Isothermal section at 1673 K of the Mo–Ru–Si diagram and crystallographic structures of ternary phases

Efforts to improve the high temperature behavior of MoSi₂ in oxidizing environments led to the investigation of the Mo–Ru–Si phase diagram. The isothermal section at 1673 K was determined by X-ray diffraction, optical and scanning electron microscopies and EPMA. Five new silicides were identified and their crystallographic structure was characterized using conventional and synchrotron X-ray as well as neutron powder diffraction. Mo₁₅Ru₃₅Si₅₀, denoted α-phase, is of FeSi-type structure, space group P2₁3, a=4.7535 (5) Å, D_x=7.90 g. cm⁻³, Bragg R=7.13. Mo₆₀Ru₃₀Si₁₀ is the ordered extension of the Mo₇₀Ru₃₀ σ-phase with space group P4₂/mnm, a=9.45940(8) Å, c=4.94273(5) Å, D_x=6.14 g. cm⁻³, Bragg R=5.75.     

The morphology and corrosion resistance of electrodeposited Co‐TiO2 nanocomposite coatings

Co‐TiO₂ nanocomposite coatings with various contents of TiO₂ nanoparticles were prepared by electrodeposition in Co sulfate plating bath containing TiO₂ nanoparticles. The influence of the TiO₂ nanoparticles concentration in the bath, of the current density and of sodium dodecyle sulfate (SDS) as anionic surfactant on the morphology, composition, texture, roughness, and microhardness of the coatings was investigated. The morphology and composition of coatings were studied by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The phase structure of coatings was analyzed by X‐ray diffraction (XRD). The results showed that the maximum codeposition of TiO₂ nanoparticles in Co matrix was around 4.5 vol% obtained in 60 g/L TiO₂ in the bath, 30 mA/cm² and 0.15 g/L SDS. The microhardness of coatings was increased up to 504 Hv by increasing TiO₂ concentration in the bath to 60 g/L TiO₂. The electrochemistry tests including potentiodynamic polarization and impedance spectroscopy revealed that by addition of TiO₂ into Co matrix, the corrosion current density, polarization resistance, and charge transfer resistance of Co‐TiO₂ coating were increased compared with Co coating.     

Molten glass corrosion resistance of MoxRuySiz compounds at 1350 °C in an alkali borosilicate glass

Corrosion of Mo-silicides and Mo_xRu_ySi_z compounds in molten glass is studied by electrochemical techniques at 1350 °C, metallographic observations (SEM) and chemical analysis (EPMA). The open circuit potential measurements (free-potential) enabled the determination of the reactions responsible for the degradation of the materials.The degradation of Mo-silicide occurs through selective oxidation of Si. Mo_xRu_ySi_z compounds were subjected to reactions leading to the successive oxidation of silicon and molybdenum. The progressive depletion of Si and Mo resulted in the more or less quick formation of a thick, porous and low adherent Ru-layer. Therefore, Ru-additions had no positive effect on the molten glass corrosion resistance of MoSi₂-based materials and should even accelerate the degradation kinetics of the later through galvanic coupling.     

Improvement of corrosion resistance in NaOH solution and glass forming ability of as-cast Mg-based bulk metallic glasses by microalloying

The influences of the addition of Ag on the glass forming ability (GFA) and corrosion behavior were investigated in the Mg-Ni-based alloy system by X-ray diffraction (XRD) and electrochemical polarization in 0.1 mol/L NaOH solution.Results shows that the GFA of the Mg-Ni-based BMGs can be improved dramatically by the addition of an appropriate amount of Ag;and the addition element Ag can improve the corrosion resistance of Mg-Ni-based bulk metallic glass.The large difference in atomic size and large negative mixing enthalpy in alloy system can contribute to the high GFA.The addition element Ag improves the forming speed and the stability of the passive film,which is helpful to decrease the passivation current density and to improve the corrosion resistance of Mg-Ni-based bulk metallic glass.     

Effect of current density on micro‐arc oxidation of Q235 steel and corrosion resistance in liquid Pb‐Bi

An attempt has been made to improve the corrosion resistance in liquid Pb‐Bi by micro‐arc oxidation (MAO), and the effects under different current densities on the corrosion resistance of the coatings were discussed. Scanning electron microscope, energy dispersive spectrometer, and X‐ray diffraction were used to analyze the surface morphology and phase constituents of the MAO coatings produced under different current densities. The corrosion resistance of the coatings was evaluated by studying the element changes and morphology evolution. The results show that the compactness of the ceramic coating decreases with the current density increasing. In contrast to the performance of matrix metal, the ceramic coating exhibited a much better corrosion resistance in liquid Pb‐Bi. Moreover, the ceramic coating produced under current density of 10 A/dm ² shows the best corrosion resistance.     

Ageing resistance and corrosion resistance of silicone‐epoxy and polyurethane topcoats used in sea splash zone

The silicone‐epoxy and polyurethane topcoats applied in a simulated sea splash environment were studied according to ISO20340 standard, and their properties were analyzed by differential scanning calorimetry (DSC), scanning electron microscopy (SEM), X‐ray photoelectron spectroscopy (XPS), and electrochemical impedance spectrum (EIS). Glass transition temperature (T_g) of silicone‐epoxy polymer is 7.7 °C higher than that of polyurethane polymer. Measurement of gloss and color difference demonstrates that anti‐ageing property of silicone‐epoxy topcoat is better than that of polyurethane topcoat. It is known from SEM and XPS analysis that silicone‐epoxy topcoat can retain the good morphological structure and ageing resistance after accelerated cycle ageing test. The results from EIS analysis indicate that the silicone‐epoxy topcoat has good anti‐corrosion properties. Measurement of contact angles indicates that surface tension of silicone‐epoxy topcoat is smaller than that of polyurethane topcoat before and after corrosion test.     

Role of chlorides in hot corrosion of a cast Fe–Cr–Ni alloy. Part II: thermochemical model studies

In many high temperature applications severe degradation of alloys is caused by thin deposits of molten salts, especially alkali metal sulfates, alkali metal chlorides and mixtures of these salts. Calculations of multi-component thermochemical equilibria in systems involving (initially) metal/salt/gas as a function of local oxygen activity can help identify the important hot corrosion reactions. Such calculations for four pure salts (KCl, NaCl, Na₂SO₄, and K₂SO₄) and a mixture of these salts in contact with an Fe–20%Cr alloy at 800 °C are presented. The results predict that the compositions of gas, oxide, (sulfide, when sulfate was input) and salt phases depend strongly upon the salt chosen and upon the local oxygen activity. In some cases the equilibrium salt composition is significantly changed by reactions with metal or oxide phases. The calculated results for the salt mixture were compared with experimental data from part I of this paper. The model calculations have led to the identification of two new factors that support faster hot corrosion rates in an alkali chloride + sulfate salt compared to that in alkali sulfate alone. First, alkali chlorides, unlike sulfates, support a continuous salt pathway from ambient to the metal/scale interface, allowing oxidant to be efficiently transported to oxidize metal. Secondly, under oxidizing conditions alkali chlorides have a higher solubility of dissolved Fe- and Cr-containing species than that in alkali sulfates. Both of these factors support higher transport rates, which according to the fluxing theory of hot corrosion will lead to faster corrosion.     

A new solid‐state mode of hot corrosion at temperatures below 700°C

Preliminary results on a single‐crystal nickel‐based superalloy indicated that hot corrosion can occur at temperatures as low as 550°C, where liquid formation, generally believed to be responsible for Type II hot corrosion, is not predicted. Additional tests were conducted on pure‐nickel samples at 650°C and below to more clearly elucidate the mechanism of this very low‐temperature hot corrosion. Environments in dry air and O₂‐(2.5, 10, 100, and 1000) ppm SO₂ were studied. Based on the results obtained, a solid‐state corrosion mechanism was inferred. The mechanism relies on the formation of a previously unreported compound phase, which was identified using transmission electron microscope analysis that indicated the stoichiometry of Na₂Ni₂SO₅. Furthermore, it was nanocrystalline in structure and metastable. It was deduced that the Na₂Ni₂SO₅ formation was responsible for the rapid nickel transport required for the observed accelerated corrosion process. Moreover, its eventual decomposition resulted in a mixed product of porous NiO with embedded particles of Na₂SO₄. Application of the proposed mechanism to nickel‐based alloys is discussed.