Einfluß von Schwefel auf die Aufkohlung einer CrNiFe-Legierung bei hohen Temperaturen

Influence of sulphur on the carburization of a CrNiFe-alloy at high temperatures Gravimetric investigations at 900, 1000 and 1100°C have been performed on the corrosion of Incoloy 800 (X 40 CrNi 21 32) in CH₄?H₂?H₂S mixtures at the carbon activity a_C = 1 and with varied ratios: 10^?6 < H₂S/H₂ < 10^?3. The carburization and internal carbide formation is increasingly retarded with increasing ratio H₂S/H₂ up to the stability limit of CrS, where sulfidation starts and carburization begins anew. The retardation of the carburization is caused by adsorbed sulphur, blocking the metallic surface for the carbon transfer from the gas phase. Selenium and tellurium also are inhibitors for the carburization. With the optimum H₂S/H₂-ratio experiments have been performed at a_C > 1, in which the graphite formation on the metallic surface was suppressed effectively. The application of these effects for inhibiting carburization is discussed.     

Corrosion of Binary Fe–Si Alloys in Reducing Oxidizing and Sulfidizing–Oxidizing Atmospheres at 700 °C

The corrosion behavior of three Fe–Si alloys containing approximately 5, 9 and 13 at.% Si was studied at 700 °C in an H₂–CO₂ gas mixture providing 10^?20 atm O₂ as well as in an H₂–H₂S–CO₂ gas mixture providing the same oxygen pressure coupled to an S₂ pressure of 10^?8 atm. All the alloys followed complex kinetics which were mostly linear for Fe–5Si, but showed one or two parabolic stages for the other two alloys. Simple oxidation produced essentially two-layered scales in which Si was confined to the alloy consumption zone in the form of silicon oxide and iron-silicon double oxide. Corrosion in the oxidizing–sulfidizing gas mixture produced scales composed of a thick external zone of pure FeS followed by an internal region containing complex mixtures of FeS with Si and Fe oxides. Internal oxidation of silicon was only observed for the oxidation of Fe–5Si in both environments. The extent of corrosion decreased in both gas mixtures with an increase in the Si content of the alloys. Finally, the addition of sulfur produced a significant increase of the overall mass gains for each alloy.     

Sulfidation properties of Fe-6.1 at.% Mo alloy at 973–1273 K

Sulfidation of an Fe-6.1 at% Mo alloy was investigated in H₂S-H₂ atmospheres, 10^?4 ? P_s2 ? 10²Pa, at 973-1273 K. The reaction kinetics are parabolic except at 1273 K as liquid sulfide formation leads to catastrophic corrosion. This solid-liquid transformation between Fe₂Mo₂S₄ and Mo₂S₃ occurs at 1214 ± 9 K. At 1073 K and P_s2 = 10^?4Pa, growth of a duplex Mo₂S₃/FeMo₂S₄ scale offers high resistance to sulfidation. At 973, 1073 and 1173 K, 10^?2 ? P_s2 ? 10²Pa, parabolic sulfidation kinetics of the same magnitude as for pure iron describe growth of a duplex scale composed of an inner (FeMo₂S₄ + Mo₂S₃) layer and at an outer FeS layer. Marker measurements indicated that growth of the inner two-phase layer was supported by inward migration of sulfur and that growth of the outer FeS layer resulted from outward migration of iron.     

Corrosion behaviour of oil well casing steel in H2S saturated NACE solution

Abstract

The corrosion behaviour of oil well casing steel in H₂S saturated NACE solution (5 wt-%NaCl+0·5 wt-%CH₃COOH) was investigated by means of electrochemical methods, X-ray photoelectron spectroscopy and scanning electron microscopy. It was found that the investigated oil well casing steel can react with hydrogen sulphide in the H₂S saturated NACE solution at 25°C. During the reaction, corrosion films of mackinawite (FeS) form on the steel surface and the hydrogen atoms diffuse into steel matrix. The further corrosion of iron with H₂S can be retarded, and hydrogen permeation flux decreased, by the presence of the mackinawite film, which acts as a protective barrier provided it remains intact.     

Einflüsse von Legierungselementen auf die Korrosion und Wasserstoffaufnahme von Eisen in Schwefelsäure – Teil III: Kinetik der Abscheidung und Aufnahme von Wasserstoff an binären Eisenlegierungen

Effects of Alloying Elements on Corrosion and Hydrogen Uptake of Iron in Sulfuric Acid – Part III: Kinetics of Proton Discharge and Hydrogen Uptake at Binary Iron Alloys The effects of C, S, P, Mn, Si, Cr, Ni, Sn und Cu on the kinetics of hydrogen uptake of iron in 1 M H₂SO₄ at 25°C are investigated by potentio-kinetic and steady state current density-potential measurements accompanied by hydrogen permeation measurements. The anodic dissolution of iron is increased mainly by S and P, and decreased by Ni, Sn and Cu. Hydrogen uptake is enhanced by S, P and Sn, and inhibited by Ni and Cu. The kinetics of hydrogen evolution on the surface of iron alloys investigated are controlled by a coupled discharge – chemical recombination mechanism.     

The corrosion of a Fe-15 wt.% Ce alloy in coal gasification type atmospheres at 600 to 800 °C

The corrosion of a Fe-15 wt.% Ce alloy and of the two pure metals have been studied at 600 to 800 °C in H₂-H₂S-CO₂ mixtures providing high-sulfur and low-oxygen activities that are typical of coal gasification atmospheres. The alloy corrodes more slowly at 600 to 700 °C than pure Fe, more rapidly than pure Ce, while at 800 °C it corrodes at about the same rate as pure Fe. The scaling kinetics of Fe-15Ce are irregular and generally intermediate between linear and parabolic. The scale formed on Fe-15Ce shows a multilayered structure, including an outermost layer of base metal sulfide (FeS), an intermediate complex layer composed of a mixture of compounds of the two metals, and finally an innermost region of internal attack of Ce by both oxygen and sulfur. Ce is not able to diffuse outward in the metal substrate and remains in the alloy consumption region. In the intermediate region, FeS forms a continuous network that allows the growth of an external iron sulfide layer. A Ce content of 15 wt.% is insufficient to prevent the sulfidation of the base metal. These results as well as the scale microstructure are interpreted by taking into account the limited solubility of Ce in Fe and the presence of Ce-rich intermetallic compounds in the alloy examined.     

The corrosion of a Fe-15 wt.% Ce alloy in coal gasification type atmospheres at 600 to 800 °C

The corrosion of a Fe-15 wt.% Ce alloy and of the two pure metals have been studied at 600 to 800 °C in H₂-H₂S-CO₂ mixtures providing high-sulfur and low-oxygen activities that are typical of coal gasification atmospheres. The alloy corrodes more slowly at 600 to 700 °C than pure Fe, more rapidly than pure Ce, while at 800 °C it corrodes at about the same rate as pure Fe. The scaling kinetics of Fe-15Ce are irregular and generally intermediate between linear and parabolic. The scale formed on Fe-15Ce shows a multilayered structure, including an outermost layer of base metal sulfide (FeS), an intermediate complex layer composed of a mixture of compounds of the two metals, and finally an innermost region of internal attack of Ce by both oxygen and sulfur. Ce is not able to diffuse outward in the metal substrate and remains in the alloy consumption region. In the intermediate region, FeS forms a continuous network that allows the growth of an external iron sulfide layer. A Ce content of 15 wt.% is insufficient to prevent the sulfidation of the base metal. These results as well as the scale microstructure are interpreted by taking into account the limited solubility of Ce in Fe and the presence of Ce-rich intermetallic compounds in the alloy examined.     

Corrosion behaviour of X52 pipeline steel in high H2S concentration solutions at temperatures ranging from 25°C to 140°C

Abstract

High temperature and high pressure immersion tests in an autoclave were employed to study the corrosion behaviour of X52 pipeline steel in aqueous solutions containing high concentrations of H₂S. The corrosion products generated were characterised using scanning electron microscope, energy dispersive spectroscopy and X-ray diffraction. It was seen that at a constant H₂S concentration of 22 g/l, the corrosion rate increased with increasing temperature up to 90°C, thereafter decreased at 120°C and slightly increased again at 140°C while the corrosion rate increased with H₂S concentration at a temperature of 90°C. When the temperature and H₂S concentration increased, the corrosion product converted from iron rich to sulphur rich products in the following sequence: mackinawite→troilite→pyrrhotite, where the microstructure and stability of the corrosion products had an important effect on the corrosion rate. The corrosion film was formed through the combination of the outward diffusion of Fe²⁺ ions and the inward diffusion of H₂S and HS^? species.     

The Sulfidation Behavior of Several Commercial Ferritic and Austenitic Steels

The sulfidation behavior of C-steel, 1Cr-0,5Mosteel, 12Cr-1Mo-0.25V steel, 18Cr-10Ni-Ti steel, thebinary alloys Fe-20Cr, Fe-25Cr, Fe-30Cr, and pure Cr wasinvestigated between 400 and 700°C in a94Ar-5H₂-1H₂S gas mixture. All steels sulfidize according tocomplex kinetics which, after a period with decreasingrate, can be approximated by a linear rate law. Thescale of the three ferritic steels consists of two layers, an outer outward-growing one of FeSwith traces of dissolved Cr and an inner, inward-growingone, which contains in addition to Fe the alloyingelements Cr and Mn. Most of the outer FeS layer is separated from the inner layer and can be splitinto several partial layers, the number increasing withincreasing sulfidation time and temperature. The scaleon the austenitic 18Cr-10Ni-Ti steel differs insofar as that of the ferritic steels as theouter FeS layer contains some Ni and that a third layerof the spinel FeCr₂S₄ is formedbetween the outer and the inner layer. This intermediatelayer is responsible for the lower sulfidation rate of this materialcompared with that of the ferritic steels. The scale ofthe binary Fe-Cr alloys is similar to that of theaustenitic steel. From AE-measurements it can be deduced that the separation of the outer FeSlayer occurs during isothermal sulfidation and isaccompanied by an increase in the AE event rate. Theseparation is a consequence of the formation and growth of pores in the region close to the inner/outerlayer interface and the development of compressivegrowth stresses in the outer FeS layer. While detachmentof the FeS layer on the ferritic steels was already observed at 400°C, the austenitic steelshowed a similar separation of the FeS layer only at600°C. The detached FeS layer is obviously rathergas tight. Differences in the sulfur partial pressure ofthe bulk gas and the gas in the cavity between theinner and separated outer layer lead to a reduction ofFeS at the inner surface of the detached FeS layer. TheFe ions and electrons, produced by this reaction, diffuse outward, forming new FeS on the outerFeS surface. This process not only shifts the detachedFeS layer continuously away from the core of thespecimen but offers also the possibility of healing cracks in the separated FeS layer. This scaledetachment does not stop scale growth. After scaleseparation the total sulfidation reaction consists of atleast seven partial reactions: phase-boundary reaction at the outer surface, diffusion of iron ionsand electrons outwards in the detached FeS layer,formation of H₂S at the inner surface of thedetached layer, gas diffusion in the cavity, formationof FeS on top of the porous inner layer, gas diffusionin the channels of the porous inner layer, FeS formationat the metal/scale interface. When the new FeS layer ontop of the porous inner layer exceeds a critical thickness, the detachment of the FeSlayer from the inner porous layer repeats. This processcan take place several times, leading to an outer FeSpartial scale, split into several layers, which are separated by relatively large cavities andkept together only locally by FeS bridges. The overallreaction rate is controlled by the phase-boundaryreaction at the outermost FeS surface.     

Die Kinetik des Rostens von Eisen in der Atmosphäre

The kinetics of the rusting of iron at the atmosphere Laboratory experiments organized to describe the kinetics of active periods of atmospheric rusting have lead to the following conclusions: At temperatures ? 0°C the corrosion velocity is independent from the SO₂- content and humidity of the atmosphere. Above 0°C the corrosion rate is controlled by reactions leading to sulphate-nests and to the hydrolytic rust-formation. Simultaneous action of high temperature (≥15°C) and higher amounts of water (≥100 g.m^?2) decreases the accelerating effect of growing SO₂-contents. The corrosion belocities found are in good agreement with data calculated from long-term tests in natural environments.     

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.     

Effect of Temperature on the Corrosion Behavior of API X120 Pipeline Steel in H<Subscript>2</Subscript>S Environment

The corrosion behavior of newly developed API X120 C-steel that is commenced to be used for oil pipelines was studied in a H₂S saturated 3.5 wt.% NaCl solution between 20 and 60 °C using potentiodynamic polarization and electrochemical impedance spectroscopy techniques. The corrosion products formed on the surface of the alloy were characterized using x-ray diffraction and scanning electron microscopy. It has been noticed that the formation of corrosion product layer takes place at both lower and higher temperatures which is mainly comprised of iron oxides and sulfides. The electrochemical results confirmed that the corrosion rate decreases with increasing temperature up to 60 °C. This decrease in corrosion rate with increasing temperature can be attributed to the formation of a protective layer of mackinawite layer. However, cracking in the formed mackinawite layer may not be responsible for the increase in the corrosion rate. More specifically, developed pourbaix diagrams at different temperatures showed that the formed protective layer belongs to mackinawite (FeS), a group of classified polymorphous iron sulfide, which is in good agreement with the experimental results. It is also noticed that the thickness of corrosion products layer increases significantly with decrease in the corrosion rate of API X120 steel exposed to H₂S environment. These findings indicate that API X120 C-steel is susceptible to sour corrosion under the above stated experimental conditions.     

On the “coke” growth in carburizing and sulfidizing atmospheres upon High temperature corrosion of iron and nickel base alloys

The graphite deposition from carbonaceous atmospheres can initiate a catastrophic deterioration of alloys in high temperature corrosion. The graphite nucleation and growth is catalyzed by metal surfaces and affected by the presence of sulfur. Gravimetric studies have been performed on the carbon transfer from CH₄? H₂ or CH₄? H₂? H₂S atmospheres to iron, nickel or ironnickel alloys at 1000°C. The carbon activities were a_c = 1 (equilibrium with graphite), a_c = 5 or a_c = 10; the sulfur pressure was in a range where the metal surfaces are nearly saturated with adsorbed sulfur. The carburization, i.e. the transfer of C into solid solution is retarded in the presence of sulfur since surface sites are blocked for the methane decomposition. In the sulfur-free environment graphite layers grow with their basal planes parallel to the metal surface – for nickel an epitaxial growth occurs which is extremely slow. In the presence of sulfur the graphite can only nucleate in small islets which grow to irregular nodules. This results in a retardation by sulfur of the graphitisation on iron, whereas the growth of graphite on nickel is accelerated by sulfur. The transition between these ways of graphitisation behaviour was studied for Fe? Ni.     

Effect of H2S on High Temperature Corrosion of Fe–Ni–Cr Alloys in Carburizing/Oxidizing Environments

This investigation involves the corrosion behavior of two Fe–Ni–Cr alloys containing different Si content at 1050?°C in carburizing-oxidizing environments (typical of ethylene pyrolysis) with varied concentration of H₂S. High-Si containing alloy could form thinner but less uniform oxide scale than low-Si alloy after pre-oxidation due to the barrier effect of continuous SiO₂ at interface of scale/substrate. Pre-oxidized alloy showed a better resistance to carburization/sulfidation attacks than the bare alloy in absence of pre-oxidation. It was found that carburization and sulfidation of the Fe–Ni–Cr alloys could be prevented in the environment with a ratio of $ P_{{{\text{H}}_{ 2} {\text{S}}}} /P_{{{\text{H}}_{ 2} }} $ at 1.7?×?10^?5. When the sulfur partial pressure was lower than this value, oxides were found to be converted to porous and non-protective carbides. When the sulfur potentials were increased, manganese or chromium sulfide on outer layer and internal sulfide stringers mixed with silicon oxide in substrate could be formed. Under high sulfur partial pressures, spallation of outer sulfide or oxide scale was observed on high-Si alloy due to less stability of oxide layer formed at surface which was converted to sulfide faster than on low-Si alloy.     

Einflüsse von Legierungselementen auf die Korrosion und Wasserstoffaufnahme von Eisen in Schwefelsäure – Teil II: Korrosion und Deckschichtbildung

Effects of Alloying Elements on Corrosion and Hydrogen Uptake of Iron in Sulfuric Acid Part II: Corrosion and Formation of Surface Layers The effects of C, S, P, Mn, Si, Cr, Ni, Sn and Cu on the formation of surface layer and hydrogen uptake of iron during corrosion in 1 M H₂SO₄/N₂ were investigated using AES, XPS, SEM and electrochemical permeation techniques. Cu, Sn, P and C are enriched on the surface of iron during corrosion in H₂SO₄. Cu is enriched in the metallic form. P forms a phosphate and phosphide containing surface layer. Ni is not enriched. Cr is preferentially dissolved. Cu, Sn and Ni inhibit the dissolution of iron and thus decrease the hydrogen activity. S, P and Mn (MnS) increase the corrosion and hydrogen activity. Cr forms traps in iron which increase the hydrogen uptake.     

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.     

Einflüsse von Mo,V, Nb,Ti, Zr und deren Karbiden auf die Korrosion und Wasserstoffaufnahme von Eisen in H2S-haltigen Lösungen

Effects of Mo, V, Nb, Ti, Zr and their carbides on the corrosion and hydrogen uptake of iron in H₂S-solutions Effects of the transition metals Mo, V, Nb, Ti, Zr and their carbides as well as of phosphorous on the corrosion and hydrogen uptake of iron in acid to weakly acid NaCl solutions with and without H₂S are discussed. Investigations were carried out on binary, ternary and quaternary iron based alloys, using electrochemical and surface analytical methods. No specific effect of one of the alloying elements or the carbides on the corrosion or hydrogen uptake is observed. Due to the experimental conditions, sulphur and oxygen enriched surface scales form, by which the kinetics of the corrosion processes are determined. The alloying elements are enriched on the iron surface only as a carbide. Phosphorous is enriched as a phosphide at low pH and as a phosphate at higher pH. H₂S and phosphides increase the corrosion rate and hydrogen uptake. In pure iron or low strength iron alloys, at the very high H₂S affected hydrogen activities new lattice defects are induced permanently resulting in extremely high hydrogen concentrations.     

Corrosion of low carbon steel weldments at 600–800 °C in N2/H2S/H2O gases

A low carbon steel was arc-welded, and corroded at 600, 700 and 800 °C for up to 20 h in 1 atm of either N₂/H₂S-mixed gases or N₂/H₂S/H₂O-mixed gases to characterize the effects of H₂S and H₂O gases on the high-temperature corrosion of welded joints. Corrosion proceeded fast and almost linearly. It increased with the increases in the corrosion temperature and with the addition of H₂S and H₂O. H₂S formed FeS, while H₂O formed iron oxides such as Fe₃O₄. Hydrogen and sulfur that were released from H₂S and H₂O made the scales fragile and nonadherent. Weld metals corroded faster than base metals because the former had coarser grains than the latter.     

Korrosionsreaktiorren von elementarem Schwefel mit unlegiertem Stahl in wäßrigen Medien

Corrosion reactions between elemental sulphur and plain carbon steel in aqueous media Plain carbon steels are rather severely attacked by elemental sulphur at room temperature in the presence of aqueous media. The corrosion occurs preferentially at the places where the two solid substances iron and sulphur are in contact with each other and results in shallow pit formation. At the same time the pH is also decreased slightly and small amounts of H₂S and sulphate ions are formed. Neutral salts stimulate the corrosion process whereas phosphates inhibit it and the alkaline media such as Na₂CO₃ and ethylamine with pH > 12 prevent it completely. The latter can be made use of for corrosion protection. At high salt concentrations (c > 1 mol/l) the corrosion rate, however, decreases with increasing salt concentration. The corrosion rate may increase with increasing flow velocity of the medium, but the corrosion takes place uniformly. The results of electrochemical investigations show that the reduction of sulphur occurs at the corrosion product FeS and is the rate controlling step. No sulphur reduction is observed on platinum electrodes when no FeS is present. It is assumed that the starting reaction to initiate corrosion in the system Fe/S/H₂O is a slight disproportionation of S to H₂SO₄ resulting in the formation of FeS.     

Effect of H2S on metal dusting of iron

A review is given on the effect of H₂S on metal dusting of iron which has been studied by gas carburisation in CO‐H₂‐H₂O‐H₂S and CH₄‐H₂‐H₂S mixtures at 500 and 700°C. The presence of H₂S in carburising gas atmospheres leads to sulphur adsorption on the iron surface, which retards carbon transfer. Segregation experiments and surface analyses have shown that sulphur segregates (and thus adsorbs) on cementite surfaces as well as on iron surfaces. The adsorbed sulphur also suppresses graphite nucleation and thus can stop the reaction sequence of metal dusting. Experiments by thermogravimetric analysis (TGA) have shown that the extent of retardation of metal dusting depends on temperature, carbon activity and H₂S content. The higher the carbon activity, the higher is the H₂S content required for suppression of metal dusting. At carbon activities a_C > a_C(Fe/Fe₃C) the metastable iron carbide, cementite (Fe₃C), occurs as an intermediate phase during metal dusting. Carburisation experiments in CO‐H₂‐H₂O‐H₂S mixtures at 500°C and X‐ray diffraction analysis (XRD) of carburised samples have revealed that at very high carbon activities a second iron carbide, Hägg carbide (Fe₅C₂), forms on the cementite surface. Microstructural investigations have shown that both metastable carbides decompose during metal dusting. Metal dusting experiments on iron at 700°C have been performed in CH₄‐H₂‐H₂S gas mixtures. By adding 15 ppm H₂S to the CH₄‐H₂ atmosphere the onset of metal dusting can be retarded for more than 350 hours. By means of Auger electron spectroscopy (AES), scanning electron microscopy (SEM) and energy dispersive X‐ray analysis (EDX) it was shown that coke contains graphite, cementite and iron particles with adsorbed sulphur.