Kinetics and mechanism of high-temperature sulfur corrosion of Fe-Cr-Al alloys

The influence of aluminium on the kinetics and mechanism of high-temperature sulfidation of Fe-Cr alloys containing 20 at.% chromium has been investigated. It has been found that the addition of aluminum greatly improves the scaling resistance of Fe-Cr alloys against attack by sulfur vapors at high temperatures.     

High temperature corrosion of Ni-20Cr and Ni-15Cr-8Fe alloys in sulfur dioxide

Two nickel-base alloys, Ni-20Cr and Ni-15Cr-8Fe, in the form of wire specimens, have been exposed to 100 mbar of sulfur dioxide between 550 and 850°C. For Ni-20Cr, an outer Cr₂O₃ layer formed only at the beginning of the reaction, but very quickly Ni₃S₂ grew preferentially at the exterior by outward diffusion of nickel. The reaction rate is regulated by an external interfacial process. A barrier effect was noted near 645°C associated with the formation of NiCr₂O₄; a new acceleration takes place above 680°C. The external growth of Ni₃S₂ is attributed to the low radius of curvature of the samples. For Ni-15Cr-8Fe, the reaction mechanism is rather similar, except that no barrier effect occurred. A protective Cr₂O₃ layer formed above 800°C in both cases.     

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.     

The oxidation behavior of an aligned Co-TaC eutectic alloy

The oxidation behavior of the directionally aligned eutectic alloy, TaC-Co50B, has been examined between 600 and 1155°C. Parabolic oxidation behavior, isotropic in nature, is found to 800°C. Above 800°C, the oxidation behavior is no longer either parabolic or isotropic. The interaction between the Co matrix alloy and the TaC fibers adjacent to the oxidizing interface is reported as a function of temperature in oxidation.     

Influence of solid-state CaS-CaO-CaSO4 deposits on corrosion of high-temperature alloys in simulated FBC environments

This study addresses questions concerning the likelihood of sulfidation attack of heat-exchanger alloys beneath deposits of sulfur-sorbent material in fluidized-bed combustors. Alloy specimens were exposed at 900°C in calcium sulfate-calcium oxide and calcium sulfide-calcium oxide mixtures, in environments in which the oxygen partial pressures were fixed at values corresponding to the equilibrium values for each solids mixture, using controlled ratios of CO and CO₂. The only source of sulfur in these systems was the calcium sulfate or sulfide. Sulfidation attack of nickel-base alloys occurred in both mixtures, the calcium sulfide-calcium oxide mixture being the more aggressive. Iron-base alloys were less susceptible to attack, although susceptibility increased with increasing nickel content. FeCrAlY-type alloys were resistant to attack. Comparison with corrosion behavior under conditions in which the oxygen and sulfur partial pressures were the same as those used here, but in which the sulfur source was in the gas phase, indicates that the form of the sulfidation attack is similar but that its progress is much slower under solid deposits.     

Effects of mode-I stresses on the oxidation and failure mechanisms of Ni-20Cr and Ni-15Cr-8Fe alloys in sulfur dioxide

Two alloys, Ni-20Cr and Ni-15Cr-8Fe, as wire specimens, were exposed to sulfur dioxide between 325 and 800°C, with applied external stresses (mode I). Their mechanical properties have been investigated, and the variation of their radius has been precised by conductivity measurements. For the Ni-20Cr alloy, below 550°C, the failure process combines cracking and corrosion: Cr₂S₃ crystals formed at the tip of the cracks facilitate their propagation. Above 550°C, no barrier effect is observed, and intergranular corrosion takes place. For the Ni-15Cr-8Fe alloy, mode-I stresses bolster intergranular corrosion.     

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 .     

Transport properties of sulfide scales and sulfidation of metals and alloys

Oxidation of Metals - Defect and transport properties of metal sulfides are discussed, showing the differences from and similarities with oxide systems. The sulfidation kinetics and mechanism of...     

The attack of Co-Cr alloys by Ar-SO2 atmospheres

Cobalt alloys containing up to 25% chromium have been exposed to Ar-10% SO₂ atmospheres at temperatures between 600 and 1000° C. The results show that, although an increase in chromium content leads to a reduction in the reaction rate, even to negligible rates in the cases of the higher chromium contents, all of the alloys are eventually subjected to rapid attack at more or less longer times, depending on the chromium content. The mechanism of the reaction appears to involve the formation of a more or less protective oxide layer which is eventually penetrated by sulfur. The sulfur forms chromium sulfides at the metal-scale interface, removing the chromium from solution and causing an expansion that cracks the protective scale, allowing both the ingress of gas and the formation of rapidly growing cobalt compounds. The process occurs rapidly with Co-5% Cr alloys, whereas, only the initial sulfur penetration is observed with Co-25% Cr alloys during the time scale of the investigation. The penetration of sulfur is thought to occur as a molecular gas species permeating through the scale down physical defects.     

High-temperature sulfide corrosion of cobalt-chromium alloys

The kinetics and mechanism of high-temperature sulfidation of cobaltchromium alloys were investigated. It has been found that the sulfidation rate of low-chromium alloys, up to 2% Cr, is higher than that of pure cobalt. The one-layer homogeneous scale on these alloys is a solid solution of chromium sulfide in cobaltous sulfide. In the case of alloys containing more chromium, up to 43%, sulfidation rate decreases gradually with chromium content, the scale consisting of two layers. The growth mechanism of scales as well as the influence of chromium concentration on sulfidation rate is discussed.This work was carried out under contract No. 03.10 coordinated by the Institute of Physical Chemistry, Polish Academy of Sciences.     

High-temperature corrosion of nickel in SO2

The reaction of nickel with SO₂ has been studied in the temperature range 650–1100°C at SO₂ pressures from 10 to 760 Torr. Reaction kinetics have been studied by thermogravimetry; the reacted specimens have been characterized by means of metallography, scanning electron microscopy, and electron microprobe analysis. The reaction involves oxidation and sulfidation except at sufficiently high temperatures and low pressures of SO₂ (e.g., 1000°C and 10 Torr SO₂) where only formation of NiO takes place. Approximately linear reaction kinetics are observed between 650 and 900°C. Reaction mechanisms are discussed, and the relative importance of oxide formation and sulfidation is interpreted in terms of the thermodynamics of the Ni-O-S system.     

The penetration by sulfur of NiO scales growing on nickel

The transport of sulfur through growing scales may occur by chemical (solution and diffusion) or physical (gas molecule permeation) mechanisms. Both possibilities are examined theoretically for the case of NiO growing on nickel. Experiments are designed and carried out to establish which mechanism plays the major role in sulfur transport. The results indicate that the physical mechanism is likely to be predominant.     

The high-temperature corrosion of Fe-Nb alloys in a H2/H2O/H2S gas mixture

The corrosion of Fe-Nb alloys containing up to 40 wt.% Nb has been studied over the temperature range 600–980°C in a mixed gas of constant composition having sulfur and oxygen pressures ranging from 10^–8 to 10^–4 atm. and from 10^–27 to 10^–18 atm., respectively. All alloys were two-phase, consisting of an Fe-rich solid solution and an intermetallic compound, Fe₂Nb. The scales formed on the Fe-Nb alloys were duplex, consisting of an outer layer of iron sulfide (FeS) and an inner complex layer of Fe_xNbS₂(FeNb₂S₄ or FeNb₃S₆), FeS and unreacted Fe₂Nb. No oxides were detected at any temperature. The addition of Nb reduced the corrosion rate. The corrosion kinetics of Fe-Nb alloys followed the parabolic rate law, regardless of alloy composition and temperature. Platinum markers, attached to the original alloy surfaces, were always located at the interface between the inner and outer scales.     

The corrosion of pure niobium in oxidizing,sulfidizing, and oxidizing-sulfidizing gas mixtures at 600–800°C

The corrosion of pure niobium has been studied at 600–800°C in various environments as part of a study of the corrosion resistance of its alloys with iron, cobalt, and nickel to atmospheres of low-oxygen and/or high-sulfur activities. The results have shown that not only the sulfidation but also the corrosion in mixed atmospheres and particularly the oxidation under low oxygen pressures of pure niobium are quite slow, with kinetics rather similar in the three types of gas mixtures used. The good corrosion resistance of niobium to attack by oxygen under low pressures is quite interesting because this element is corroded very rapidly by oxygen under high oxygen pressures, due to the formation of the nonprotective highest oxide Nb₂O₅ as a main corrosion product.     

The high-temperature corrosion behavior of Co-Nb alloys in mixed-gas atmospheres

The corrosion of Co-Nb alloys containing up to 30 wt.% Nb in H₂-H₂S-H₂O gas mixtures was studied over the temperature range of 600–800°C. The gas composition falls in the stability region of cobalt sulfide and Nb₂O₅ in the phase diagrams of the Co-O-S and Nb-O-S systems at all temperatures studied. Duplex scales, consisting of an outer layer of cobalt sulfide and a complex, heterophasic inner layer, were formed at all temperatures studied. In addition to cobalt sulfide and CoNb₃S₆, a small amount of NbO₂ was found in the inner layer. The reason for the formation of NbO₂ over that of Nb₂O₅ in the scale is that the outer sulfide scale lowers the oxygen activity within the scale into the NbO₂-stability region. Two-stage kinetics were observed for all alloys, including an initial irregular stage usually followed by a steady-state parabolic stage. The steady-state parabolic rate constants decreased with increasing amounts of Nb, except for Co-20Nb corroded at 700°C. Nearly identical kinetics were observed for Co-20Nb corroded at 600°C and 700°C. The presence of NbO₂ particles leads only to a limited decrease of the available cross-section area for the outward-diffusing metal ions. The activation energies for all alloys are similar and are in agreement with those obtained in a study of the sulfidation of the same alloys. The primary corrosion mechanism involves an outward Co transport.     

The high-temperature oxidation resistance of Co-Al alloys

Most Ni and Co-base alloys used for high-temperature service rely on the production of a compact, stable Cr₂O₃ scale for their oxidation resistance. However, as operating temperatures have risen above 900–950° C, the loss of Cr₂O₃ as the volatile CrO₃ has led to an inadequate life span of these alloys, particularly in rapidly flowing, turbulent gas streams. As a result of this, it has been necessary to examine the possibility of using Al₂O₃ as the protective scale. Al₂O₃ has a lower growth rate than Cr₂O₃, it is nonvolatile, and, unlike Cr-containing systems, it is less likely to form compound oxides such as spinels. In this study, the amount of Al which must be present in the Co-Al system to form a continuous layer of Al₂O₃ in the temperature range 800–1000° C has been determined. The quantity was found to rise from about 7–10 wt. % at 800° C to 10–13 wt. % at 900° C and 13 wt. % at 1000° C. Notice has also been taken of the abilities of the alumina-forming alloys to re-form a protective oxide in the event of spalling, blistering, or any other disruptions of the scale, and some cyclic-oxidation checks have been conducted on the Co13Al alloy at 900 and 1000° C.This work has been partly supported by the Science Research Council and one of us (G.N.I.) wishes to thank them for the award of a Science Research Council Research Studentship     

Observations on the effect of low sulfur activity on the oxidation of chromium-depleted zones in a stainless steel

Coupon specimens of 20% Cr-25% Ni-Nb-stabilized steel have been oxidized in an atmosphere of CO ₂-2%CO-670 vppb COS at 0.1 MPa pressure for periods up to 500 hr at 1123 K. Standard specimens, annealed at 1203 K prior to testing, showed an enhancement of iron in the surface scale and a much increased propensity to spall compared with control tests in a sulfur-free atmosphere. The main purpose of the work was to examine the effect of sulfur on the ease of formation of a healing layer under duplex attack produced by depleting specimens in chromium by prior vacuum annealing. It is shown that although a chromium-rich layer had formed, extensive breakdown occurred in the sulphidizing atmosphere leading to continued internal oxidation. Sulfur was found to partition at the base of this attack and to be associated with a large concentration of nickel. Spaliation was also enhanced in the depleted specimens, the favored site being the interface between the spinel and outer iron-rich oxide of the duplex structure. Partitioning of both sulfur and carbon was observed at this interface in those regions of the specimen showing healing layer breakdown.     

Sulfidation and sulfidation-oxidation of nickel- and cobalt-base alloys containing dispersions of stable oxides

The presence of a fine dispersion of a stable oxide is known to have a beneficial effect on the oxidation resistance of nickel- and cobalt-base heat-resisting alloys. This paper presents some preliminary experimental results relating to the hot-corrosion resistance of these alloys. Alloys forming Cr ₂O₃ scales appear to be resistant to oxidation when coated with sodium sulfate, whereas an alloy normally forming an Al ₂O₃ scale suffers accelerated attack. During sulfidation some of the alloys suffer an accelerated degradation, with sulfur penetrating rapidly along what appear to be grain boundaries. The same effect is noted in sulfidation-oxidation experiments, when the Cr ₂O₃-forming alloys suffer accelerated oxidation, the effect of the dispersoid being apparently removed. An Al ₂O₃-forming alloy resists this form of attack well. The sodium sulfate-coated test is probably a good guide to the behavior under weakly corroding conditions, whereas the sulfidation-oxidation test may give a better indication of the behavior under highly aggressive conditions.     

Mechanism of the simultaneous formation of oxide and sulfide at the scale surface during the oxidation of a pure metal in mixed atmospheres

During the reaction of a pure metal with complex atmospheres containing both oxygen and sulfur, the formation of sulfide mixed with oxide is often observed at high temperature, contradicting thermodynamic predictions. The mechanism proposed so far to explain the formation of a duplex scale at the scale surface assumes a change in composition of the gas phase in a boundary layer next to the scale-gas interface and a reaction of the metal with the molecules of the two elements. This model is shown to be unable to explain the observed amounts of the less stable phase and the reaction rates when sulfur dioxide is the prevailing reacting species and is substituted by the assumption of a direct reaction with the SO₂ molecules. The thermodynamic equivalence of the two approaches is also pointed out.     

The high-temperature oxidation of Ni-20%Cr alloys containing various oxide dispersions

The oxidation behavior of Ni-20%Cr alloys containing approximately 3 vol.% Y₂O₃, ThO₂, and A1₂O₃ as dispersed particles has been examined in the temperature range 900 to 1200° C in slowly flowing oxygen at 100 Torr. The results show that the oxidation behavior of the Y₂O₃-, ThO₂-, Al₂O₃-, and Ce0₂-containing alloys is very similar and that some anomalies in the behavior of the ThO₂-containing alloy might be explained by the slower rate of chromium diffusion in this coarse-grained alloy. Two Al₂O₃-containing alloys were studied. One with a relatively coarse dispersoid size behaved in a manner analogous to a dispersion-free Ni-30% Cr alloy at 1100°C. The other alloy contained a dispersion of fine Al₂O₃ particles and behaved exactly like the Y₂O₃-containing alloy at 1000 and 1100°C, but at 1200° C oxidized at a faster rate. It has been shown that the adherent scales on dispersion-containing alloys have a stabilized fine grain size, whereas the nonadherent scales on dispersion-free alloys undergo grain growth.This work has been supported by the Naval Air Systems Command under Contract No. N00019-72-C-0190.