Einfluß der Hydrodynamik auf die Korrosion von Eisen und Stahl in neutralen belüfteten NaCl- und Na2SO4-Lösungen

Influence of hydrodynamics on the corrosion of iron and steel in neutral aerated NaCl and Na₂SO₄ solutions The corrosion behaviour of iron and steel in aerated neutral NaCl and Na₂SO₄ solutions is mainly determined by the transport-controlled cathodic oxygen reduction. Its reaction zone depends on the physical and chemical properties of the 3-D layer formed on the corroding surface. The influence of hydrodynamics is restricted to the transport of the dissolved oxygen to the reaction zone. The obtained results are independent of laminar or turbulent flow conditions.     

Die Verzunderung von Eisen zwischen 700 und 900 °C in CO/CO2-Gemischen mit kleinen Zusätzen von COS,SO2 und H2S

Scaling of iron between 700 and 900°C in CO/CO₂mixtures with minor additions of COS, SO₂ and H₂S Scaling of iron in CO/CO, mixtures containing less than 1.6% COS, H₂S or SO₂follows initially a linear kinetic law. The transition from the linear to the parabolic law is displaced toward shorter periods with increasing sulfur contents in the gas and with decreasing temperature. At 800 and 900°C the rate of the reaction between iron and the sul-fur compound in the gas is controlled by the mass transfer in the gas phase. In this conditions the reaction rates with COS and H₂S are practically identical, while the reaction with SO₂yields al-most double the weight increase because in this case not only sulfur, but also part of the oxygen of SO₂ react with iron. At 700°C there is a transition of the control mechanism in CO/CO₂C/S mixtures with increasing COS contents, namely from control by mass transfer in the gas phase to control by the phase boundary reaction. Some consequences concerning the heating of steel in technical furnaces are discussed.     

Morphological development of oxide-sulfide scales on iron and iron-manganese alloys

Pure iron and alloys containing 2, 15, 25, and 50 wt. % manganese have been reacted at 1073 K in controlled gas atmospheres of SO ₂-CO ₂-CO-N ₂.Equilibrium gas compositions were such that (1) FeS was stable but not FeO, or (2) both FeS and FeO were stable, or (3) FeO was stable but not FeS; in all cases, both MnS and MnO were stable. Under all reaction conditions, pure iron corroded to produce both sulfide and oxide. The resultant scale morphologies were consistent with local solid-gas equilibrium for the case in which both oxide and sulfide were stable but in the other cases indicated that equilibrium was not achieved and that direct reaction with SO ₂ (g) was responsible for corrosion. Additions of manganese did not greatly alter the scale morphologies. Under reaction conditions that were oxidizing and sulfidizing, very high levels of manganese were required to reduce the corrosion rate. On the other hand, relatively low levels had a beneficial effect both when FeO but not FeS was thermodynamically stable and similarly when FeS but not FeO was stable.     

Die Verzunderung von Eisen in O2?SO2-Inertgas-Gemischen bei 900° C

Scaling of iron in O₂? SO₂-inert gas mixtures at 900 °C . Kinetic and metallorgraphic investigations into the oxidation of iron in O₂So₂-inert gas mixtures show, that SO₂ increases the scaling rate of iron when the oxidation in the SO₂ free gas follows a linear kinetic law; in these cases the transport of oxygen from the flowing medium to the specimen surface is the rat-controlling step. Such conditions exist at 900 °C and linear flow velocities of 5.8 cm/s at oxygen contents below about 7% At constant oxygen pressure the constant of the linear kinetic law is a linear function of the SO₂.     

Amounts and Distribution of Phases in Sulfide Plus Oxide Scales on Iron

Pure iron was exposed at 800°C to flowing, catalyzed-gas mixtures of N₂/CO₂/CO/SO₂ adjusted to control the partial pressures of SO₂, S₂ and O₂. The equilibrium gas compositions were such that iron oxide was thermodynamically stable with respect to sulfide. The reaction product scale was invariably a mixture of oxide plus sulfide, and grew according to parabolic kinetics at high P_SO2 values and by linear kinetics in dilute gases. In both cases the reactant gas species was SO₂, not molecular oxygen or sulfur. The relative amounts of sulfide and oxide corresponded to stoichiometric reaction of SO₂ at high P_SO2 values, but not in dilute gases. At low P_SO2 values, the relationship between scale-sulfide volume fraction and P_SO2 corresponded to two independent scale-SO₂ reactions leading to oxide and sulfide growth. The two-phase mixture was lamellar, with platelets oriented approximately parallel to the mass-transfer direction. An inverse relationship between lamellar spacing and linear scaling rate is interpreted as evidence of a cooperative (cellular) growth mechanism.     

The Effect of Traces of SO2 on Iron Oxidation: A Microstructural Study

Pure iron has been exposed to pure O₂ and O₂ with 100 ppm SO₂ at 525 °C for 1 and 24 h. The samples were investigated by FIB, SEM, TEM, EDX and EBSD. The oxide scales formed on iron at 525 °C in O₂ and in O₂ + 100 ppm SO₂ are dense and adherent and consist of three layers. The outermost layer consists of hematite. Beneath it there is a duplex-magnetite scale. The two magnetite layers are separated by a straight interface. It is concluded that the inner-magnetite layer grows inward while the outer magnetite layer grows outwards. In the presence of SO₂ the inner-magnetite layer is much thinner, iron sulphate forms at the oxide surface and discrete iron sulphide grains nucleate at the metal/oxide interface. The amount of sulphide at the metal/oxide interface increases with exposure time. The oxidation of iron in oxygen at 525 °C is inhibited by 100 ppm SO₂. The inhibitive effect of SO₂ is attributed to iron sulphate that blocks active sites on the hematite surface, slowing down the formation of oxygen ions. This explains the strong inhibition of the inward growth of magnetite by SO₂. There is also a marked effect on the morphology of the outer oxide, producing hematite whisker growth and a less porous surface in the presence of SO₂.     

Accelerated oxidation of iron induced by Na2SO4 deposits in oxygen at 750°C—A new type low-temperature hot corrosion

Na ₂ SO ₄ -induced accelerated corrosion of iron in oxygen at 750°C was observed. EDX, XRD, SEM, EPMA and some chemical examinations were carried out to understand the corrosion mechanism. The accelerated oxidation was attributed to the formation of abundant sulfide which has a highly defected lattice and allows rapid diffusion of iron ions. The sulfide resulted in turn from the formation of a liquid phase which was a eutectic melt of Na ₂ SO ₄ and Na ₂ O. The formation of and other possible effects of the melt were discussed. The accelerated oxidation was compared with the usual low-temperature hot corrosion, showing that it has most of the characteristics of low-temperature hot corrosion except that it occurred under basic conditions developed by the removal of sulfur from the sulfate deposits instead of the usual acidic conditions established by the SO ₃ in the atmosphere.     

Research on the Post-reaction Strength of Iron Coke Hot Briquette Under Different Conditions

The utilization of iron coke hot briquette (ICHB) prepared by carbonizing iron ore–coal composite agglomerate made from hot-pressing the mixture of iron ore and blended coal has been considered to be an effective countermeasure to improve blast furnace ironmaking reaction efficiency and to reduce carbon emissions. The strength of ICHB after gasification reaction is overestimated by the Chinese National Standard (GB/T 4000/2008, equivalent to the Nippon Steel Corporation method) and should be evaluated by different methods. In this study, the post-reaction strength of ICHB with the addition of different ratios of iron ore under various conditions was experimentally investigated to illuminate the degradation mechanism of ICHB reacted with CO₂. The results showed that, with increasing the iron ore addition ratio from 0% to 20%, the reactivity of ICHB reacted with CO₂ at 1100°C for 2 h is remarkably increased, due to the catalytic effect of metallic iron in ICHB, while the post-reaction strength is distinctly decreased. Furthermore, stopping at the weight loss ratio of 20%, the strengths of ICHB after reaction at 1100°C under a CO₂ atmosphere and a CO₂/CO=1/1 atmosphere are clearly reduced, from 89.74% to 75.93% and from 85.24% to 73.65%, respectively. Meanwhile, the post-reaction strength of ICHB under CO₂ is greater than that obtained under CO₂/CO=1/1 atmosphere, since there is more time for the reaction gas to diffuse from the exterior to the interior of the ICHB under the latter condition. Additionally, the post-reaction strength of ICHB decreases with increasing weight loss ratio regardless of the reaction gas composition; however, it can be maintained at a high level.     

Kinetic and morphological development of oxide—sulfide scales on manganese at 1073 K

The corrosion behavior of manganese in controlled gas atmospheres of SO₂-CO₂-CO-N₂ at 1073 K was studied. Under all conditions, the gas phase was slow to equilibrate, and catalysis of the gas affected the corrosion mechanism and resulting scale morphologies. Product scales invariably became detached from the metal during reaction, but the high manganese vapor pressure meant that no slowing of reaction resulted. Corrosion under conditions where MnS was the equilibrium reaction product led to the formation of a sulfide scale. At low values, this scale grew by reaction with either COS or SO₂ according to parabolic kinetics. Gases with equilibrium compositions calculated to produce MnO, in fact corroded manganese to produce an inner layer of oxide plus sulfide, and an outer layer of MnO. The tendency to form sulfide was more marked at lower SO₂ partial pressure and higher sulfur activities, the latter resulting from gas catalysis. These effects are due to the fact that SO₂ is the principal reactant species.     

High temperature corrosion of cobalt in SO2

The reaction of cobalt in SO₂ has been studied in the temperature range 800–1000°C and at SO₂ pressures from 10 to 760 torr. Reaction kinetics have been studied by thermogravimetry, while the reacted specimens have been characterized by means of optical metallography, scanning electron microscopy, and electron microprobe analysis. The reaction involves formation of cobalt oxide (CoO), cobalt sulphides, and probably cobalt sulphate. The latter compound is formed at the lower temperature due to the presence of oxygen impurities in the SO₂. The relative importance of formation of the different reaction products is a function of temperature and the partial pressure of SO₂ (and O₂). At sufficiently high temperatures and reduced SO₂ pressure, CoO is the only reaction product. Reaction kinetics vary with reaction conditions. The amount of reaction goes through a maximum at about 920°C at 1 atm.SO₂. The reaction mechanism is interpreted in terms of the stability diagram of the Co-O-system.     

Quantum chemistry study on the effect of Cl ion on anodic dissolution of iron in H2S-containing sulfuric acid solutions

The effect of Cl⁻ ion on the anodic dissolution of iron in H₂SO₄ solutions containing low H₂S level has been studied by electrochemical polarization curve measurements. The total energy and binding energy of the competitive adsorption for Cl⁻ and HS⁻ ions have been calculated with CNDO/2 method, as well as the net charge distribution of iron atoms at an anodic potential. The results showed that certain concentration of Cl⁻ ion inhibit the anodic reaction of iron accelerated by HS⁻. However, when Cl⁻ ion reached saturated adsorption, it began to promote the anodic reaction of iron due to the increased negative charge of iron atoms.     

The reaction of nickel with SO2 + O2/SO3 at 500–900°C

The reaction of high purity nickel with SO₂ + O₂ mixtures at 500–900°C has been studied. Measurements have been done in gas mixtures with different SO₂/O₂ ratios and as a function of the total gas pressure of the system. Rapid corrosion rates are observed under conditions where NiSO₄ may be formed on the scale surface and the primary reaction products are NiO and Ni₃S₂. Corrosion rates are faster when the Ni specimens are surrounded by a Pt catalyst. It is concluded that the reaction mechanism involves an SO₃ adsorption equilibrium on the surface followed by formation of NiSO₄. The sulfate, in turn, reacts with nickel diffusing rapidly in a sulfide network in the scale to give NiO and Ni₃S₂.     

Über den beschleunigenden Einfluß von Schwefeldioxid und Wasser auf die atmosphärische Korrosion von verrosteten Eisen

The accelerating effect of sulphur dioxide and water on the atmospheric corrosion of rusty iron The atmospheric corrosion process of rusty steel was observed in the laboratory in an atmosphere with 1, 10 and 100 p.p.m. SO₂, respectively, at a temperature of 30°C. It was found that the correlation of the corrosion rate with humidity can, in the range between critical humidity and nearly 100 per cent. Relative humidity, be represented by a rising quadratic parabolic equation. The differences in the three SO₂ concentrations had no influence on the corrosion kinetics, which is explained by the fact that, under the testing conditions, the rust was fully saturated with SO₄^2?. On the strength of these and earlier results, the authors submit a new working theory concerning the atmospheric corrosion of already rusty steel, introducing the theories of Heusler and Florianovitsch-Kolotyrkin into the sphere of atmospheric corrosion.     

Inhibitory effects of manganeous,cadmium and zinc ions on hydrogen evolution reaction and corrosion of iron in sulphuric acid solutions

The presence of metal ions (Cd²⁺, Mn²⁺, Zn²⁺), more electronegative than the cathodic potential for the hydrogen evolution reaction on iron in a 0.25M H₂SO₄ solution, inhibits the hydrogen evolution reaction and corrosion of iron. This effect has been explained as the under-potential deposition of the adatoms of these metals on iron.     

Der Oxidationsmechanismus von Szomolnokit im intermittierend befeuerten Warmwasserkessel

The oxidation mechanism of szomolnokite in intermittently fired domestic boilers Using thermogravimetry and exposure experiments on initially formed iron(II)-sulfate hydrates the consecutive reactions of the primary corrosion product szomolnokite under the typical operating conditions of domestic boilers were investigated. The reaction mechanism thus deduced is in accordance with thermodynamic equilibrium considerations in the system Fe₂O₃? H₂SO₄? H₂O/crystalline phase. The reaction mechanism consists of the following steps: With this reaction mechanism the most frequently occurring crystalline phases in corrosion samples from oil fired domestic boilers can be explained.     

Kinetic conditions for the simultaneous formation of oxide and sulphide in reactions of iron with gases containing sulphur and oxygen or their compounds

The scaling of pure iron has been investigated in N₂O₂SO₂ and COCO₂COS mixtures between 700 and 900°C. Simultaneous formation of FeO and FeS at the scale/gas phase boundary is observed when the diffusion in the aerodynamic boundary layer or the reaction at the scale/gas phase boundary is the rate-controlling step of the oxidation in O₂N₂ mixtures or of the sulphidation in COCOS mixtures. In those cases the addition of the second oxidant (SO₂ to O₂N₂ mixtures, and an increased CO₂ to COCO₂COS mixtures) increases the rate of the oxidation or sulphidation reactions. When, however, the diffusion of iron ions and electrons through the oxide or sulphide layer respectively, or the reaction at the metal/scale phase boundary are rate-determining, the thermodynamically stable phase (oxide or sulphide) is formed exclusively and the addition of the second oxidant has no influence on the scaling rate. These results may be understood from an evaluation of the equilibria prevailing at the scale/gas phase boundary.     

Study of iron oxidation in sulfur dioxide atmospheres by means of the35S radioisotope

Results of investigations of the mechanism of iron oxidation in atmosphere containing sulfur dioxide are presented. Experiments were carried out both by the platinum marker method and by means of the two -stage oxidation method using sulfur dioxide labeled with the³⁵S radioisotope. SO₂ partial pressures applied were 0.03 and 1 atm at 800°C. The reaction occurred by outward diffusion of iron ions at both sulfur pressures. In addition to cation diffusion there was also some inward sulfur penetration which occurred, however, not by volume diffusion but by short-circuit paths. At the lower SO₂ partial pressure these paths are probably discontinuities in the scale (microfissures), whereas, forp = 1 atm, the inward penetration paths are probably large cavities produced by dissociation at the edges of the specimen when the reaction time is long enough.     

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.     

Interfacial reactions of chalcopyrite in ammonia–ammonium chloride solution

The interfacial reactions of chalcopyrite in ammonia–ammonium chloride solution were investigated. The chalcopyrite surface was examined by scanning electron microscopy and X-ray photoelectron spectroscopy (XPS) techniques. It was found that interfacial passivation layers of chalcopyrite were formed from an iron oxide layer on top of a copper sulfide layer overlaying the bulk chalcopyrite, whereas CuFe_1–xS₂ or copper sulfides were formed via the preferential dissolution of Fe. The copper sulfide layer formed a new passivation layer, whereas the iron oxide layer peeled off spontaneously and partially from the chalcopyrite surface. The state of the copper sulfide layer was discussed after being deduced from the appearance of S^2–, S^2?₂, S^2?_n, S⁰ and SO^2?₄. A mechanism for the oxidation and passivation of chalcopyrite under different pH values and redox potentials was proposed. Accordingly, a model of the interfacial reaction on the chalcopyrite surface was constructed using a three-step reaction pathway, which demonstrated the formation and transformation of passivation layers under the present experimental conditions.