Plasma-sprayed self-sealing ceramic coatings: Materials chemistry and high temperature protective properties

The principal theoretical and experimental data relating to the structure and the protective action of self-sealing ceramic coatings (SSCCs) used to prevent non-aqueous corrosion phenomena (T ? 900 K) on metallic surfaces in chemically aggressive environments (oxidation, carburization, nitridation, sulphidation, attack by molten metals etc.) are reviewed. The plasma spray process used to deposit SSCCs is described and some of their properties are summarized. The results obtained in an investigation of the protective efficiency of SSCCs on Incoloy 825 and titanium substrates, which are particularly susceptible to metal dusting and oxidation-nitridation corrosion phenomena, are reported. It is hoped that the work described in this paper will stimulate new approaches to the development and characterization of these novel protective coatings as it is necessary to optimize the prevention of non-aqueous corrosion in many applications.     

Self-fluxing coatings for stationary gas turbines

Self-fluxing alloys are becoming more and more interesting for hardware applications in stationary gas turbines. This is due to their ease of application which involves plasma spraying and sintering. These coatings are basically Cr₂O₃ formers which provide protection against sulphidation.As a result of the demand for greater efficiency in gas turbines and the consequent increase in operating temperature oxidation has become the basic form of corrosion attack and therefore oxides other than Cr₂O₃, i.e.Al₂O₃, are required.In this paper we shall describe an attempt that was made to modify a self-fluxing alloy (Ni-Cr-Fe-Si-B) with aluminium in order to produce protective Al₂O₃ scales. The coating is applied using air plasma spraying. The variation in the sinter temperature and the oxidation and corrosion resistance properties due to aluminium are given. The effect of aluminium on the tensile properties is illustrated using data from a ductile-brittle transition temperature test.     

Mechanism of carburization of high-temperature alloys

An investigation of the mechanism of gaseous carburization in a reducing environment was conducted for selected Fe- and Ni-base alloys. Carburization kinetics were measured as functions of temperature in the range 870–980 °C. Scanning electron microscopy, analytical electron microscopy and X-ray diffractometry were employed for microstructural characterization and microchemical analysis. Changes in mechanical strength produced by carburization were determined from microhardness and tensile property measurements. Kinetic studies indicated that the carburization reaction followed a parabolic rate law. Depending upon the nature of surface scale formed in the presence of a carburizing environment, the rate-determining step of the reaction varied from C diffusion into the alloy in the presence of a carbide scale to that in the presence of an oxide scale. Under reducing carburizing conditions, alloys inherently protected by Cr₂O₃-base scale were found to develop a surface carbide scale which allowed C to penetrate into the alloy with relative ease and, thus, the carburization kinetics was accelerated. In contrast, an alloy capable of forming Al₂O₃ developed and maintained a protective surface oxide scale which acted as an effective barrier to C diffusion into the alloy. Degradation of mechanical strength due to precipitation of carbides in the alloy was correlated with the rate of attack and consequently the nature of the surface scale.     

The corrosion of INCONEL alloy 740 in simulated environments for pulverized coal-fired boiler

The corrosion of a new nickel base superalloy, INCONEL alloy 740, has been studied at 550 and 700 °C on exposure to the synthetic coal ash/flue gas environments by means of XRD, SEM, and EDX. Low temperature hot corrosion of the new alloy occurred at two temperatures. The corrosion started to form the thin Cr₂O₃ scale on the alloy at 550 °C and developed as pitting attack resulted from sulfidation. The frontal attack at 700 °C consisted of two successive stages in which the corrosion mechanism started from the sulfidation and ended up in the fluxing of oxide. The compact and protective Cr₂O₃ scale formed and the internal sulfidation took place during the initial stage. The severe hot corrosion occurred due to the presence of the molten CoSO₄ during the propagation stage. The loose and porous outer layer and the compact inner layer consisted of spinels and oxides, respectively. The sulfides of Cr, Ti, and Nb formed on the front of oxide scale and in Cr-depletion zone. The rapid degradation of corrosion resistance of the alloy can be attributed to the dissolution of both cobalt and cobalt oxide on the surface. The alloy of 25% Cr exhibited better resistance to coal ash/flue gas corrosion as compared to the alloy of 23% Cr in the present case.     

Microstructural investigations of Ni and Ni2Al3 coatings exposed in biomass power plants

Abstract

The present work investigates the corrosion resistance of Ni and Ni₂Al₃ coated austenitic stainless steel (TP347H) tubes, which were exposed in a biomass-fired boiler with an outlet steam temperature of 540 °C for 6757 h. The Ni₂Al₃ coating was produced by electroplating Ni followed by low temperature pack cementation. After exposure, microstructural investigations were performed by light optical and electron microscopy (SEM-EDS). Electroplated Ni coatings were not protective in straw firing power plants and exhibited similar corrosion morphology as uncoated tubes. For Ni₂Al₃ coatings, the nickel aluminide layer was no longer adherent to the tube and was only found within the deposit. However, Ni₂Al₃ coatings had provided some protection compared to uncoated and Ni coated tubes. The formation of nickel chloride binds aggressive chlorine and slows down the active oxidation mechanism. In local areas, sulphidation corrosion attack of Ni was detected.     

Design of sulphidation resistant alloys

Abstract

The interaction of metallic materials with sulphur and sulphur bearing gases at elevated temperatures leading to the formation of sulphide corrosion products – sulphidation – is generally an extremely rapid process, much more so than oxidation. Conventionally developed high temperature alloys with adequate oxidation performance generally have poor resistance to sulphidation and the design of alloys or coatings to withstand such aggressive environments represents a major technological challenge. The basic process and mechanisms of sulphidation are reviewed and contrasted with oxidation, leading to an enunciation of the principles underlying the development of sulphidation resistance in model alloys involving ‘barrier layer’ formation of protective sulphide(s) to inhibit the usually extraordinarily fast diffusion processes responsible for rapid wastage. Underlying principles for the development of sulphidation resistant alloys are shown with reference to the behaviour of various experimental model MX type alloys in which the limitations of addition elements (X) such as chromium or aluminium, used successfully to control oxidation, are emphasised. The potential offered by the use of various group IV–VI semirefractory and refractory elements such as Ti, Zr, Hf, V, Nb, W, and Mo is discussed. The actual greatly enhanced performance of various experimental alloys containing such elements is illustrated and interpreted, leading to suggestions for projected future areas of research.

MST/1078     

Degradation of mechanical strength of pyrolysis furnace tubes by high-temperature carburization in a petrochemical plant

Pyrolysis furnace tubes made of HP45 heat-resistant steel casting to handle carbonaceous gases at about 850 °C in a petrochemical plant developed longitudinal cracks after less than one-third of the expected life. Detailed microstructural analysis showed that the cracks were developed by a ductile intergranular mode. This was correlated with high-temperature carburization attack leading to massive precipitation of intergranular carbides and a mixture of carbides and sigma phase within the matrix. Experimental results suggested that the highly reducing nature of the environment precluded the tube material from developing and maintaining a surface protective oxide scale, which facilitated the rapid inward diffusion of carbon.     

High-temperature corrosion of superalloys

The performance of gas turbines has been improved by the development of alloys with progressively increasing high-temperature capabilities. While both strength and corrosion resistance are important, the strength requirements have a higher priority, and alloy developments which led to higher strengths also had the effect of reducing the corrosion resistance, particularly with nickel-base alloys. The most important form of corrosion is the accelerated oxidation which takes place when the air or fuel is contaminated with certain impurities, of which alkali metal salts are the most important. This type of attack is generally known as ‘hot corrosion’. Two different forms of hot corrosion have been distinguished. Type I, which is present over a temperature range of about 800–950°C, and type II, which is present over the range 700–800°C. Both processes involve an incubation period, an initiation step, and a propagation stage. Most attention has been given to the propagation stage but, from a technical point of view, the initiation step is the most important process. Mechanisms suggested include the salt fluxing model, the electrochemical model, and the sulphidation–oxidation model. Both the practical and theoretical aspects of the problem will be reviewed.     

NaCl-induced accelerated oxidation of 304 stainless steel and Fe-Mn-Al alloy at 900°C

The high-temperature corrosion behavior of cold-rolled and annealed 304 stainless steel (304SS) and Fe-29Mn-8Al-2.5Si-2Cr-0.74C alloy coated with 0.002 g cm^–2 NaCl initially were studied at 900°C in air. The corrosion kinetics of the two alloys follow the parabolic rate law. The initial NaCl coating accelerates oxidation of these alloys by oxychlorination and chlorination/oxidation cyclic reactions, and catalytic actions of chloride or chlorine are thought to be the principal causes. A bulky, layered scale as well as some intergranular attack is noted on the annealed 304SS, and intergranular attack distributes over the alloy substrate of the cold-rolled 304SS during a 144 h exposure. With the formation of a compact Al₂O₃ scale to decrease further chlorine attack, the corrosion resistance of Fe-Mn-Al alloy is superior to that of 304SS in this study.     

Verhalten von Gasturbinenschaufelwerkstoffen bei mechanischer Langzeitbeanspruchung unter Heißgaskorrosion. Teil 2: Metallkundliche Nachuntersuchungen

Behaviour of gas turbine blade materials under mechanical long term loading and hot gas corrosion. Part 2: Metallographical and microanalytical investigations Results of metallographical and microanalytical investigations on selected specimens after mechanical long term creep and LCF-loading under hot gas corrosion conditions will be presented. Uncoated and MCrAlY or Al-diffusion coated specimens of the polycristalline materials IN-738 LC and IN-939 and the single crystal alloys M 002 mod., CM-SX-6 and B 1914 were investigated. The corrosion attack increases with increasing impurities of the hot gas. Internal nitridation and sulphidation of base material or coating is observed after deterioration of the protective scales rich in Cr₂O₃ and/or Al₂O₃.     

Carburization of high-temperature steels: A simulation-based ranking of carburization resistance

Carburization is a failure mechanism affecting equipment, such as furnace tubes, operating at high temperatures. Carburization simulations were carried out for the heat-resistant steels referred to the API-530 standard by applying a model for carbon diffusion with the concurrent formation of alloy carbides. The calculated carbon and carbide volume fraction profiles were validated experimentally. The carburization layer is composed from M₂₃C₆ and M₇C₃ carbides. The time required for the carburization front to reach the mid-thickness of the tubes was used to characterize carburization resistance. The austenitic grades exhibit a higher carburization resistance than the ferritic grades at all temperatures. In the ferritic grades, alloy composition has a stronger effect at lower service temperatures (600 °C) where carburization resistance increases with Cr and Mo content. The acceleration of diffusion at high temperatures (800 °C) dominates the composition effects on carbon diffusion, and the carburization front is controlled by the formation of carbides, which in turn depends on the available amount of Cr in the steel. In the austenitic grades, the highest carburization resistance is exhibited by the stabilized grades 321 and 347 due to formation of TiC or NbC carbides respectively. Regarding the non-stabilized grades, carburization resistance is raised by addition of Mo (316 vs 304) and lower carbon (316L vs 316). The results of this study can be used for material selection for carburization resistance and for planning maintenance procedures for the timely replacement of tubes.     

The behaviour of commercial FeCrAlRE alloys in nitrogen-oxygen – water vapour bioxidant environments

Abstract

The corrosion behaviour of a range of commercial FeCrAlRE alloys (MA956, ODM751, PM2000, Kanthal AF, Kanthal APM and Aluchrom YHf) have been examined in nitrogen–oxygen-H₂O or N₂–H₂–H₂O bioxidant environments, at temperatures between 1100°C and 1350°C. The corrosion behaviour is governed by the competition between oxidation leading to protective alumina formation/ maintenance and nitrogen ingress leading to nitridation of the matrix alloy. Key issues addressed by four series of experiments, have included: the influence of a pre-formed protective alumina scale; the oxidant level required to form/reheal a protective oxide scale; the role of mechanical failure of the scale above the critical thickness for cracking/spallation in oxygen rich environments; chemical failure of the protective oxide scale leading to breakaway (non-protective) attack and in particular, the potential roles in such failure processes of nitridation concurrent with, and following defective oxide scale formation, and of oxidation following nitridation.

Detailed characterisation of the chemical composition and physical microstructure of the attack of the respective alloys was undertaken using a range of surface analytical techniques, including X-ray diffraction, optical and scanning electron microscopy and energy dispersive X-ray analysis.     

TEM Studies of High Temperature Corrosion Behaviour of TiAl Intermetallics with Surface Modifications

The oxidation/sulphidation behaviour of a Ti‐46.7Al‐1.9W‐0.5Si alloy with a TiAl₃ diffusion coating was studied in an environment of H₂/H₂S/H₂O at 850^oC. The kinetic results demonstrate that the TiAl₃ coating significantly increased the high temperature corrosion resistance of Ti‐46.7Al‐1.9W‐0.5Si. The SEM, EDX, XRD and TEM analysis reveals that the formation of an Al₂O₃ scale on the surface of the TiAl₃‐coated sample was responsible for the enhancement of the corroison resistance. The Ti‐46.7Al‐1.9W‐0.5Si alloy was also modified by Nb ion implantation. The Nb ion implanted and as received sampels were subjected to cyclic oxidation in an open air at 800^oC. The Nb ion implantation not only increased the oxidation resistance but also substantially improved the adhesion of scale to the substrate.     

Sulphidation resistance of composite boiler tube materials

Abstract

A lab-based testing program was undertaken to generate data to better define the sulphidation resistance of composite tubes installed in the lower-furnace section of black liquor recovery boilers. All composite tube cladding alloys tested were observed to have an acceptable corrosion rate at normal operating temperatures (up to 400°C) in the synthetic lower-furnace gaseous environment tested (1% H₂S–99% N2). This acceptable corrosion resistance is due to the expected formation of a relatively protective chromium-rich inner sulphide scale. An increase in temperature up to 560°C was found to significantly increase the corrosion rate. Of the various alloys tested, Alloy HR11N exhibited the lowest corrosion rate at each of the three temperatures tested. Moreover, the corrosion rate was found not to be strongly dependent on the fabrication route (weld overlay versus co-extruded). To minimize corrosion, operating conditions that promote prolonged exposure to elevated temperatures in the lower-furnace section of black liquor recovery boilers should be avoided, regardless of the type of composite tube installed.     

Sensitization induced stress corrosion failure of AISI 347 stainless steel fractionator furnace tubes

Austenitic stainless steel tubes are used as furnace tubes in petrochemical industries mainly because of their corrosion resistance and mechanical strength. AISI 347 grade stainless steel is used as furnace heater tubes in the fractionator of hydrocracker unit. Even though this stainless steel is stabilized with the addition of niobium thus preventing sensitization related corrosion failures, operational and maintenance errors may result in premature failures if conditions prevail. The present work reports the premature failure of AISI 347 grade fractionator furnace tubes after nearly 8 years of service. The failure occurred after shutdown. Carbonaceous deposits were found on the inner walls of the tube and circumferential cracks were found beneath the deposit. The service exposed 347 SS alloy tube was in the sensitized condition as confirmed by microstructure and double loop electrochemical potentiodynamic reactivation test. The tube material got sensitized possibly by localized overheating at the carbon layer deposited site. During shutdown of hydrocracker unit polythionic acid formation occurred possibly due to errors in shutdown procedures. Sensitized alloy 347 tube undergone polythionic acid induced intergranular stress corrosion cracking (PASCC).     

Corrosion by carbonaceous gases,carburization and metal dusting,and methods of prevention†

Abstract

Exposure of metallic materials to carbonaceous gases can lead to (i) carburization, i.e. internal carbide formation causing embrittlement and failure especially at high temperatures of cracking tubes for olefine production, (ii) metal dusting, a disintegration of metals to a dust of carbon and fine metal particles, occurring at intermediate temperatures, e.g. in synthesis gas for ammonia or methanol production or in direct reduction of iron ores, and (iii) coking, the carbon deposition which is an annoying phenomenon in many processes. The mechanisms of these phenomena are well understood now and they can be suppressed or effectively retarded either by the presence of some sulphur in the process gases and/or by a protective dense oxide scale. This review describes the mechanisms and kinetics of corrosion by carbonaceous gases and the fundamentals of prevention.     

Sulphidation and oxidation of the Ni22Cr10Al1Y alloy in H2/H2S and SO2 atmospheres at high temperatures

The Ni22Cr10Al1Y alloy was exposed in H₂/H₂S gas mixture under the sulphur pressure 10^–3 and 1 Pa as well as in SO₂ at 1173 and 1273 K. At p_s = 1 Pa the sulphidation rate was relatively high and the reaction obeyed the linear rate law. Under these conditions a nickel/nickel sulphide eutectic was formed. At p_s = 10^–3 Pa nickel sulphides became unstable and the sulphidation rate was significantly lower. The reaction obeyed the parabolic rate law. The oxidation rate of the alloy in SO₂ was lower than that in any of the H₂/H₂S atmospheres. The sulphide scales formed during sulphidation in H₂/H₂S had complex microstructures and compositions, with sulphospinel and sulphide phases being present, e.g. NiCr₂S₄, Ni₃S₂, Cr_xS_y. As the temperature increased and the sulphur pressure decreased, these phases were replaced by the chromium-rich sulphide phase. Various oxides formed during oxidation of the alloy in SO₂.     

Sulphidation of cobalt at high temperatures

The kinetics and mechanism of cobalt sulphidation have been studied as a function of temperature (773–1023 K) and sulphur partial pressure (1–104 Pa) by means of thermogravimetric, SEM and X-ray techniques, and also using inert-marker and ratio-tracer methods. It has been shown that the sulphidation process is diffusion controlled, the rate-determining step being the outward volume diffusion of cations. According to the phase diagram of the Co–S system, the sulphide scale on cobalt is heterogeneous. At sulphur pressures higher than the dissociation pressure of the CoS2 phase, the sulphidation rate is pressure independent, and at lower pressures it increases with rising pressure, in agreement with theoretical predictions. The apparent activation energy of sulphidation is considerably higher for multilayer than for double-layer scale formation, because the main part of multilayer scale is growing at the dissociation pressure of the CoS2 phase, which increases with increasing temperature. Over the whole temperature and pressure range studied, the rate of cobalt sulphidation is more than three orders of magnitude higher than the oxidation rate of this metal. Rapid degradation of cobalt in a sulphur atmosphere results mainly from a very high defect concentration in Co1-yS and Co9S8 sulphides, participating in comparable amounts in the scale formation on this metal at T>900 K. The only sulphide of cobalt in which the defect concentration may be very low is CoS2, the growth rate of this sulphide layer being more than two orders of magnitude lower than that of other cobalt sulphides. © 1998 Chapman & Hall     

Failure Analysis of T12 Boiler Re-Heater Tubes During Short-Term Service

The failure of T12 re-heater tubes that had been in service only for 3000 h was investigated. The wall thickness of the tubes was visibly reduced by heavy oxidation corrosion on the outer and inner walls. The original pearlite substrate completely decomposed. Uniform oxide scale appearance on the inner wall shows obvious vapor oxidation corrosion characteristics. Corrosion originated in the grain boundary, and selective oxidation occurred due to ion diffusion in the substrate. The layered oxide scale formed on the inner wall is related to the different diffusion rates for different cations. Exposure to high temperature corrosive flux accelerated the corrosion on the outer wall. The microstructure degradation and corrosion characteristics of the tubes indicate that the tubes failed primarily because of overheating, as evidenced by calculations.     

Effect of oxygen partial pressure on the oxidation behaviour of an yttria dispersion strengthened NiCr-base alloy

An yttria dispersion strengthened NiCr-base alloy was studied with respect to isothermal oxidation behaviour at 1000 °C and 1050 °C in high- and low-pO₂ gases, i.e. Ar–O₂ and Ar(−H₂)–H₂O. The scale growth kinetics, morphology and composition were studied by thermogravimetry in combination with SEM/EDX and SNMS. Due to Y doping the surface scale is very protective and initially grows predominantly by inward oxygen diffusion. Local formation of mainly outwardly growing oxide nodules occurs after longer oxidation times and is related to metallic protrusions formed as a result of internal oxidation of the minor alloying addition aluminium. The differences in scale morphology in the various environments are related to the effect of the gas composition on scale grain size and on the relative amounts of inward scale growth. Possibly the pO₂ dependence of the Ti-solubility in the chromia scale and/or hydrogen doping of the oxide plays an additional role in the scale growth process.