The corrosion of chromium and nickel-chromium alloys in oxygen-sulfur-carbon gases at 800°C

The simultaneous oxidation and sulfidation of Cr, Ni-10Cr, Ni-20Cr, and Ni-30Cr was studied at 800°C in three gases falling within the Cr₂O₃ stability field of the Cr-S-O system. The sulfur partial pressure remained constant at 1×10^?6 atm, whereas the oxygen partial pressure varied from 5×10²¹ to 5×10^?20 atm, and the carbon activity varied from 0.108 to 0.416. Reaction kinetics were measured, and the reaction products were characterized by X-ray diffraction, metallography, scanning electron microscopy, and X-ray energy dispersive analysis. Reaction rates decreased with increasing oxygen partial pressure and decreased with increasing chromium content of the alloys. Sulfides always formed along with Cr₂O₃, even though the gases fell within the oxide stability field. No carburization was observed even though carbon activities were sufficiently high to form carbides. The reaction mechanisms are discussed, and the absence of carburization is analyzed on the basis of a three-dimensional stability diagram.     

The corrosion of ferritic stainless steels and high-purity Fe-Cr alloys in basaltic lava and simulated magmatic gas

Three ferritic stainless steels, types 410, 430, and 446, containing 12, 17, and 26% Cr, respectively, and two high-purity binary alloys, Fe-19Cr and Fe-24Cr, were subjected to molten tholeiitic basaltic lava with a cover gas simulating magmatic gas at 1150°C for periods up to 400 hr. The oxygen and sulfur partial pressures were 9.8×10^?10 and 7.0×10^?3, respectively. All alloys formed Cr₂O₃ scales. Internal sulfidation occurred in the commercial alloys resulting in the formation of chromium and manganese sulfides. Internal oxidation of silicon also occurred. The extent of internal sulfidation decreased with increasing chromium content. There was a “critical” chromium content between 12 and 17%, above which internal sulfidation did not occur in 96 hr. However, the “critical” chromium level increased with exposure time to nearly 26% for 400 hr. Little internal sulfidation was observed in the high-purity alloys. The different behavior between the commercial and high-purity alloys may be attributed to (i) the formation of more perfect scales on the latter, which inhibited the inward migration of sulfur, and (ii) changes in the sulfur activity gradient across the scale caused by the presence of silicon and manganese.     

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.     

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⁺.     

Sulfidation kinetics and scale structures of chromium at temperatures of 973–1173 K and 104–10−6 Pa sulfur pressures

The sulfidation behavior of chromium was investigated over a temperature range of 973–1173 K in H₂S-H₂ gas mixtures of 10⁴–10^–6 Pa sulfur partial pressures using thermogravimetry, X-ray diffractometry, optical and scanning electron microscopy, and electron-probe microanalysis. Sulfidation kinetics are rapid for short periods and obey a linear rate law at low sulfur pressures, whereas at high sulfur pressures sulfidation tends to be parabolic. The surface morphologies can be divided into four types: at high sulfur pressures a petal-like crystal of Cr₂S₃(rho. and tri.) (type 1), at intermediate sulfur pressures a twinlike structure of Cr₃S₄ (type 2), at low sulfur pressures a flat surface with numerous hexagonal pits of Cr_1–xS (type 3), and a fine twinlike structure of ordered Cr_1–xS (type 4). At 973 K, the sulfur pressure ranges are type 1 at > 10^–4, type 2 at , and type 3 at . The critical sulfur pressure where type 2 was formed, 10^–5 Pa at 973 K, shifts toward higherressures at higher temperatures and becomes 10^–3 Pa at 1073 K and 10^–1 Pa at 1173K. Type 4 is observed at 1173K and 10^–6 Pa sulfur pressure. Thesulfide scale is composed of two distinct layers: an external layer, which is dense with a fine columnar structure, and an inner layer, which is porous with a layered structure of sulfides and voids. The external scale is composed offour layers at high sulfur pressures: at the scale-gas interface Cr₂S₃(rho.), next Cr₂S₃(tri.), third Cr₃S₄, and innermost Cr_1–xS. With decreasing sulfur pressures,the number of layers in the external scale was reduced. Pt markers were positioned between the external and inner scales.Emeritus Professor.     

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.     

The influence of sulfur pressure on Sulfidation behaviour of NiCoCrAl(Y) alloys at high temperature

Sulfidation of alloy having nominal composition Ni-23Co-19Cr-12Al (wt%) with and without the addition of 0.6% yttrium was studied at temperatures 1073–1273 K in sulfur vapor at atmospheric pressure and in H₂/H₂S gas mixtures at sulfur pressure of 10^?3 and 10^?1.5 Pa. Sulfidation runs were followed thermogravimetrically. Phase and chemical composition of sulfide scales and scale morphologies were determined by means of XRD, EDX, EPM and SEM analyses. After certain initial period sulfidation of both materials followed approximately a parabolic rate law. The estimated sulfidation rates for each alloy increased with sulfur pressure and temperature. The sulfide scales on both materials showed complex microstructures and compositions, depending on sulfidation conditions, with several sulfide and sulfospinel phases present, such as (Ni,Co)S, (Ni,Co)₃S₄, (Ni,Co)Cr₂S₄, (Cr,Ni,Co)Al₂S₄ or (Cr,Ni,Co)S and (Cr,Ni,Co)₃S₄. There was no evidence of yttrium segregation either to the grain boundary regions in the scale or to the alloy/scale interface. Yttrium dissolved in the sulfide phases and accelerated the sulfidation process. This behaviour was ascribed to the doping effect.     

High-temperature sulfidation of Fe-Mn-Cr alloys

Alloys of composition (in weight percent) Fe-10Mn-10Cr, Fe-10Mn-25Cr, and Fe-25Mn-10Cr were reacted at temperatures of 973 and 1073 K with flowing hydrogen-hydrogen sulfide mixtures corresponding to equilibrium sulfur partial pressures of 10^?3 and 8 Pa. Sulfide-scale-growth kinetics and morphologies were compared with those found on pure iron and on the binary alloys Fe-25Cr and Fe-25Mn. All alloys reacted according to parabolic kinetics after an initial period of slow approach to this steady state. Of the materials examined, the binary Fe-25Mn showed the slowest sulfidation rates, except at 973 K and a sulfur pressure of 8 Pa, where Fe-10Mn-25Cr had the best performance. Ternary alloys provided improved performance only when a scale layer of Cr₃S₄ was formed, an event dependent on temperature and sulfur activity. Multilayered scales were always formed on the ternary alloys, and the role of these layers in controlling sulfidation rates is discussed.     

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.     

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.     

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.     

Hydrogen doping effects in the sulfidation of molybdenum

The kinetics of molybdenum sulfidation have been studied in H₂S/H₂ gas mixtures at 1173 K. Sulphur partial pressures were controlled by the equilibrium reaction between H₂S, H₂, and S₂. Pure molybdenum metal was sulfidized at a fixed value of 133 Pa with varying H₂S and H₂ partial pressures, and at fixed H₂S partial pressures of 5.06×10 pa⁴ and 5.06×10³ Pa with varying hydrogen and sulfur partial pressures. The gravimetric sulfidation kinetics were parabolic under all conditions. X-ray diffraction analysis of the reaction product scale revealed the presence of MoS₂ only. The sulfide scales were of uniform thickness, had a compact morphology, and were tightly adherent to the metal. The sulfidation rates at a fixed sulfur partial pressure increased with increasing hydrogen partial pressure. At fixed values, the rate was almost constant at high values, but increased substantially as was decreased. Defect models for hydrogen dissolution in MoS₂ are developed and compared with the experimental results. It is concluded that the sulfidation rate effects are due to hydride anion occupation of interlayer sites in MoS₂.     

Sulfidation behavior of Fe-25Cr-20Ni-1.5Al2O3 in simulated coal combustion/gasification environments

The effect of minor addition of -Al₂O₃ dispersoids on the sulfidation behavior of Fe-25Cr-20Ni was investigated over a range of pO₂, 1.13×10^–20 to 1.18×10 ****^–22 atm. at constant pS₂=1.22×10^–8 atm. Fe-25Cr-20Ni and Fe-25Cr-20Ni 1.5 Al₂O₃ with and without preformed oxide scales were exposed to bioxidant gas mixtures H₂/H₂O/H₂S/Ar at 700° C. Both isothermal and cyclic exposures were included. Scales were characterized by a combination of several surface analytical tools. A remarkable improvement in sulfidation resistance is observed in Fe-25Cr-20Ni-1.5Al₂O₃ under the conditions investigated here. This is attributed to the ability of the alloy to form and maintain a predominantly Cr₂O₃ scale with reduced Fe-diffusion and content. Possible scientific reasons for such improvement are discussed. The base alloy, Fe-25Cr-20Ni, fails to develop and retain such a Cr₂O₃ scale and undergoes sulfidation within a few minutes of exposure. The scale breakdown process by sulfidation is explained qualitatively. Experimental evidence suggests that sulfur in the environment enhances Fe-diffusion and content in the scale.     

Sulfur solubility in NiO and CoO at 1000°C

The solubility of sulfur in NiO and CoO at 1000°C has been investigated over a wide range of oxygen and sulfur partial pressures using CO, CO₂, and SO₂ as input gases. The concentration of dissolved sulfur increases regularly with sulfur partial pressure but appears to be insensitive to oxygen partial pressure. Dissolution of sulfur did not affect the electrical conductivity of NiO samples. It was concluded that sulfur probably dissolves, in the range 10^–2 –10^–3%, either by exchange with O^–2 ions occuping O^–2 sites as S^–2 ions, or as neutral sulfur on interstitial sites. The scatter in results prevented a more definite conclusion being drawn.     

Effect of yttrium on the sulfidation behavior of Ni-Cr-Al alloys at 700°C

The sulfidation of Ni-10Cr-5Al, Ni-20Cr-5Al, and Ni-50Cr-5Al, and of the same alloys containing 1% Y, was studied in 0.1 atm sulfur vapor at 700°C. The sulfidation process followed linear kinetics for all the alloys except Ni-50Cr-5Al-1Y, and possibly Ni-50Cr-5Al, which followed the parabolic law. The reaction rates decreased with increasing chromium content in alloys without yttrium, and the addition of yttrium reduced the rates by at least a factor of two for the alloys containing 10 and 20% Cr and by an order of magnitude for Ni-50Cr-5Al. Alloys containing 10 and 20% Cr (with and without yttrium) formed duplex scales consisting of an outer layer of NiS_1.03 and an inner lamellar layer of a very fine mixture of Cr₂S₃ and A1₂O₃ in a matrix of NiS_1.03. The two alloys containing 50% Cr formed only a compact layer of Cr₂S₃, which was brittle and spalled during cooling. The lamellae in the duplex scales were parallel to the specimen surface and bent around corners. The lamellae were thicker than those on Ni-Al binary alloys. The lamellae were also thicker in scales on the 20% Cr alloy than on the 10% Cr alloy. The presence of yttrium refined the lamellae and increased the lamellae density near the scale/metal interface in the 10% alloy, but in the 20% Cr alloy the lammellae were thicker and more closely spaced. Platinum markers were found in the inner portion of the exterior NiS_1.03 layer close to the lamellar zone. A counter-current diffusion mechanism is proposed involving outward cation diffusion and inward sulfur diffusion, although diffusion was not rate controlling for alloys containing 10 and 20% Cr. Auger analysis of scales formed on Ni-50Cr-1Y showed an even distribution of yttrium throughout the layer of Cr₂S₃, suggesting that some yttrium dissolved in the sulfide. The reduced sulfidation rate of samples containing yttrium is explained by the possible dissolution of yttrium as a donor. The presence of Y⁴⁺ would then decrease the concentration of interstitial chromium ions in the N-type layer of Cr₂S₃, which would decrease the reaction rate.     

Stability of MnCr2O4 spinel and Cr2O3 in high temperature carbonaceous environments with varied oxygen partial pressures

In this investigation, Cr₂O₃ and MnCr₂O₄ were comparatively tested at 1050 °C in carbonaceous environment with varied oxygen partial pressures. MnCr₂O₄ exhibits much better resistance to carbonaceous attack than Cr₂O₃. The carburization rate of MnCr₂O₄ decreases sharply with increasing oxygen partial pressures. The oxygen partial pressures have less effect on the carburization resistance of Cr₂O₃. The increased resistance of MnCr₂O₄ to carburization is attributed to the dissolution of MnO into Mn-Cr-O spinel lattices with elevated oxygen partial pressures, which retards the decomposition and carburization of Mn-Cr-O spinel. The thermodynamic equations defining the carburization stability of MnCr₂O₄ and Cr₂O₃ are modified.     

Sulfidation properties of an Fe-23.4Cr-18.6Al alloy at temperatures 1073 and 1173 K in H2S-H2 atmospheres of sulfur at pressures 104-10−5 Pa

An investigation is reported on the sulfidation properties of an Fe-23.4Cr-18.6Al(at.%) alloy at 1073 and 1173 K in H₂S-H₂ atmospheres, 10⁴ > P_{S 2} 10⁵Pa. The sulfidation kinetics exhibited an early transient period before onset of parabolic kinetics. Values of the parabolic sulfidation rate constants increased by as much as 10⁵ from their smallest values at low sulfur pressures, P_{S 2} 10^–4 Pa, to maximum values at sulfur pressures P_{S 2} 10² Pa. Multilayered scales were formed, the number and types of layers dependent on sulfur pressure. A fully developed scale at sulfur pressures P_{S 2} > 10^–3 Pa.     

The corrosion of iron and of three commercial steels in H2?H2S and in H2?H2S-CO2 gas mixtures at 400–700 °C

The corrosion behavior of three commercial steels including a carbon, a low-chromium (2.25Cr-1Mo) and a medium-chromium (9Cr-1Mo) steel in H₂? H₂S and in H₂? H₂S-CO₂ mixtures has been investigated at 400–700 C under two sulfur pressures at each temperature. The ternary mixtures had the same sulfur pressure as the binary gases, but also a small partial pressure of oxygen. The corrosion of pure iron in the same H₂? H₂S mixtures was also studied for comparison. The scales formed on the steels were always composed of sulfides only: they showed a typical duplex structure as well as a strong tendency to crack and spall off during cooling. The corrosion kinetics of the steels were generally irregular, presenting an initial period of decreasing rate and a second approximately linear stage. On the other hand, the scales formed on iron were compact and well adherent to the metal, while the corrosion kinetics appeared to be generally controlled by a surface reaction step, leading to a transition from linear to parabolic behavior. The kinetics and mechanisms of scale growth for both iron and the steels are examined and discussed.     

Roles of aluminium and chromium in sulfidation and oxidation of sputter-deposited Al- and Cr-refractory metal alloys

Our recent results of the sulfidation and oxidation behavior of sputter-deposited Al- and Cr-refractory metal alloys at high temperatures are reviewed, and the roles of the aluminum and chromium in sulfidation and oxidation of these alloys are discussed in this paper. Niobium, molybdenum and tantalum are highly resistant to sulfide corrosion. Their sulfidation resistance is further enhanced by alloying with aluminum. Although Cr-refractory metal alloys also reveal high sulfidation resistance, their sulfidation rates do not become lower than those of the corresponding refractory metals. The sulfide scales formed on the Al-refractory metal and Cr-refractory metal alloys consist of two layers, comprising an outer Al₂S₃ or Cr₂S₃ layer and an inner refractory metal disulfide layer. The inner layer has a columnar structure, and the growth direction of the refractory metal disulfides is perpendicular to 0 0 1 direction. Intercalation of Al³⁺ ions into NbS₂ and a decrease in the sulfur activity at the outer layer/inner layer interface by the presence of the Al₂S₃ layer are probably responsible for the improvement of the sulfidation resistance by the addition of aluminum. The oxidation resistance of niobium and tantalum is improved more effectively by the addition of chromium rather than aluminum. Although preferential oxidation of chromium does not occur, an outer protective Cr₂O₃ layer in the oxide scales is formed on Cr-rich Cr-Nb and Cr-Ta alloys due to outward diffusion of Cr³⁺ ions. In contrast, continuous alumina layer cannot be formed on the Al-Nb and Al-Ta alloys, and the alloys reveal a pest phenomenon at 1073 K, and at higher temperatures rapid oxidation occurs. Concerning the oxidation of molybdenum, the addition of aluminum, which has higher activity for oxidation than chromium, is more effective in improving the oxidation resistance of molybdenum than chromium addition, since preferential oxidation of aluminum suppresses the formation of volatile molybdenum oxide.     

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.