and graphite deposition were retarded or prohibited by dense and well adherent oxide layers.     

Isothermal oxidation behavior of Ni3Al-0.1B base alloys containing Ti,Zr, or Hf additions

The isothermal oxidation behavior of Ni₃Al-0.1B and of this alloy containing additions of approximately 2% Ti, Zr, or Hf was studied in purified oxygen at atmospheric pressure over the temperature range 1300 to 1500 K and for periods up to 400 ks. Ni₃Al was also studied similarly for comparison. The oxidation of Ni₃Al and Ni₃Al-0.1B resulted in an extensive formation of geometric voids on the substrate surface leading to poorly adherent scales. The addition of B to Ni₃Al did not change the oxidation behavior much, however, both the activation energy of oxidation and the relative thickness of the Al₂O₃ layer in the scale were increased. The addition of Ti led to the formation of adherent scales at 1300 K, but the oxidation was accelerated after an initial period at 1300 and 1400 K. At higher temperatures the scale was protective, and the parabolic rate constant, k_p, decreased, however, the scale adherence was impaired by the formation of interfacial voids. The addition of Zr resulted in very adherent scales at all temperatures, but at the same time it increased k_p considerably. The addition of Hf also resulted in very adherent scales at all temperatures and decreased k_p except at 1300 K. The improved scale adherence can be accounted for by the keying effect, since the additives resulted in roughened scale/alloy interfaces. A complex of oxide particle and an associated void was found on the exposed surface of the Hf-containing alloy. This supports the vacancy-sink mechanism.     

Initial oxidation of Cr at 300–730 K in H2 18O/H2 16O/16O2 gas mixtures studiedin situ with18O/SIMS and XPS

The initial oxidation of pure, polycrystalline-chromium metal with gas mixtures of oxygen-labeled water, H₂ ¹⁸O, and oxygen,¹⁶O₂, at temperatures 300–730 K has been studied in situ with secondary ion mass spectrometry, SIMS. The fraction of O in secondary ion species: CrO⁺, Cr₂O⁺, CrOH⁺, Cr₂OH⁺, CrOH₂ ⁺, and Cr₂OH₂ ⁺ originating from water and oxygen was found. The reaction of water and oxygen in the oxygen/water gas mixture with Cr is then revealed. The dissociation of water in its reaction with Cr in such a gas mixture was analyzed. Oxide growth on cleaned Cr surfaces during the reaction with H₂O, O₂ and H₂O/O₂ gas was also studied in situ with X-ray photoelectron spectroscopy, XPS. A correlation between a chemical shift in XPS and the OH content in the oxide is discussed.     

The influence of sulfur on adherence of Al2O3 grown on Fe-Cr-Al alloys

Recently a new theory was proposed to explain the effect that reactive elements have on oxide adherence. Based on data obtained on Ni-Cr-Al-Y material, this theory stated that trace quantities of sulfur in the alloy degrade adherence by weakening the metal-Al₂O₃ bond. The work presented here extends this concept to Fe-Cr-Al alloys by examining Al₂O₃ adherence on foil samples with various bulk sulfur levels obtained using high-temperature vacuum anneals. Results show that long-time vacuum anneals dramatically increase the adherence of the subsequently grown aluminum oxide, concurrent with removal of sulfur from the matrix. This evidence shows that the Al₂O₃-metal bond is intrinsically strong without the presence of reactive elements such as Y or rare earths in the alloy. Sulfur in the alloy, and not void formation, was found responsible for oxide spalling. In addition, voids were eliminated by reducing the sulfur concentration near the oxide-metal interface.     

Self-Repairing Metal Oxides

The oxidation of Cu, Zr, and alloys forming chromia, alumina, and zirconia was studied in a closed reaction chamber in O₂ gas near 20 mbar. Information on the position of oxide growth has been gained from the ¹⁸O/SIMS technique. Rates of O₂ dissociation on metal oxides, Au, and Pt have been evaluated from measurements in labeled O₂. The experimental results indicate that hydrogen in the metal substrates induces increased metal-ion transport in internal oxide surfaces during oxidation, which leads to increased oxide growth at the oxide–gas interface. Experiments also show that oxides of rare-earth metals (REM) and Pt catalyze the dissociation of O₂. An increased rate of O₂ dissociation can lead to increased transport of oxygen ions in the oxides and increased oxide growth at the substrate–oxide interface. A balanced transport of metal and oxygen ions in metal oxides that leads to oxide growth at both the metal–oxide and at the oxide–gas interface is found to be favorable for the formation of protective oxides with good adherence to the metal substrate. Depending on the original proporation of metal–to–oxygen ion transport in the oxide, an addition of hydrogen will increase or decrease the oxidation kinetics. In analogy, an addition of REM will increase or decrease the oxidation kinetics, depending on the original proportion of metal-to-oxygen ion transport.     

Phase Diagram of Ternary Calcium Acetate—Magnesium Acetate—Water System at 298 K, 313 K and 323 K

The solubility data of the calcium acetate-magnesium acetate-water system at 298, 313, and 323 K were measured using the Schreinemaker’s wet residue method, the corresponding phase diagram for the system were constructed. The solid phase in the system at different temperatures was confirmed by the Schreinemaker’s wet residue method, which correspond to Mg(CH₃COO)₂·4H₂O and Ca(CH₃COO)₂·2H₂O. At the studied temperature, no double salt was formed. The crystalline region of Ca(CH₃COO)₂·2H₂O is larger than that of Mg(CH₃COO)₂·4H₂O. The solubility of Mg(CH₃COO)₂ increases with the increasing of the temperature, while the solubility of Ca(CH₃COO)₂ decreases with the increasing of the temperature.