Protective Alumina Scale Research Articles

T91 cladding tubes with and without modified FeCrAlY coatings exposed in LBE at different flow, stress and temperature conditions

Corrosion tests of 2000 h duration are conducted on tubes consisting of the steel T91 in liquid metal loops containing eutectic lead–bismuth melt with 10 −6 wt% oxygen in solution. The experiments include tests at temperatures of 480–600° C, at liquid metal flow velocities of 1, 2 and 3 m/s and under mechanical stress due to an internal pressure of 15 MPa. The surface of tubes exposed to 600 °C and to different flow velocities are coated with a FeCrAlY alloy to examine its suitability as a protective coating for high loaded parts like cladding tubes. The coating was remelted by an electron pulse of GESA to homogenize the coating and improve its bonding to the bulk material. In all of the tests no liquid metal attack was observed. As received steel specimens developed multilayer oxide scales of a thickness increasing with temperature and internal pressure, while coated tubes had a thin protective alumina scale. Flow velocities above 2 m/s permanently removed formed magnetite at 550 °C. No influence of the flow velocity was observed for the coated surfaces which keep their stable thin alumina scale. The internal pressure of 15 MPa caused a strain of 0.7% in the tube wall, which obviously increases iron diffusion and enhances magnetite formation.

Alloying effects on creep and oxidation resistance of austenitic stainless steel alloys employing intermetallic precipitates

Microstructure evolution during creep testing at 750 °C and 100 MPa of Fe–20Cr–30Ni–2Nb (at.%) alloys with and without 0.4 Si, 0.2 Zr or 5.0 Al additions has been studied, in order to explore the viability of Fe-rich austenitic stainless alloys strengthened by intermetallic phases. Fine Fe 2Nb Laves phase dispersions with the size of less than 1 μm within the γ-Fe matrix were obtained in the base and Si-modified alloys after aging at 800 °C. The addition of Si helped to refine and stabilize the size of particles, resulting in finer and denser Fe 2Nb dispersion than that in the base alloy. The alloys with only solution heat-treatment exhibited superior creep resistance to the aged ones, which is due to dynamic precipitation of the Fe 2Nb Laves phase during creep testing with a size of 300–400 nm, resulting in more effective pinning of dislocations. The base alloy also showed the meta-stable γ″-Ni 3Nb phase with a size of 50 nm during the transient state of the creep testing. The Zr-modified alloy achieved significant improvement of creep properties. This is hypothesized to be due to the stabilization of δ-Ni 3Nb phase relative to Fe 2Nb, resulting in the formation of multiple fine dispersions of stable intermetallic compounds of δ and Fe 2Nb within the γ-Fe matrix. A small amount of a {Ni, Zr and Nb}-enriched unidentified phase was also observed. The addition of Al significantly improved the oxidation resistance because of the formation of protective alumina scales on the surface. The alloy also showed superior creep resistance to that of the base alloy due to the formation of a dense dispersion of spherical Ni 3Al, typically 30 nm in diameter. Collectively, these findings provide the alloy design basis for creep and oxidation-resistant austenitic stainless steel alloys via intermetallic precipitates.

電析法による Ni アルミナイド/Ni-Hf 合金コーティングとその耐サイクル酸化性

The formation of a bi-layer coating consisting of Ni aluminide/Ni-Hf alloy on a Ni substrate was attempted by the electrodeposition of Hf on the Ni substrate, followed by the electrodeposition of Ni and Al. Before the formation of this coating, it was confirmed by the diffusion couple test that the Ni-Hf alloy consisting of Ni7Hf2 acted as a diffusion barrier of aluminum. Upon forming this bi-layer coating, Hf and Al were deposited by molten salt electrolysis at 1023 K. The cyclic oxidation resistance for the Ni covered with this coating was then evaluated in air at 1373 K. The electrodeposition of Hf on the Ni substrate led to the formation of a layer consisting of the Ni-Hf alloy. The electrodeposition of Ni and Al on the Ni-Hf alloy layer led to the formation of a layer consisting of Ni2Al3 as a surface layer. The cyclic oxidation test showed that the mass gain due to oxidation for the sample covered with the bi-layer coating consisting of the Ni2Al3/Ni-Hf alloy was lower than that for the sample covered with a single-layer coating consisting of Ni2Al3. The analytical results of a cross-section of the sample covered with the bi-layer coating after the 5-cycle oxidation test showed that the chemical composition of the Ni-Hf alloy layer was uniform, and this layer consisted of Ni7Hf2. Furthermore, it was confirmed that the aluminum concentration in the Ni aluminide layer was constant at about 40 at%, and the Ni-Hf alloy layer depressed the diffusion of aluminum from the Ni aluminide layer to the Ni substrate. For the sample covered with the bi-layer coating after the 100-cycle oxidation test, the aluminum concentration in the Ni aluminide layer was constant at about 20 at%, and a protective alumina scale containing small amounts of hafnium oxide was formed. On the contrary, for the sample covered with the single-layer coating consisting of Ni2Al3, the aluminum concentration in the Ni aluminide layer was only about 10 at%, and an internal oxide was largely formed in the Ni aluminide layer.

Accelerated Corrosion of Fe–xCr–10Al Alloys Containing 0–20 at.% Cr Induced by Sulfur and Chlorine in a Reducing Atmosphere at 600 °C

The corrosion behavior of five Fe–xCr–Al alloys with a constant Al content of 10 at.% and Cr contents ranging from 0 at.% to 20 at.% was examined at 600 °C in a H2–HCl–H2S–CO2 gas mixture providing 3.7 × 10−22 atm O2, 2.4 × 10−14 atm Cl2 and 3.9 × 10−9 atm S2. All the alloys formed duplex scales containing an outermost layer of iron oxide plus an inner layer composed of mixtures of the oxides of all the alloy components. Besides, a region of internal attack of Al or Al + Cr, whose depth decreased with increasing Cr content, formed in all the alloys. The simultaneous presence of chlorine and sulfur in the gas mixture significantly accelerated the corrosion of all the alloys with respect to their oxidation in a simpler H2–CO2 mixture providing the same oxygen pressure, by forming thick and cracked scales. The effect was particularly large for the high-Cr alloys due to their inability to form external protective alumina scales in the present gas mixture.

Effects of minor alloy additions and oxidation temperature on protective alumina scale formation in creep-resistant austenitic stainless steels

Niobium was found to be beneficial for alumina scale formation in newly developed creep-resistant austenitic stainless steels. Additions of both Ti and V degraded alumina scale formation, whereas alloys with individual additions of Ti or V were able to form protective alumina scales. The ability to form external alumina scales was lost in the lower Al- and Nb-containing alloys between 800 and 900 °C. Compositions with the potential to form alumina at 900 °C were identified.

Examination of the platinum effect on the oxidation behavior of nickel-aluminide coatings

Oxidation resistant nickel-aluminide coatings are designed to develop a protective alumina scale during high temperature exposure. It is well established that platinum additions, typically about 6–8 at.%, provide substantial improvements in oxidation resistance of such coatings, yet the nature of the platinum effect is still not fully understood. In this work, the oxidation behavior of two commercial NiAl and NiPtAl coatings deposited on the same Ni-base single crystal alloy CMSX-4 was analyzed. Cyclic and isothermal oxidation tests were conducted at 1150 °C in air. Microstructure development and alumina/coating interface chemistry were studied as a function of oxidation time. Numerous voids developed at the Al 2O 3/NiAl interface, and sulfur was found to segregate at the void surfaces and at the contact interface, leading to spallation of the scale over the convex areas along ridges on the coating surface. The presence of platinum prevented sulfur segregation and void formation at the Al 2O 3/NiPtAl interface. As a result, the Al 2O 3 scale on the NiPtAl coating remained adherent and virtually no spallation was observed even after prolonged cyclic oxidation.

Improving the oxidation resistance of TiAl-alloys with fluorine

The technical TiAl-alloy γ-MET (Ti–46.5Al–4(Cr, Nb, Ta, B)) was oxidised thermocyclically (24 h-cycle-test) at 900 °C in wet and dry air. In this paper results of untreated and fluorine treated TiAl-samples are presented. Their oxidation behaviour will be shown. Several methods were used to apply the halogens to the surface, e.g. beamline ion implantation, spraying of or dipping into a halogen containing liquid. A pure protective alumina scale was found, e.g. after treatment with a fluorine containing polymer and thermocyclic oxidation up to 1 year at 900 °C in laboratory air. On the other side thick fast growing and non-protective mixed oxide scales were found on the untreated samples.

Oxidation resistant coatings in combination with thermal barrier coatings on γ-TiAl alloys for high temperature applications

Titanium aluminide alloys based on γ-TiAl are considered of growing interest for high temperature applications due to their attractive properties. To extend the service temperatures above 750 °C, the oxidation behaviour has to be improved predominantly by protective layers. In the present study environmental and thermal protection coatings on gamma titanium aluminides were investigated. Nitride and metallic overlay coatings based on Ti–Al–Cr–Y–N and Ti–Al–Cr, respectively, were produced by magnetron sputtering techniques. Thermal barrier coatings (TBCs) of partially yttria stabilized zirconia were deposited onto Ti–45Al–8Nb, either pre-oxidized or coated with protective layers, applying electron beam physical vapour deposition (EB-PVD). Cyclic oxidation tests were performed at 900 °C and 950 °C in air. The nitride coating exhibited poor oxidation resistance when exposed at 900 °C providing no protection for γ-TiAl. The oxidation behaviour of the Ti–Al–Cr coating was reasonable at both exposure temperatures. During prolonged exposure the coating was depleted in chromium, resulting in the breakdown of the protective alumina scale. EB-PVD zirconia coatings deposited on γ-TiAl exhibited promising lifetime, particularly when specimens were coated with Ti–Al–Cr. The adherence of the TBC on the thermally grown oxide scales was excellent; failure observed was associated with spallation of the oxide scale. At 950 °C, TBCs on specimens coated with Ti–Al–Cr spalled after less than 200 thermal cycles caused by severe oxidation of γ-TiAl and reactions between the zirconia coatings and the thermally grown oxides.

Characterization of chemical and microstructural evolutions of a NiPtAl bondcoat during high temperature oxidation

Bondcoats used to protect turbine blades, such as platinum modified NiAl alloys, are designed to develop a protective alumina scale during exposure conditions at high temperatures. However, during high temperature oxidation, the system is subjected to chemical and microstructural changes that arise from the consumption of aluminium to ensure alumina growth and interdiffusion between the underlying nickel-based superalloy and the bondcoat. The aim of the present work is to report experimental results concerning the chemical and microstructural evolutions of a NiPtAl bondcoat, deposited on a single crystal nickel-based superalloy, during isothermal oxidation tests at 1100 °C, up to 50 h. Analytical STEM studies were carried out, in conjunction with Auger experiments, in order to follow the various changes that occur in the bondcoat and at the Al 2O 3/bondcoat interface. Efforts were concentrated on the effect of interfacial sulfur segregation (at both intact interface and void surface) as a function of the oxidation time, as well as its dependence on phase transformations in the external layer of the bondcoat. Strong S segregation at some bondcoat NiPtAl/Al 2O 3 interface, especially at γ′/Al 2O 3, was found in co-segregation with Cr, which has diffused from the substrate.

Improvement of the oxidation behaviour of TiAl-alloys by treatment with halogens

Titanium aluminides are of great interest for several structural high temperature applications because of their low specific weight (about 4 g/cm 3) and their excellent high temperature strength. They could replace the much heavier high temperature steels or Ni-based superalloys (up to 9 g/cm 3) which are usually in service. The implementation of this new group of intermetallic alloys in e.g. the aerospace or automotive industry is therefore due to economic and ecologic reasons. The use of TiAl-based alloys is still limited to a temperature of about 750 °C because of their poor oxidation resistance despite of their good mechanical properties which would allow the use at higher temperatures. The oxidation resistance can be improved significantly by small amounts of halogens such as fluorine, chlorine, bromine and iodine (so called halogen effect). A defined dose of these halogens has to be provided at the metal/oxide interface of the component. The halogens promote the selective formation of gaseous Al-halides at temperatures above 700 °C which are oxidised to Al 2O 3 during their outward diffusion through the naturally grown oxide scale. So finally a protective alumina scale is formed which is stable for long times even under thermocyclic exposure and wet atmospheres. In this paper the results of isothermal and thermocyclic high temperature oxidation tests of technical TiAl-alloys with and without halogen treatment are shown. Additionally the results of high temperature creep tests of halogen treated TiAl-alloys are presented and compared with the untreated alloys.

The use of the 18O2‐exposure + SIMS‐based approach to investigate the spallation mechanisms of alumina scales

AbstractSpallation of the protective alumina scale accelerates the oxidation‐induced degradation of ‚alumina formers’︁. Despite of the vast evidence of this process its mechanism has not been elucidated, so far.This paper reports the results of the scale spallation mechanism investigation using a novel 18O2‐exposure + SIMS‐based approach. The FeCrAl‐type alumina‐forming alloys, without and with Zr additions, were subjected to high temperature exposure at 1473 K in 18O2‐rich atmosphere and, subsequently, cooled down in air. The surfaces exposed after spallation were observed using SEM, while high resolution SIMS was used to determine the elemental distributions.The obtained results indicate that: (i) in all cases the spallation occurs according to the adhesive‐mode; (ii) for reactive‐element‐free alloys the scales spalls away at temperatures high enough to enable the re‐oxidation of the exposed surface already during cooling; (iii) for the Zr‐containing alloys the spallation occurs at fairly low temperatures.Two practical implications of the obtained results should be noted: (i) oxide layers and not ‘bare substrate’ result from the scale spallation process during cooling; (ii) the formation of oxide layers during cooling may affect the behaviour of the system during subsequent heating to the reaction temperature.

Determination of Corrosion Layers and Protective Coatings on Steels and Alloys Used in Simulated Service Environment of Modern Power Plants

The development of modern power generation systems with higher thermal efficiency requires the use of constructional materials of higher strength and improved resistance to the aggressive service atmospheres. In this paper, the following examples are discussed. (i) The oxidation behavior of 9% Cr steels in simulated combustion gases: The effects of O2 and H2O content on the oxidation behavior of 9% Cr steels in the temperature range 600-800°C showed that in dry oxygen a protective scale was formed with an oxidation rate controlled by diffusion. In contrast, that in the presence of water vapor, after an incubation period, the scale became nonprotective as a result of a change in the oxidation mechanism. (ii) The development of NiCrAlY alloys for corrosion-resistant coatings and thermal barrier coatings of gas turbine components: The increase of component surface temperature in modern gas turbines leads to an enhanced oxidation attack of the blade coating. Considerable efforts have been made in the improvement of the temperature properties of MCrAlY coatings by the additions of minor elements, such as yttrium, silicon, and titanium. The experimental results show the positive, but different influence of the oxidation behavior of the MCrAlY coatings by the addition of these minor elements. (iii) The development of lightweight intermetallics of TiAl-basis: TiAl-based intermetallics are promising materials for future turbine components because of the combination of high-temperature strength and low density. These alloys, however, possess poor oxidation resistance at temperatures above 700°C. The experimental results showed that the oxidation behavior of TiAl-based intermetallics can be strongly improved by minor additions of 1-2at.% silver. (iv) The oxide-dispersion-strengthened (ODS) alloys provide excellent creep resistance up to much higher temperatures than can be achieved with conventional wrought or cast alloys in combination with suitable high-temperature oxidation/corrosion resistance. The growth mechanisms of protective chromia and alumina scales were examined by a two-stage oxidation method with O18 tracer. The distribution of the oxygen isotopes in the oxide scale was determined by secondary ion-mass spectroscopy and SNMS. The results show the positive influence of a Y2O3 dispersion on the oxidation resistance of the ODS alloys and its effect on growth mechanisms.

Static and dynamic aspects of coupling between creep behavior and oxidation on MC2 single crystal superalloy at 1150 °C

Creep tests were performed on thin wall specimens made of MC2 single crystal superalloy at 1150 °C and under controlled atmosphere. The results highlight the deleterious oxidation effect on creep properties. The assumption that oxidation leads to a non-load-bearing affected zone is insufficient to explain the difference in creep rate that was noticed between tests performed under synthetic air and under hydrogenated argon, and cannot explain the decrease of the strain rate during the tests that were carried out with a change of atmosphere from synthetic air to hydrogenated argon. On the other hand, these experimental results are consistent with vacancy injection due to partial cationic oxidation, which accelerates the creep rate by promoting creep mechanisms controlled by diffusion. The anionic protective alumina scale formed under hydrogenated argon prevents this vacancy flux. The integration of this dynamic and long range oxidation effect in creep strain models is discussed.

Protective coatings on orthorhombic Ti2AlNb alloys

The oxidation behaviour of an orthorhombic Ti–22Al–25Nb alloy, bare and with protective coatings, was investigated at 750°C in air under quasi-isothermal and thermal cycling conditions. As found by post-oxidation analysis, the uncoated substrate material was severely degraded by formation of spalling oxide scales and ingress of oxygen and nitrogen causing nitride precipitation, internal oxidation and interstitial embrittlement. Metallic Ti–51Al–12Cr coatings as well as nitride coatings based on Ti–Al–Cr–Y–N, either monolithically grown or with superlattice structure, provided an effective diffusion barrier against oxygen. The excellent oxidation resistance of the TiAlCr coatings was associated with the ternary Ti(Al,Cr)2 Laves phase promoting the formation of a protective alumina scale. The different intermetallic phases formed in the interdiffusion zone caused neither cracking nor spallation of the protective coating. Both, monolithically grown TiAlCrYN and superlattice TiAlYN/CrN coatings, exhibited slow, but nearly linear oxidation kinetics at 750°C in air. In the subsurface region of the substrate a niobium rich phase and the α2-phase formed. At the coating/substrate interface pores and a thin, fine-grained TiN layer were found.

Characterisation studies of chromia formation on commercial FeCrAlRE alloy foils following chemically induced failure at 900°C

The chemical failure of commercial FeCrAlRE (where RE is a reactive element) alloys, such as Aluchrom YHf and Kanthal AF foils (30–50 μm thick), is induced by oxidation, in air, at 800–950°C. This is also the life limiting process of this family of alloys at higher temperatures (≥1100°C) and occurs when the Al activity in the alloy becomes depleted below a critical level, as a result of oxidation. At this junction, the integrity of the protective alumina scale is not sustainable but continued ‘pseudo-protection’ can be provided by chromia formation beneath the alumina layer. Oxidation continues at a slow linear rate controlled by transport through the alumina scale. The duration of the pseudo-protection region can extend to several hundred hours and has considerable technological implications, if it could also be used in defining component lifetimes.Greater understanding is, therefore, required concerning the formation and growth of the chromia subscale, as this has received only limited attention to date. Of particular possible significance is the mode of formation of the subscale. At temperatures above 1000°C, this forms a continuous, essentially uniform layer. In contrast, at ∼900°C scalloped shaped pits of chromia develop beneath the outer alumina scale. Hence a detailed characterisation study has been undertaken of the formation and development of this chromia attack; the results of which form the basis of this paper. The study has involved the deployment of a range of microscopy and analytical techniques, including optical and scanning electron microscopy, electron microprobe analysis (EPMA) and energy dispersive X-ray analysis (EDX).

Effect of carbon content on the oxidation behaviour of FeCrAlY alloys in the temperature range 1200–1300°C

Two FeCrAlY alloys with different carbon contents (90 and 500 ppm respectively) were investigated in respect to their oxidation behaviour at 1200 and 1300°C in air. Oxidation tests, with exposure times ranging from a few hundred to several thousands of hours, revealed that the growth rate of the protective alumina scale was hardly affected by the alloy C-content. However, the time to occurrence of breakaway oxidation for the specimens (1 mm thickness) was substantially shorter for the high-than for the low-C alloy. This was primarily caused by poorer oxide scale adherence but additionally by a higher critical Al-content for occurrence of breakaway of the high-C alloy compared to the low-C alloy.Extensive microstructural studies revealed formation of Cr-carbides at the grain boundaries in both alloys. The high-C alloy additionally showed carbide formation at the scale/metal interface, thus deteriorating scale adhesion. Furthermore, inter- and intra-granular carbide precipitation is considered to induce strengthening of the metal, thus hindering relaxation of the thermally-induced oxide stresses by substrate creep. In a series of experiments with variations in the cooling rates, it was verified that carbide formation very likely occurs during specimen cooling.

High temperature scaling of Ni-15Cu-5Al alloy in 1×105 Pa pure oxygen

The oxidation of the ternary alloy Ni-15Cu-5Al in 1×105 Pa pure oxygen at 700 °C and 800 °C was studied. The results show that the behavior of the Ni-rich alloy is similar to that of the binary Ni-Al alloy with the same Al content in the form of an external NiO layer coupled to the internal oxidation of aluminium. The presence of 15% (mole fraction) Cu cannot modify substantially the values of relevant parameters affecting the transition from the internal to the external oxidation of aluminium. The presence of 5% Al reduces the oxidation rate of the corresponding Ni-Cu alloy during the whole oxidation stages, though 5% Al is still insufficient to form protective external alumina scales.

The oxidation behavior and mechanical performance of Ti60 alloy with enamel coating

In the present paper, the influence of enamel coating, compared to the traditional TiAlCr coating, on oxidation behavior and oxygen contamination of Ti60 alloy at 700 and 800 °C was studied. A continuous protective alumina scale formed on TiAlCr coating during oxidation at the two temperatures; but a rather heavy interdiffusion layer appeared at the interface of TiAlCr/Ti60 during oxidation at 800 °C. The uniform and dense enamel coating could provide excellent protectiveness to Ti60 due to its thermal chemical stability and good compatibility in terms of thermal expansion coefficient to the substrate Ti60 alloy. According to the microhardness measurement results, there exists a layer of contamination of about 30 μm into the alloy after the enamel was vitrified for 30 min at 900 °C in air; but the depth of oxygen contamination into the alloy changed little after oxidation for 1000 h at 600 °C. The strength and the elongation at ambient temperature of Ti60 alloy with enamel coating decreased about 7.4% and 3.4% in comparison to the original bare alloy, respectively. From the results, the enamel coating could protect Ti60 alloy from oxidation and oxygen embrittlement.

Effect of Combined Yttrium and Zirconium Additions on Protective Alumina Scale Formation on High Purity FeCrAl Alloys during Oxidation in the Temperature Range of 1200 to 1300°C

Effect of Combined Yttrium and Zirconium Additions on Protective Alumina Scale Formation on High Purity FeCrAl Alloys during Oxidation in the Temperature Range of 1200 to 1300°C

Alumina forming high temperature silicides and carbides

Aluminium containing heat-resistant silicides and ternary carbides (Maxthal ®), have been evaluated in terms of their high temperature behaviour in various atmospheres including vacuum. The novelty of these materials is in their ability to form a stable and adherent protective alumina scale at low partial pressures of oxygen as well as in air. Recently developed heating elements, for oxidising and reducing atmospheres, made of molybdenum aluminosilicide (Kanthal Super ER) have been evaluated by thermogravimetric analysis (TGA) studies in air and other atmospheres. The performance up to 1575 °C has been evaluated at vacuum levels down to 10 −4 mbar. In addition to its remarkable thermal shock resistance, damage tolerance and machinability, Ti 2AlC (Maxthal ®) showed a parabolic and stable oxide growth rate up to 1400 °C in air. Cyclic testing confirmed that the oxide layer is adherent to the bulk. It has been concluded that both Maxthal ® and Kanthal Super ER are capable of operating in air, up to 1400 and 1575 °C, respectively.