Transport of sulfur in the scale growing on nickel in sulfur dioxide atmosphere at 873 K

SO₂ labeled with³⁵S radioisotope was used to study the diffusion of sulfur in scales growing on nickel in sulfur-dioxide atmosphere at 873 K. It was observed that in addition to the inward molecular transport of SO₂, outward diffusion of sulfur also occurred. This diffusion occurred mainly through the grain boundaries of the scale [D_G873=3×10^–7cm²/s]. The liberation of sulfur inside the scale was caused by the reaction of Ni₃S₂ with oxygen or with sulfur dioxide.     

The reaction of preoxidized nickel in SO2 at high temperatures

The reaction between preoxidized nickel and SO₂ was studied at 800–1000° C. The experimental methods consisted of thermogravimetry, metallography, scanning electron microscopy, and electron microprobe analysis. Sulfur accumulates in the surface layer of the metal during the SO₂ exposure. It is concluded that this is due to transport of SO₂ through the oxide scale. This leads in turn to formation of a Ni-S liquid solution which eventually ruptures the scale and causes an accelerated, breakaway reaction behavior. The induction period for the accelerated reaction increases with increased preoxidation and furthermore is influenced greatly by the microstructure of the oxide scale.     

On the reaction mechanism of nickel with SO2+O2/SO3

High-purity nickel has been reacted with 96% O₂+4% SO₂ at 700–900°C. The reaction has been studied at 700°C as a function of the total gas pressure (0.06–1 atm) and at 1 atm as a function of temperature (700–900°C). The reaction mechanism changes with the effective pressure of p(SO₃) in the gas. When NiSO₄ (NiO + SO₃ = NiSO₄) is formed on the scale surface, the scale consists of a two-phase mixture of NiO + Ni₃S₂; in addition, sulfur is enriched at the metal/scale interface. A main process in the reaction is rapid outward diffusion of nickel through the Ni₃S₂ phase in the scale; the nickel reacts with NiSO₄ to yield NiO, Ni₃S₂, and possibly NiS as an intermediate product. When NiSO₄ cannot be formed, the scale consists of NiO, and small amounts of sulfur accumulate at the metal/scale interface. It is proposed that the reaction under these conditions is primarily governed by outward grain boundary diffusion of nickel through the NiO scale, and in addition, small amounts of SO₂ migrate inward through the scale—probably along microchannels.     

Studies of the mechanism of reaction of nickel with SO2

The reaction of nickel with SO₂ has been studied. The composition and morphology of the scale formed in sulfur dioxide (1.013×10⁵ Pa) at 600°C and the transport phenomena occurring in the growing scale have been investigated. The experimental methods consisted of metallography, scanning electron microscopy, and electron microprobe analysis. The transport phenomena have been studied by the marker method and with the use of a³⁵S radioisotope. The scale was composed of a NiO and Ni₃S₂ mixture and grew by the outward diffusion of nickel and inward transport of SO₂ molecules through the discontinuities of the scale. It has been shown that outward transport of sulfur originating from grains of sulfide occurs.     

Corrosion Behavior of Fe–25Cr in a N2–0.1% SO2 Atmosphere at 973 K Under Mechanical Loading

The corrosion behavior of an Fe-25Cr alloy was investigated in a N₂-0.1 vol.% SO₂ atmosphere at 973 K with and without stress. Without stress, the surface scale formed at the very initial stage consisting of Cr oxides and sulfides, which later changed to a Cr₂O₃-rich scale with the initially formed sulfides remaining. Under relatively low constant stress, +15 and +20 MPa, as well as cyclic stress of ±30 or ±40 MPa, the total strain for 36 ks was less than 0.1% and there was little cracking of the external scale, which consisted of Cr₂O₃ and (Fe, Cr)₃O₄ with small amounts of sulfide, growing faster. The external scale here was poorly adhesive. Under high stress, +25 and +35 MPa, cracks formed in the external scale and both oxides and Fe sulfide grew rapidly through the cracks to form nodules. The nodules consisted of an FeS-rich core surrounded by Fe₃O₄. With increasing strain, the preformed Cr₂O₃-rich scale changed drastically to a multilayered scale with an alternating oxide layer-sulfide layer structure.     

Mechanism of Chromium Oxidation in SO2 at 3 × 104 Pa and 105 Pa

The kinetics, phase composition, and morphology of scales growing on chromium in SO₂ atmospheres were studied over the temperature range 1073–1273 K and SO₂pressures of 3×10⁴ Pa and 10⁵ Pa. It was found that the scales consist mainly of Cr₂O₃, with only small amounts of sulfur (probably CrS) detected next to the metallic substrate. Oxidation proceeds according to the linear rate law at 10⁵ Pa SO₂ whereas at 1173 and 1273 K at 3×10⁴ Pa SO₂ the parabolic rate law is followed. The transport phenomena were studied by means of radiotracer techniques as well as marker techniques. The oxide–sulfide scales grew mainly by outward diffusion of metal; however, inward transport of S₂ or SO₂ molecules was also observed. The mechanisms of sulfide and oxide formation are discussed on the basis of the experimental results.     

The effect of K2SO4 additive in Na2SO4 deposits on low temperature hot corrosion of iron-aluminum alloys

To understand the effect of K₂SO₄ additive in an Na₂SO₄ deposit on low temperature hot corrosion, the corrosion behavior of Fe-Al alloys induced by Na₂SO₄+K₂SO₄ was compared to that by Na₂SO₄ alone, and sulfation of Fe₂O₃ in the presence of either Na₂SO₄ or Na₂SO₄+K₂SO₄ was studied. It was found that K₂SO₄ additive promoted the low temperature hot corrosion, but did not change the corrosion-mechanism. Experimental results refuted the prior suggestions that the accelerated hot corrosion resulted either from the formation of K₃Fe(SO₄)₃ or from the stimulation of sulfation of Fe₃O₃. The earlier formation of the eutectic melt caused the accelerated hot corrosion, or in other words, the K₂SO₄ additive shortened the induction stage of hot corrosion.     

Sulfide-forming features during oxidation of predeformed nickel in SO2

Loaded parts are exposed to hot corrosion to a greater extent than unloaded components. Pure nickel predeformed to various degrees by compression (up to 27%) has been oxidized in SO₂ at 600°C for different periods (22 to 95 hr). It has been shown that transport properties of the scale, formed on the initial metal surface containing physical defects, depend on their surface density. A general behavior was established for the same exposure (>70 hr): the higher the preliminary strain, the greater the amount of Ni₃S₂ in the scale. Nickel predeformed 21% and 27%, oxidized in SO₂ over 70 hr, formed scales consisting mainly of a single-phase Ni₃S₂ layer. An increase of the scale defectiveness accelerated attainment of heterogeneous equilibrium in a gas-scale system and intensified the formation of Ni₃S₂—the stable phase for the conditions used.     

Influence of SO2 on the Oxidation of 304L Steel in O2 + 40%H2O at 600 °C

The effect of sulphur dioxide on the oxidation of alloy 304L in O₂ + 40%H₂O has been investigated at 600 °C. A protective chromium-rich corundum-type oxide forms in clean dry O₂. Exposure to O₂ + 40%H₂O environment results in chromium vaporization in the form of CrO₂(OH)₂. This causes local failure of the protective oxide and the formation of 10 μm thick oxide islands on the alloy grain centers. The oxide islands are layered, the outer part consisting of hematite while the inner part is FeCrNi spinel oxide. The addition of 100 ppm SO₂ to O₂ + 40%H₂O reduces the corrosion rate compared to O₂ + 40%H₂O. SO₂ is suggested to influence oxidation by two separate effects. Firstly, SO₂ forms surface sulfate on the oxide surface that impedes the vaporization of chromium from the protective oxide. This slows down the breakdown of the protective oxide. Secondly, SO₂ also influences the rapid oxidation that ensues once the protective oxide has been destroyed. In this case, the presence of surface sulfate interferes with the surface reactions involved in oxidation. In this way, SO₂ slows down the growth of the oxide islands.     

Effect of alloy grain size and silicon content on the oxidation of austenitic Fe-Cr-Ni-Mn-Si alloys in a SO2-O2 gas mixture

Austenitic Fe-18Cr-20Ni-1.5Mn alloys containing 0, 0.6, and 1.5 wt.% Si were produced both by conventional and rapid solidification processing. The cyclic oxidation resistance of these alloys was studied at 900°C in a SO ₂-O ₂ gas mixture to elucidate the role of alloy microstructure and Si content on oxidation properties in bioxidant atmospheres. All the large-grained, conventionally processed alloys exhibited breakaway oxidation during cyclic oxidation due to their poor rehealing characteristics. The rapidly solidified, fine-grained alloys that contained less than 1.5 wt.% Si exhibited very protective oxidation behavior. There was considerable evidence of sulfur penetration through the protective chromia scale. The rapidly solidified alloys that contained 1.5 wt.% Si underwent repeated scale spallation that led to breakaway oxidation behavior. The scale spallation was attributed to the formation of an extensive silica sublayer in the presence of sulfur in the atmosphere.     

Sulfidation properties of Cr23C6 at 1073 K in H2S-H2 atmospheres of 103.5−10−6 Pa sulfur pressure

The sulfidation behavior of chromium carbide, Cr₂₃C₆, was investigated in H₂S-H₂ gas mixtures over a sulfur partial pressure range of 10^3.5–10^–6 Pa at 1073 K using thermogravimetry, optical and scanning electron microscopy, X-ray diffraction analysis, and electron-probe microanalysis. The kinetics were rapid for short time periods and followed a linear rate law at low sulfur pressures, whereas sulfidation tends to obey a parabolic rate law at high pressures. Sulfidation rates decreased with increasing carbon content in the carbide. Surface morphologies could be divided into three groups: (I) at high sulfur pressures, petal-like. crystals (Cr₂S₃); (II) at intermediate pressures, a twinlike structure (Cr₃S₄); (III) and at low pressures, a flat surface with numerous hexagonal pits (Cr_1–xS). The scale consisted of two distinct layers: an external scale with a single or multilayer structure and an inner scale with a mixture of Cr_1–xS, Cr₃C₂, and Cr₇C₃. These higher carbides, Cr₃C₂ and Cr₇C₃, may be formed by the sulfidation-carburization of Cr₂₃C₆. Pt-marker experiments indicated that the external scale grew by chromium diffusion and that sulfur migration played an important role in the growth of the inner scale.     

Corrosion in SO2 of pure and preoxidized copper at high temperature

A kinetics and morphological study of the reaction of pure SO₂ with copper over the temperature range 500–950°C showed that only Cu₂S formed despite the fact that thermodynamic ally its formation is not expected. Alternatively, the formation of Cu₂O, expected from the Cu-O-S diagram did not occur during sulfidation; however, its evaporation was observed in an atmosphere of pure SO₂ at high temperature. Thus copper differs from others metals such as nickel or cobalt by its low reactivity with SO₂ compared to the oxidation reaction; therefore, it was possible to follow the beginning of sulfidation.     

Interfacial segregation during oxidation in O2 at 1173 K of CeO2-coated Fe-20Cr alloys

The interfacial segregation of sulphur and carbon during the oxidation in 1 torr O₂ at 1173 K of Fe-20Cr alloy, which was either free of Ce, alloyed with 0.078 wt.% Ce, or sputter-coated with a 4 nm-thick CeO₂ layer, was studied using polyatomic SIMS. Oxidation periods were up to 19 hr. During oxidation, sulphur and carbon accumulated at the alloy-oxide interface region of both uncoated and coated alloys. The amount of segregated sulphur did not vary appreciably with time, whereas carbon increased with time. The total amount of segregants was similar for both uncoated and coated alloys, although the scales formed on the sputter-coated alloy maintained adhesion and were about 10 times thinner than those on the uncoated alloys.     

18O Tracer studies of Al2O3 scale formation on NiCrAl alloys

Diffusion processes in Al ₂ O ₃ scales formed on NiCrAl + Zr alloys were studied by the proton activation technique employing the ¹⁸ O isotope as a tracer. The ¹⁸ O profiles identified a zone of oxide penetration beneath the external scale. Both this subscale formation and the outer Al ₂ O ₃ scale thickness were shown by this technique to increase with Zr content in the alloy. Estimated k _p 's from scale thicknesses were in agreement with gravimetric measurements for various Zr levels. Alternate exposures in O and ¹⁸ O revealed that oxygen inward transport was the primary growth mechanism. A qualitative analysis of these ¹⁸ O profiles indicated that the oxygen transport was primarily via short-circuit paths, such as grain boundaries.     

Long term corrosion of X70 steel and iron in humid supercritical CO2 with SO2 and O2 impurities

Abstract

The corrosion of X70 steel and iron in supercritical CO₂/SO₂/O₂/H₂O environment were investigated after a 454 h exposure. Optical microscopy was applied to observe the morphology of etch pits and synthesise the three-dimensional morphology. X-ray diffraction and X-ray photoelectron spectroscopy were employed to detect the composition of product scales. Experimental results verified that the localised corrosion occurred on the X70 steel sample under corrosion product deposits. Ferrous sulphate, sulphur and iron sulphide were detected as the corrosion products.     

Thermodynamics of the Na2SO4-K2SO4-CoSO4 system and their relevance to low-temperature hot corrosion

Thermodynamic calculations and experiments were performed to determine the SO₃ partial pressures and temperatures at which K₂SO₄-CoSO₄ binary mixed liquid phases form on CoO and Co₃O₄ in the presence of K₂SO₄. The calculations and experiments are in excellent agreement. Similar calculations were also made of the compositions at the liquidus surface and the associated SO₃ partial pressures for the K₂SO₄-Na₂SO₄-CoSO₄ ternary system. These calculations show that the presence of K₂SO₄ substantially reduces the SO₃ partial pressures required to stabilize a liquid salt phase on the surface of oxidized cobalt alloys at 600–800°C. Consequently, at these temperatures the hot corrosion in coal-fired systems, where K levels are high, is expected to be worse than in oil-fired systems, where K levels are low. This prediction was confirmed by experiments in a pressurized fluidized bed coal combustor and in an atmospheric pressure burner rig.     

High-temperature oxidation of 1.25Cr-0.5Mo steel in SO2

The oxidation kinetics of 1.25Cr-0.50Mo steel under different thermal cycling conditions in the range of 520–700°C in a 2000 ppm SO ₂-argon gas mixture have been studied by thermogravimetric techniques. The influence of the number of thermal cycles experienced by the specimen has been determined on the oxidation process. Characterization of the scales was performed by scanning electron microscopy, and it was found that the nature of the scale is a function of oxidation temperature.     

Strength and Corrosion Behavior of Fe–25Cr Alloys under Various Strain Rates in Ar and N2–0.1%SO2 at 973 K

The mechanical strength and corrosion behavior of Fe–25Cr alloys were studied in Ar and in N₂–0.1SO₂ at 973 K under strain rates of 2.7 × 10⁻⁴–10⁻⁶/s. A 0.1μm thick adhering Cr₂O₃ layer is formed on the alloy by pre-treatment in Ar. Scales formed in N₂–0.1SO₂ are thicker with poorer adherence to the metal substrate. The tensile strength is higher, about 9–19 MPa, in Ar than in N₂–0.1SO₂. However, the strain-rate sensitivities show similar values in both environments. Deformation is concentrated near the grain boundaries of the alloy, and it is more conspicuous at lower strain rates. Scale growth is rapidly increased by the deformation in N₂–0.1SO₂, and the scale growth rate increases with the increase in strain rates. The scale growth rate is higher at higher strain rates and the scale increases in sulfides.     

Corrosion of nickel with small alloy additions of Si,Fe, and Mn in SO2+O2/SO3 at 700°C

The corrosion of nickel with alloy additions of Si, Fe, and/or Mn up to 4 wt% has been studied in SO ₂+O₂/SO₃ at 700°C. All alloy additions greatly improve the corrosion resistance of nickel in oxygen-rich atmospheres (O₂ with about 4% SO₂); the best improvements are achieved with Si, Fe+Si, and Fe+Mn+Si additions. High-purity nickel corrodes rapidly under these conditions; the scale then consists of NiO+Ni₃S₂, and the sulfide forms a three-dimensional network along the grain boundaries of the NiO grains and serves as the diffusion path for rapid outward migration of nickel. From studies of the microstructure and distribution of the alloying elements in the protective scales, it is proposed that the alloying additions exert their beneficial effects by accumulating/segregating at the grain boundaries of NiO (e.g., as silicates) and thereby influence the wetting characteristics and disrupt the sulfide network.