Kinetics and mechanisms of the oxidation of chromium in H2-H2O-H2S gas mixtures

The oxidation of pure chromium in H₂-H₂O-H₂S gas mixtures was studied as a function of gas composition at 900°C. Oxidation kinetics were measured using a thermogravimetric apparatus, and the oxidation products were characterized using scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM). Chromia scales formed when the H₂O/H₂S ratio was about 10 or greater. Scales that comprised a mixture of Cr₂O₃ and chromium sulfides formed when the H₂O/H₂S ratio was about 3, even though Cr₂O₃ was the thermodynamically stable phase under these conditions; i.e., a kinetic boundary exists for pure chromium in H₂-H₂O-H₂S gas mixtures. The transition from chromia scale formation to the formation of scales containing both oxide and sulfide with a change in gas composition (decrease in the H₂O/H₂S ratio) is associated with an inhibition of the overgrowth of growing, metastable sulfide nuclei by the thermodynamically stable Cr₂O₂ phase. Presulfidation experiments confirmed that metastable chromium sulfide can continue to grow after H₂O is added to the gas phase when the H₂O/H₂S ratio in the gas phase is less than a critical value at the temperature of interest.     

High temperature corrosion of Fe-Cr-Mn-alloys in H2-H2S and H2-H2O-H2S gas mixtures

The sulfidation of Fe-20% Cr-30% Mn, Fe-25%Cr-20%Mn and Fe-25% Cr was studied at 700°C in H₂-H₂S and the oxidation and sulfidation in H₂-H₂O-H₂S after preoxidation in H₂-H₂O. The sulfidation rate is strongly increased for the Mn-containing alloys, layers of (Mn,Cr)S and (Mn,Fe)Cr₂S₄ are formed. Also the oxidation rate is enhanced compared to Fe-25% Cr by formation of MnCr₂O₄ instead of Cr₂O₃. The sulfidation after preoxidation leads to internal and external sulfidation of the Mn-containing alloys. With increasing oxygen pressure p(O₂) = 10^?26…10^?22 atm. of the H₂-H₂O-H₂S mixtures the sulfidation is suppressed, for the higher oxygen pressure 10^?23 and 10^?22 atm. fast oxidation prevails under formation of MnCr₂O₄. Manganese cannot increase the sulfidation resistance of alloys, in spite of the stability and low degree of disorder of its sulfide, since the mixed sulfide (Mn,Cr)S is formed which has a high degree of disorder, high diffusivities and high growth rate according to the doping effect of trivalent Cr³⁺.     

Influence of H2S on metal dusting

Presence of H₂S in a carburizing atmosphere causes S-adsorption which retards carbon transfer and deposition and can suppress metal dusting of iron and steels. In the latter process cementite Fe₃C is an intermediate, graphite deposition would initiate its decomposition but graphite nucleation is prevented by adsorbed sulfur. Thus continued Fe₃C growth can be observed in the presence of H₂S. Thermogravimetric studies in CO-H₂-H₂O-H₂S mixtures have been conducted at 500°C at various carbon activities a_C and H₂S/H₂-ratios. With increasing a_C higher H₂S/H₂-ratios are needed to suppress metal dusting, with increasing H₂S/H₂-ratio the kinetics of Fe₃C growth change from diffusion controlled parabolic kinetics to linear carbon transfer controlled kinetics. At very high a_C≥1000 besides Fe₃C also the Hägg carbide Fe₅C2 was observed as an outer layer on the cementite.     

The scaling of a Fe-20Cr alloy in H2-H2O-H2S mixtures

The scaling of an Fe-20Cr alloy has been studied in H₂-H₂O-H₂S mixtures between 973 and 1223 K. According to a simplified phase diagram, Cr₂O₃ and FeS should be the thermodynamically stable compounds in the gas mixtures chosen. The reaction followed a mixed rate law between linear and parabolic, indicating that the reaction rate at the scale-gas interface was comparable with the diffusion rate in the scale. At a constant H₂/H₂S ratio the scaling rate initially decreased slightly with increasing water-vapor pressure. A further increase of the water-vapor pressure resulted in localized sulfide formation, while the other parts of the surface were covered with a Cr₂O₃ film. Only Cr₂O₃ formed above a critical water-vapor content. Three zones could be distinguished when a sulfide scale is formed. The outer zone consisted of practically pure FeS; the intermediate zone was a solid solution of (Fe,Cr)S, partially decomposed to FeCr₂S₄ and metal during cooling; and the inner zone contained small Cr₂O₃ inclusions in an (Fe,Cr)S matrix.     

Transient state scale formation on Fe 32 Ni 20 Cr and Fe 20 Cr in H2-H2O-H2S atmospheres

The transient state of simultaneous oxidation and sulfidation of Fe-32 Ni-20 Cr and Fe-20 Cr was studied at 700°C for short time exposures in H₂-H₂O-H₂S. After heating the specimens in pure, dry hydrogen they were corroded by introduction of the oxidizing and sulfidizing atmosphere for 2, 4 or 15 min. After quenching the layer was investigated by SEM, AES, X-ray and electron diffraction. Four different gas compositions were applied: pS₂ = 10^?12 bar and pO₂ = 10^?25, 10^?26, 10^?27, 10^?28 bar, all within the thermodynamic stability range of Cr₂O₃. After the short time exposures oxides and sulfides were present on the surface, Cr₂O₃ and Cr₃S₄ had grown side by side and in case of the alloy Fe-32 Ni-20 Cr Fe- and Ni-containing sulfides formed patches on top of the scale. The amount of sulfides was higher for the lower oxygen pressures. After a longer time exposure, 120 min, all sulfides had vanished. Simultaneous formation of oxides and sulfides occurs in the transient state during phase boundary reaction or transport control. Upon transition to diffusion control the sulfides vanish by dissolution into the alloy and reaction with the gas atmosphere. This is valid for low pS₂ where no iron and nickel sulfides are stable.     

The influence of aluminum on the transition from oxide to sulfide of aluminum-rich Fe-25Cr alloys with low-oxygen and high sulphur pressures

The simultaneous oxidation and sulfidation of Fe-25Cr, Fe-25Cr-5Al and Fe-25Cr-10Al alloys were studied at 1023, 1123, and 1223 K in H₂-H₂O-H₂S gas mixtures. Fe-25Cr and aluminum-rich alloys with 0–10 wt.% Al show, in H₂H₂O-H₂S gas mixtures at high temperatures, a transition from protective oxide-scale formation to the formation of a sulfide-rich corrosion product. The kinetics boundary, which indicates the transition from oxide formation with slow weight gains to sulfide formation with rapid weight gains, has been found in these three alloys. The critical oxygen partial pressures to stabilize oxide formation at the constant-sulfur partial pressures of aluminum-rich Fe-25Cr alloys were systematically below those of Fe-25Cr alloy. When the oxygen partial pressure is much higher than the critical one, the oxide scale formed on the Fe-25Cr alloy was mainly Cr₂O₃ with a small amount of FeCr₂O₄; on the other hand, the oxide scale formed on the aluminum-rich Fe-25Cr alloys was mainly Fe(Cr,Al)₂O₄ with a small amount of Al₂O₃ and Cr₂O₃. The thermodynamic stability diagrams for (Fe, Cr, Al) -S-O systems were constructed, and the experimental results which show the existence of Fe(Cr, Al)₂O₄ in the simultaneous sulfidation and oxidation of aluminum-rich Fe-25Cr alloys are explained by these diagrams. The reaction kinetics were measured by a stainless-steel spring balance, and the reaction products were characterized by x-ray diffraction, Auger spectroscopy, and scanning electron microscopy. The reaction rate usually decreased with an increase of the oxygen partial pressure at a constant sulfur partial pressure. The existence of aluminum plays an important role to suppress the sulfidation of Fe-25Cr alloys.     

Transition from oxidation to sulfidation of Fe-Cr alloys in gases with low oxygen and high sulfur pressures

Fe-Cr alloys with 17–30% Cr show in H₂-H₂O-H₂S mixtures at 1273 and 1073 K a transition from protective oxide scale formation to rapid sulfidation. The critical oxygen pressure to stabilize the oxide formation increases with increasing sulfur pressure of the gas and decreasing Cr content of the alloy. Cr₂O₃ with traces of Fe₂O₃ is formed under these conditions. Below the critical oxygen pressure, a primarily formed Cr₂O₃ film becomes overgrown by (Fe, Cr)S. The kinetic boundary of oxidation-sulfidation, which lies in the stability field (Fe, Cr)S + spinel Fe_1+xCr_2–xO₄ of the Fe-Cr-O-S phase diagram, is explained with the help of the Fe-Cr-O-S phase diagram and the assumption that Fe diffuses faster through the (Cr, Fe)₂O₃ solid solution than does Cr.     

Die Möglichkeiten einer Wasserstoffversprödung von Spannstahl bei Einwirkung von Kohlensäure auf sulfidhaltigen Zementstein

The possibility of hydrogen embrittlement of reinforcing steel during carbon dioxide attack ou sulfide containing concrete Cement mortar (furnace cement, alumina-silicate cements) has been carbonated and the H₂S being generated has been quantitatively determined. The gas volumes measured are by orders of magnitude below the values theoretically expected. During the carbonate formation in mortar tubes advancing from the external surface only a small proportion of the H₂S penetrates into the interior of test specimens, e. g. 5·10^?6 H₂S (with reference to the mortar weight) in the case of a high furnace mortar containing 1,19% S, while a H₂S concentration by three orders higher was found in a tube of transformed alumina-silicate cement stone. This goes to show that the mechanism of reinforcing steel embrittlement as described in literature for alumina-silicate cements cannot be correct.     

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.     

Effect of 1 wt.% yttrium addition on high-temperature sulfidation behavior of Fe-15Cr-4Al alloy

High-temperature sulfidation studies have been carried out on Fe-15Cr-4Al with and without 1% Y in the temperature range 700–1000°C in an H ₂-H₂ S environment over the sulfur pressure range of 10 ^–9–10^–3 atm. Two-layered and three-layered sulfide scales were observed in both alloys at low and high sulfur pressures, respectively. The pegging phenomenon, similar to that occurring in high-temperature oxidation, across the innermost layer and substrate was observed in the case of the yttrium-containing alloy. Yttrium was found to be associated with aluminum and chromium sulfides. The role of yttrium was more evident at low than at high sulfur pressures and was found to reduce the parabolic rate constants by a factor of about one-half to one-seventh, respectively.     

The sulfidation of manganese at low sulfur pressures at 700–950°C

The kinetics of manganese sulfidation has been studied in H ₂-H₂ S gas mixtures as a function of temperature (973–1223 K) and sulfur pressure (7×10 ^–9 to 3×10 ^–4 Pa), using a thermogravimetric technique. The sulfidation of manganese at low pressures follows the parabolic rate law similar to the behavior at high sulfur pressures (10 ^–4–10⁵ Pa), although an initial nonparabolic incubation period, longer at lower sulfur pressures, was observed. The sulfidation rate constant increased with sulfur pressure and temperature according to the following empirical equation: k_P=const P(S ₂)^1/n exp(–E/RT)However, in disagreement with the results at high sulfur pressures, the exponent 1/n and the activation energy changed considerably with temperature and sulfur pressure. The results are analyzed in terms of a point-defect model of the single corrosion product—MnS—and of the possibility of a doping effect of MnS by hydrogen.     

Sulfidation behaviour of nickel aluminides

The sulfidation behaviour of four nickel aluminium alloys containing 25 to 45 at.% Al was studied over the temperature range of 750 to 950°C in a gas mixture of H₂-H₂S (0.1 to 10 vol.%). The sulfidation kinetics were determined using a continous weight gain system. The corrosion products were examined by SEM, EDX and XRD. Sulfidation in H₂-H₂S gas mixtures formed bilayered scales consisting of an outer layer of Ni₃S₂ and an inner layer of NiAl_3,5S_5,5 on all alloys regardless of the different aluminium contents. In H₂-H₂S gas mixtures the sulfidation kinetic generally followed the parabolic rate law for all alloys. The influence of aluminium content on corrosion rate was relatively low. The influence of low oxygen partial pressure on sulfidation was investigated in H₂-H₂S-H₂O mixtures. In these atmospheres the corrosion mechanism is completely different. Severe attack by rapid internal oxidation destroyed all the alloys except Ni25Al (25 at.%Al). The internal oxidation zone consisted of a mixture of γ-Ni₃Al and Al₂O₃. On the alloys containing 36 and 45 at.% Al local attack occurred, fast growing pocks were observed after an incubation period. Nickel aluminides show this corrosion phenomena only in H₂-H₂S-H₂O mixtures. An interruption of the H₂S gas flow stops the running internal oxidation. In flowing H₂-H₂O atmospheres no internal oxidation was observed. These facts prove that H₂S is necessary for starting and maintaining the internal oxidation of the nickel aluminides.     

High-temperature corrosion and creep of Ni-Cr-Fe alloys in carburizing and oxidizing environments

The carburization of NiCr 32 20 and NiCrSi 60 16 has been studied in CH₄-H₂ mixtures in the temperature range 900–1100°C. The methods included thermogravimetric measurements and studies on reacted specimens by X-ray diffraction, metallographic, and chemical analysis. Upon carburization internal carbides M₇C₃ and M₂₃C₆ are formed (M=mainly Cr); the rate of carburization is determined by carbon diffusion in the Fe-Ni matrix with carbide precipitations. The effect of the alloying elements Ni and Si on the carburization resistance of austenitic alloys is explained. By the same methods the oxidation and carburization in CO-H₂O-H₂ mixtures have been studied. The important role of a stable chromium oxide layer for the carburization resistance was confirmed. Creep tests at 1000°C in a CO-H₂O-H₂ atmosphere where Cr₂O₃ is stable showed carburization occurring through cracks in the oxide layer. At high strain rates premature failure occurs by carburization, which is followed by internal oxidation and formation of cracks, voids, and holes.     

Thermodynamic Properties of CS and Solutions of Sulfur in Carbon-Saturated Liquid Iron

A study has been made of the equilibria, at 1600° and 1800°C, between dilute solutions of sulfur in carbon-saturated liquid iron and gas mixtures consisting, initially, of argon and CS₂. By comparing the results obtained with those of previous workers on the corresponding H₂-H₂S-S in carbon-saturated liquid iron equilibria, the equilibrium sulfur pressures over these melts and the standard free energy of formation of gaseous CS have been obtained.     

On the sulphidation mechanism of niobium and some of Nb-alloys at high temperatures

The kinetics and mechanism of niobium sulphidation have been studied as a function of temperature (700-1000 °C) and sulphur pressure (10⁻⁴-10⁴ Pa) in pure sulphur vapour and H₂-H₂S gas mixtures, using microthermogravimetric technique. It has been found that in both different sulphidizing atmospheres the sulphidation process follows parabolic kinetics, being thus diffusion controlled. Marker experiments have shown that the slowest step of the overall reaction rate is the outward diffusion of cations. No influence of small amounts of impurities on the sulphidation rate has been observed in this study. Excellent agreement between calculated and experimentally determined parabolic rate constants has been obtained under the assumption, that the correct formula of the sulphide scale on niobium is Nb_2±yS₃ and not Nb_1+xS₂, as suggested by Gesmundo.It has been found that the rate of niobium sulphidation in H₂-H₂S gas mixtures is much higher than in pure sulphur vapour, strongly suggesting that the dissolution of hydrogen in the growing scale influences the defect structure in this sulphide.     

Plasma nitriding of austenitic stainless steel in N2 and N2-H2 dc pulsed discharge

AISI 302/304 austenitic stainless steels have been nitrided in N₂ and N₂-H₂ glow discharge plasmas powered by a high frequency dc pulsed power supply. In a pure nitrogen plasma, no increase in surface hardness from the initial value has been observed. In a N₂-H₂ gas admixture, a 3-4 times increase in hardness has been found, which confirms the necessity of hydrogen gas as a powerful reagent. Grazing incidence X-ray diffraction (GIXRD) shows the presence of three phases of nitride (Fe₃N, Fe₄N and CrN) along with iron oxide (Fe₂O₃). It is found in a N₂ plasma at a sample temperature of 540 °C that at 200 kHz, maximum peak intensity of iron oxide/nitride decreases. In N₂-H₂ plasma treated sample the nitride peak intensity increased in comparison to the intensity in sample nitrided in N₂ plasma. Optical emission spectroscopy (OES) has been used to investigate the active species present during nitriding in the near-cathode region. Emission bands of neutral and ionic molecular species and ionic atomic species of nitrogen have been detected in a nitrogen plasma. In a N₂-H₂ gas admixture, the H_α and H_β lines of Balmer series in addition have been observed. At 20% H₂ addition in nitrogen plasma, a few vibrational state intensities of N₂ and N₂⁺ are observed to be optimized.     

The sulfidation of pure Co, Y, and of a Co-15 wt.% Y alloy in H2-H2S mixtures at 600–800°C

The corrosion of pure Co and Y and of a Co-15 wt.% Y alloy in H₂-H₂S mixtures providing a sulfur pressure of 10^–8 atm. at 600–800°C and also of 10^–7 atm. at 800°C was was studied to examine the effect of yttrium on the sulfidation resistance of pure cobalt. The alloy was nearly single phase, containing mostly the intermetallic compound Co₁₇Y₂ plus a small amount of cobalt solid-solution. For all conditions except for 800°C under 10^–8 atm. S₂, the alloy formed multilayered scales consisting of an outer region of pure cobalt sulfide, an intermediate region of a mixture of cobalt sulfide with yttrium oxysulfide and finally an innermost layer of a mixture of yttrium oxysu fide with cobalt metal. At 800°C under 10^–8 atm. S₂, below the dissociation pressure of cobalt sulfide, the alloy formed only a single layer composed of a mixture of metallic cobalt with yttrium oxysulfide. Pure yttrium produced only the oxysulfide Y₂O₂S, as a result of the large stability of this compound and of the presence of some impurities in the gas mixtures used. The corrosion kinetics were generally rather complex, but except at 800°C under 10^–8 atm. S₂, the addition of yttrium reduced the sulfidation rate of cobalt, even though the formation of a continuous protective external layer of a pure yttrium compound was never achieved. Finally, when the gas-phase sulfur pressure was above the dissociation of cobalt sulfide the corrosion rate of yttrium was significantly lower than that of Co-15 Y. The internal sulfidation of Y in Co-15 Y was not associated with depletion of Y in the alloy. This difusionless kind of internal attack is typical of binary A-B alloys presenting a very small solubility of the most-reactive component B in the base metal A, which restricts severely the flux of B from the alloy toward the alloy-scale interface.     

Temperature dependence of oxide scale formation on high-Cr ferritic steels in Ar–H2–H2O

Four high chromium ferritic steels were oxidized in Ar/H₂/H₂O at temperatures between 500 and 900 °C. Polished specimens of all steels formed iron-rich oxides at temperatures below 600 °C, whereas increasing the temperature resulted in local formation of protective chromia scales. As the temperature was raised further, the specimens were totally covered with chromia scales. For the higher chromium steels this was also observed at 900 °C but not for the steel with a chromium content of 16%. The temperature dependence of the oxidation rates is governed by the competing diffusion processes in the alloy and the growing scales.     

The corrosion of copper by atmospheric sulphurous gases

The sulfurization of copper by atmospheric gases is widely recognized, but the importance of the potential causative agents of sulfurization and the mechanisms involved have remained unresolved. In this work, polycrystalline copper has been exposed to the atmospheric gases hydrogen sulfide (H₂S), carbonyl sulfide (OCS), carbon disulfide (CS₂), and sulfur dioxide (SO₂) in humidified air under carefully controlled laboratory conditions. At room temperature, the rates of sulfurization by H₂S and OCS are comparable, and are some two orders of magnitude greater than those by CS₂ and SO₂. Given the atmospheric concentrations of these gases, it is clear that OCS is the principal cause of atmospheric sulfurization of copper except near sources of the gases where high concentrations may render H₂S (and possibly SO₂) important. At constant absolute humidity, the sulfurization rate of copper by OCS is found to be inversely proportional to temperature over the range 21–80°C, a property attributed to reduced quantities of surface water at high temperatures and the subsequent decrease in the rate of hydrolytic transformation of OCS into a reactive form. In a final series of experiments, the initial sulfurization of copper by 2.2 ± 0.2 ppm H₂S in humidified air at 22°C has been studied in detail. The first stages of sulfurization involve rapid attack by H₂S at surface defect sites. As these corrosive mounds spread and merge, diffusion of copper to the surface is impeded and the fraction of H₂S molecules striking the surface that become incorporated into the corrosion film drops sharply from ～ 5 × 10^?5 (at t = 5 s) to ～ 8 × 10^?7 (at t = 72 h).     

The Sulfidation of a Co-15 wt.% Ce Alloy in H2-H2S Mixtures at 600-800°C

The sulfidation behavior of a Co-Ce alloycontaining approximately 15 wt.% Ce has been studied at600-800°C in H₂-H₂S mixturesproviding a sulfur pressure of 10^-8 atm, butalso of 10^-7 atm at 800°C. At 600 and 700°C the alloy corrodes moreslowly than pure cobalt, but more rapidly than purecerium. At 800°C under 10^-8 atmS₂, which is below the stability of thecobalt sulfides, the alloy corrodes quite slowly, but under 10^-7 atmS₂ it corrodes very rapidly and practicallyat the same rate as pure cobalt. The sulfidationkinetics are generally irregular, except for a few casesof nearly parabolic behavior. The sulfidation of the alloy produces duplexscales, containing an outermost layer of practicallypure cobalt sulfide and an inner complex layer where thetwo components are simultaneously present. Cerium is not able to diffuse out of thealloy-consumption region, where it forms a ceriumsulfide mixed with cobalt sulfide and an innermostregion where cerium sulfide is mixed with cobalt metal.The cobalt sulfide forms a continuous network which allowsthe growth of the external CoS_y layer, eventhough at rates reduced with respect to pure cobalt.Thus, a cerium content of 15 wt.% is not sufficient toprevent the sulfidation of the base metal. Theseresults as well as the details of the microstructure ofthe scales grown on the alloy are interpreted by takinginto account the limited solubility of cerium in the base metal and the presence in the alloy ofan intermetallic compound rich in cerium.