The high-temperature corrosion of alloy 800 in carburizing,oxidizing, and sulfidizing environments

This paper reports the results of corrosion experiments of a commercial iron-nickel-chromium alloy in multiple oxidant gases at 800, 900, 1000, and 1100°C. Two sets of experiments were performed at low oxygen activities; one with high sulfur activities and the other with high sulfur and with high carbon activities. The scale growth and the morphologies are interpreted in terms of the calculated thermodynamic stability of the product phases. The calculated chromium sulfide-chromium oxide equilibrium coexistence lies at systematically lower oxygen potential than the experimentally observed transition. The transition from sulfidation to oxidation occurs over a range of at constant .     

High temperature corrosion of alloy Haynes 556 in carburizing/oxidizing environments

The iron based alloy Haynes 556 has been recently used in hydrocarbon and carbonaceous environments due to its excellent resistance to carburization. This alloy outperforms stainless steels and some of the best commercial carburization‐resistant nickel‐based alloys. This paper is concerned with the behavior of alloy Haynes 556 in high temperature carburizing environments containing trace amounts of oxygen. Thermal cyclic exposures were conducted in 2% and 10% CH₄/H₂ gas mixtures at 800, 900, 1000, and 1100°C for 10 cycles, 50 h each. Carbon activities, oxygen partial pressures, and stabilities of oxides and carbides were used to identify the role played by the reaction products in providing protection. Thermodynamic analyses, weight changes, and microstructural characterization were correlated with environmental parameters and alloy composition to elucidate the causes of its marked resistance to carburization. The results indicate protective character in both gas mixtures under all exposure conditions except the most aggressive, namely 1100°C in 10% CH₄/H₂ gas mixture. Below 1000°C, the formation of Cr, Al, and Si oxides along with Cr carbides provides the primary means of protection. Catastrophic failure at 1100°C was manifested by extensive fracture and crack development within the outer substrate surface in the 10% CH₄/H₂ gas mixture resulting in a dramatic increase in weight gain. This has been attributed to the increased carbon pick‐up, coupled with the loss of the protective outer scales.     

High-temperature corrosion in mixed gas environments

Scaling reactions between pure metals and multiple oxidant gases are reviewed briefly. It is recognized that elemental oxidant activities are usually so low that the actual reactant species are heteronuclear molecules such as SO₂, CO₂, etc. The formation of duplex, sulfide-oxide scales on iron and manganese, even when sulfide is unstable with respect to oxide, is attributed to direct reaction with SO₂. The persistence of the metastable sulfide is due to its preservation by the rapidly growing scale. The reaction of pure chromium with a number of mixed gases is also discussed. The continued formation of carbides and nitrides beneath an external Cr₂O₃ scale layer indicates that the latter material is permeable to gas species. Interaction among different gas species is observed, and is attributed to selective adsorption on internal surfaces within the chromium oxide. New work on the reaction of alloys with mixed gases is reported. Several austenitic heat-resistant alloys were exposed at 1000°C to gases containing one, two or all of the oxidants carbon, sulfur and oxygen. Gases containing two or more oxidants produced multiple zones of internal precipitation. The precipitates were chromium-rich oxides, sulfides and carbides arranged in order of thermodynamic stability: oxides beneath the external scale, carbides deepest within the alloys and sulfides in an intermediate zone overlapping the oxide zone. Each precipitate zone widened according to parabolic kinetics. This finding confirms the as yet untested prediction made by J. L. Meijering in 1971. However, the rate at which a particular zone grows changes according to presence of other oxidants. Interactions between the oxidants can be large and reaction rates are currently not predictable.     

High-temperature oxidation of Al-Mg alloys

Surface-analysis techniques (x-ray photoelectron spectroscopy and Auger electron spectroscopy) and electron microscopy (SEM and TEM) have been used to study the mechanism of steady-state high-temperature oxidation of Al-Mg alloys. Two high-purity alloys containing 0.4 and 2.0 wt.% Mg were heat-treated in dry air at 550°C up to 90 hr. It was found that the oxide layer was composed of MgO and spinel (MgAl₂O₄), the major constituent being MgO. The molar concentration of MgO decreased with increasing depth, while that of spinel increased. The rate-controlling mechanism for the growth of the oxide layer in the Al-0.4Mg alloy was the solid-state diffusion of Mg in the MgO-spinel constituents. For alloys of higher-magnesium content, the growth of the oxide layer was controlled by solid-state diffusion of Mg through the adherent protective oxide areas and by the transport of Mg vapor across voids formed between the alloy substrate and the oxide layer.     

Mixed oxidant corrosion in nonequilibrium gasifier environments

The major use of high-temperature steel alloys in gasifiers is in heat exchangers for cooling hot syngas, consisting mainly of CO and H₂ with lesser amounts of H₂O and CO₂ and minor quantities of H₂S and HCl. Metal temperatures range from 250 to 600°C, gas temperatures from 250 to 1200°C. Because of rapid cooling the composition of the gas does not change with temperature. Therefore the gas is not in equilibrium at the metal surface. Calculations show that such gases have lower oxygen and sulfur pressures than equilibrated gases at the same temperature. This makes the results of previous laboratory studies less appropriate for predicting mixed oxidant corrosion in gasifiers. For this reason the present study was carried out using nonequilibrium gas mixtures, similar to gases, produced in entrained-slagging gasifiers. Most corrosion experiments were carried out at 540°C, as this is a common temperature for superheaters and hot-gas cleanup systems. Iron-base model alloys containing 35% Ni, 20% Cr, and various minor alloying additions were studied. Three corrosion regimes were identified over the range of conditions studied, depending on the sulfur-to-oxygen pressure ratio of the gas and the alloy composition. At high P_{S 2}/P_{O 2} ratios a somewhat protective FeCr₂S₄ scale formed on all alloys. Below this layer internal oxidation and sulfidation occurred at a slow rate. At lower P_{S 2}/P_{O 2} ratios nonprotective Fe(Ni, Cr)S external scales formed. These allow rapid internal oxidation of the chromium in the alloy, resulting in high corrosion rates. Under the same conditions very low corrosion rates are obtained when silicon is added to the alloy, because the presence of SiO₂ precipitates makes the internal-oxidation layer protective. Thus, the same corrosion model is operative in all three corrosion regimes: external sulfidation of iron and nickel, together with internal oxidation of chromium and other strong-oxide formers.     

High-temperature corrosion performance of plasma-sprayed CrNiMoSiB coatings

This study examined the effects of application parameters for plasma spraying and CCh- laser glazing of two types of chromium-base coatings. Coatings were deposited by low-pressure and atmospheric plasma spraying. The high-temperature corrosion resistance of Cr-Ni-2.5Mo-lSi-0.5B (55 and 58% Cr) coatings was evaluated with respect to structural and compositional changes both in the as- sprayed condition and after CCb- laser glazing. Coatings that were deposited by atmospheric plasma spraying and subsequently laser glazed showed excellent resistance to oxidation and sulfate- vanadate attack at 900 °C due to the formation of a protective chromia film and a high silica concentration on the top layers of the oxide.     

Investigations of the degradation of high-temperature alloys in a potentially oxidizing-chloridizing gas mixture

The degradation of high-temperature alloys in argon-5.5% oxygen-0.96% hydrogen chloride-0.86% sulfur dioxide at 900°C under isothermal and thermal cycling conditions has been investigated. All the alloys showed reasonable resistance under isothermal conditions, although the Al₂O₂ ***-forming material, alloy 214, gave the lowest amount of corrosion, consistent with Al₂O₃ being a more effective barrier than Cr₂O₃ to inward penetration of chlorine or sulfur-containing species from the environment. Significant internal corrosion was observed for some alloys. Degradation of all the alloys was much more severe under thermal cycling conditions because of the failure of the protective scales. In all cases, formation of volatile chlorine-containing compounds was observed. Degradation of the alloys resulted from the penetration of chlorine-containing species through the initially formed oxide scale and formation of chlorides or, possibly, oxychlorides at the alloy-scale interface or in the subjacent alloy. The sulfur dioxide did not play any obvious role in the process.     

The initial stages of high-temperature corrosion of Fe-Cr-Ni and Cr-Ni alloys in a carburizing atmosphere of low oxygen partial pressure

The initial corrosion reactions on a number of Fe-Cr-Ni and Cr-Ni alloys when exposed in a H₂-CH₄ gas mixture ofa _c =0.8 and 10^–30 bar at1098 K have been investigated. All alloys were studied with surfaces in boththe cold-worked and electropolished conditions. The experiments, in whichexposure times up to 400 rain were used, were carried out in a conventionalcorrosion rig and also in a small reaction chamber contained within an X-rayphotoelectron spectrometer. The use of XPS, SEM, and TEM showed thattransient -Cr₂O₃ layers formed on all alloy surfaces during the heat-up period.On the electropolished surfaces, the oxide layers developed nonuniformly withrespect to microstructure and thickness, whereas homogeneous, compact, andfine-grained -Cr₂O₃ layers formed on the cold-worked specimens. Followingan incubation period, M₇C₃ carbides nucleated on the surfaces; they grew byconsuming the less stable -Cr₂O₃ and also through diffusion of metal fromthe alloy substrate. The mode of nucleation of the M₇C₃ and its rate of growthwere strongly dependent on the character of the transient -Cr₂O₃ layers. Afterremoval of -Cr₂O₃, carbon ingressed into the substrate where M₂₃C₆ wasformed internally. These results are broadly in line with the thermodynamicstability predictions and are discussed in terms of simple models.     

铸造Mg-2%Ca合金的拉伸蠕变和抗腐蚀性能(英文)

对分别添加Zn和Al的Mg-2%Ca合金的拉伸蠕变和抗腐蚀性能进行比较。结果表明:添加Zn比添加Al能更加显著地提高Mg-2%Ca合金在170°C下的蠕变性能。然而,添加Al对提高合金的耐腐蚀性能更为有效。由于添加Zn的合金的凝固区间更为宽大,因此,为了生产高质量的铸件产品,对铸造工艺的控制要求更加严格。     

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.     

High-temperature corrosion of Co-15Mo alloys containing ternary additions of Al in mixed-gas environments

The corrosion behavior of Co-15 at.% Mo alloys containing up to 20at.% Al in gaseous H ₂ -H ₂ O-H ₂ S mixtures was studied over the temperature range of 600–900°C. The corrosion kinetics of all alloys followed the parabolic rate law over the temperature range of interest. Corrosion resistance increased with increasing aluminum content. Complex scales formed on the alloys, consisting of an outer layer of cobalt sulfide and a heterophasic inner layer. Al ₂ O ₃ formed only at high temperatures in alloys having aluminum additions of 15at.% or more. The absence of Al ₂ O ₃ in some cases is due to the small volume fraction of the intermetallic phase CoAl in the alloys and the nature of the slow growth rate of Al ₂ O ₃.Improvement in corrosion resistance is attributed to the presence of a ternary sulfide, Al _0.55 Mo ₂ S ₄,and Al ₂ O ₃ in the inner layer.     

Observations on the role of oxide scales in high-temperature erosion-corrosion of alloys

The results are presented of exposure to a controlled high-temperature erosive gas stream of a series of alloys, which were selected to represent the range of microstructures and mechanical properties available in commercial high-temperature alloys. Analysis of the kinetic and morphological data suggested that the high-temperature oxidation behavior of a given alloy plays a very important role in determining its erosion-corrosion behavior under the conditions studied. In terms of relative behavior, alloys which are weak but ductile at temperature, and which form tenacious oxide scales, exhibited the highest resistance to high-temperature erosion-corrosion. Simple models were developed to describe the expected interaction between high-temperature oxidation and erosion.     

Internal oxidation of Ag-in alloys: Stress relief and the influence of imposed strain

The kinetics of internal oxidation of silver-indium alloys containing 3.5, 5.9, and 9.8 at.% In in air at temperatures 773 to 973 K were established by TGA with no load applied to the specimens. Silver nodules free of oxide particles were observed to form at the surface during internal oxidation. The volume of these silver nodules was comparable to the total volume increase caused by internal oxidation. The alloys were also creep tested during oxidation in air at creep rates varying from 10^–7 to 5×10^–5 s^–1 at 773, 873, and 973 K. The parabolic rate constants k_p for the internal oxidation of the solute were determined from the measured widths of the internal oxidation zones. A small increase in k_p was observed with increased strain rate. The large volume change associated with internal oxide formation resulted in a stress gradient between the stress-free surface and the internal oxidation front which is under a high compressive stress. Stress relief occurred by transport of silver to the surface. A Nabarro-Herring creep type mechanism based on lattice diffusion of Ag cannot account for the high rate of silver transport to the surface. Pipe-diffusion controlled creep is proposed as the mechanism of stress accommodation by silver diffusion.     

High-temperature oxidation of Fe-Cr alloys in wet oxygen

Fe-Cr binary alloys have been oxidized in a stream of oxygen containing different amounts of water vapor at 900–1000°C to study the effects of water vapor. The Fe-Cr alloys exhibit an initial protective behavior due to formation of a Cr-rich scale and followed by a nonprotective breakaway oxidation due to formation of iron-rich scale. The appearance of the breakaway oxidation was very sensitive to the water vapor content in the atmosphere. The higher the water vapor content, the earlier the breakaway oxidation takes place. Increasing the oxidizing temperature or decreasing the Cr content in the alloys facilitate an earlier breakaway oxidation. The breakaway oxidation was inhibited effectively by surface-applied CeO₂ particles before oxidation. The oxide scales were examined and analyzed by optical metallography, X-ray diffraction, SEM, and EPMA. A mechanism of the effects of water vapor has been proposed.     

Effects of electric parameters on corrosion resistance of anodic coatings formed on magnesium alloys

Anodic coatings were obtained by micro-arc oxidation on AZ91HP magnesium alloys in a solution containing 10 g/L NaOH and 8 g/L phytic acid. The effects of electric parameters including frequency, final voltage, duty cycle and current density on the corrosion resistance of anodic coatings formed on the magnesium alloys were investigated by using an orthogonal experiment of four factors with three levels. The results show that the final voltage plays a main role on the coating properties. The orders of affecting corrosion resistance and coating thickness are separately ranked from high to low as, final voltage>duty cycle>current density>frequency and final voltage>current density>frequency>duty cycle. The final voltage influences the corrosion resistance of the anodized samples mainly by changing the surface morphology and coating thickness.     

Correlation between microstructure and corrosion behaviour of Bi-Zn solder alloys

ABSTRACT

Bi-Zn solder alloys with different Zn contents (1.5, 2.7 and 5wt-%) were prepared by casting and the correlation between the microstructure and corrosion behaviour by mean of direct current electrochemical tests was studied. The surface analysis of the hypoeutectic Bi-Zn alloy samples revealed the presence of needle-like ZnO and very small agglomerated spherical particles. By contrast, eutectic alloy presented the formation of a uniform and compact film of ZnO, which is related to the better distribution of Zinc in the Bi-rich matrix. Despite the increase in Zn content compared to the hypoeutectic alloy, the corrosion rate showed similar values regardless of its content in the alloy. The Bi-5wt-% Zn alloy presented the highest limiting current density, and consequently, the highest degree of corrosion of the studied alloys. The pro-eutectic phase consisting of large and thick Zn fibres is preferentially dissolved, promoting a selective attack that penetrates inside the sample.