Formation Of Oxide Scale Research Articles

Effect of microstructure on the oxidation behavior of a β-solidified γ-TiAl based alloy after nitrogen ion implantation

The microstructure of the oxide layer of a β-solidifying γ-TiAl alloy (Ti-43.2Al-1.9V-1.1Nb-1.0Zr-0.2Gd-0.2B) after nitrogen ion implantation was studied after cyclic oxidation tests at 800 °C. Oxidation resulted in the formation of the continuous oxide scale, consisting of thin inner and outer TiO2, dense Al2O3 and mixed Al2O3 + TiO2 layers. It was revealed that the nitrogen ion implantation considerably inhibited the growth of the oxide scale, internal oxidation and Al-depleted zones. Thick Al-depleted zones and a mixture of Al2O3 and TiO2 were formed along the colony boundaries. Tooth-shaped Al-depleted zones propagated along the lamella boundaries. In addition, the presence of Gd-rich particles in the oxide scale promoted the formation of Al2O3 particles in surrounding areas and increased the depth of the oxide scale around.

Characterisation and oxidation behaviour of plasma surface alloyed on γ-TiAl alloy

ABSTRACT The double glow plasma surface alloying process (DGP) is an emerging alternative technology for fabricating thermal barrier coating. In spite of its merits such as gradient metallurgical bond and thickness controllability, DGP method is less well known than electron beam-physical vapour deposition and plasma spraying technique for the bond coatings. This paper serves as a preliminary study into the use of this technology for preparation of gradient MCrAlY bond coating on titanium alloys, with a focus on the oxidation performance at 750, 850 and 950°C. In this study, a gradient MCrAlY bond coating was successfully synthesised on γ-TiAl alloy by DGP. The as-deposited coating was composed of γ′-Ni3Al, γ-TiAl, AlCr2 and σ-NiCoCr. The maximum load that the as-deposited coating could bear was 60 N. The MCrAlY coating exhibited much better oxidation resistance than the bare TiAl alloy due to the formation of multilayer oxide scales. The research opportunities for further optimisation of this type of MCrAlY coatings are also discussed.

TEM investigation of pre-oxidised Fe–Al with improved aqueous corrosion resistance

The protectiveness of an oxide scale on iron aluminides against aqueous corrosion has been controversially discussed in the literature. For clarification, Fe–25.8 at.% Al was oxidised in air at 1000 °C for 188 h and then immersed in H2SO4 (0.0126 M, pH 1.6). This experiment showed that pre-oxidation can substantially improve the aqueous corrosion behaviour of Fe-Al [31]. The present detailed investigation by transmission electron microscopy on the structure and constitution of the oxide scale after aqueous corrosion reveals why the oxide scale improved aqueous corrosion behaviour. During oxidation, a two-layered oxide scale formed. Corrosive attack by the acid is limited to the outer layer, consisting of equiaxed α-Al2O3 and a small amount of spinel. The inner layer of columnar α-Al2O3 grains with few grain boundaries apparently inhibits further ingress of the acid to the scale/metal interface. Faceting of the columnar α-Al2O3 grains at the scale/metal interface explains the good adherence of the oxide scale to the underlying alloy. Differences in oxide scale formation after short-time and prolonged oxidation are likely the reason for the discrepancy in previous observations.

Degradation of AlSi10Mg powder during laser based powder bed fusion processing

Knowledge concerning powder degradation during additive manufacturing (AM) processing is essential to improve the reusability of the powder in AM and hence maximize feedstock powder reuse and economy of the process. AlSi10Mg powder degradation in Concept Laser XLINE 2000R machine over the total period of 30 months was analyzed in order to understand the extent and mechanism affecting powder aging. Thereby, detailed analysis of the powder morphology, microstructure and surface chemistry was performed by SEM, TEM and XPS. The results show an increase in volume fraction of heavily oxidized spatter particles up to 3% in 30 months. XPS analysis of the powder surface chemistry indicates that powder particles are covered by uniform oxide layer, formed by Mg- and Al-based oxides, average thickness of which increased from ~4 nm in case of the virgin powder up to about 38 nm in case of the reused for about 30 month powder, established by XPS. Analysis of the oxide characteristics were consistent with the observed oxygen content in the sampled powder. Columnar oxide scale formation on spatter particles was revealed as well, reaching up to 125 nm in thickness measured using STEM. Results of the XPS and STEM-EDX analysis of oxide composition are shown to be in agreement with the thermodynamic calculations confirming that oxide scale on sputter particles is formed by MgAl2O4 spinel and Al2O3 (corundum) oxides.

Insight into high temperature performance of magnetron sputtered Si-Ta-C-(N) coatings with an ion-implanted interlayer

Challenges related to the application of wear resistant coatings at high temperatures require the development of novel materials with an exceptional combination of mechanical, chemical and tribological properties. The present paper is focused on understanding of relationships between structure, composition and high-temperature performance of the Si-Ta-C-(N) coatings. The coatings were produced using combined magnetron sputtering (MS) and ion implantation (CMSII) technique. It was found that ion implanted coatings demonstrated better thermal shock resistance compared to MS Si-Ta-C-(N) coatings. The Si-Ta-C-(N) coatings revealed a nanocomposite structure consisting of 2–3 nm fcc TaC(N) grains and amorphous a-Si and a-SiC(N) phases. The composition and structure of amorphous matrix and nanocrystallites strongly affected tribological performance of the Si-Ta-C-(N) coatings. The N-doped coatings exhibited exceptionally good tribological performance due to a higher ductility of N-rich amorphous a-SiCN and a-SiNx matrix, and fcc Ta(C,N)-based crystallites compared with the a-Si + a-SiC, and fcc TaC-based phases in N-free coating. The Si-Ta-C-(N) coatings easily withstood oxidation annealing at 800 °C due to the formation of a 200 nm protective TaSiOx amorphous layer. Oxidation annealings revealed that under thin protective TaSiOx layer crystalline components of coatings did not change when Si and C from the amorphous matrix started to diffuse towards the substrate at 800 °C but even after redistribution of elements and formation of oxide scale the coatings demonstrated reasonably high hardness – 13–16 GPa. Triboactivated formation of TaSiOx fibers which could slide/roll against the same TaSiOx tribolayer during high-temperature tribotests resulted in low coefficient of friction values (0.23 at 800 °C) and absence of wear.

Investigation of graphite/CNTs on the equiatomic AlCoNi alloy: Hardness and surface morphology of the oxide scale

Equiatomic AlCoNi alloys with 0.1 wt% of carbon addition in different forms such as graphite and carbon nanotubes were fabricated by powder metallurgy. The effect of carbon addition on the microstructure and oxide scale formation was investigated as a function of carbon type. Evidence showed that both carbon forms addition reduced the porosity of the sintered AlCoNi alloy. A mixture of Al and Ni-Co oxide scale was formed in all samples after a heat treatment at 1000 °C for 2 h in air. After subsequent oxidation at 900 °C for 100 h, graphite induced the Ni-Co oxide growth; meanwhile, carbon nanotubes favored the Al oxide growth. The dispersion of carbon nanotubes produced the lowest growth rate of the oxide scale.

Mechanism of adding rhenium to improve hot corrosion resistance of nickel-based single-crystal superalloys

Hot corrosion behavior in sulfate salt at 950 °C of Rene N5 single-crystal superalloys with 3 wt% rhenium (NSR) was investigated compared with that of nickel-based single-crystal superalloys without rhenium (NS). After 30-h corrosion, the surface of the NS superalloy is seriously corroded. Many holes and exfoliation appear on the surface. The NSR superalloys exhibit better hot corrosion resistance than the NS superalloys. After 30-h corrosion, a continuous and compact Al2O3 film is observed on its surface. The Al2O3 film with dense structure formed on the surface provides protection for the matrix. The characterization results show that Al is aggregated in the γ′ phase, while Re is aggregated in the γ phase during the formation of oxide scale. Considering that Re can inhibit the diffusion of Al in the nickel matrix, it is inferred that Re can inhibit the outward diffusion of Al and prevent the decrease of Al concentration in the γ′ phase. High concentration of Al hinders the decomposition of Al2O3 due to the reaction of acid and basic dissolution. Al2O3 keeps its structure intact and provides protection for the matrix.

Prevention of High-Temperature Surface Degradation in SiMo Cast Irons by Cr and Al Alloying

High silicon molybdenum-alloyed cast irons (SiMo) with spherical graphite are widely used for cast thin-wall exhaust manifolds, worked in high-temperature oxidation environment; therefore, prevention of surface degradation during service is important for such common application. The base SiMo and two alloyed by 1 pct Cr and 3 pct Al cast irons were tested at high temperatures in air and a combustion atmosphere to study the microstructural processes of surface degradation. It was shown that surface degradation of SiMo cast irons involves several microstructural processes, including the formation of multilayered oxide scale and development of internal porosity, resulting in decarburization of graphite nodules. Monitoring weight change of the specimens and Leco C analysis were used for decoupling the rates of surface oxidation and metal matrix decarburization. SEM/EDX analysis was used to study the phase composition of scale and high-resolution TEM analysis was performed to determine the detailed information about the atomic structure of protection layers formed on scale/metal matrix interface. The results were used to qualify and link different surface degradation mechanisms. In the base SiMo cast iron, the protection efficiency of formed amorphous silica layers is limited by intensive decarburization at temperatures above 700 °C. It was shown that Cr and Al alloying prevents surface degradation and suppresses decarburization by developing thermally stable oxide films on metal-scale interface, thus allowed to increase a safe working temperature up to 800 °C in air and the combustion atmosphere.

Relationship analysis on particle-coating-ablation property of UHTC coatings fabricated by plasma spray technique

The relative high porosity of plasma spray ultra-high temperature ceramic (UHTC) coatings has been an important limiting factor for the ablation property and one of the reasons is the original pores of feedstocks. In this work, dense and spherical ZrC–SiC powders were fabricated by induction plasma spheroidization (IPS) and were used to prepare ZrC–SiC coating by vacuum plasma spray. The effect of microstructure of feedstock particles on the microstructure and ablation property of coating was studied. The results showed that the spherodized particles were composed of eutectic-like phase and ZrC granules, exhibiting high degree of compactness and sphericity, which had higher deposition rate and generated splats with flattened structure to overlap compactly then produce highly dense coating compared with those of spray drying (SD) particles. The deposition rate was doubled and the porosity of the coating was reduced by half. The ablation resistance of ZrC–SiC coating was greatly improved due to the formation of dense oxide scale.

Corrosion of AISI 310 and 316 in Molten Hydroxide Under Steam Electrolysis Conditions

Steam electrolyzers, utilizing molten hydroxide electrolyte, offer potential for improvement in electrochemical efficiency, cost reduction, use of conventional materials of construction and process scale up for large scale hydrogen production. Stainless steels, used for the fabrication of cell components (current collector, gas separator, wet seals, and manifolds) experience accelerated corrosion in the presence of molten hydroxide electrolyte in both oxidizing (anodic) and reducing (cathodic) atmospheres. In this study, the corrosion behavior of AISI 310 and 316 has been studied at 600 °C using melt immersion and cyclic voltammetry tests. Melt immersion test revealed formation of the porous lithium iron oxide at the melt-oxide interface for both AISI 310 and 316. Oxide scale cross-sectional study conducted by FIB/TEM showed multi-layer oxide scale formation. Inductively Coupled Plasma (ICP) Spectroscopy analysis showed the presence of Cr in the electrolyte melt obtained from samples exposed to oxidizing atmosphere. Cyclic voltammetry tests showed the breakdown of passive metal-oxide under anodic overpotential in molten hydroxide. Mechanisms for hydroxide-induced corrosion is proposed and discussed.

Scale Formation on HSLA Steel during Continuous Casting Part II: The Effect of Surface Conditions

The present research addresses the effect of surface condition on oxide scale formation at high temperatures such as those experienced during secondary cooling in Continuous Casting. Tests were carried out in clean, as-cast and surfaces covered with casting powder to replicate the oxidation/re-oxidation after the mould. Specimens oxidized at 1000, 1100 and 1200 °C under dry air and water-vapour conditions revealed that the oxide scale formation is strongly influenced by temperature, environmental and surface conditions. The oxide scale thickness increases with temperature alterations in the surface (e.g., as-cast and covered with powder) where oxides and carbonates from the casting powder accelerate oxidation kinetics leading to thick and unstable scales. A high amount of carbon is present on surfaces covered with casting powder where it diffuses through the oxide scale forming CO and CO2 which lead to stress accumulation that makes scales prone to defects such as pores, voids and micro-cracks. Ultimately, if wüstite remains attached to the steel surface or inside oscillation marks, it may disturb heat transfer during secondary cooling which has deep industrial implications for crack formation and overall casting yield. Therefore, accurate insights on scale type and growth mechanisms could lead to accurate control of its formation during casting.

Precipitation and Growth of Ni-Rich Substances in the Oxide Scale of Steel in Mixed C2h5oh-H2o Atmosphere

Nickel is an iconic alloying element in the piercing plug steel. In order to study the formation mechanism of Ni-rich substances in oxide scale of steel under mixed C2H5OH-H2O atmosphere, the high-temperature oxidation experiment was carried out by employing steels with varied Cr and Ni contents, and the oxidation characteristic was explored by using scanning electron spectroscope (SEM), energy dispersive X-ray spectroscopy (EDS), metallographic microscope (OM) and Image-Pro Plus. Under the experimental conditions, it is found that Ni promotes the formation of denser inner oxide scale while Cr inhibits inner oxidation effectively. Congregation of Ni-rich substances is a diffusion process strongly relying on the Ni content of the steel. The total Ni-rich substances dependence of Ni content follows the parabolic law. It is proposed that the Ni-rich substances precipitate once the oxidation of Fe or Cr atoms leaves vacancies for the nearby Ni atoms to fill in to form a substitute Ni-rich crystal structure, and then the Ni-rich substances congregate and grow up as oxidation proceeds. The results are meaningful for the reasonable design of piercing plug steel.

Investigation of oxide scale formation and internal oxidation of an Fe-based coating at 500 °C and 600 °C

In this study, we investigated an Fe-based coating that was produced on a cylindrical surface via an internal rotating plasma spray and was oxidized at different temperatures. For microstructural characterization, scanning electron microscopy and electron probe microanalysis were combined with energy dispersive X-ray spectroscopy and X-ray diffraction analyses. The Fe-based coating exhibits a dense structure with small amounts of internal oxides and pores after internal rotating plasma spraying. During the oxidation process, an iron oxide scale formed on the surface of the Fe-based coating, and the thickness of the oxide scale increased with increasing oxidation temperature and time. In addition, internal oxidation occurred in the alloy matrix and formed mixed oxides, mainly at the pores and the alloy particles' interface during the oxidation process. Compared to the Fe-based alloy, the oxidation behaviour of the Fe-based coating presented different results. The mechanisms of oxidation behaviour at different temperatures and time, including oxide scale formation and in-situ internal oxidation in the Fe-based coatings matrix, are discussed.

Ablation resistance of tungsten carbide cermets under extreme conditions

A cobalt-free tungsten carbide cermet (WC-FeNi) has been subjected to oxyacetylene flame tests to simulate extreme operating conditions such as a worst-case fusion reactor accident. In such an accident, air-ingress to the reactor may impinge on components operating at surface temperatures in excess of 1000 °C, leading to tungsten oxide formation and its subsequent hazardous volatilisation. Here, the most challenging accident stage has been simulated, where the initial air-ingress could lead to extremely rapid air-flow rates. These conditions were simulated using an oxidising oxyacetylene flame. The separation between flame nozzle and sample was varied to permit peak surface temperatures of ~950–1400 °C. When the peak temperature was below 1300 °C, the cermet gained mass due to the dominance of oxide scale formation. Above 1300 °C, the samples transitioned into a mass loss regime. The mass loss regime was dominated by liquid-phase ablation of the scale rather than its volatilisation, which was confirmed by performing a systematic thermogravimetric kinetic analysis. The result was unexpected as in other candidate shielding materials, e.g. metallic tungsten, volatilisation is considered the primary dispersion mechanism. The unusual behaviour of the cermet scale is explained by its relatively low melting point and by the lower volatility of its FeWO4 scale compared to tungsten's WO3 scale. The substantially lower volatility of the WC cermet scale compared to metallic W scales indicates it may have a superior accident tolerance.

Oxidation Resistance Failure of Dilute CuAl alloys at 800 °C

Oxidation is a major failure mode for the application of Cu in industry. To address this issue, alloying Cu with Al presents a potential solution due to the Al2O3-rich thin film formed during a pre-annealing process. The low oxidation resistance (OR) of dilute CuAl alloys that are oxidized at high temperatures is a critical issue due to their long-term exposure to an aerobic environment. Thus, the ORs of dilute CuAl alloys with 0.2–2.0 wt.% Al pre-annealed in a hydrogen atmosphere were examined at 800 °C and compared with the case at 400 °C. The OR of dilute CuAl alloys at 800 °C can be distinctly improved at the initial oxidation stage as well as 400 °C. However, at 800 °C, the OR gradually declines as the oxidation progresses, which considerably differs from the observation at 400 °C with the stable OR. The results show that the decline of the OR at 800 °C starts from the oxide button formation at the surface, and the growth and coarsening of these buttons cause nonuniform oxide scale formation. As the oxidation proceeds, the remnant Al2O3-rich thin film does not move with the Cu2O/CuAl interface, which is induced by the impeded outward diffusion of Cu atoms. The slowed diffusion of Cu atoms originates from the remnant Al2O3-rich thin film and the Al2O3 particles that are newly formed in the CuAl alloys due to the internal oxidation of Al.

Consequences of Different Mechanical Surface Preparation of Ni-Base Alloys during High Temperature Oxidation.

The influence of surface roughness on its high temperature oxidation for an Ni-base superalloy was studied using laser profilometry, atomic force microscopy, mass change measurements, glow-discharge optical emission spectrometry, scanning electron microscopy, X-ray diffraction, and positron annihilation methods. The isothermal and cyclic air oxidation tests were performed at 1000 °C and showed dependence of oxidation behavior on surface roughness. Smoother surfaces oxidation resulted in the formation of a multilayered oxide scale consisting of NiO, Cr2O3, and internally oxidized Al2O3 while a rougher surface formed protective Al2O3 scale. The factors responsible for different oxidation behaviors were determined as higher concentration of vacancies and increased residual stresses in the near-surface region of studied alloys.

High-temperature oxidation behavior of modified 4Al alumina-forming austenitic steel: Effect of cold rolling

The oxidation behavior and mechanism of as-received and 30 % cold-rolled alumina-forming austenitic (AFA) steel were investigated in dry air at 700 °C. The results show that the mass gain per unit area curves of as-received and 30 % cold-rolled steels subject to near-parabolic law before 100 h oxidation time. Two samples both show higher high-temperature oxidation resistance due to the formation of dense Al2O3 oxide scale. Gradual spallation of outer scale results in the formation of continuous and dense alumina scale. Dislocations can act as short-circuit diffusion channel for the diffusion of Al from alloy matrix to surface, and also provide nucleation sites for B2-NiAl phase, which ensure the continuous formation of Al2O3 scale.

High-temperature oxidation behavior of a Ti5Si3-incorporated MoSiBTiC alloy

The oxidation behavior of a Ti5Si3-incorporated MoSiBTiC alloy (38Mo–30Ti–17Si–10C–5B, at.%) was systematically studied in the temperature range of 700–1000 °C. Temperature dependence of oxidation kinetics and oxide scale formation was revealed for this alloy. The oxidation kinetics at 700 °C featured an initial transient stage of weight gain and then switched to weight loss. At 800 °C, the oxidation kinetics featured an initial transient stage with fast weight loss, a steady stage with small weight loss, and an acceleration stage leading to catastrophic failure. At 900 and 1000 °C, the alloy exhibited a rapid transient weight loss as followed by a decreased weight loss fitting to parabolic kinetics. The scales that formed at 700–1000 °C mainly consisted of rutile TiO2 and amorphous SiO2. The high viscosity of SiO2 at 700 and 800 °C caused the formation of porous and microcracked oxide scales while the occurrence of viscous flow during oxidation at 900–1000 °C promoted the formation of dense and protective scales.

Quantitative µ-CT Analysis of Scale Topology Formed During Oxidation of High SiMo Cast Iron

High SiMo cast iron components in automotive exhaust systems are exposed to high-temperature oxidation over time. Quantitative analysis of formed oxide scale is therefore important for the assessment of a component durability. The brittle nature of multilayered scale and thermal stress limits capture of a true topology using traditional 2D destructive cut and polish methods. In this study, nondestructive high spatial-resolution 3D µCT analysis was performed on 2.90-mm-diameter oxidized specimens which permitted direct observation with 3.5 µm pixel resolution. The specimens were oxidized in three sequential time steps for a total 100 h at 700 °C and 800 °C in air and combustion gas atmospheres. A MATLAB-coded algorithm was used to quantify the topology, thickness variation in internal and external scale layers, and scale/metal interface unevenness. Scale topology was linked to oxidation temperature and gas atmosphere. A water vapor environment increases scale/metal interface unevenness and scale layer thickness irregularity which were related to an accelerated oxidation rate.

Improving the high-temperature oxidation resistance of TiB2 thin films by alloying with Al

Refractory transition-metal diborides (TMB2) are candidates for extreme environments due to melting points above 3000 °C, excellent hardness, good chemical stability, and thermal and electrical conductivity. However, they typically suffer from rapid high-temperature oxidation. Here, we study the effect of Al addition on the oxidation properties of sputter-deposited TiB2-rich Ti1-xAlxBy thin films and demonstrate that alloying the films with Al significantly increases the oxidation resistance with a slight decrease in hardness. TiB2.4 layers are deposited by dc magnetron sputtering (DCMS) from a TiB2 target, while Ti1-xAlxBy alloy films are grown by hybrid high-power impulse and dc magnetron co-sputtering (Al-HiPIMS/TiB2-DCMS). All as-deposited films exhibit columnar structure. The column boundaries of TiB2.4 are B-rich, while Ti0.68Al0.32B1.35 alloys have Ti-rich columns surrounded by a Ti1-xAlxBy tissue phase which is predominantly Al rich. Air-annealing TiB2.4 at temperatures above 500 °C leads to the formation of oxide scales that do not contain B and mostly consist of a rutile-TiO2 (s) phase. The resulting oxidation products are highly porous due to the evaporation of B2O3 (g) phase as well as the coarsening of TiO2 crystallites. This poor oxidation resistance is significantly improved by alloying with Al. While air-annealing at 800 °C for 0.5 h results in the formation of an ~1900-nm oxide scale on TiB2.4, the thickness of the scale formed on the Ti0.68Al0.32B1.35 alloys is ~470 nm. The enhanced oxidation resistance is attributed to the formation of a dense, protective Al-containing oxide scale that considerably decreases the oxygen diffusion rate by suppressing the oxide-crystallites coarsening.