Oxidation behaviour of Ti–Al–C films composed mainly of a Ti2AlC phase

This paper addresses the oxidation behaviour of Ti–Al–C films composed mainly of a Ti₂AlC phase. The films exhibited rather low oxidation rates at 600 and 700 °C, with an oxygen-rich zone or a thin oxide layer appearing on the film surfaces. Much faster oxidation rates were observed at 800 and 900 °C. The Ti₂AlC phase was quickly consumed by oxidation. From the film surface to the inner zone, TiO₂-rich layer, Al₂O₃-rich layer, and TiO₂ + Al₂O₃ mixed layer was observed, respectively. The oxidation mechanism of the Ti–Al–C film is discussed based on the experimental results.     

Novel multilayer structure design of metallic glass film deposited Mg alloy with superior mechanical properties and corrosion resistance

A novel multilayer structure of metallic glass film deposited Mg alloy was designed and fabricated by combining surface mechanical attrition treatment and magnetron sputtering. The multilayer structure consists of amorphous layer and gradient structure sequentially from surface, following with the coarse grains inside. Gradient structure significantly decreases hardness mismatch between the amorphous layer and the matrix of Mg alloy. Wear resistance is increased by 2 orders for this novel multilayer structure with the simultaneous improvement of corrosion resistance.     

Corrosion mechanism of Mo/Nd16Fe71B13/Mo film in a simulated marine atmosphere

This study describes the corrosion mechanism of Mo/Nd₁₆Fe₇₁B₁₃/Mo film induced by sodium chloride particles in 80% relative humidity (RH) environment. The deliquescence of sodium chloride particles on the Mo/Nd₁₆Fe₇₁B₁₃/Mo film caused the step by step attacks. Initial loosening of the Mo layer allows permeation of electrolyte into Nd–Fe–B layer, resulting in cavitations of electrolyte and subsequent film failure. The second failure step involves corrosion of Nd element in the Nd–Fe–B layer, with Fe element remaining beneath the corrosion product. Corrosion of Fe constitutes the third-step failure, forming a mixture of Nd and Fe corrosion product.     

Short-term oxidation resistance and degradation of Cr2AlC coating on M38G superalloy at 900–1100 °C

High temperature oxidation behavior of the Cr₂AlC coating was investigated at 900–1100 °C. During the oxidation, a continuous Al₂O₃ scale formed, resulting in the improvement of the oxidation resistance of the substrate. Meanwhile, the oxidation induced depletion of Al within the Cr₂AlC coating resulted in the transformation of Cr₂AlC to Cr–C phases. Compared with bulk Cr₂AlC, the Cr₂AlC coating possessed similar oxidation behavior, but with higher oxidation rate. This is because a great number of columnar grain boundaries existed in the as-deposited coating, through which oxygen and nitrogen could diffuse inwardly, resulting in the internal oxidation and nitridation.     

Electrolytic MgO/ZrO2 duplex-layer coating on AZ91D magnesium alloy for corrosion resistance

By a two-step fabrication process of electrolytic deposition and annealing treatment, an MgO/ZrO₂ duplex-layer coating has been prepared on AZ91D magnesium alloy as a protective film against corrosion. Owing to the chemical bonding formed after the condensation of precursory hydroxides, the adhesion strength, thickness and compactness of MgO coating on the substrate are significantly enhanced by the intermediate ZrO₂ layer which prevents the formation of corrosion product Mg₂(OH)₃Cl·4H₂O. As a result, the MgO/ZrO₂ duplex-layer coated specimen reveals relatively high corrosion resistance and superior stability in 3.5 wt% NaCl solution with respect to the MgO single-layer coated specimen.     

Magnetron-sputtered oxidation protection coatings for Mo–Si–B alloys

Oxidation protective Mo–70Al, Mo–37Si–15B and Mo–46Si–24B (at.%) coatings with 5–10 μm thickness were deposited on Mo–9Si–8B alloys by magnetron sputtering; and their oxidation behavior was studied at 800, 1000 and 1300 °C in air. On the Mo–70Al layer a dense aluminum borate scale grew at 800 °C; however, this coating rapidly degraded at 1000 °C linked to substrate oxidation at uncoated areas. The Mo–37Si–15B and Mo–46Si–24B layers provided oxidation protection to the Mo–Si–B alloy at 800 and 1000 °C for up to 100 h due to formation of a borosilicate scale. The latter coating was protective for short times even at 1300 °C.     

Influence of solid corrosion by-products on the consumption of dissolved oxygen in copper pipes

Research on corrosion of copper pipes has given little consideration to the influence of solid corrosion by-products on the processes occurring at the metal-liquid interface. Consequently, the effect of such solid phases on the rate of dissolved oxygen (DO) consumption remains poorly understood. In-situ experiments were performed in copper pipes under different carbonate concentrations and ageing times. Our results show that the amount of solid corrosion by-products and concentration of hydrogen ions affect the rate of DO consumption during stagnation. Furthermore, our findings support the existing hypothesis that the available concentration of hydrogen ions, rather than DO, is the limiting factor for copper release into drinking water.     

Microstructure and corrosion resistance of modified 2024 Al alloy using surface mechanical attrition treatment combined with microarc oxidation process

A top ceramic coating was fabricated on the surface mechanical attrition treatment (SMAT) modified nanocrystalline layer of 2024 Al alloy by microarc oxidation (MAO) process. The corrosion resistance of the SMAT-MAO composite coating was studied by EIS. The results show that SMAT-MAO composite coating with 10 μm top ceramic coating exhibited better corrosion resistance, while the SMAT-MAO coating with the thickness of 15 μm showed worse corrosion resistance compared with those simple MAO coatings with same thickness. The formation of a dense passive film at the damaged region caused by the bottom nanocrystalline interface contributed to the improved corrosion resistance property.     

Microstructure and pitting corrosion resistance of annealed duplex stainless steel

The transition from metastable to stable pitting was studied in 0.5 M NaCl water solution for two cast duplex stainless steels under different microstructural conditions achieved by annealing in the range from 900 °C to 1200 °C. The ensuing microstructural changes in heat treated steels were defined and correlated with established pitting potentials (E_p) and sites of corrosion damage initiation. The variations in E_p have been discussed in terms of secondary phases precipitation. The critical condition for pit stability was quantified and used to select an appropriate microstructural state, resulting in the higher potential at which stable pit growth is first observed.     

Oxidation of Ti2AlC bulk and spray deposited coatings

The oxidation behaviour of Ti₂AlC bulk and high velocity oxy-fuel spray deposited coatings has been investigated for temperatures up to 1200 °C. X-ray diffraction and electron microscopy show that bulk Ti₂AlC forms a continuous layer of α-Al₂O₃ below a layer of TiO₂ at temperatures as low as 700 °C. Oxidation of the Ti₂AlC coatings is more complex, and also involves the phases Ti₃AlC₂, TiC, and Ti_xAl_y, formed during the spraying process. α-Al₂O₃ is observed, however, it is unevenly distributed deep into the material, and does not form a continuous layer essential for good oxidation resistance.     

Characterisation and corrosion resistance of the electrodeposited hydroxyapatite and bovine serum albumin/hydroxyapatite films on Ti–6Al–4V–1Zr alloy surface

A new titanium base Ti–6Al–4V–1Zr alloy covered with hydroxyapatite or bovine serum albumin/hydroxyapatite was characterized in this paper in order to be used as implant material. Following techniques were used: linear polarization, electrochemical impedance spectroscopy, scanning electronic microscopy, Fourier transform infrared spectroscopy and atomic force microscopy. For HA or BSA/HA covered alloy, the electric equivalent circuit contains two time constants (for the passive film and for coatings). The resistance of the protective films increased in time and BSA/HA coating was slightly rougher than HA coating, this situation being favourably to the cell adhesion.     

Modelling the corrosion behaviour of Al2CuMg coarse particles in copper-rich aluminium alloys

The corrosion behaviour of 2024 aluminium alloy in sulphate solutions was studied; attention was focused on the influence of coarse intermetallic Al₂CuMg particles on the corrosion resistance of the alloy. Model alloys representative of the aluminium matrix and of Al₂CuMg coarse intermetallics were synthesized by magnetron sputtering. Open-circuit potential measurements, current-potential curve plotting and galvanic coupling tests were performed in sulphate solutions with or without chlorides. Further explanations were deduced from the study of the passive films grown on model alloys in sulphate solutions. The results showed that model alloys are a powerful tool to study the corrosion behaviour of aluminium alloys.     

Comparison of the oxidation of high-sulfur Ni-25Cr-5Al alloys in as-cast and as-sputtered states

To understand the effect of sulfur on the oxidation of nanocrystalline (NC) alloys, a high-sulfur alloy having a chemical composition similar to a coarse-grained (CG) cast alloy of Ni-25Cr-5Al-1S (wt.%) was fabricated using magnetron sputtering. The oxidation of the two alloys in isothermal and cyclic conditions in air at 1000 °C shows that the alumina scale formed on the cast alloy was susceptible to spallation, whereas the alumina scale on the sputtered alloy was intrinsically adhesive. The result indicates that the nanocrystallization of alloys helps to eliminate the detrimental “sulfur effect” on oxidation, through minimizing the segregation of sulfur to the scale/alloy interface.     

Pitting corrosion of 304ss nanocrystalline thin film

Pitting corrosion behavior of coarse crystalline (CC) 304ss and its nanocrystalline (NC) thin film have been investigated by electrochemical measurement and in situ AFM observation in 3.5% NaCl solution. Results show two effects of nanocrystallization on pitting corrosion behavior: (1) more frequent occurrence of metastable pits, but with lower probability of transition to stable pits, which is attributable to differences in morphologies of sulfur and manganese as well as outstanding repassivation ability of NC thin film; (2) nanocrystallization decreases stable pit generation rate and its propensity to form larger pit cavities, and modifies the morphology of stable pit cavity.     

High temperature corrosion behaviour of a gradient NiCoCrAlYSi coating I: Microstructure evolution

A gradient NiCoCrAlYSi coating was prepared by a combined method of arc ion plating and subsequent diffusion treatment. The microstructure of the gradient coating was investigated in the as-annealed condition and after oxidation at 1000 °C. During the oxidation, an in situ diffusion barrier composed of Cr(W)-rich σ phases was formed in the interdiffusion zone, resulting in an effective inhibition of inward diffusion of Al. The formation of the in situ diffusion barrier layer is also discussed.     

Corrosion characteristics of LiBH4 film exposed to a CO2/H2O/O2/N2 mixture

LiBH₄ films were prepared by pulsed laser deposition using a LiB target in a background pressure of hydrogen. The corrosion characteristics of LiBH₄ films were measured by exposing them to a gas mixture of CO₂/H₂O/O₂/N₂ at ambient temperature for 1–24 h. Scanning electron microscopy images show some cracks on the surface of corrosion films, which could act as easy paths for H₂O and CO₂ to further react with Li⁺ and B³⁺. The X-ray photoelectron spectroscopy results and theoretical analysis show that LiBH₄ tends to react with H₂O and CO₂ to form Li₂B₄O₇, Li₂CO₃ and LiOH during the corrosion process.     

Oxidation behavior of molten magnesium in atmospheres containing SO2

The microchemistry and morphology of the oxide layer formed on molten magnesium in atmospheres containing SO₂ were examined. Based on the results and the thermodynamic and kinetic calculations of oxide-growth process, a schematic oxidation mechanism is presented. The results showed that the oxide scales with network structure were generally composed of MgO, MgS, and MgSO₄ with different layers, depending on the SO₂ content, the time and the temperature. The formation of MgSO₄ was important for the formation of the protective oxide scales. The growth of the oxide scales followed the parabolic law at 973 K and was controlled by diffusion.     

The growth mechanism of CO2 corrosion product films

Carbon steel usually has high corrosion rate in water with dissolved CO₂ however the steel surface can be covered by protective corrosion product film. Therefore the carbon steel can be widely used in oil and gas industry. This paper discusses the structure, electrochemistry performances, initiation and growth of protective corrosion product films. A duplex layer structure of FeCO₃ films was found by electrochemical impedance spectrum (EIS). The physical and chemical situations of solution near carbon steel surface were investigated by ultraviolet spectrophotometry and chronopotentiometry. According to crystallography the growth mechanisms of protective CO₂ corrosion product films were discussed.     

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.     

High temperature oxidation of Fe-Al and Fe-Cr-Al alloys: The role of Cr as a chemically active element

Good high-temperature corrosion resistance of Fe-Al alloys in oxidizing environments is due to the α-Al₂O₃ film which is formed on the surface provided temperature is above 900 °C and the Al-content of the alloy exceeds the critical value. Ab initio calculations combined with experiments on Fe-13Al, Fe-18Al, Fe-23Al and Fe-10Cr-10Al alloys show that the beneficial effect of Cr on the oxidation resistance is significantly related to bulk effects. The comparison of experimental and calculated results indicates a clear correlation between the Fe-Cr chemical potential difference and the formation of the protective oxide scales.