Sulphidation Behaviour Research Articles

Comparison of the combined oxidation and sulphidation behavior of nickel- and cobalt-based alloys at high temperature

The combined oxidation-sulphidation behaviour at high temperature of two nickel-based and two cobalt-based cast alloys is analysed and discussed. These materials are evaluated in view of their possible application for building the components of the high temperature heating furnace up to 1380 °C. Typical service conditions combine extremely high temperatures with the simultaneous presence of sulphur-bearing impurities, due to the combustion of fuels derived from the coke production. The main requirements for this demanding application are simultaneous resistance to high thermal levels and to degradation caused by oxidation and sulphurization. The atmosphere chosen to simulate this harsh environment consists of atmospheric air enriched by 12% of sulphur. The testing procedure has been performed at three temperatures (1080 °C, 1180 °C and 1280 °C) and three times of exposure (24 h, 48 h and 96 h). Macroscopic observations have been coupled to microstructural investigations, performed by SEM-EDS. The analysis is mainly focused on the identification of the microstructural alterations experienced by each alloy and on the extent of internal attack caused by oxygen and sulphur. The combination of the macroscopic experimental data and the information from the microstructural observations alloys have been shown to point out the performances of alloy grade throughout the testing trials in order to define the chemical compositions associated to the moderate weight loss and limited depth of surface attack. The new chemical composition of a Ni-superalloy points out performance of corrosion resistance that are even better than the cobalt based alloys usually selected for such working conditions.

Degradation of thermocouple in a temperature programmed sulphidation reactor

Sulphidation due to sulphur bearing gases such as H2S with refinery materials is a serious problem in the petroleum industry. Measuring instruments such as thermocouples (TCs) used in the refining industry to measure and control temperatures in critical areas are also susceptible to sulphidation when exposed to the sulphur bearing process media. The paper presents an investigation carried out to understand the failure of a TC in a temperature programmed sulphidation (TPS) reactor. The TC was incorrectly installed in the reactor without the external protective sheath. As a result of incorrect installation, the TC was exposed directly to sulphidizing gas of H2S (5% H2S in H2) at a programmed temperature profile (150–1000°C). The qualitative H2S consumption with a change in temperature indicated that the H2S reaction with the TC started at about 400°C, which corresponds to a physical phenomena on the metal surface. The TPS result indicated that the first H2S consumption peak in the range 600–780°C corresponded to the surface phenomena, mostly due to the H2S chemisorption reaction on the metal surface. However, the bulk phenomena of sulphidation or corrosion require diffusion as a function of further increase in temperature, hence a mass consumption peak in the range of 780–900°C is recorded. Detailed investigations of corroded TC were carried out using TPS, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The investigation revealed that the failure of the TC was due to catastrophic sulphidation taken place as a result of incorrect installation. The results further suggest that the TPS technique has the potential to study the sulphidation behavior of metals and alloys.

Sulphidation Behavior of a Non Harmful Water-Based Al and Al–Si Slurry Coating on CM247 Superalloy

Slurry aluminide coatings are widely applied to protect metallic surfaces from oxidation and corrosion. They are frequently used in gas turbine engine nozzles because of economical advantages and a straight-forward manufacturing route. A variety of commercial slurries are available to aluminize the surfaces of nickel-based superalloys, however, they have two main disadvantages. First, the phosphates and chromates or halides used as binders and to activate the diffusion species are environmentally harmful; second, the conventional systems have to be heat-treated in an inert atmosphere. As an outcome of the PARTICOAT project the variety of slurry derived coatings has been extended by tailoring the particle size of the metallic source. By doing that, environmentally friendly water-based slurries were developed to produce in a one-step process und atmospheric conditions, a thermal barrier system based on an aluminum diffusion layer and an alumina foam layer which serves as bond coat as well as top coat (TC). CM 247 nickel base superalloy was coated and heat-treated in air using newly developed Al and Al–Si slurries. The oxidation behavior was investigated at 1,000 °C and then compared to pack-cemented aluminide coatings. The sulphidation behavior was investigated at 1,000 °C in an atmosphere of 1.5 vol% SO2 in synthetic air for Al and Al–Si slurry coated samples with and without the alumina foam TC layer. PARTICOAT Al-based slurries,, after the initial stabilization of the TC, showed similar oxidation kinetics as pack cemented aluminides when exposed to air. When the coatings were exposed to sulphide-containing atmospheres, their oxidation rates increased, producing typical type I corrosion damage. Coatings without TC produced more protective oxide scales. The weight gain and coating area affected by corrosion were slightly lower for the Al-based slurries after 1,000 h of exposure than for the Al–Si based ones. The new coating presented here offers unique advantages in comparison to state-of-the-art slurry and pack cemented coatings by opening a potential way to manufacture a complete thermal barrier coating system by a simple, inexpensive and environmentally safe deposition and heat-treatment in air.

High Temperature Sulphidation Behavior of Fe20Cr6Al-Y,Hf Steel

The high temperature corrosion behavior of Fe20Cr6Al-Y,Hf steel was studied in the range of temperature 800-1000oC in H2/H2S atmospheres at pS2=10-2 ,10-3 and 10-4 Pa sulphur vapor pressures. Kinetics depend on the temperature and sulphur vapor pressure. After 24 hours the whole specimen was practically consumed because the samples were 0,06 cm thick. Morphology of the scales have been performed by SEM techniques. Phase and chemical composition have been studied by EDX and XRD techniques. It was found that scale formed on Fe20Cr6Al-Y,Hf alloy was built with porosity sulphides layer. EDX analysis of the scale surface show that the any aluminum, hafnium and yttrium sulfides were found in the formed scale layer, however small amounts of Al2S3 was detected in scale/steel interface. Also internal sulphidation was observed. A phase analysis of the formed scale revealed that it is composed mainly of an FeS, Fe7S8 phases and CrS, Cr5S6. Result were compared with data obtained on the pure Fe and Cr samples.

Design and Development of Sulphidation Resistant Coatings

This paper discusses the design and development of sulphidation resistant coatings. The first section discusses the rationale for designing and developing such coatings. The rationale for selecting the coating systems has been derived from our work on the HT sulphidation behaviour of MCrAlYX (M=Co, or Co-Fe and X=V, Nb, Mo and W) type of coating alloys. This work has allowed the establishment of the mechanisms of sulphidation and led to the choice of the coating systems discussed here. The second section describes the sulphidation performance of several selected coatings, single and multilayer, deposited on Ti, TiAl and Ni-based alloys at 750-850oC in environments of low oxygen and high sulphur potentials.

The Influence of Lithium on the Sulphidation Behavior of Molybdenum at High Temperatures

The Influence of Lithium on the Sulphidation Behavior of Molybdenum at High Temperatures

High-temperature characterization of reactively processed nanostructure nickel aluminide intermetallics

A detailed study of the high-temperature oxidation behavior of reactively powder processed nanostructure nickel aluminides Ni 3Al and NiAl (with and without microalloying additions) starting from elemental nanopowders in air at two different temperatures (800 and 1000 °C) for 200 h was carried out, as well as the sulphidation behavior (in 10% SO 2, 90% He atmosphere) at 800 °C for 100 h. The weight gain per unit area (mg/cm 2) due to oxide scale formation at a given exposure time t (h) and temperature was discussed. Thermodynamics and kinetics were used to explain the oxidation mechanism. The high temperature oxidation/or sulphidation resistance was improved due to nanostructure grains formation. Also, additions of a small amount of Nb (up to 4 wt%) as microalloying element resulted in a significant increase in the high temperature oxidation/ or sulphidation resistance compared to the microalloying free intermetallics. Also, the results indicated that nanostructure NiAl (with and without micro-alloying elements) showed superior high temperature oxidation/or sulphidation resistance compared to the nanostructure Ni 3Al (with and without micro-alloying elements). Both XRD and SEM/EDS-analysis were used to identify the oxide phases present after the high temperature oxidation/or sulphidation exposure of nanostructure aluminides at 1000 °C after 200 h, and at 800 °C after 100 h, respectively.

Effect of alloying additions and preoxidation on high temperature sulphidation resistance of iron–chromium alloys

High temperature corrosion of structural alloys in sulphur bearing environments is many orders of magnitude higher than in oxidising environments. Efforts to increase sulphidation resistance of these alloys include addition of alloying elements. This paper reports the effect of yttrium, aluminium and yttrium plus aluminium addition on the sulphidation behaviour of an Fe–20Cr alloy in H2–2%H2S at 700 and 800°C. The effect of preoxidation of these alloys on sulphidation behaviour was also investigated. The initial stages of sulphidation of these alloys were observed and overall sulphidation kinetics was determined. In the presence of yttrium, the sulphidation rate of the alloy decreased by an order of magnitude, and the sulphide layer on the FeCrY alloy was significantly thinner than that formed on the FeCr alloy. Aluminium addition increased sulphidation resistance even more. The FeCrAl and FeCrAlY alloys exhibited parabolic sulphidation kinetics at the two temperatures. On both alloys, initial chromium sulphide (Cr2S3) formation at grain boundaries and Al2S3 and FeCr2S4 formation over the grains was followed by formation of an outer Cr2S3 layer over a mixed sulphide layer. Preoxidation of the two alloys resulted in an ‘incubation period’ with practically no sulphidation during subsequent exposure to S bearing environments. After this incubation period, the sulphidation behaviour of the FeCr alloy was similar to that when the alloy was not preoxidised. The incubation period of the FeCrAl alloy was significantly longer, over 45 h, compared with 2 h for the Al free alloy. Based on thermogravimetric as well as SEM, energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) data, sulphidation mechanisms have been proposed for the alloys before and after preoxidation.

Effect of tin on high-temperature sulphidation of Fe–Cr alloys in H 2/H 2S atmospheres

The high temperature sulphidation behaviour of Fe–46Cr– xSn ( x = 0; 0.2; 0.5; 1; 2) alloys has been studied at temperatures of 1073, 1173 and 1273 K in H 2/H 2S mixtures with different sulphur vapour partial pressures of 10 −1, 10 −3 and 10 −5 Pa. Thermogravimetric studies in combination with scanning electron microscope (SEM), with energy dispersive spectrometer (EDS), and X-ray diffraction (XRD) techniques, have displayed a significant influence of the sulphur partial pressure on the composition and growth rate of the sulphide scale. The results have shown that addition of tin increases the sulphidation rate of Fe–46Cr alloys but not considerably (except at temperatures of 1073 and 1173 K combined with sulphur partial pressure of 10 −5 Pa). The metallic core of the studied samples was enriched in tin and iron, moreover tin was found in the internal layer close to the metallic core as metallic Fe x Sn y inclusions with tin concentrations of up to 12 at.%.

The sulphidation behaviour of hard chromium plated coatings on steel

The sulphidation behaviour of high contraction (HC) and low contraction (LC) chromium plated steel was studied in sulphur with a partial pressure of 1 atm at 1, 3, 5 and 7 h and 0.1 and 0.01 atm at 5 h. The temperature was 600°C throughout the work. It was found that HC and LC chromium plating imparted significant sulphidation resistance to steel. It should be mentioned that by increasing sulphidation time and sulphur partial pressure, the hardness of chromium coating decreased. LC chromium coating had a lower thermal shock resistance than the HC coating. Thus, it was concluded that the HC chromium coating had the better sulphidation resistance.

Sulphidation Behaviour of FeAl Intermetallics in an Atmosphere Containing Low Concentration of H2S

Article Sulphidation Behaviour of FeAl Intermetallics in an Atmosphere Containing Low Concentration of H2S was published on July 1, 1999 in the journal High Temperature Materials and Processes (volume 18, issue 5-6).

Sulphidation of NiAl-Cr Materials at Elevated Temperatures

The paper is a comprehensive overview of sulphidation-related problems in the β-NiAl-αCr system. It presents many physico-chemical data on intermetallic phases in the Ni-Al-Cr ternary system and on the respective binary and ternary sulphides together with rich experimental material concerning the sulphidation behaviour of the NiAl-Cr intermetallics in He/S 2 and H 2 /H 2 S gas atmospheres at high temperatures.

Influence of plasma-sprayed Mo coating on sulphidation behaviour of Inconel 600 and Nimonic PE11 alloys

Sulphidation is a serious problem in many energy conversion systems. Sulphidation attack is particularly severe in environments of low oxygen ( p O2~10 −18 Pa) and high sulphur ( p S2 ~ 10 −1_10−3 Pa) potentials at temperatures above 700°C. Recognizing that refractory metals have a high resistance to sulphidation in reducing environments, their use as overlay coatings in sulphur-containing environments deserves serious consideration. In the adoption of such an approach, Mo has been recognized as a highly sulphidation-resistant metal with a Kp value of 10 −12 g 2 cm −4 s −1 in an atmosphere of p S2 ~ 10 −1 Pa at 750°C. In this study, Mo was deposited on two superalloys, Inconel 600 and Nimonic PE11, using air plasma spraying. The coated specimens were tested at 750°C for up to 168 h in an environment comprising p S2 ~ 10 −1 Pa and p O2 ~ 10 −18 Pa. The sulphidation kinetics (this is not real through-layer kinetics, since the specimens had open edges) were determined by a discontinuous gravimetric method. The exposed samples were characterized and analysed using SEM, EDX and XRD techniques. For uncoated Inconel 600, a duplex scale was formed which consisted of an outer Ni 3S 2 layer and an inner Cr 2S 3 layer. The scale developed on uncoated Nimonic PE11 comprised three sub-scale layers—an outer (Fe, Ni) 9S 8 layer, a mid Cr 2S 3 layer and an inner MoS 2 layer. However, after prolonged exposure, the outer layer contained both (Fe, Ni) 9S 8 and Ni 3S 2 on the surface of the uncoated Nimonic PE11. The kinetics results demonstrated that Mo coating enhanced the corrosion resistance of both alloys, and particularly so for Nimonic PE11. For both substrates, Mo coating led to the development of two sub-scale layers at the early stages of exposure (e.g. up to 5 h). The outer layer consisted of Ni 3S 2 for Inconel 600 and (Fe, Ni) 9S 8 for Nimonic PE11. A MoS 2 layer constituted the inner layer for both materials. After prolonged sulphidation, a Cr 2S 3 layer gradually developed between the outer layer and the inner MoS 2 layer. It is apparent that the use of Mo coating hindered the formation of the Cr 2S 3 layer.

Influence of plasma-sprayed Mo coating on sulphidation behaviour of Inconel 600 and Nimonic PE11 alloys

Sulphidation is a serious problem in many energy conversion systems. Sulphidation attack is particularly severe in environments of low oxygen ( p O2 ~10 −18 Pa) and high sulphur ( p S2 ~ 10 −1_10−3 Pa) potentials at temperatures above 700°C. Recognizing that refractory metals have a high resistance to sulphidation in reducing environments, their use as overlay coatings in sulphur-containing environments deserves serious consideration. In the adoption of such an approach, Mo has been recognized as a highly sulphidation-resistant metal with a Kp value of 10 −12 g 2 cm −4 s −1 in an atmosphere of p S2 ~ 10 −1 Pa at 750°C. In this study, Mo was deposited on two superalloys, Inconel 600 and Nimonic PE11, using air plasma spraying. The coated specimens were tested at 750°C for up to 168 h in an environment comprising p S2 ~ 10 −1 Pa and p O2 ~ 10 −18 Pa. The sulphidation kinetics (this is not real through-layer kinetics, since the specimens had open edges) were determined by a discontinuous gravimetric method. The exposed samples were characterized and analysed using SEM, EDX and XRD techniques. For uncoated Inconel 600, a duplex scale was formed which consisted of an outer Ni 3 S 2 layer and an inner Cr 2 S 3 layer. The scale developed on uncoated Nimonic PE11 comprised three sub-scale layers—an outer (Fe, Ni) 9 S 8 layer, a mid Cr 2 S 3 layer and an inner MoS 2 layer. However, after prolonged exposure, the outer layer contained both (Fe, Ni) 9 S 8 and Ni 3 S 2 on the surface of the uncoated Nimonic PE11. The kinetics results demonstrated that Mo coating enhanced the corrosion resistance of both alloys, and particularly so for Nimonic PE11. For both substrates, Mo coating led to the development of two sub-scale layers at the early stages of exposure (e.g. up to 5 h). The outer layer consisted of Ni 3 S 2 for Inconel 600 and (Fe, Ni) 9 S 8 for Nimonic PE11. A MoS 2 layer constituted the inner layer for both materials. After prolonged sulphidation, a Cr 2 S 3 layer gradually developed between the outer layer and the inner MoS 2 layer. It is apparent that the use of Mo coating hindered the formation of the Cr 2 S 3 layer.

An RBS study of the sulphidation behaviour of niobium and NbAl alloys

The growth mechanisms of the sulphide scales on niobium and niobium-aluminium alloys at a temperature of 1073 K and sulphur pressure of 1 kPa have been studied by means of Rutherford backscattering (RBS) with the aid of a marker technique. A gold marker film of about 1 nm thickness was deposited on the surface of niobium as well as on Nb-0.6 at% Al, Nb-15.2 at% Al and Nb-63.4 at% Al alloys by vacuum evaporation. After sulphidation the gold marker distribution in the scale has been determined by RBS. It has been found that sulphidation of niobium occurs by the outward diffusion of niobium cations since the gold marker has been located at the metal/scale interface. The scale structure for the Nb-0.6Al alloy is almost the same as that for niobium because the alloy aluminium content is too low to form a separate phase of aluminium sulphide. Other niobium-aluminium alloys form a double layer scale with an outer aluminium sulphide layer and an inner niobium sulphide layer, both of which are formed by the outward diffusion of cations.

The Sulphidation and Oxidation Behavior of New Sputter-Deposited Alloys at High Temperatures

The Sulphidation and Oxidation Behavior of New Sputter-Deposited Alloys at High Temperatures

The sulphidation behavior of MoAl alloys with low aluminum contents

The beneficial effect of alloying with aluminum to enhance the sulphidation resistance of molybdenum using sputter-deposited amorphous Mo-Al alloys has been studied as a function of temperature (1073–1273 K), aluminum content (1.7, 6.2 and 10.7 at% Al) and sulphur vapor pressure (20–4000 Pa) in He-S 2 gas mixtures. It has been shown that the sulphidation process follows parabolic kinetics, being diffusion controlled. The reaction rate of Mo-Al alloys with low aluminum contents is lower than that of pure molybdenum and decreases with increasing aluminum content in the alloy. Because sulphide scales formed on Mo-Al alloys containing small amounts of aluminum are homogeneous and composed only of MoS 2, the lower sulphidation rate of those alloys is explained in terms of the aluminum doping effect in the MoS 2 scale. It is believed that the better protective properties of the sulphide scale on Mo-Al alloys in comparison with those of the MoS 2 scale on pure molybdenum result from a lower defect concentration in aluminum-doped MoS 2 phase. The experimentally obtained dependence of the sulphidation rate of Mo-1.7Al alloy on the sulphur pressure at 1173 K is in accordance with that predicted from theoretical considerations.

Sulphidation behaviour of FeCoCrAlY alloys containing refractory metals

The corrosion behaviour of three FeCoCrAlYX (where X = V and Nb)-type alloys has been studied in a sulphur-bearing atmosphere ( pS 2 ∼ 10 −1 Pa and pO 2 ∼ 10 −18 Pa) at 750°C. Weight change data revealed protective kinetics and followed a parabolic rate law. The alloys containing higher levels of V and Nb showed better corrosion resistance. Replacement of Co by Fe improved the sulphidation behaviour. Analysis of the exposed specimens by means of SEM, EDAX and XRD demonstrated that the scales formed were duplex in nature. The outer layer of the scale mainly consisted of sulphides of Fe and Cr (Fe 3S 4, Co 9S 8 and Cr 3S 4). The inner layer principally containing sulphides of V, Nb and Cr was compact, adherent and protective, and substantially resisted the outward diffusion of the base alloying elements.

The sulphidation and oxidation behaviour of sputter-deposited amorphous AlMo alloys at high temperatures

The sulphidation behaviour of sputter-deposited amorphous AlMo alloys has been studied as a function of temperature (973–1273 K) and alloy composition (34–46 at%Mo) in pure sulphur vapour at 10 3 Pa pressure. It has been shown that under these conditions the sulphidation process follows parabolic kinetics, being diffusion controlled. No influence of the alloy composition on the reaction rate has been observed. The scales were heterogeneous and composed of two layers. The outer scale layer was built of aluminium sulphide, while the inner layer was heterogeneous and composed of a mixture of Al 0.5Mo 2S 4, Al 2S 3 and MoS 2 phases, the latter being the major component. It is believed that the slowest step, determining the overall reaction rate, is the inward diffusion of sulphur through the inner barrier layer of the scale. Over the whole temperature range studied the alloys showed excellent resistance to sulphide corrosion, their sulphidation rates being comparable with, or even lower than the oxidation rates of chromia forming materials. The better protective properties of the sulphide scale on AlMo alloys in comparison with those of the MoS 2 scale on pure molybdenum result probably from lower defect concentration in aluminium-doped MoS 2 phase, constituting the major part of the inner barrier layer. Preliminary results indicate that the alloys under investigation show satisfactory oxidation resistance in the temperature range not exceeding about 1123 K. Under these conditions a protective oxide scale is formed composed virtually only of Al 2O 3 and the oxidation rates are lower than those of chromia formers. At higher temperatures aluminium activity in the alloy is too low to suppress the formation and evaporation of molybdenum oxides.

Sulphidation of someMCrAlYX-type alloys

The sulphidation behavior of a series ofMCrAlYX-type alloys, where M is Co or Co and Fe, and X is V, Nb, Mo, or W, added in combination at various levels, has been studied at 750°C in an atmosphere of\(p_{S_2 } \)∼ 10−1 Pa and\(p_{O_2 } \)∼10−18 Pa. Sulphidation kinetics followed a parabolic rate law for each alloy over prolonged periods of exposure (up to 240 hr). Inclusion of Mo in the alloy containing both V and Nb, and particularly the combined additions of Mo and W, greatly enhanced sulphidation resistance. Partial replacement of Co by Fe also provided superior sulphidation resistance. The scales formed on the Co-based alloys consisted of three subscale layers-an outer layer of Co9S8 and Co3S4, a mid-layer of Cr3S4 and an inner layer of chromium and refractory-metal sulphides. In the case of (Fe, Co)-based alloys, the scales were duplex in nature with an outer layer of Fe3S4, Co9S8 and Cr3S4 and a compact inner layer consisting of sulphides of chromium and refractory metals. For both categories of alloys, the inner layer of refractory-metal sulphides was decorated by an alumina film which inhibited the outward migration of the base elements.