Improved Scale Adhesion Research Articles

Polymer-derived SiAlOC coating to improve the high-temperature resistance of chromium

Cr and its alloys are interesting materials for applications at temperatures higher than 1000 °C. However, challenges such as limited oxidation resistance and nitridation still restrict their usage as high-temperature structural materials. The present work investigates the influence of a 0.6 μm thin SiAlOC sol-gel coating on the oxidation resistance and kinetic of Cr at 950 °C for 50 h and 1050 °C for up to 100 h of exposure time in dry synthetic air using thermogravimetric analysis. The resulting oxide scale and microstructure were extensively studied by combining different techniques such as EPMA, XRD, SEM, Raman spectroscopy, FT-IR, and STEM. Overall, a reduction in mass gain and an improved scale adhesion was observed. The interaction of coating and substrate causing nanosized Cr2O3 and cristobalite formation in the SiAlOC coating was further supported by investigations of bulk materials.

Effects of Zr Distribution on High‐Temperature Oxidation Resistance of a FeCrAlZr Alloy

Two FeCrAlZr alloys are fabricated by twin‐roll strip casting (TRSC) and ingot casting (IC), and different Zr distributions are formed. After cyclic oxidation tests at 1200 °C for 200 h in air, it is found that the scale spallation resistance in the FeCrAlZr alloy is improved by homogenizing the Zr distribution by TRSC. To understand the improvement in oxidation resistance, the composition and microstructure of the thermally grown scales are investigated. During oxidation, Zr diffuses toward the scale and forms ZrO2 particles in the scale and at the surface of the oxide scale. Meanwhile, the outward diffusion of Zr inhibits the formation of interfacial voids and S segregation regardless of Zr distribution. However, the TRSC sample with a more uniform Zr distribution can form the high‐density oxide penetrations, keying the scale to the matrix and improving scale adhesion. The Zr fluxes toward the scales are calculated to explain the difference in the distribution of oxide penetrations in the TRSC and IC samples during oxidation.

A Study of the Effect of 5 at.% Sn on the Micro-Structure and Isothermal Oxidation at 800 and 1200 °C of Nb-24Ti-18Si Based Alloys with Al and/or Cr Additions.

This paper presents the results of a systematic study of Nb-24Ti-18Si based alloys with 5 at.% Sn addition. Three alloys of nominal compositions (at.%), namely Nb-24Ti-18Si-5Cr-5Sn (ZX4), Nb-24Ti-18Si-5Al-5Sn (ZX6), and Nb-24Ti-18Si-5Al-5Cr-5Sn (ZX8), were studied to understand how the increased Sn concentration improved oxidation resistance. In all three alloys there was macrosegregation, which was most severe in ZX8 and the primary βNb5Si3 transformed completely to αNb5Si3 after heat treatment. The Nbss was not stable in ZX6, the Nb3Sn was stable in all three alloys, and the Nbss and C14-NbCr2 Laves phase were stable in ZX4 and ZX8. The 5 at.% Sn addition suppressed pest oxidation at 800 °C but not scale spallation at 1200 °C. At both temperatures, a Sn-rich area with Nb3Sn, Nb5Sn2Si, and NbSn2 compounds developed below the scale. This area was thicker and continuous after oxidation at 1200 °C and was contaminated by oxygen at both temperatures. The contamination of the Nbss by oxygen was most severe in the bulk of all three alloys. Nb-rich, Ti-rich and Nb and Si-rich oxides formed in the scales. The adhesion of the latter on ZX6 at 1200 °C was better, compared with the alloys ZX4 and ZX8. At both temperatures, the improved oxidation was accompanied by a decrease and increase respectively of the alloy parameters VEC (Valence Electron Concentration) and δ, in agreement with the alloy design methodology NICE (Niobium Intermetallic Composite Elaboration). Comparison with similar alloys with 2 at.% Sn addition showed (a) that a higher Sn concentration is essential for the suppression of pest oxidation of Nb-24Ti-18Si based alloys with Cr and no Al additions, but not for alloys where Al and Cr are in synergy with Sn, (b) that the stability of Nb3Sn in the alloy is “assured” with 5 at.% Sn addition, which improves oxidation with/out the presence of the Laves phase and (c) that the synergy of Sn with Al presents the “best” oxidation behaviour with improved scale adhesion at high temperature.

Undoped and ytterbium-doped titanium aluminum nitride coatings for improved oxidation behavior of nuclear fuel cladding

In an effort to develop coatings to increase the oxidation resistance of nuclear fuel cladding during a loss-of-coolant-accident (LOCA), undoped and ytterbium-doped titanium aluminum nitride (TixAl1-xN) coatings were deposited onto ZIRLO® and grade-5 titanium substrates using a hybrid coating technique incorporating both resistance evaporation and cathodic arc physical vapor deposition (CA-PVD). The coating systems consisted of a titanium bond layer, an undoped TiAlN layer, and an outer TiAlN layer doped with varying levels of ytterbium (0.44 to 33.24at.%). The coated materials were subjected to high temperature atmospheric oxidation testing and differential scanning calorimetry (DSC) to evaluate the effect of different Yb dopant concentrations on the TixAl1-xN coating oxidation resistance. The results showed that the coatings evaluated in this study protected the tubular ZIRLO® cladding material from oxidation when exposed to air at temperatures up to 1200°C. DSC data showed that the corrosion resistance of coatings with Yb dopant concentrations of 0.44 and 0.64at.% was better than that of undoped TixAl1-xN coatings and of TixAl1-xN coatings with Yb concentrations ≥4.78at.%. Yb doping and the 0.44 and 0.64at.% level also resulted in a lower rate of oxide scale growth and improved scale adherence, which further slowed oxidation kinetics. Peak oxidation resistance was observed in the TiAlN coating with a 0.44at.% ytterbium dopant concentration. Ytterbium concentrations of 4.78at.% and greater led to accelerated oxidation rates for the testing conditions studied compared to undoped TiAlN. These results show that ytterbium doped coatings are possible candidates for high temperature resistance of accident tolerant fuel coatings.

Synergistic effect of reactive element co-doping in two-phase (γ’ + β) Ni-Al alloys

To propose reactive element (RE) co-doping strategy, the oxidation behaviour of Dy, Hf, Y, Zr single- and co-doped Ni-30Al alloys was comparatively studied. All the alloys containing Dy or Y exhibited improved scale adhesion due to the pinning effect of the pegs at the scale/alloy interfaces and the reduced residual stress by the flaky oxides formed on the alloy surfaces. Compared to the corresponding RE single-doping, Dy/Hf, Dy/Zr, Hf/Y and Y/Zr co-doping further reduced the scale growth rate, revealing a synergistic effect. The role of reactive element co-segregation in the formation of synergistic effect was analyzed.

Microstructure and cyclic oxidation behaviour of low-Pt/Dy co-doped β-NiAl coatings on single crystal (SC) superalloy

Abstract The low-Pt/0.1 at.% Dy co-doped β-NiAlCrSi coating was produced on single crystal (SC) superalloy IC21 by electro-plating of a ~ 3 μm thick Pt film followed by electron beam physical vapour deposition (EB-PVD) of a β-NiAlDyCrSi over layer. For comparison, the single low-Pt doped and single Dy-doped β-NiAlCrSi coatings were also prepared, respectively. The cyclic oxidation behaviour of the above coatings at 1200 °C was investigated. Minor Dy-doping led to refinement of grains of the as-deposited coating. The addition of Pt effectively suppressed precipitation of topologically close-packed phase (TCP) in the interdiffusion zone (IDZ) during heat-treatment. The coupled effects of Pt/Dy co-doping led to lower oxidation rate, less scale rumpling and improved scale adherence than the single-doping cases and the related mechanisms were discussed.

The Effect of Grain Size on the Cyclic Oxidation of NiCoCrAlY

NiCoCrAlY samples were fabricated via spark-plasma sintering with and without prior cryomilling of the as-received powder. Thermal cycling oxidation tests were conducted at 1000°C in air for up to 200 1-h cycles. The mass gain data indicate a notably lower mass gain for the cryomilled NiCoCrAlY sample. In both the conventional and cryomilled samples, scanning electron microscopy and x-ray diffraction data indicate a single-layer oxide scale of α-alumina has formed, having undergone a transition from θ-alumina during oxidation testing. Spallation in the conventional sample allowed a significant amount of internal spinel oxide formation, and this was not found in the cryomilled sample. The difference in behavior is attributed to increased pathways for aluminum diffusion to the surface and nucleation sites for the α-alumina transition, as well as improved scale adhesion through nanostructuring and stress relief through the accelerated θ–α transition.

Influence of CeO2 superficial coating on the high temperature corrosion behavior of 2.25 Cr-1 Mo Steel in SO2 + O2 atmospheres

The influence of superficial CeO2 coatings on isothermal and cyclic high temperature corrosion behavior of 2.25 Cr-1 Mo steel in oxidation and sulfidation (SO2 + O2) atmosphere have been reported in this paper. The CeO2 coating of three different depositions rate (0.15 mg/cm2, 0.30 mg/cm2 and 0.50 mg/cm2) were prepared for experimental investigations. The isothermal corrosion study of uncoated and CeO2 coated specimens was carried out in SO2 + O2 environment at 973K for 150 hours. The results clearly indicate that CeO2 coated specimen improves the high temperature corrosion resistance than uncoated specimens. The post corroded CeO2 coated specimen shows reduction of scale growth and improved scale adhesion to the substrate. The cyclic corrosion study was also carried out in both coated and uncoated specimen in same environment at 973K for 100 h. The CeO2 coated specimens on thermal cycling also shows significant improvement of the corrosion resistance without scale cracking and spallations. In contrary, the uncoated specimen in both isothermal as well as cyclic environment shows higher corrosion rate along with scale cracking and spallations. The results of the reaction kinetics have been substantiated by post corrosion analysis of alloy/scales combinations by SEM and XRD. The details mechanism of the corrosion in presence of CeO2 coating was further discussed in the paper.

Improved alumina scale adhesion of electron beam physical vapor deposited Dy/Hf-doped β-NiAl coatings

The cyclic oxidation behavior of Dy/Hf-doped β-NiAl coatings produced by electron beam physical vapor deposition (EB-PVD) was investigated. For the undoped NiAl coating, numerous voids were formed at the alumina scale/coating interface and large rumpling developed in the scale, leading to premature oxide spallation. The addition of Dy and Hf both improved scale adhesion and the alumina scale grown on the NiAl-Hf coating showed better adhesion than that on the NiAl-Dy coating, although the suppressing effect on interfacial void formation and the scale rumpling resistance were stronger in the NiAl-Dy coating. It is proposed that the segregation of Dy and Hf ions at the scale/coating interfaces not only prevent interfacial sulfur segregation but also may directly enhance interfacial adhesion by participating in bonding across the interfaces, and this strengthening effect is relatively stronger for Hf ionic segregation.

Microstructures and cyclic oxidation behaviour of Pt-free and low-Pt NiAl coatings on the Ni-base superalloy Rene-80

Aluminizing of bare and 3μm-Pt-electroplated specimens has been utilised to prepare NiAl and low platinum (Ni,Pt)Al coatings. Cyclic oxidation of the coatings was investigated by exposing samples to 1h cyclic oxidation at 1100°C. The modified coating exhibited an external layer of NiAl-25 vol.% PtAl2 above a three-zone structure. This structure endured over the whole testing time, while the NiAl coating failed after 77cycles. The (Ni,Pt)Al coating did not reduce the scale growth rate, but it improved scale adhesion. In addition, Pt limited the outward diffusion of Ti from substrate and hence prohibited formation of undesirable TiO2.

Improved oxidation resistance and diffusion barrier behaviors of gradient oxide dispersed NiCoCrAlY coatings on superalloy

NiCoCrAlY has been used as the bond coat material in thermal barrier coating (TBC) to protect the superalloy substrate from oxidation and hot-corrosion. Inter-diffusion of elements between the coating and substrate could degrade the oxidation resistance of the coating and the mechanical properties of the superalloy. In this work, a gradient oxide dispersed (OD) NiCoCrAlY coating was produced onto DZ125 superalloy using electron beam-physical vapor deposition (EB-PVD). For comparison, conventional NiCoCrAlY (OD free) coated specimens were also produced by EB-PVD. The oxidation and inter-diffusion behaviors of the coated specimens at 1373 K were investigated. As compared to OD free coating, the OD coating exhibits not only a lower oxidation rate but also an improved oxide scale adherence because outward diffusion of the elements such as Ta, W and Hf from the superalloy was effectively blocked by the OD zone. Meanwhile, the presence of minor Hf in the OD coating contributes to the improved oxide scale adherence by reactive element mechanism.

High-Temperature Oxidation Resistant Nanocoatings on Austenitic Stainless Steels

ABSTRACTIn recent years, the increasing energy costs have lead to power utility industries to seek/develop high efficiency systems of production and of energy utilization. In addition, environmental concerns regarding greenhouse gas emissions are playing a major role in the development of clean energy systems. The development of metallic materials that can withstand elevated temperatures is among the viable alternatives to increase energy efficiency. Nevertheless, for this to happen, the corrosion and oxidation resistance of Fe- and Ni-based alloys needs to be significantly improved. Among the possible ways to enhance the life of high temperature alloys is the application of protective ceramic coatings. Conventional coatings are expensive and the protective effects controversial at times. An alternative which offers a great potential is the application of nano-ceramic coatings. Hence, in this work nanocrystalline coatings based on nano-CeO2 are applied to an austenitic stainless steel 304L and then exposed to elevated temperatures. Weight changes are monitored as a function of time and the results are compared with uncoated alloys tested under similar conditions. In addition, computer simulations of possible rate limiting diffusion mechanisms are carried out. It is found that the nanocoatings provided remarkable high-temperature oxidation resistance and improved scale adhesion. In particular, it is found that the smaller the nanoparticles are, the more effective the nanocoatings in providing oxidation resistance.

Role of molybdenum on the AISI 316L oxidation at 900 °C

In situ X-ray diffraction was used to study the oxide formation on AISI 316L stainless steel (SS) specimens during isothermal oxidation at 900 °C in air. Results were compared with those obtained on AISI 304 SS to determine the role of molybdenum on the oxidation process for the AISI 316L SS specimens. Our results show that molybdenum plays a major protective role during steel oxidation. This element is found in a NiMoO4 phase at the internal oxide–metal interface. The high molybdenum content of the alloy hinders the outward diffusion of iron and leads to a lower growth rate and better scale adherence. The oxide scale is then composed of Cr2O3 with a small amount of Mn1.5Cr1.5O4 at the external interface. The improved scale adherence appears to be due to a keying effect at the scale/alloy interface promoted by molybdenum.

FeCrAl Alloys Produced by Roll-Bonding and Annealing of Al(RE)-Clad Stainless Steel: How Addition of Hf and Zr Affects their Oxidation Behaviour

The process involving cold rolling and annealing of Al-steel-Al sandwiches allows to produce an alloy containing more than 7 wt.% of Al. Unfortunately many α-Al2O3 scales tend to spall during thermal cycling because of thermal expansion mismatch between the alloy and the scale. Small additions to the ferritic stainless steel of oxygen reactive elements are known to improve the adhesion of α-Al2O3 scales. In this work hafnium and zirconium were added to the aluminium foil before roll bonding and their behaviour during the diffusion bonding was evaluated. This novel doping method was proven to be effective in improving scale adhesion and alloy lifetime. Moreover it allows to overcome problems related to steel alloying with reactive elements and reduces the quantity of expensive reactive elements that needs to be added to the steel foil.

Thermally grown Al 2O 3 on a H 2-annealed Fe 3Al alloy: Stress evolution and film adhesion

Stress evolution in Al 2O 3 films grown on a Fe 3Al-based alloy, with or without a prior H 2-anneal heat treatment, was followed in situ with a synchrotron X-ray source at 1000 and 1100 °C and during cooling. These results are complimented by studies of oxidation kinetics, the scale/alloy interfacial chemistry, and the oxide, alloy and interface microstructures. The H 2-anneal reduced the sample sulfur impurity to less than 1 ppm, significantly improved scale adhesion, maintained a planar scale/alloy interface, but did not demonstrably affect the oxidation rate or the growth stress development. On the untreated alloy, interface wrinkling was a major stress relief mechanism, but for the H 2-annealed samples it was oxide creep with contribution from alloy grain boundary sliding. The interface strength and the extent of oxide spallation were closely related to the amount of sulfur segregated at the interface.

Effects of Surface-Deposited Nanocrystalline Chromite Thin Films on the Performance of a Ferritic Interconnect Alloy

-based nanocrystalline thin films, substituted with Sr and Zn and coated onto a Cr Ebrite alloy were evaluated at 750–900°C as potential enhancers of oxide properties in ferritic alloy interconnects of a solid oxide fuel cell. It was found that at 750°C the coatings provided lower oxidation rates, increased electrical conductivity, protection during cycling and at least of isothermal oxidation, and a fine-grained and uniform oxide microstructure. Although after oxidation at 850°C the grains grew substantially and the scale seemed to be overgrowing the film, conductivity and oxidation rates still matched that of the uncoated sample. Tensile tests showed that the coating improved scale adhesion after a oxidation at 850°C. At 900°C, the film was still capable of blocking 70% of the Cr that evaporated from an uncoated Ebrite substrate.

Oxidation of an Fe-Cr-Al-Y-Zr alloy forming an alumina scale in a Ga2O atmosphere at 1473 K

The oxidation of an Fe-Cr-Al-Y-Zr alloy exposed to a Ga atmosphere at 1473 K has been investigated by an encapsulation method. A Ga/Ga2O3 mixture was placed within the quartz ampoules to establish the Ga2O and the O2 partial pressures enveloping a Fe-Cr-Al-Y-Zr alloy (specifically, 72.8Fe-22Cr-5Al-0.1Y-0.1Zr by mass pct), which contains minor additions of yttrium and zirconium to improve scale adherence. A deoxidized Ar atmosphere surrounded the ampoules to control oxygen permeation through the silica wall. The Fe-Cr-Al-Y-Zr alloy developed an Al2O3 scale growing parabolically and protecting the substrate from gallium attack for 5, 10, 20, 40, and 50 days, as examined with a scanning electron microscope (SEM) and energy dispersive X-ray (EDX) analysis. The Ga2O atmosphere decomposes to liquid Ga and Ga2O3 on the alloy surface, but gallium species do not penetrate the oxide layer according to the limits of detection for EDX analysis. Defect equilibria incorporating Ga2O are presented with neutral interstitial oxygen to explain the oxidation of the Fe-Cr-Al-Y-Zr alloy.

Effect of water vapour on growth and adherence of chromia scales formed on Cr in high and low pO2-environments at 1000 and 1050°C

The oxidation behaviour of pure Cr at 1000 and 1050°C was studied in Ar–O2 and Ar–H2–H2O mixtures. It was found that in the low-pO2 gases the oxide scales exhibited higher growth rates than in the high-pO2 gases. The scales formed in the low-pO2 gases showed substantially better adherence during cooling, than scales formed in the high-pO2 gases. These differences in growth rate and adherence can be correlated with differences in size and location of the in-scale voids formed during the isothermal exposure. Exposures in Ar-O2-H2O mixtures revealed that the differences in scale growth rates as well as in scale void formation and growth are not primarily related to differences in the oxygen partial pressure of the atmosphere but to the presence of water vapour in the test gas. At sufficiently high H2O/O2-ratios, water vapour promotes oxide formation at the scale/metal interface thereby suppressing excessive growth of existing voids, and also as a consequence improved scale adherence. Whether the enhancement of inward scale growth is related to transport of H2O- or H2-molecules or due to OH− ions, cannot be derived with certainty from the present results.

Improved oxidation life of segmented plasma sprayed 8YSZ thermal barrier coatings

Unconventional plasma sprayed thermal barrier coating (TBC) systems were produced and evaluated by interrupted or cyclic furnace oxidation life testing. First, approximately 250 µm thick 8YSZ coatings were directly sprayed onto grit blasted surfaces of PWA 1484, without a bond coat, to take advantage of the excellent oxidation resistance of this superalloy. For nominal sulfur (S) contents of 1 ppmw, total coating separation took place at relatively short times (200 h at 1100°C). Reductions in the S content, by melt desulfurization commercially (0.3 ppmw) or by hydrogen (H2) annealing in the laboratory (0.01 ppmw), improved scale adhesion and extended life appreciably, by factors of 5–10. However, edge-initiated failure persisted, producing massive delamination as one sheet of coating. Secondly, surfaces of melt desulfurized PWA 1484 were machined with a grid of grooves or ribs (∼250 µm wide and high), resulting in a segmented TBC surface macrostructure, for the purpose of subverting this failure mechanism. In this case, failure occurred only as independent, single-segment events. For grooved samples, 1100 °C segment life was extended to ∼1000h for 5 mm wide segments, with no failure observed out to 2000 h for segments ≤2.5 mm wide. Ribbed samples were even more durable, and segments ≤6 mm remained intact for 2000 h. Larger segments failed by buckling at times inversely related to the segment width and decreased by oxidation effects at higher temperatures. This critical buckling size was consistent with that predicted for elastic buckling of a TBC plate subject to thermal expansion mismatch stresses. Thus, low S substrates demonstrate appreciable coating lives without a bond coat, while rib segmenting extends life considerably.

Influence of Hydrogen in Iron and in Two Stainless Steels on Aqueous and Gaseous Corrosion

The influence of hydrogen in Fe, 301 stainless steel (SS 301), and Avesta 353MA on corrosion resistance in aqueous solutions was studied. The oxidation kinetics of uncoated and Pt-coated Fe was also investigated in at 500 and 700°C. Electrochemical and weight loss measurements were used to evaluate the corrosion resistance in aqueous solutions, and the high-temperature oxidation rate was determined by measuring the pressure decrease in a closed volume. Observations of a high dissolution rate of Fe and SS 301 in deionized water, as well as a high oxidation rate in can be explained by an increased metal cation transport in the oxide film induced by hydrogen in the metal. However, hydrogen up to a certain concentration in Avesta 353MA was found to increase the resistance to localized corrosion in chloride solution. Pt coating on Fe was found to decrease the oxidation rate and improve scale adherence. The presented results are discussed in view of recent findings of balanced metal cation and oxygen anion transport in protective metal oxides. © 2002 The Electrochemical Society. All rights reserved.