Multilayered Oxide Scale Research Articles

The oxidation behavior of Cu42Zr42Al8Ag8 bulk metallic glasses

The oxidation behavior of Cu42Zr42Al8Ag8 bulk metallic glass was studied in synthetic air over the temperature range of 330–460 °C. The oxidation kinetics of the metallic glass follows a single parabolic rate law at 330 and 390 °C and a two-stage parabolic rate law from 420 to 460 °C. Silver precipitates on the topmost oxide layer of the metallic glass were revealed by energy-dispersive X-ray spectroscopy together with X-ray diffraction pattern. Observation using scanning electron microscopy shows that silver metal precipitated at all temperatures and some islands were formed on the outermost copper oxide layer at high temperatures. Multilayered oxide scales were also observed with silver precipitates sandwiched between copper- and zirconium-rich oxides layers.

Characterisation of oxide scale adherence after the high temperature oxidation of nickel-based superalloys

The scratch test technique was used to characterise the adherence of multi-layered oxide scales formed during the high temperature isothermal oxidation (1100°C) of a single crystal Ni-based superalloy AM1 with varying sulfur concentrations (0.22 – 3.2 ppmw). Results were discussed in relation to cyclic oxidation behaviour and microstructural evolution of oxidised samples. The most commonly used elastic energy-based models were applied to calculate the work of adhesion. The obtained values of the work of adhesion were then analysed with regard to the mode-dependence of the fracture behaviour and the possible sources of errors in the scratch test results interpretation. This analysis allowed us to understand the limits of validity of these models and of the experimental value of the work of adhesion.

Cyclic oxidation behavior of plasma surface chromising coating on titanium alloy Ti–6Al–4V

The cyclic oxidation behavior of plasma surface chromising coating on titanium alloy (Ti–6Al–4V) was researched in air at 650°C, 750°C and 850°C. A NiCrAlY coating was prepared by multi-arc ion plating as a comparison. The surface morphologies, microstructures and phases of both coatings before and after oxidation were investigated using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffractometry (XRD). The results show that the chromising coating consisted of an outer layer of loose Cr deposition, an intermediate layer of compact Cr deposition and an inner Ti–Cr mutual diffusion layer. The multilayer oxide scales formed in the oxidation process, which has the better cyclic oxidation resistance compared with NiCrAlY thermal barrier coating. However, the brittleness of Ti(Cr, Al)2 laves phase resulted in spallation of oxide scales at 750°C and 850°C.

Strength of hot pressed ZrB2–SiC composite after exposure to high temperatures (1000–1700 °C)

Abstract Residual strength (room temperature strength after exposure in air at high temperatures) of hot pressed ZrB2–SiC composites was evaluated as function of SiC contents (10–30 vol%) as well as exposure temperatures for 5 h (1000–1700 °C). Multilayer oxide scale structures were found after exposures. The composition and thickness of these multilayered oxide scale structure was dependent on exposure temperature and SiC contents in composites. After exposure to 1000 °C for 5 h, the residual strength of ZrB2–SiC composites improved by nearly 60% compared to the as-hot pressed composites with 20 and 30 vol% SiC. On the other hand, the residual strength of these composites remained unchanged after 1500 °C for 5 h. A drastic degradation in residual strength was observed in composites with 20 and 30 vol% SiC after exposure to 1700 °C for 5 h in ZrB2–SiC. An attempt was made to correlate the microstructural changes and oxide scales with residual strength with respect to variation in SiC content and temperature of expsoure.

Scaling and internal oxidation of α-CuSn alloy

α-CuSn alloy containing 8.2 wt% tin oxidized in laboratory air at 610°C is studied for its microstructural evolution with time. A cross-sectional study of the sample after oxidation shows the presence of a multilayered oxide scale on the surface and internal oxidation. Oxide morphology is expected to give an insight on undergoing oxidation mechanism. Analysis shows that SnO2 is present in porous morphology close to the substrate as clusters and not as a continuous layer; and as an internal oxide, occurring non-uniformly throughout the sample surface.

Investigation of Isothermal and Cyclic Oxidation of Plasma-Nitrided Hot-Work Tool Steel at Elevated Temperatures

The oxidation of a plasma-nitrided, hot-work tool steel at temperatures that cover a range of operations from post-plasma-nitriding oxidation to steel thixoforging processing was investigated. Thermal exposure at 500 °C led to the formation of a thin Fe–Cr spinel layer and an even thinner outermost layer of hematite. The former is the only oxide that grew on samples exposed to oxygen-lean conditions at 750 °C. A thick, multi-layered oxide scale formed on the surface when the plasma nitrided hot-work tool steel was held at 750 °C under atmospheric conditions. In this scale, the outermost hematite layer and the inner Fe–Cr spinel were separated by a magnetite layer. The oxide scale produced during thermal cycling at 750 °C was also multi-layered with an identical oxide scale configuration to that formed during isothermal exposure at 750 °C. The hematite layer, which retained its integrity during isothermal exposure at 750 °C, suffered small cracks that were instrumental in its fracture and spallation during thermal cycling. The distinct feature resulting from cyclic oxidation, however, was the wide gap that formed along the magnetite–spinel interface. Thermal expansion mismatch produced compressive stresses which in turn led to buckling of the magnetite layer and to its detachment; while, the spinel layer adhered to the tool steel substrate and survived throughout thermal cycling. Enrichment of nitrogen and the subsequent precipitation of N2 gas were also believed to have contributed to the gap formation. Formation of such a gap poses a serious threat to the integrity of the oxide scale and was shown to be responsible for the spallation of the magnetite layer upon thermal cycling.

The effect of Mn on the oxidation behavior and electrical conductivity of Fe–17Cr alloys in solid oxide fuel cell cathode atmosphere

Four Fe–17Cr alloys with various Mn contents between 0.0 and 3.0 wt.% are prepared for investigation of the effect of Mn content on the oxidation behavior and electrical conductivity of the Fe–Cr alloys for the application of metallic interconnects in solid oxide fuel cells (SOFCs). During the initial oxidation stage (within 1 min) at 750 °C in air, Cr is preferentially oxidized to form a layer of Cr 2O 3 type oxide in all the alloys, regardless the Mn content, with similar oxidation rate and oxide morphology. The subsequent oxidation of the Mn containing alloys is accelerated caused by the fast outward diffusion of Mn ions across the Cr 2O 3 type oxide layer to form Mn-rich (Mn, Cr) 3O 4 and Mn 2O 3 oxides on the top. After 700 h oxidation a multi-layered oxide scale is observed in the Mn containing alloys, which corresponds to a multi-stage oxidation kinetics in the alloys containing 0.5 and 1.0 wt.% of Mn. The oxidation rate and ASR of the oxide scale increase with the Mn content in the alloy changes from 0.0 to 3.0 wt.%. For the application of metallic interconnects in SOFCs, Mn-free Fe–17Cr alloy with conducting Cr free spinel coatings is preferred.

Quantification of growth kinetics and adherence of oxide scales formed on Ni-based superalloys at high temperature

Cyclic and isothermal oxidation behaviors of first and fourth-generation superalloys AM1 and MCNG were investigated to evaluate the ability of the scratch test to quantify the adhesion of multi-layered oxide scales. Effects of sulfur content and of scale thickness were studied independently. Available models lead to large discrepancies in the calculated work of adhesion values with the evaluation of the residual stress being the largest source of error. Nevertheless, models can assess the effect of sulfur content and the scratch test can be used to correlate the long-term cyclic oxidation behavior and the adhesion of oxide scales.

Hot Corrosion Behavior of a Novel Enamel-Based Thermal Barrier Coating

A novel thermal barrier coating comprised two layers: sputtered bond coat and enamel-zirconia composite top layer. The hot corrosion tests were carried out at 900 °C for 100 h. The salt used in the tests was 25wt% NaCl + 75wt% Na2SO4. Mass loss and spalltion of oxide scales of uncoated K444 specimens was obvious. For the coated specimens, no oxide scale spallation and no weight loss were observed. The TGO formed on the coated specimens was thin layer of mixture oxides of Al, Ti and Cr, while thick multi-layered oxide scales formed on the uncoated specimens. Besides, deep internal oxidation zone was observed on the uncoated specimens. The coatings after hot corrosion tests contained t-ZrO2 and NaAlSi3O8, while the oxide scales formed on the uncoated K444 were TiO2, Cr2O3, NiCr2O4 and Na2Cr2Ti6O16.

Formation of Protective Intermediate Phase in Ta Addition TiAl during High Temperature Oxidation

The mechanical properties and the oxidation resistance of -TiAl at elevated temperatures have to be improved to be used in the severe environmental conditions. It has become clear that the addition of more than 4at.%Ta in TiAl demonstrates a superior oxidation to the other TiAl-X compounds, according to the weight gain results of cyclic oxidation experiments at 1173 and 1273K. Oxidation behaviors are strongly influenced by the Ta concentration in TiAl. XRD, SEM-EDS, and TEM-EDS observations have been carried out to determine the microstructures and the surface compositions of multi-layered oxide scales. It was revealed that a protective intermediate phase simultaneously formed between the substrate γ-TiAl and the oxide scale layer. The Ti53Al32Ta15 ternary compound exists as an equilibrium phase at 1373K, according to the published Ti-Al-Ta ternary phase diagram. This ternary compound can work as a barrier to some extent. It contributes to decelerating the diffusion of Ti and Al atoms and to decreasing the oxidation rate. The formation mechanism of the intermediate phase has been discussed in conjunction with diffusion in TiAl.

Fabrication and oxidation behavior of Cr 2AlC coating on Ti6242 alloy

An ∼ 5 µm Cr 2AlC coating was synthesized on near-α titanium alloy Ti6242 using an industrially sized magnetron sputtering coater. Isothermal oxidation at 700 °C and 800 °C, and cyclic oxidation at 700 °C of the bare alloys and coated specimens were investigated in air. The results indicated that the Ti6242 alloy faced serious oxidation problems at 700 °C and 800 °C. Repeated formation and spallation of the multilayered oxide scale on the Ti6242 alloy occurred during oxidation testing. The coated specimens exhibited much better oxidation behaviour as compared to the bare alloy. A continuous Al-rich oxide scale formed on the coating surface during the initial oxidation stages. The oxide scale and coating itself acted as diffusion barriers blocking the further ingress of oxygen and protected the substrate alloy from oxidation. The oxidation mechanisms of the bare alloy and the coated specimens were investigated based on the experimental results.

High temperature corrosion of pure Fe, Cr and Fe–Cr binary alloys in O 2 containing trace KCl vapour at 750 °C

Pure Fe, Cr and Fe–Cr binary alloys were corroded in O 2 containing 298 ppm KCl vapour at 750 °C. The corrosion kinetics were determined, and the microstructure and the composition of oxide scales were examined. During corrosion process, KCl vapour reacted with the formed oxide scales and generated Cl 2 gas. As Cl 2 gas introduced the active oxidation, a multilayer oxide scales consisted of an outmost Fe 2O 3 layer and an inner Cr 2O 3 layer formed on the Fe–Cr alloys with lower Cr concentration. In the case of Fe–60Cr or Fe–80Cr alloys, monolayer Cr 2O 3 formed as the healing oxidation process. However, multilayer Cr 2O 3 formed on pure Cr.

Optical Emission Spectroscopy During Plasmatron Testing of ZrB2-SiC Ultrahigh-Temperature Ceramic Composites

Optical emission spectroscopy is used to investigate the oxidation of a hot-pressed ZrB 2 -SiC ultrahigh-temperature ceramic composite tested in the 1.2 MW Plasmatron facility at the von Karman Institute for Fluid Dynamics. Time-resolved spectra enable the in situ detection and temporal characterization of electronically excited B, BO, and BO 2 species concentrations directly adjacent to the oxidizing sample surface. The evolution of these boron species correlates well with the transient formation of a complex multilayer oxide scale containing a silica-rich glassy outer layer that limits oxide growth.

Oxidation Behavior of Ti50Ni40Cu10 Shape-Memory Alloy in 700–1,000 °C Air

The isothermal-oxidation behavior of Ti50Ni40Cu10 shape memory alloy (SMA) in 700–1,000 °C air was investigated by TGA, XRD, SEM and EPMA. Experimental results indicate that a multi-layered oxide scale formed, consisting of an outermost Cu2O(Ni,Ti) layer, a layer of the mixture of TiO2, TiNiO3 and irregular small pores, a layer of the mixture of Ni(Ti,Cu), TiO2 and irregular large pores, a Ti(Ni,Cu)3 layer and an innermost Ti30Ni43–47Cu27–23 layer. The apparent activation energy for the oxidation reaction of Ti50Ni40Cu10 SMA is determined to be 180 kJ/mol, and the oxidation rate follows a parabolic law. A schematic oxidation mechanism of Ti50Ni40Cu10 SMA is proposed to explain the observed results.

Oxidation of single crystal PWA 1483 at 950°C in flowing air

Abstract The oxidation behaviour of single crystal PWA 1483 at 950 °C was investigated by means of XRD, SEM and EDS. The parabolic oxidation behaviour, as defined by mass gain and the respective oxide layer thicknesses, is characterized by a parabolic rate constant of about 4 × 10−6 mg2/(cm4 × s) and the formation of a multi-layered oxide scale. An outer scale contains a Ti-bearing thin film composed of TiO2 and NiTiO3 but mostly Cr in Cr2O3 and (Ni/Co)Cr2O4 besides NiTaO4. This outer scale is connected to a discontinuous layer of Al2O3 and an area of γ′-depletion within the base material.

Nano-indentation measurement of oxide layers formed in LBE on F/M steels

Ferritic/martensitic (F/M) steels (T91, HT-9, EP 823) are candidate materials for future liquid lead or lead bismuth eutectic (LBE) cooled nuclear reactors. To understand the corrosion of these materials in LBE, samples of each material were exposed at 535 °C for 600 h and 200 h at an oxygen content of 10 −6 wt%. After the corrosion tests, the samples were analyzed using SEM, WDX and nano-indentation in cross section. Multi-layered oxide scales were found on the sample surfaces. The compositions of these oxide layers are not entirely in agreement with the literature. The nano-indentation results showed that the E-modulus and hardness of the oxide layers are significantly lower than the values for dense bulk oxide materials. It is assumed that the low values stem from high porosity in the oxide layers. Comparison with in-air oxidized steels show that the E-modulus decreases with increasing oxide layer thickness.

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.

High-Temperature Oxidation of Cr-Mo Steels and Its Relevance to Accelerated Rupture Testing and Life Assessment of In-Service Components

Use of accelerated creep rupture testing to assess the remaining life of components operating at elevated temperatures, such as pipes and tubes, is a common practice. At high temperatures, oxide growth can affect the creep results by diameter reduction and thus can increase the stress. However, the nature of oxide layer and hence oxidation behavior can be affected by minor changes in alloying composition of steels. This article presents the study of oxide-scale growth and specimen diameter reduction kinetics during oxidation of two Cr-Mo steels used in the manufacture of boiler tubing. Oxidation tests were carried out on 1.25Cr-0.5Mo and 2.25Cr-1Mo steels at 600 °C and 700 °C for times up to 1000 hours, using cylindrical specimens (similar to those used for creep testing). At 600 °C, the oxidation resistance of 2.25Cr-1Mo steel was superior to 1.25Cr-0.5Mo steel. However, the oxidation resistance of the two steels at 700 °C was similar in spite of the difference in their Cr contents. Multilayer oxide scales of oxides with various compositions were observed to have formed over the two steels. The similarity in oxidation kinetics of the two steels at 700 °C (in spite of differences in Cr contents) is ascribed to their Si contents and the predominant role of Si in oxidation at this temperature. The article also discusses implications of the variation in the oxidation kinetics to the stress enhancement in creep specimens due to scaling losses, and possible inaccuracies in creep data, as a result of minor variations in alloying composition.

Isothermal Oxidation Behavior of Ni Base Superalloy DM02 for High Temperature Dies

The isothermal oxidation behavior of a new developed Ni base superalloy named DM02 for high temperature dies was studied in this paper. The dynamic curve was achieved by monitoring weight gain of the alloy as a function of time. The results showed that the alloy had fairly good oxidation resistance at 1050°C and 1100°C. The oxidation kinetics at both 1050°C and 1100°C followed parabolic rules in segment. It has been found that the oxidation of the alloy was controlled by multi-oxides of (Ni, Co)O, (Ni, Co)Al2O4, and NiWO4, growth mechanism in the primary stage, and by Al2O3, NiAl2O4 growth mechanism in the following stage. After oxidation at 1050°C for 100h, the oxide scale of the alloy was mainly composed of two areas. Some were thin uniform (Ni， Co)Al2O4(outer)/Al2O3 (inner) composites scale and others were multi-layer oxide scale of ( Ni,Co)O / multi-oxides (mainly NiWO4、NiO and NiAl2O4.) /Al2O3.

Amorphous sol–gel SiO 2 film for protection of Ti6Al4V alloy against high temperature oxidation

Amorphous SiO 2 thin films were deposited on Ti6Al4V alloy by sol–gel processing. Isothermal and cyclic oxidation tests of the coated and uncoated specimens were performed at 700 and 800 °C. The SiO 2 film exhibited beneficial effects on the oxidation resistance of the alloy. Titania scales formed on the uncoated specimens, and severe spallation and stratification of the scales were observed. The oxidation rates of the silica coated specimens were decreased significantly. The silica film shrunk to about a quarter in thickness, probably by mechanism of crystallization of silica and evaporation of the organic additments. The oxide scales formed on the coated specimens were multilayered. Beneath the silica film, formation of a thick rutile titania layer followed by a thin alumina layer occurred. Above the silica film, alumina plus minor titania layer formed. It is deduced therefore that the growth of the multilayered and mixed oxide scales was dominated by both outward diffusion of metal and inward diffusion of oxygen.