Steam oxidation resistance of Ni-aluminide/Fe-aluminide duplex coatings formed on creep resistant ferritic steels by low temperature pack cementation process

Steam oxidation resistance and thermal stability were studied at 650 °C for a coating with an outer Ni₂Al₃ layer and an inner Fe₂Al₅ layer formed on P92 steel surface. The parabolic rate law of oxidation was obeyed only in less than 2000 h with positive deviations occurring at longer oxidation times. The outer layer of the coating was transformed to NiAl during oxidation, but it remained stable once it was formed. The mechanisms for the enhanced thermal stability were discussed and a simple approach to enhancing the lifetime of the coating was proposed.     

Simulation and experimental approach to CVD-FBR aluminide coatings on ferritic steels under steam oxidation

The ferritic steels used to produce structural components for steam turbines are susceptible to strong corrosion and creep damage due to the extreme working conditions pushed to increase the process efficiency and to reduce pollutants release. The response of aluminide coatings on the P-92 ferritic steel, deposited by CVD-FBR, during oxidation in a simulated steam environment was studied. The analyses were performed at 650 °C in order to simulate the working conditions of a steam turbine, and 800 °C in order to produce a critical accelerated oxidation test.The Thermo-Calc software was used to predict the different solid phases that could be generated during the oxidation process, in both, coated and uncoated samples. In order to validate the thermodynamic results, the oxides scales produced during steam tests were characterized by different techniques such as XRD, SEM and EDS. The preliminary results obtained are discussed in the present work.     

Resistance to oxidation and ablation of SiC coating on graphite prepared by chemical vapor reaction

A thick SiC coating was prepared on graphite by chemical vapor reaction. The coating reveals a typical crystalline structure with limited porosity and combines well with the substrate. Oxidation tests demonstrate that the coating has a weak self-healing ability at 1100 K and good self-healing ability at temperatures from 1623 to 1823 K. An oxyacetylene torch test verifies that the prepared coating can effectively protect graphite from ablation for 50 s. After the ablation test, the silica microspheres and other interesting silica structures such as microwires, microparticles, microflowers, nanowires and nanoparticles are formed at the ablation center and its surroundings.     

Early stages of oxidation observed by in situ thermogravimetry in low pressure atmospheres

A highly sensitive set-up for in situ mass-gain measurements during high temperature reaction is presented. By removing the inert gas components of a corrosive atmosphere and keeping the flows and partial pressures of the reactive species constant, the experiment is preformed in a low-pressure environment. A better signal-to-noise ratio which increases sensitivity down to 10 μg/cm² is achieved. The random behaviour of the mass-gain signal, caused by turbulences in the gas stream, could be reduced by a factor of 10.Compared to theoretical simulations, the mass-change during oxidation in binary iron-based model alloys is slower than expected from pure bulk diffusion behaviour.     

The role of microstructure in the high temperature oxidation mechanism of Cr3C2-NiCr composite coatings

Composites of Cr₃C₂-NiCr provide superior oxidation resistance to WC-Co composites, which has seen them applied extensively to components subjected to combined high temperature erosion and oxidation. This work characterises the variation in oxidation mechanism of thermally sprayed Cr₃C₂-NiCr composites at 700 °C and 850 °C as a function of heat treatment. Carbide dissolution during spraying increased the Ni alloy Cr concentration, minimising the formation of Ni oxides during oxidation. Compressive growth stresses resulted in ballooning of the oxide over the carbide grains. Carbide nucleation with heat treatment reduced the Ni alloy Cr concentration. The oxidation mechanism of the composite coating changed from being Cr based to that observed for NiCr alloys.     

Influence of salt spray corrosion on subsequent isothermal oxidation behaviours of AIP NiCoCrAlYSiB coatings

The influence of salt spray corrosion on subsequent oxidation behaviours of arc ion plated (AIP) NiCoCrAlYSiB coatings is investigated in this work. The results indicate impact of the salt spray corrosion on successive oxidation behaviour depends greatly on the degree of the previous corrosion attack. Accelerated oxidation kinetics and unsatisfactory oxide morphologies are observed on the as-deposited and as-annealed NiCoCrAlYSiB coatings that suffered serious salt spray attacks. Pre-oxidised coatings prove excellent performances during both salt spray tests and the subsequent oxidations. The mechanism of the corrosion attack influencing the isothermal oxidation behaviour is discussed.     

Effect of Ce and La additions in low temperature aluminization process by CVD-FBR on 12%Cr ferritic/martensitic steel and behaviour in steam oxidation

Two different coatings based of iron aluminide on 12% Cr ferritic-martensitic steel have been developed by CVD-FBR technique, which is modified by the introduction of Ce and La as powder in the fluidized bed. These elements change the gaseous environment, which composition is predicted by a thermodynamic approximation. Partial pressures of all gaseous precursors are drastically modified; in consequence AlCl has the highest partial pressure in the system leading to an increment of the coating thickness. Coatings are composed by (Fe, Cr)₂Al₅ or (Fe, Cr)₂Al₅ and (Fe, Cr)Al₃ intermetallic phases. On the other hand, steam oxidation test at 650 °C was performed in order to observe improvements in the HCM12A oxidation resistant.     

Semiconducting properties of thermal scales grown on a chromia-forming alloy under controlled oxygen partial pressures

The present work is focused on the oxidation at 1050 °C of a model chromia-forming alloy, paying special attention to the control of the oxygen partial pressure. Taking benefit of a recently developed approach to describe and fit photocurrent energy spectra, photoelectrochemical characterizations allowed to reveal the presence, in the scale formed at low P(O₂) (10⁻¹⁵ atm), of an unexpected high number of oxide phases. This result underlines the complexity of the scale in term of chemical composition as well as of the semiconducting types of the individual phases.     

High temperature oxidation of AlCrFe complex metallic alloys

Isothermal oxidation of Al₆₅Cr₂₇Fe₈ and Al₈₀Cr₁₅Fe₅ was studied in the 600–1080 °C range. Formation of transient alumina layers is obtained up to 900 °C. On Al₆₅Cr₂₇Fe₈ transient to α-phase transformations occur when performing oxidation at 1000 °C, together with the possible appearance of (Al_0.9Cr_0.1)₂O₃. At 1080 °C, direct formation of α-alumina is obtained. On Al₈₀Cr₁₅Fe₅, spallation of the oxide layer during the cooling stage is observed following oxidation at 800 and 900 °C, revealing thermal etching of the underneath alloy surface. At 1050 °C the α-Al₂O₃ scale is directly formed but plastic deformation and recrystallization of the underneath alloy into several intermetallic phases is observed.     

The isothermal and cyclic oxidation behaviour of two Co modified aluminide coatings at high temperature

The isothermal and cyclic oxidation behaviour of two Co modified aluminide coatings together with the simple aluminide coating were performed at 1000 °C and 1100 °C. All the three coatings show a much lower oxidation rate compared with the bare alloy. Results also indicate the addition of Co to the aluminide coating decreases the oxidation resistance slightly. It can be ascribed to that Co is easier to be oxidized than Ni at high temperature, and the Cr(W) rich phases which could act as a diffusion barrier are less in the coating with higher Co content.     

Corrosion of barium aluminosilicates by water-vapour: An investigation from first principles

Theoretical studies on the water-vapour corrosion resistance of barium aluminosilicates were carried out using Mulliken analysis based on first principles. The Mulliken population of Si–O bonds in different barium aluminosilicates was calculated. The water-vapour corrosion resistance of them was predicted based on these calculation results. In order to verify the prediction results, four barium aluminosilicate powders were synthesized by sol–gel method, and the water-vapour corrosion behaviour of these materials was studied at 1250 °C in an atmosphere of 50%H₂O–50%O₂ water-vapour flowing at a rate of 0.85 mm/s. The experimental results were consistent with the predictions by first principles. This work suggested a methodology for design and selection of silicate materials with good water-vapour corrosion resistance.     

Improving the corrosion and tribocorrosion resistance of Ni–Co nanocrystalline coatings in NaOH solution

Ni–Co nanocystalline coatings were electrodeposited from a modified Watts bath. Increasing the deposition current density had no significant effect on structure, corrosion and tribocorrosion behavior of the coatings. Adding saccharin into the bath reduced the grain size, increased the hardness, changed the texture component from (2 0 0) to (1 1 1), smoothed the surface morphology, increased the corrosion resistance and improved the tribocorrosion behavior of coating. Presence of sodium lauryl sulfate in the bath increased the corrosion resistance of coating by producing a more compact surface morphology. However, the coating showed low tribocorrosion resistance, probably due to its lower hardness.     

Hot corrosion behaviour of low Al NiCoCrAlY cladded coatings reinforced by nano-particles on a Ni-base super alloy

Three NiCoCrAlY coatings with Al content lower than 5 wt.% reinforced by different kinds of nano-particles with the same addition and one without nano-particles were prepared on a Ni-base super alloy using laser cladding technique. Hot corrosion of the NiCoCrAlY coatings in Na₂SO₄/K₂SO₄ (75:25, wt./wt.) mixture was performed at 1050 °C in static air. Results indicate that the hot corrosion resistance of the coatings with nano-particles is better than that of the one without nano-particles, among which the one with nano-CeO₂ presented the best hot corrosion resistance. Effects of nano-particles on the hot corrosion behaviour were also discussed.     

Sol–gel coating to reduce 1.25Cr–0.5Mo steel oxidation at 700 °C: Catalyst type effect

With the use of sol–gel it is possible to apply zirconia coatings to reduce high temperature oxidation of steel. Some limitations of this technique are high hydrolysis rates and the formation of cracks during the drying stage. In this work, the mole ratio of zirconium butoxide to ethylacetate, and the nature of the catalyst were varied. SEM-EDX was used to evaluate the continuity of the zirconia coatings, and thermo-gravimetric analysis for the oxidation rate of coated samples. Thin and continuous coatings were obtained. With a basic catalyst, the coatings were crack-free and presented lower oxidation rate than acid-catalyzed coatings.     

Synergistic corrosion behavior of coated Ti60 alloys with NaCl deposit in moist air at elevated temperature

In the present paper, the corrosion behavior of Ti60 alloys with an aluminide, TiAlCr, and enamel coatings in moist air containing NaCl vapor at 700-800 °C were studied. The results showed that the TiAlCr and aluminide coatings failed to protect the substrate from corrosion due to the cyclic formation of volatile products during corrosion at 800 °C. However, an uneven continuous protective Al₂O₃ scale could form on the aluminide coating during corrosion at 700 °C. And the enamel coating could protect Ti60 from corrosion due to its high thermochemical stability and matched thermal expansion coefficient with substrates of Ti-base alloys during corrosion.     

High temperature corrosion of coatings and boiler steels below chlorine-containing salt deposits

High temperature corrosion tests were performed on low-alloy ferritic steel and austenitic stainless steel, five high velocity oxy-fuel (HVOF) coatings, a laser cladding, and a diffusion chromized steel. Test conditions simulated superheater conditions of biofuel-fired boiler. The samples were exposed to synthetic salt containing 40 wt% K₂SO₄, 40 wt% Na₂SO₄, 10 wt% KCl, and 10 wt% NaCl. Exposures were carried out in oxidizing and in reducing atmospheres. The test temperature was 550 °C and the test duration was 100 h. Corrosion was extremely severe in oxidizing conditions because of active oxidation. In reducing atmosphere corrosion was retarded due to depletion of chlorine in the scales by evaporation of metal chlorides, and formation of a layer rich in chromium, sodium, sulfur, and oxygen adjacent to the metal surface. The corrosion resistance of coatings was determined by composition and microstructure. Oxides at splat boundaries were attacked by chlorine, and chlorine was able to penetrate through the coatings along splat boundaries.     

An investigation on oxidation wear mechanisms of Ti-46Al-7Nb-0.7Cr-0.1Si-0.2Ni intermetallic-based alloys

The results of investigation on oxidation wear mechanism of Ti-46Al-7Nb-0.7Cr-0.1Si-0.2Ni-based intermetallic alloy are presented. Oxidation was carried out in air at temperatures: 900 °C, 925 °C and 975 °C taking into account the micro-geometry of surfaces being heated and oxidized. It was determined that the rise of surface roughness to Ra = 5.8 μm definitely reduces scale of chipping in higher temperature (975 °C). Investigation of the material structure of the specimen and chemical composition of oxidation products was performed. Possibilities of an increase in the heat resistance of the tested alloy by means of application of high roughness surfaces were shown.     

Characterization of microstructures and oxidation behaviour of Nb–20Si–20Cr–5Al alloy

Static oxidation in air was performed on Nb–20Cr–20Si–5Al alloy at high temperatures ranging from 700 to 1400 °C. Pesting occurred at 700 °C while internal oxidation took place at 1300 and 1400 °C where Al₂O₃ initiated at the interface between NbCr₂ and Nb₉Cr₃Si₂ phases. Phases present were Nb₅Si₃, NbCr₂, Nb solid solution and Nb₉Cr₃Si₂ depending on the temperature. The aluminium content on each of the phases was analyzed. Al content in Nb₉Si₂Cr₃ has been found to be as high as 5–6 atomic percent. SEM, EDS and XRD techniques were utilized in order to characterize the specimens.     

Structural and compositional transformations in the near-surface layers of Fe–Cr based steels exposed to lithium – Effect of alloying and corrosion-assisted substructure coarsening

The corrosion behaviour of RAF/M RUSFER-EK-181 and F/M (EP-823, SUS410) steels was investigated in static “pure” (0.004 wt.% N) and N-added Li (0.5 wt.% N) at 600 °C up to 750 h. The weight losses of samples increased with time and nitrogen content in Li. The corrosion-assisted coarsening was observed in the near-surface layers of RAF/M steels. It is caused by the dissolution of Cr and decomposition of carbides during exposure to both melts. The coarsening mechanism and the influence of alloying on the structural and compositional transformations in steels exposed to Li are discussed.