Protective Alumina Scale Research Articles

Effect of β phase size on the microstructure of thermally grown oxide in water vapor atmosphere at 1100 °C

The arc-melted and vacuum-casted CoNiCrAlHf alloys with a 7YSZ top coat were designed to develop a protective alumina scale at 1100 °C in water vapor content. Arc-melted CoNiCrAlHf alloys provided substantial improvements in oxidation resistance. In this work, the oxidation behavior and void distribution of thermally grown oxide (TGO) formed on CoNiCrAlHf alloys were analyzed. The Al2O3 crystal structure and void distribution were studied as a function of the grain size of the CoNiCrAlHf substrate. Numerous voids developed in the oxide scale, and coarse HfO2 particles segregated at the Al2O3 grain boundaries of vacuum-cast CoNiCrAlHf alloys. The uniformly distributed HfO2 particles prevented void formation in the oxide scale. As a result, the phase transformation of Al2O3 was effectively impeded in water vapor.

High-temperature corrosion and oxidation properties of borided CoCrFeNiAl0.5Nb0.5 HEA

This study is focused on a detailed investigation of high-temperature corrosion and oxidation behavior of borided CoCrFeNiAl0.5Nb0.5 HEAs, considering their use in advanced engineering applications. CoCrFeNiAl0.5Nb0.5 HEA was produced by arc melting. XRD and SEM-EDS analysis before boriding determined that the alloy consisted of four different phases with different chemical compositions. Powder-pack boriding of a CoCrFeNiAl0.5Nb0.5 HEA was performed at 1000 °C for 3 h in a boriding media containing 90 % boron carbide and 10 % sodium tetrafluoroborate. As a result of the boriding process, complex boride layers consisting of (CoFe)B2, CrFeB, CoNbB, FeB and NiB phases were obtained on the surface with a thickness of 40 μm and hardness of 3004 HV. it was determined that the microstructure with cauliflower appearance evolved towards a dense and non-porous appearance around 20 μm as the boron diffusing into the HEA microstructure filling the gaps of intermetallic compound in the single structure, HEA and borided HEAs were each subjected to a cyclic hot corrosion test at 900 °C in molten corrosion salts of Na2SO4 and V2O5. After hot corrosion tests, long rod-like structures were observed in both samples due to the excessive corrosion, while borided alloys were more resistant to hot corrosion damage. After oxidation tests, HEA consists of a compact protective alumina scale that provided better oxidation resistance, while non-protective mixed oxides with cracks were dominant in the borided HEA.

Microstructures, mechanical properties, and oxidation behavior of in-situ ultra-fine duplex carbides reinforced Ni-based superalloy composites

A Ni-based superalloy composite (FTSC) was successfully prepared by spark plasma sintering with Ti3SiC2 (15 vol%) and FGH97 alloy powders as precursors. During sintering, the weakly bonded Si layers de-intercalate from Ti3SiC2 and the remaining TiC is eroded by the elements of the matrix, which promoted in-situ precipitation of MC and M6C carbides. MC with core-shell structure builds up a semi-coherent interface with the alloy matrix: (1¯11¯)γ/γ'‖(11¯1)MC with [110]γ/γ'‖[110]MC. The Zener pinning effect of ultra-fine duplex carbides results in refinement of alloy grains and notably improvement in mechanical strength. Comparing to the alloy FGH97, the composite FTSC possesses 55% higher microhardness, 100% higher compressive yield strength and 92% higher flexural strength. On the other hand, the refined grains ensured fast formation of a protective alumina scale at surface, which provides high oxidation resistance for the composite.

Oxidation-Limited Lifetime of ODS-FeCrAl Alloys: Observations on Influence of Surface Shape

Oxide dispersion-strengthened (ODS) FeCrAl ferritic steels constitute a class of alloys with a combination of mechanical strength capabilities and environmental resistance that permits operation at temperatures significantly higher than available classes of wrought alloy. A significant issue for these ODS alloys is that, because of the extreme operating temperatures, unexpected failure of the protective scale in service could lead to catastrophic degradation of high-temperature, high-pressure components. In the absence of handbook data for quantification of their high-temperature environmental degradation, significant efforts have been made to develop approaches for predictive oxidation lifetime modeling. The main basis for such models is an aspect of oxidation behavior peculiar to these alloys at very high temperatures, specifically, their ability to maintain a protective alumina scale until practically all available Al in the alloy has been consumed. At the high temperatures involved the concentration profile for Al throughout the component remains flat while the alumina scale is intact, so that knowledge of the oxidation kinetics provides a basis for calculating the time to exhaustion of the alloy’s Al reservoir, hence service life. Difficulties arise when unanticipated gradients in the Al concentration profile are introduced following, for instance, premature local mechanical failure of the protective scale. The intent of this work was to consider if and how scale failures change with differences in surface shape, and how related mechanistic understanding of the actual oxidation behavior may be used to extend the utility of the modeling approaches. Experimental results for coupons of different shapes were very largely in agreement with the basic tenets of the lifetime modeling approaches, but a notable finding was that cylindrical shapes reproducibly exhibited a reduction in unexpected early scale failures, leading to more consistent oxidation-limited lifetimes.

A Comparison of the Oxidation and Nitridation Properties of Selected Chromia- and Alumina-Forming Alloys at 800 °C

Three FeCrAl alloys and two chromia-formers (a stainless steel, and a Ni-base alloy) have been exposed in four environments (dry air, air + 20% H2O, 20% H2 + 20% H2O + Ar and 95% N2 + 5% H2) for 168 h at 800 °C. The corroded samples were investigated by SEM/EDS, XRD and gravimetry, and the formation of CrO2(OH)2(g) was measured as a function of time using a denuder technique. The Fe-base alloy formed a Cr-rich protective oxide scale in dry air and wet air but suffered break-away oxidation in 20% H2 + 20% H2O + Ar. In contrast, the Ni-base alloy suffered extensive NiO formation and internal oxidation in dry air and wet air but formed a protective chromia scale in 20% H2 + 20% H2O. All three FeCrAl alloys formed protective alumina scales in dry air, wet air and 20% H2 + 20% H2O + Ar. The FeCrAl alloy Kanthal APMT was severely nitrided in the 95% N2 + 5% H2 environment due to defects in the oxide scale associated with RE-rich inclusions which allowed nitrogen to enter the alloy. In contrast, the two Cr-lean FeCrAl alloys Kanthal EF101 and Kanthal EF100 did not suffer nitridation at all.

Synthesis of self-healing NiAl-Al2O3 composite coatings by electrochemical way

Nickel aluminide diffusion coatings act like an aluminium reservoir to form protective alumina scales at high temperatures. However, interdiffusion phenomena play a critical role in the metallurgical integrity of these coatings. This work explores a new type of self-healing coating for high temperature oxidation applications. Micro-reservoirs consisting of an aluminium-rich intermetallic core (Al3Ni2) and an aluminium oxide shell are embedded in a nickel aluminide matrix (NiAl). This new type of coating has been synthesized by an electrochemical chemical route to trap the micro-reservoirs in an electrodeposited nickel matrix followed by aluminization via a slurry route. Under high temperature oxidation conditions, the micro-reservoirs allow to form a diffusion barrier and to release Al into the coating matrix. These two simultaneous phenomena make it possible to maintain a sufficient amount of aluminium and ensure the unique formation of α-alumina at the surface of the coatings, hence to possibly increase the lifespan of the aluminium diffusion coatings.

Role of Cr Content in Microstructure, Creep, and Oxidation Resistance of Alumina-Forming Austenitic Alloys at 850–900 °C

Creep-rupture properties and oxidation behavior of a series of alumina-forming austenitic (AFA) alloys with variations of Cr contents, based on Fe-(13.5-18)Cr-25Ni-4Al-1.5Nb-0.1C in weight percent, have been evaluated at 850–900 °C. The study investigates material responses in the properties and microstructure through compositional modifications in AFA alloys, targeting performance optimization of alloys under high-temperature, corrosive industrial environments. The creep-rupture life of the alloys at 850 °C and 30MPa monotonically decreased with increasing Cr content, which was correlated with changes in secondary phase volume fractions, such as the reduction in B2-NiAl + Laves-Fe2Nb and increase in Sigma-FeCr with Cr content. The oxidation test at 900 °C in a water-vapor containing environment revealed a range of Cr content from 13.9 to 15.7 wt.%, enabling the formation of stable, protective external alumina scale as well as preventing internal oxidation/nitridation for up to total 7000 h exposure. On the other hand, the alloys with >16.7 wt.% Cr formed Sigma precipitates, which caused a reduction in not only Cr but also Al in the austenite matrix, resulting in less oxidation resistance than other alloys. The findings will guide the further optimization of material performance in the AFA alloy series.

Development of low-Cr wrought FeCrAl cladding alloys and its irradiation tolerance and steam oxidation resistance at 1200 °C

FeCrAl alloy with a low-Cr content (10%) and higher amount of Al (5%) was developed. The effects of Cr content on the mechanical properties, irradiation tolerance and the steam oxidation resistance of FeCrAl alloy were systematically investigated. The low-Cr FeCrAl alloys exhibit much better steam oxidation resistance at 1200 °C and improved radiation resistance, while maintaining comparable mechanical properties to the FeCrAl alloy with 13Cr. Stable and protective alumina scale was formed in the surface of the low-Cr alloy after the 1200 °C steam oxidation, which is different from the FeCr2O4 and Al2O3 in the surface of 13Cr alloy.

Exploring the high-temperature steam oxidation behaviors of the lean-Cr (7–10 wt%) FeCrAl alloys

The high-temperature steam oxidation behaviors of the four lean-Cr (7–10 wt% Cr) FeCrAl alloys containing minor elements and the Fe13Cr4.5Al alloy without minor elements (as a reference) were systematically investigated at 1100 ℃. The cross-sectional microstructure of the products after oxidation was investigated with FIB + TEM. The results show that the oxidation kinetics of these alloys except for 7Cr alloys obeyed the sub-parabolic rate law. The ability to form a protective alumina scale was worsened by the reduction of Cr content. The content of Cr should be higher than 7 wt% to promote alumina scale formation at 1100 ℃.

Protective Alumina Scale Growth at 900\xa0\xb0C for a Ni- and Cr-Free Co-Base Model Alloy with \u03b3/\u03b3\u2019-Microstructure: Synergistic Effects by Combining Shot-Peening and Halogenation

The oxidation resistance of novel γ/γ’-strengthened Co-base superalloys is clearly outmatched by their Ni-base counterparts within the high-temperature regime. Therefore, surface modification strategies to foster protective alumina growth seem auspicious. This study elucidates the impact of fluorination and shot-peening on protective alumina formation at 900 °C for a quaternary Co-base model alloy (Co-Al-W-Ta system) which is well known for an exceptionally low inherent oxidation resistance. Time-resolved isothermal gravimetric analysis (TGA) in synthetic air, detailed electron microscopic analysis, and X-ray diffraction (XRD) were used. For polished samples, no pronounced enhancement of oxidation resistance could be obtained by halogenation. However, in case of shot-peened samples (halogen-free), an increased tendency for alumina formation is found compared to polished surfaces. The very early stages of oxidation were identified to be especially crucial with respect to sustainable protective scale growth. Most noteworthy is the observation of a strong synergistic effect derived by a combination of halogenation and shot-peening, leading to significantly increased oxidation resistance.

High-temperature oxidation of AlCrFeNi-(Mn or Co) high-entropy alloys: Effect of atmosphere and reactive element addition

The effects of atmosphere (oxygen and steam) and reactive element addition (Zr and Y) on high-temperature oxidation behavior of Al0.5CrFeNiMn and Al0.5CrFeNiCo high-entropy alloys at 1000 °C and 1200 °C were studied. The Al0.5CrFeNiMn alloy displayed different oxidation mechanisms in oxygen and steam owing to differing oxygen partial pressures and preferential oxidation of Mn. A protective alumina scale established on Al0.5CrFeNiCo alloy in both atmospheres and the beneficial effects of Y addition are seen by remarkably improved oxide scale adherence and suppression of convoluted oxide scale morphology. However, doping with 1 wt% Zr deteriorates the oxidation resistance with substantial Al+Zr internal oxidation.

Carburization effect of Austenitic alloys with various Cr and Al additions under the methane/hydrogen atmosphere on the corrosion behaviors of steels

The corrosion behaviors of six Fe-19Ni-13/21Cr-xAl (x = 0, 2, 6 at. %) alloys in 10% CH4/H2 at 800oC were investigated. 2 at. % Al did not affect the corrosion resistance obviously, while 6 at. % Al reduced the carbon attack completely for Fe-19Ni-13Cr-6Al but was still insufficient to form protective alumina scales for alloys with 21 at. % Cr. An increase of Cr content changed the appearance of the internal carburization zone under the optical microscope. Stability diagrams of M-C-O system（M= Cr, Fe）were established to estimate the diffusion paths of carbon in the alloys.

Effect of Nb and Cu Additions on High Temperature Oxidation Behavior and Mechanical Properties of Al2O3-Forming Austenitic Alloys

The addition of Cu to austenitic steels was found to be effective for decreasing the critical Al content and promoting protective alumina scale formation from our recent studies. It was also reported that Nb addition to the alloy promotes the formation of alumina. These two elements are also known to improve the mechanical properties of steels by forming fine precipitates of Cu and intermetallics such as Laves and γ” phases, thus combined addition of these elements is expected to improve both the oxidation and mechanical properties of steels, however, there is no systematic study on the effects of the combined addition of these elements on the oxidation resistance and mechanical properties of austenitic heat-resistant alloys. The aim of this study was to investigate the effects of Nb and Cu on the oxidation behavior and mechanical properties of austenitic heat-resistant alloys containing Nb and Cu.Fe-25Ni-17Cr-(5~8)Al-(0, 0.5, 1)Nb-(0, 3, 5)Cu (in at.%) alloys were prepared by Ar-arc melting pure constituent metals. The ingots were cold-rolled by 90% and heat-treated at 1200ºC for 1 h, followed by aging at 800 ºC or 600 ºC for up to 500 h. Oxidation samples were cut from the cold-rolled plates and polished down to a 3 µm diamond paste. Oxidation tests were performed at 800°C in a box furnace in air for up to 100 h. The alloys were also subjected to a Vickers hardness test at room temperature and tensile tests at room temperature and 600ºC. The tensile test was performed in Ar at a strain rate of 0.005mm/s. Samples were analyzed by SEM, STEM, TEM, EPMA, GD-OES and XRD.Fig. 1 shows the oxidation kinetics and room temperature hardness of the alloys with/without different Nb and Cu contents. The oxidation kinetics of the alloys with/without different Nb and Cu contents shown in Fig. 1(a) and (b) reveals that the oxidation mass gain of the alloys without Cu decreased significantly by the addition of Nb and it further decreased with increasing Nb content. The oxidation mass gain of the 1Nb doped alloys tended to increase slightly with increasing Cu content. From the STEM-EDS analysis, it was confirmed that the protective alumina scale was formed on the Cu and/or Nb added alloys. An α-alumina was confirmed to develop on the alloys with Nb addition at about 600 ºC during heating, which is much lower temperature than that reported for formation of an α-alumina, suggesting that Nb addition promotes α-alumina formation. Vickers hardness of all alloys at room temperature increased with aging time, however, the maximum hardness was higher for the alloys aged at 600 ºC than 800 ºC. The hardness of the 5Cu alloy was higher than 0Cu alloy, which might be attributed to a formation of very fine Cu precipitates confirmed by TEM observation. Figure 1

(Invited) New Insights into the Formation and Failure Modes of Alumina Scales

Alloys forming external alumina scales can be used at very high temperature because of the excellent ability of alumina to protect against further reaction with the environment. Traces, ~0.1 at. %, of the so-called reactive elements (REs) are added into alumina- (and chromia-) forming alloys in order to enhance their oxidation performance. In this talk, the formation mechanism of protective alumina scales on several FeCrAl and FeNiCrAl alloys for high temperature application will be first discussed. As demostrated in our paper [1], the interaction between water and REs faciliates the formation of the ”protective” alumina scales via the formation of a nanocrystalline alumina layer with yttrium-decorated grain boundaries at the early stages of oxidation. Using state-of-the-art analytical tools such as STEM-EDXS-EELS and NanoSIMS, it has also been revealed that the water/REs interplay leads to the formation of hydride- and hydroxide-nanodomains in the alumina scale. In this context, findings of our latest research on the failure modes of alumina scales will be also presented with the emphasis on the key role of RE particle size distribution and the alloy grain orientation in the protective charactrestics of the oxide scales. Controlled high-temperature oxidation exposures in conjunction with quasi-in-situ experiments revealed that failure of the protective oxide scale occurs at specific, predictable (deterministic) sites on the alloy surface. The new undrestaning that will be discussed in this talk are expected to open up new avenues of research on the development of the next generation of high temperature alloys with superiour properties.

Forming protective alumina scale for ductile refractory high-entropy alloys via aluminizing

Refractory high-entropy alloys (RHEAs), or multi-principal-element refractory alloys, have been intensively studied in recent years, due to their attractive potential for ultrahigh-temperature structural applications. One formidable challenge facing the materials development for RHEAs though, is to simultaneously achieve good oxidation resistance and good mechanical properties, which unfortunately often impose contradictory microstructural requirements. So far, there exist no ductile RHEAs that possess reasonable oxidation resistance, and the formation of protective oxide scales such as alumina in them is never seen. Here we report, for the first time, a strategy to form protective alumina scale in ductile RHEAs upon high temperature exposure, using a tailor-designed two-step pack aluminizing process. Very importantly, the oxidation resistance of ductile RHEAs improves tremendously, as evidenced from harsh cyclic oxidation tests, thus providing a huge impetus to push forward the further development of RHEAs, towards ultrahigh-temperature applications.

Cathodic Protection Using Aluminum Metal in Chloride Molten Salts as Thermal Energy Storage Material in Concentrating Solar Power Plants

The new generation of concentrated solar power (CSP) plants to be developed presents a great challenge related to the increase in maximum operating temperature since molten salt CSP technologies require alternative salt chemistries such as chloride. The cathodic protection strategy involves the addition of a sacrificial metal to prevent corrosion of the alloy tested as container material in a CSP plant. In this paper, aluminum (Al) metal was analyzed as a corrosion inhibitor in OCT and HR224 alloys, obtaining corrosion rates of 4.37 and 0.27 mm/y, respectively. It has been confirmed that the use of Al metal can reduce the anodic current which is directly related to the corrosion rate. The formation of protective alumina scales (Al2O3) was assessed by scanning electron microscopy (SEM) and X-ray diffraction (XRD), confirming the corrosion model results from electrochemical impedance spectroscopy monitoring tests.

Effect of Cr addition on the evolution of protective alumina scales and the oxidation properties of a Ta stabilized γ'-strengthened Co-Ni-Al-Mo-Ta-Ti alloy

The paper reports the role of Cr addition (5 and 8 at.%) on the mechanism of oxidation of γ'-strengthened Co-30Ni-10Al-5Mo-2Ta-2Ti(at.%) alloy at 900 °C. The mass gain curves follow parabolic growth for all the three alloys. Cross-sectional studies reveal the formation of (Co1-x,Nix)O phase as an outermost layer. The intermediate layer consists of complex oxides (Co2Mo3O8, NiO6Ta2, CoCr2O4, TiO2, Al2O3) of other alloying elements. The formation of spinel CoCr2O4 with the addition of Cr leads to phase transformation of discontinuous κ-Al2O3 to protective and continuous α-Al2O3. Cr containing alloys show the presence of TiN in the γ' free zone.

Microstructure-oxidation resistance relationship in Ti3AlC2 MAX phase

Spark Plasma Sintering and Hot Isostatic Pressing were used to synthesize coarse-grained and fine-grained Ti3AlC2 specimens. Moreover, Spark Plasma Sintering processing parameters were modified in order to vary the TiC, Al2O3 and TixAly impurity and the porosity contents in the fine-grained samples. The influence of the Ti3AlC2 microstructure on the oxidation resistance was assesed. It is demonstrated that the grain size can drastically modify the oxidation resistance. The higher density of grain boundaries, in fine-grained specimens, increases the number of Al diffusion paths and leads to the formation of a protective alumina scale. In coarse-grained sample, Al diffusion is the rate limiting step of the α−Al2O3 formation and TiO2 is formed simultaneously to alumina. TiC impurities and porosity are demonstrated to be detrimental to the oxidation resistance in the 800 °C–1000 °C temperature range by favouring TiO2 formation. Finally, it is also shown that, for fine-grained specimens, the oxide scale grows very slowly for oxidation times in the range 20–40 days.

Failure analysis of diesel engine glow plugs

The implementation of start-stop technology in modern cars requires improvements of several elements of the internal combustion engine start-up system. In the present study, we have been focused on the failure phenomena of diesel engines glow plugs and attempts to extend their lifespan. The investigation revealed that the most sensitive element of the glow plug is a heating coil made of FeCrAl (Kanthal AF®) wire. Despite excellent anti-oxidation properties of these alloys, provided by the formation of a protective alumina scale on the alloy surface, the oxidation resistance is limited when exposed to thermal cycling up to 1050 °C. The reason is cracking and constant re-formation of the alumina protective scale and consequent consumption of the aluminium in the subsurface area of the wire. To prevent premature failure of heating coils, powder pack aluminization was implemented to increase the aluminium content in the subsurface of the alloy and thus the possibility of easier re-formation of the protective alumina scale. Phenomena that lead to failure of the glow plug were studied by X-ray computerized tomography (CT), optical microscopy (OM), scanning electron microscopy (SEM-EDXS), and microhardness measurements (HV). Comparison between aluminized and untreated samples is outlined and microstructural changes during thermal cycling and isothermal annealing discussed.

Effect of Cyclic Reaction on Corrosion Behavior of Chromium-Containing Alloys in CO2 Gas at 650 °C

In this work, seven commercial alloys (602CA, 310SS, 253MA, F321, F316L, 800H and 304SS) were investigated in Ar–20% CO2 gas at 650 °C under a cyclic condition (1-h reaction and 0.25-h cooling in each cycle) for up to 310 cycles. The corrosion behavior of these alloys in isothermal reaction condition was also carried out for a purpose of comparison. The results showed that nickel-based 602CA alloy stayed protective in both isothermal and cyclic reaction conditions by forming a thin protective alumina scale. However, alloys F321, F316L, 800H and 304SS all formed thick multilayered oxides with external iron-rich oxides and internal spinel oxides in all reaction conditions. Alloys 310SS and 253MA behaved protective in isothermal reaction condition but formed pitting corrosion in cyclic reaction condition. The high corrosion resistance of 310SS and 253MA was attributed to the high Cr content and the effect of other alloying elements, e.g., Si and Mn, forming additional oxide layers to enhance chromia protection. Cyclic reaction created stress on oxide scale during cooling and heating which accelerated the initiation of breakaway corrosion of these alloys. Carburization due to CO2 reaction was identified for F321, F316L and 304SS, but not for other alloys because of the formation of highly protective alumina or chromia scales.