Oxide networks, graph theory, and the passivity of Ni–Cr–Mo ternary alloys

The passivity of Ni–Cr–Mo ternary alloys is considered in terms of a continuous network of –Cr–O–Cr– bridges in the oxide film (which also contains Ni²⁺ and Mo⁴⁺ ions), or in terms of a continuous network of –Ni–O–Ni– bridges in the oxide film (which also contains Cr³⁺ and Mo⁴⁺ ions). The structure of the oxide is represented by a mathematical graph, and graph theory is used to calculate the connectivity of the oxide. This approach shows that a continuous network of –Cr–O–Cr– bridges occurs when the cation fraction of Cr³⁺ ions in the oxide is 0.35 or greater. A continuous network of –Ni–O–Ni– bridges occurs when the cation fraction of Ni²⁺ ions in the oxide is 0.55 or greater. Experimental results support the –Cr–O–Cr– model.     

Comparison of the oxidation behaviour of WC–Co and WC–Ni–Co–Cr cemented carbides

Thermogravimetric experiments confirm that the oxidation resistance of WC–Co and WC–Ni–Co–Cr alloys increases with their metallic content. This is due to the fact that, as the metallic content increases, the oxide scales present higher MWO₄ to WO₃ ratios and lower porosity. The good correlation found between the activation energies calculated by either the isothermal or the isoconversion method suggests that oxidation is controlled by surface chemical reaction. Activation energies increase with temperature between 650 and 800 °C for both WC–Co and WC–Ni–Co–Cr alloys. This increment is higher for WC–Co materials due to their tendency to form scales with higher tungstate contents.     

Selective corrosion of 550 °C aged Cu10Ni–3Al–1.3Fe alloy in NaCl aqueous solution

This work studied the electrochemical behavior of a solution treated or 550 °C aged Cu10Ni–3Al–1.3Fe alloy, in 0.01 M NaCl aqueous solution, through potentiodynamic polarization in both stagnant condition or under erosion process. Results showed the occurrence of a passivity break potential (E_pb), related to the beginning of the denickelification process, which occurred as a localized attack under stagnant electrolyte. Under erosion conditions localized denickelification was not observed, despite of the presence of E_pb. This could indicate that selective corrosion of Ni, which caused the observed E_pb, occurred as a dissolution–redeposition process, with removal of the Cu deposits during erosion process.     

Effects of bovine serum albumin on the corrosion behaviour of AISI 316L, Co–28Cr–6Mo, and Ti–6Al–4V alloys in phosphate buffered saline solutions

The corrosion behaviour of AISI 316L, wrought Co–28Cr–6Mo and Ti–6Al–4V was studied in aerated solutions of phosphate buffered saline (PBS) at various concentrations of bovine serum albumin (BSA) at 37 °C. Open circuit potential, potentiodynamic polarization, linear polarization resistance (LPR) and electrochemical impedance spectroscopy (EIS) experiments along with X-ray photoelectron spectroscopy (XPS) on Co–28Cr–6Mo oxide layer were conducted to study the interaction of BSA and passive layers and to measure the corrosion rates. Ti–6Al–4V alloy had the lowest corrosion rate and the highest breakdown potential. It was shown that BSA has enhanced the alloy passive film stability at higher concentrations.     

Hot corrosion of a novel (Ni,Co)O/(Ni,Co)Fe2O4 composite coating thermally converted from an electrodeposited Ni–Co–Fe2O3 composite coating

Ni–Co–Fe₂O₃ composite coatings were successfully developed by sediment co-deposition. In order to improve their hot corrosion resistance, a pre-oxidation treatment was conducted at 1000 °C for 6 h. The corrosion behaviour of the oxidised composite coating was investigated at 960 °C in an atmosphere consisting of a mixture of Na₃AlF₆–AlF₃–CaF molten salts and air. They exhibited good hot corrosion resistance due to not only the pre-formed oxide scale with (Ni,Co)O and (Ni,Co)Fe₂O₄ phases after pre-oxidation, but also the formation of (Ni,Co,Fe)Al₂O₄ phases in the outer layer and a well-distributed NiFe₂O₄-enriched phase along the grain boundaries in the subscale area during the corrosion process.     

Interfacial reactions between molten Al and a Co–Cr–Mo alloy with and without oxidation treatment

The effect of the formation of an oxide layer on the interfacial reactions between a Co–Cr–Mo alloy (CCM alloy) and molten Al is investigated. In the absence of an oxide layer, the CCM alloy reacted rapidly and an interfacial two-layer structure formed between the CCM alloy and molten Al. Upon oxidation, a uniform (Cr,M)₂O₃ (M: doped elements) oxide layer formed on the CCM alloy surface, which effectively inhibited the reaction between the CCM alloy matrix and molten Al for more than 2 h.     

Hot corrosion of a novel NiO/NiFe2O4 composite coating thermally converted from the electroplated Ni–Fe alloy

A novel NiO/NiFe₂O₄ composite coating thermally converted from an electroplated Ni–Fe alloy was successfully fabricated. The composite coating consisted of a NiO matrix and homogeneously distributed criss-cross intragranular and intergranular NiFe₂O₄ precipitates, with a very dense and flat structure. The composite, compared to bare Ni metal, exhibited increased hot corrosion resistance under an atmosphere of Na₃AlF₆–AlF₃–CaF molten salts and air at 960 °C, mainly because of the dense structure and well-adhered, homogeneously dispersed intragranular and intergranular NiFe₂O₄ precipitates.     

Microbial corrosion of Ni–Cu alloys by Aeromonas eucrenophila bacterium

The corrosion behavior of Ni–Cu alloy in Nutrient Broth medium (peptone from meat 5.0 g, meat extract (N source) 3.0 g, and sodium chloride 6.0 g in 1 L) and in the presence of Aeromonas eucrenophila, isolated from water treatment system, has been studied using electrochemical techniques. Results showed that the polarization curves exhibiting a cathodic shift of the corrosion potential confirmed the increase in the corrosion rates. According to EIS data lower Q_CPE value for 5 h indicates surface inhomogeneity resulting from surface metal roughening, which is the likely cause of the enhanced dissolution of the metal also observed in SEM micrographs. In addition, the pH values of the medium did not change throughout the experiment; however, after immersion the pH value decreased to acidic value and caused corrosion.     

High temperature oxidation behaviour of Ni–Fe–Co anodes for aluminium electrolysis

The oxidation behaviour of ternary Ni_xFe_yCo_z alloys (where x/y (wt) = 1 and 1.85; z = 0, 10, 30 and 50 wt.%) was studied in air at 800 °C. Alloys were found to follow complex oxidation kinetics, with the highest oxidation rates observed for alloys having 50 wt.% Co. Significant improvements in oxidation resistance were achieved by addition of 10 and 30 wt.% Co to the Ni–Fe system. The decrease in oxidation rate was associated with suppression of Fe₂O₃ formation in preference for (Co,Ni)_xFe_3−xO₄ growth. The results were discussed in light of the materials requirements for inert anodes for aluminium electrolysis.     

Passivation of Al–Cr–Fe and Al–Cu–Fe–Cr complex metallic alloys in 1 M H2SO4 and 1 M NaOH solutions

The corrosion mechanisms of Al–Cr–Fe and Al–Cu–Fe–Cr complex metallic alloys have been investigated by potentiodynamic and potentiostatic polarization. Very good passivation of the Al–Cr–Fe surface is exhibited from 1 M H₂SO₄ to 1 M NaOH solutions, which was confirmed by ICP-OES analysis over a period of 273 days. Potentiostatically formed passive films analysed by XPS revealed chromium enrichment in the outermost layer of the aluminium oxy-hydroxide film. Although Al–Cu–Fe–Cr showed passivation during potentiodynamic polarization, heavy active corrosion at OCP was revealed by ICP-OES. For the Al–Cu–Fe–Cr alloy, the 10% content of Cr is insufficient to maintain a protective “chemically stable” Cr oxide/hydroxide.     

Synthesis and properties of open-cell Ni–Cr–Fe–Al alloy foams by pack co-deposition process

A pack cementation process for the co-deposition of Cr, Fe and Al onto open-cell nickel foam was developed. The reticulated open-cell Ni–Cr–Fe–Al foams were annealed to homogenize the material with 18.8 wt.% Cr, 11.3 wt.% Fe and 7.7 wt.% Al. The microstructure and phase composition of the Ni–Cr–Fe–Al foams were examined by scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive analysis (EDS). The results show that the coating is uniform and dense along the perimeter of the Ni strut, and consists of three layers: a Cr–Fe outer layer, an inner layer containing mostly Al and a transition zone. After homogenization annealing, the alloyed foams retain the hollow struts structure of the original pure nickel foams and the low relative densities. The Ni–Cr–Fe–Al alloy foams exhibit enhancement in absolute strength as compared to the pure nickel and Ni–35.2Cr foams. Furthermore, the Ni–Cr–Fe–Al alloy foams show excellent oxidation resistance and outperform the chromia-forming Ni–35.2Cr alloy foam after oxidation at 900 and 1000 °C, which is mainly due to its high aluminum and chromium content leading to the formation of a continuous and adherent duplex oxide layer.     

Phase stability,oxidation, and interdiffusion of a novel Ni−Cr−Al−Ti−Si bond-coating alloy between 900 and 1100°C

A novel, low-expansion experimental Ni–Cr–Al–Ti–Si bond-coating alloy was investigated in the as-cast state concerning its phase stability, oxidation resistance in air, and interdiffusion with single-crystal IN-100 at 900, 1000, and 1100°C. Isothermal oxidative thermogravimetry was employed up to 500 hr. Interdiffusion was compared to a commercial Ni–Co–Cr–Al–Y alloy on IN-100. Oxidized Ni–Cr–Al–Ti–Si specimens and diffusion couples were characterized by metallography, SEM, EDX, XRD, and XRF. The Ni–Cr–Al–Ti–Si alloy provides good oxidation resistance in air at least up to 1000°C. The alloy is an alumina former. Due to its coarse microstructure, other oxides (e.g., rutile) may form and considerably dominate the oxidation behavior. The kinetics of oxidation were correlated with temperature, formation of phases, and morphology of oxides. Interdiffusion fluxes between Ni–Cr–Al–Ti–Si and IN-100 were mainly directed to the superalloy. They were faster than in Ni–Co–Cr–Al–Y/IN-100 diffusion couples.     

Semiconducting properties of passive films formed on electroplated Ni and Ni–Co alloys

The semiconducting properties of the passive films on electroplated Ni and Ni–Co alloys were investigated using Mott–Schottky analysis. The cyclic voltammetry (CV) behaviors of Ni indicate that the film thickness will not increase unless the applied potential is higher than 0.504 V (vs. HgO/Hg). The passivity samples treated with CV reveal their behaviors are that of a p-type semiconductor. The three semiconductive parameters, the acceptor density (N_A), the flatband potential (E_fb) and the thickness of the space charge layer (L_sc) were calculated from the Mott–Schottky plots, respectively. The relationship between the parameters and the protective characters against corrosion was analyzed.     

Internal oxidation and phase transformations of multi-phase Fe–Ni–Al and Fe–Ni–Al–Cr alloys induced by KCl corrosion

The corrosion of two multiphase Fe–Ni–Al and Fe–Ni–Al–Cr alloys is studied at 650 °C in KCl-contaminated air. The oxidation rate of the alloys in air alone is low. When KCl is introduced, the corrosion is accelerated, producing a thick external scale of iron oxides, an intermediate layer of spinel, and a region of internal oxidation of Al. Potassium chromate is detected on Fe–Ni–Al–Cr surface that accounts for the degradation of protective chromia. An Al-depleted single phase region is observed in the front of the internal oxidation, due to the selective consumption of Al via an “active oxidation” process.     

STUDY ON HOT CORROSION RESISTANCE OF A NEW DIRECTIONAL SOLIDIFICATION Ni–BASED SUPERALLOY

研究了新型定向凝固镍基高温合金DZ68的抗热腐蚀性能, 并与K438合金进行了比较. 结果表明: 热处理态DZ68合金组织中几乎没有(γ+γ') 共晶, 碳化物尺寸小,其整体组织比较均匀; 在热腐蚀过程中发生比较均匀的腐蚀, 其外腐蚀层的腐蚀产物主要是(Ni, Co)Cr₂O₄, 内腐蚀层的腐蚀产物主要是Al₂O₃. 热处理态K438合金组织中存在较多的 (γ+γ'共晶和数量较多、尺寸较大的长条状碳化物, 组织均 匀性较差; 在热腐蚀过程中发生不均匀腐蚀, 其外腐蚀层的腐蚀产物主要是NiO, 内腐蚀层的腐蚀产物主要是CrS.两种合金中Ti元素的偏析有促进其它元素偏析的倾向, 使合金组织的均匀性恶化,热腐蚀均匀性变差. 在本实验条件下, DZ68合金的抗热腐蚀性能略好于K438合金.     

Effect of an Enamel Coating on the Oxidation and Hot Corrosion Behavior of an HVOF-Sprayed Co–Ni–Cr–Al–Y Coating

The oxidation and hot-corrosion behavior of a Co–Ni–Cr–Al–Y coating produced by high-velocity oxygen fuel (HVOF) with and without an enamel coating were investigated in air at 900°C and in molten 75 wt.% NaCl+25 wt.% Na₂SO₄ at 850°C. The results show that the enamel coating possesses excellent hot corrosion resistance in the molten salt, in comparison with the HVOF-sprayed Co–Ni–Cr–Al–Y coating alone. In the hot-corrosion test, breakaway corrosion did not occur on the samples with the enamel coating and the composition of the enamel did not significantly change. The oxidation resistance of the Co–Ni–Cr–Al–Y coating, which had good adhesion, was also improved by the enamel coating.     

Microstructure and mechanical properties of a directionally solidified and aged intermetallic Ni–Al–Cr–Ti alloy with β-γ′-γ-α structure

Microstructure and mechanical properties were investigated in a directionally solidified (DS) Ni–21.7Al–7.5Cr–6.5Ti (at.%) alloy. The dendrites of the as-grown alloy were composed of β(B2)-matrix (NiAl), coarse γ′(L1₂)-particles (Ni₃Al), fine γ′-needles and spherical α(A2)-precipitates (Cr-based solid solution). The majority of fine γ′-precipitates was found to be twinned. The interdendritic region contained γ(A1)-matrix (Ni-based solid solution) separating ordered domains of γ′-phase and fine lath-shaped α-precipitates. Ageing in the temperature range 973–1373 K decreased the volume fraction of dendrites from about 50 vol.% measured in the as-grown material to about 38 vol.% in the material aged at 1373 K for 300 h. During ageing in the temperature range 973–1273 K the γ-phase transformed to the γ′-phase in the interdendritic region. This transformation was connected with precipitation of lath-shaped α-precipitates. Ageing at higher temperatures of 1373 and 1473 K resulted in stabilisation of the γ-phase and precipitation of spherical γ′-particles in the interdendritic region. Ageing at 973 K significantly increased the microhardness, hardness and decreased room-temperature tensile ductility. Neither ageing nor finer dendritic microstructure were found to be effective in increasing the ductility of the alloy. The measured tensile ductility up to 1.1% can be attributed to the effect of extrinsic toughening mechanisms operating in the β-phase such as blunting and bridging of cracks by the α- and γ′-precipitates.     

Corrosion of nanocrystalline Ni–W alloys in alkaline and acidic 3.5 wt.% NaCl solutions

The corrosion behaviour of reverse-pulse electrodeposited nanocrystalline nickel tungsten alloys (nc Ni–W) in pH 3 and 10 3.5 wt.% NaCl solutions is investigated and analysed as a function of grain size. A potentiodynamic polarisation study reveals that the corrosion rate of nc Ni–W generally increases with the reduction of grain size in alkaline condition, but decreases with the reduction of grain size in acidic environment. Furthermore, for both environments, nc Ni–W alloys exhibit superior localised corrosion resistance than a microcrystalline Ni control specimen. Factors controlling the corrosion behaviour of these materials, including grain size, tungsten content, passivation and crystallographic texture are addressed.     

Electrochemical behaviour of Ti–6Al–4V alloy and Ti in azide and halide solutions

Electrochemical techniques including open circuit potential measurement, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were used to evaluate the corrosion and passivation behaviour of Ti–6Al–4V alloy in sodium azide (NaN₃) solutions compared to the behaviour of its pure base metal Ti. The results showed that increasing azide concentration increases the rate of corrosion (i_corr) and shifts negatively the rest potential (E_f), as well as decreases the spontaneous thickening rates of the inner and outer layers constituting the passive oxide film on each sample. These effects are more accentuated for the alloy than for the metal. Moreover, the electrical resistance (R_ox) and the relative thickness (1/C_ox) of the oxide films on the two samples exhibit an almost linear decrease vs. NaN₃ concentration. The results suggested that addition of Al and V to Ti, although improves its machinability, yet it decreases the performance of its surface oxide film to protect the degradation of the metal. The alloy was found to be more susceptible to corrosion than its base metal, since Ti expresses higher apparent activation energy (E_a) for the corrosion process than Ti–6Al–4V. Electrochemical behaviour of Ti in azide medium was also compared with that in various halide solutions. It was found that Ti has a stronger propensity to form spontaneous passivating oxide layers in bromide more than in azide and other halide media, where the positive shift in the value of E_f and the simultaneous increase in the oxide film resistance (R_ox) follow the sequence: Br⁻ > > Cl⁻ > I⁻ > F⁻.     

Effect of B and Cr on the high temperature oxidation behaviour of novel γ/γ′-strengthened Co-base superalloys

Isothermal oxidation was carried out on new γ/γ′-strengthened Co-base superalloys of the system Co–Al–W–B and Co–Al–W–B–Cr at 800 and 900 °C in air. ToF-SIMS investigations show that boron accumulates in the inner oxide layer which improves the oxidation behaviour and oxide layer adhesion considerably. B-rich precipitations can be observed in the alloy matrix, as well. Appropriate amounts of chromium as additional alloying element lead to further improvement of the oxidation resistance due to the formation of protective inner Al₂O₃ and Cr₂O₃ scales.