High-temperature corrosion of nickel in SO2

The reaction of nickel with SO₂ has been studied in the temperature range 650–1100°C at SO₂ pressures from 10 to 760 Torr. Reaction kinetics have been studied by thermogravimetry; the reacted specimens have been characterized by means of metallography, scanning electron microscopy, and electron microprobe analysis. The reaction involves oxidation and sulfidation except at sufficiently high temperatures and low pressures of SO₂ (e.g., 1000°C and 10 Torr SO₂) where only formation of NiO takes place. Approximately linear reaction kinetics are observed between 650 and 900°C. Reaction mechanisms are discussed, and the relative importance of oxide formation and sulfidation is interpreted in terms of the thermodynamics of the Ni-O-S system.     

The reaction of Ni-20Cr with SO2 at 600-900°c

A Ni-20Cr alloy has been reacted in SO₂ at 600–900°C. Reaction kinetics have been studied as a function of temperature, pressure and experimental procedure. Reaction products have been characterized by X-ray diffraction, scanning electron microscopy, and differential thermal analyses. Depending upon the reaction conditions, the primary reaction products are NiO and Ni₃S₂, and Cr₂O₃ and (Ni-S)_hq. Under conditions where Ni₃S₂ is formed, reactions are rapid, while the formation of (NiS)_liq (>800°C) leads to slow reactions. The molten sulfide phase facilitates transport of chromium to the scale-gas interface where a protective layer of Cr₂O₃ is formed. Reaction mechanisms are discussed in terms of thermodynamic stability of the reaction products.     

Corrosion in SO2 of pure and preoxidized copper at high temperature

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

Effect of sulfur dioxide on the oxidation of thin films of tin

The oxidation of thin films of tin in SO₂-air atmospheres containing 10%, 20%, 30%, 40%, 50%, and 60% SO₂ have been investigated at room temperature. Reaction kinetics were studied by the variation in resistance of thin films and growth patterns by metallography. Addition of SO₂ causes an increase in the oxidation rate of thin films of tin. Probable reactions at lower and higher percentages of SO₂ have been discussed.     

利用固液扩散偶研究Mg-40Al与Mg-20Ce的界面反应

本文主要研究了Mg-40Al与Mg-20Ce固液界面在475°C, 500°C和525°C下保温5min至30min的界面反应和扩散层的生长动力学。结果发现，在扩散层中由于Al元素和Ce元素反应生成Al₁₁Ce₃, Al₃Ce and Al₂Ce金属化合物。金属化合物的体积分数随着扩散温度的升高而增加。扩散层的生长满足抛物线生长规律，扩散层的扩散激活能为42±3.7kJ/mol。实验研究的固液扩散为理解合金熔炼过程中金属化合物的形成提供了理论基础。     

The corrosion of nickel in 1 atm of pure SO2 at 600–1000°C and the mechanism of formation of the two-layered scales

The reaction of pure nickel with 1 atm SO₂ at 600–1000°C follows an irregular type of kinetics, tending to become parabolic at the highest temperature. The reaction rate tends to decrease with an increase of temperature while the scale has the same type of overall structure at all temperatures. The scales consist of an inner region of sulfide free from oxide and containing metal particles and an outer region composed of an oxide matrix containing dispersed particles of sulfide, generally extending up to the outer scale surface. The main mechanism of reation is considered to be the simultaneous formation of a mixture of oxide plus sulfide at the scale-gas interface by direct reaction of the metal with molecules of SO₂. The formation of an inner layer of sulfide not containing oxide is attributed to an inward migration of sulfur through the outer duplex layer, most likely inside the network of sulfide particles.     

The oxidation of nickel-chromium alloys in atmospheres containing sulphur dioxide

The simultaneous attack of oxygen and sulphur on a range of NiCr alloys containing 2–30%Cr has been investigated between 500 and 790°C in Ar-10% SO₂ atmospheres. Reaction kinetics were followed using an automatic recording balance and scale structures were examined by metallography, scanning electron microscopy and microprobe analysis.Compared with pure Ni, the addition of up to 5%Cr at 500°C, or 15%Cr at 700°C, has little effect on kinetics. At higher Cr levels (8–10% at 550°C or 15–20% at 700°C) initial protective behaviour is followed by a period of rapid attack. Even with Cr contents as high as 30% the protection shown is not as good as in the absence of sulphur from the atmosphere. The failure to form protective oxides is thought to be due primarily to the penetration of initially formed Cr₂O₃ scales by sulphur, possibly by solution and diffusion, leading to the formation of duplex (oxide + sulphide) scales which grow rapidly by nickel ion migration.     

The Oxidation of cobalt-tantalum base alloys containing carbon

The oxidation of cobalt-tantalum carbon alloys, containing 10 and 15 wt.% Ta and carbon in the range 0–1 wt%, was carried out in oxygen and air at atmospheric pressure at 900, 1000 and 1100°C. The alloys oxidised according to the parabolic rate law with activation energy of about 38 Kcal/mole. In general, the addition of tantalum decreases the oxidation rates, in comparison with cobalt and with the same mass of chromium added to cobalt. Again, the presence of carbon in the Co-Ta alloys decreases its oxidation rates in comparison with carbon-free alloys. The scales formed on Co-Ta and Co-Ta-C alloys consist mainly of an outer layer of cobalt oxide, CoO, and an inner porous layer of mixture of oxides: cobalt oxide; CoO, tantalum oxide; Ta₂O₅, and solid solution of these two oxides; CoTaO₄ at all temperatures in the range of 900°-1100°C. The binary Co ?10% Ta and Co ?15% Ta show an internal oxidation along the internal phase, increasing of alloy tantalum content increases the density of the internal phase. The presence of carbon in the ternary Co-Ta-C alloys has little effect and there is no apparent preferential penetration along the tantalum carbide network. In contrast to carbide present in Co-Cr-C alloys, where these carbides were preferentially attacked, the outer scale was disrupted, due to the formation of carbon gaseous oxides.     

The corrosion of iron in pure SO2 under different pressures at 700°C

The reaction between iron and pure SO₂ under different pressures at 700 °C has been studied. The experiments carried out under static conditions showed a large scatter of kinetic data while the runs in flowing SO₂ at atmospheric pressure produced fairly reproducible results under convenient conditions. They differ, however, in some aspects from previous results for the same system; possible reasons for this effect are discussed. Finally, the formation of iron sulfide beyond the conditions of thermodynamic stability is examined by considering the possibility of the direct reaction of the metal with SO₂ rather than with the dissociation products of the latter.     

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.     

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

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

Investigations on role of HVOF sprayed Co and Ni based coatings to combat hot corrosion

Abstract

This paper aims to investigate the hot corrosion resistance of high velocity oxy-fuel (HVOF) sprayed cobalt based (Stellite-6) and nickel based (Ni–20Cr) coatings deposited on the superalloy Superni-718 (Ni–19Cr–18˙5Fe–5˙13Ta–3˙05Mo–0˙9Ti–0˙5AI–0˙18Mn–0˙18Si–0˙15Cu–0˙04C) in the Na₂SO₄–60%V₂O₅ salt environment at 900°C under cyclic conditions. The X-ray diffractometry, scanning electron microscopy/energy dispersive analysis and electron probe microanalyser techniques were used to study the corrosion products with respect to their morphology, phase composition and element concentration. The thermogravimetric technique was used to establish the kinetics of corrosion. The bare alloy underwent severe hot corrosion attack. The Ni–20Cr coating shows excellent hot corrosion resistance with negligible spallation, whereas Stellite-6 coating reveals less hot corrosion resistance and more spallation. The hot corrosion resistance of Ni–20Cr coating has been attributed to the formation of oxides of chromium, nickel and spinel of nickel chromium. The oxides of silicon, chromium, cobalt and spinels of cobalt–chromium and nickel–chromium have contributed for hot corrosion resistance of Stellite-6 coatings.     

Evaluation of High Temperature Oxidation Behaviour of Nanostructured Cr/Co–Al Coatings

The high temperature oxidation behavior of sputtered Cr/Co–Al coatings fabricated by DC/RF magnetron sputtering on a superalloy substrate has been studied in the present work. The microstructural features and phase formation of the as-deposited coatings were characterized by FE-SEM, AFM, and XRD, respectively. Weight-change measurements were made to calculate the cyclic oxidation kinetics of the coated superalloy exposed to air at 900 °C. It was observed that the corrosion rate of sputtered Cr/Co–Al coated superalloy is lower than that of the uncoated superalloy owing to the formation of continuous, dense, adherent and protective oxide scales over the surface of the coatings. The protective oxide scales in the corroded coatings were basically the thin layer of Cr₂O₃, CoO, Al₂O₃ and CoCr₂O₄, which provide protection to the base superalloy at high temperature.     

Mapping of the oxidation products in the ternary Co-Cr-Al system

The kinetics and products of oxidation of alloys in the Co-Cr-Al system have been studied and four mechanisms of oxidation identified. For the first mechanism, the rate-controlling process is cation diffusion of cobalt cations through CoO, and the main oxidation product is CoO. In the second mechanism, cation diffusion through CoO is still rate controlling, but the oxidation is strongly inhibited by an inner discontinuous spinel layer. The major oxidation products are CoO and CoCr₂O₄. The third mechanism of oxidation consists of preferential oxidation of chromium coupled with internal oxidation of aluminum while the fourth mechanism is the preferential oxidation of aluminum with the subsequent formation of Al₂O₃ scales, providing the best oxidation resistance. From the oxidation data obtained, an oxide map from which the oxidation behavior of various alloys may be deduced is drawn for the Co-Cr-Al system at 1100°C.     

The high temperature corrosion of nickel in SO2 at 500–800°C

The reaction of nickel with SO₂ has been studied at 500–800°C and different pressures of SO₂ (0.4–100 kPa). The reaction products are NiO and nickel sulphides. The reaction rate goes through a maximum at about 600°C at and above 13 kPa SO₂, while the maximum is absent at lower SO₂ pressures. It is concluded that the reaction takes place through different reaction paths: (i) the direct reaction of Ni with SO₂ and (ii) a reaction path via NiSO₄ as an intermediate reaction product. The latter path is the more rapid one and gives rise to the very rapid reaction rates and maximum in the reaction rate at 600°C.     

Oxidation mechanism of cobalt based alloy at high temperatures (800–1100°C)

Abstract

A cobalt based Phynox alloy has been oxidised in the 800–1100°C temperature range. Kinetic results show that the parabolic behaviour is followed under isothermal conditions. The scale growth mechanism of cobalt based Phynox alloy in air is consistent with a growth mechanism limited by the diffusion process in a growing Cr₂O₃ oxide scale. Thermal cycling tests show that the best scale adherence is found at 1000°C. This temperature permits a rapid chromium supply from the substrate to form a continuous chromia scale. A keying effect at the internal interface is promoted by the presence of silicon and molybdenum. At 900°C, CoCr₂O₄ cobalt containing oxide formation is favoured and leads to a bad scale adherence. At 1100°C, thermal cycling conditions lead to scale spallation and chromium depletion. Then, important weight losses are registered corresponding to the oxidation of cobalt and molybdenum to induce CoCr₂O₄ and CoMoO₄ formation.     

Sulfidation-oxidation of nickel and cobalt—reactions between the metals and their sulfates

A differential thermal, thermogravimetric analysis (DTA-TGA) study shows that both nickel and cobalt react with their respective sulfates to form oxide (NiO or CoO) and sulfide (Ni₃S₂ or Co₉S₈ and Co₄S₃). In the case of the Ni+NiSO₄ reaction, it is clear that the reaction occurs at T < 530°C and may be a solid-state process. A similar process is found for the cobalt+CoSO₄ reaction. These reactions are briefly discussed with respect to studies of the oxidation-sulfidation of Co or Ni under conditions which produce the sulfate as a stable phase. It is proposed that the rate-determining step in these studies is the formation of the sulfate.     

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.     

Influence of hydrostatic pressure on pitting of aluminium in sea water

Abstract

The effect of hydrostatic pressure on the corrosion of aluminium in sea water oj pH 8·2 containing 7ppm dissolved oxygen has been investigated at 20°C. On increasing the pressure from 1 to 300 atm, the corrosion rate increased, an effect attributable to an increase in the rate of the anodic reaction while the cathodic rate remained unchanged. The observed increased susceptibility to pitting corrosion with increasing pressure was found to correlate with the presence of increasing quantities of SO^2?₄ and Cl^? ions in the oxidation layer. The introduction of electron acceptors such as SO^2?₄ and Cl^? into n type compounds such as aluminium oxides increases the density of lattice dejects and promotes breakdown oj the barrier provided by the oxidation layer.