Electrochemical behavior of Ni3Al-based intermetallic alloys in NaOH

The corrosion behavior of Ni₃Al-based intermetallic alloys in a 0.5 M NaOH solution was studied at 25 °C. The open circuit potential, cathodic and anodic potentiodynamic polarization, Tafel plots and linear polarization resistance measurements were used to characterize the corrosion behavior. For the Ni₃Al(B, Zr) alloy, potentiodynamic polarization curves showed a wide passive region that can be found between about ?0.220 V_SCE and 0.520 V_SCE. On the other hand, a narrow passive region, in the range of potentials from about ?0.180 V_SCE to 0.180 V_SCE, was observed for the Ni₃Al(B, Zr, Cr, Mo) alloy. Chromium, as an alloying element in the Ni₃Al(B, Zr, Cr, Mo) alloy, contributes to transpassive dissolution of the passive film at much lower anodic potentials and remarkably reduces the passivation region. The experiments indicated also that damaged passive films on alloys repairs itself and pits do not initiate. The surface of both alloys and passive films possess extremely high corrosion resistance in a studied solution. However, Tafel and linear polarization tests revealed that freshly exposed surfaces of the Ni₃Al(B, Zr) alloy exhibited better corrosion resistances than the Ni₃Al(B, Zr, Cr, Mo) alloy. Both methods, used for the determination of corrosion rates gave very similar results. The calculated corrosion rates are about 2.8 ·10^?3 and 6.0·10^?3 mm year^?1 for the Ni₃Al(B, Zr) alloy and B, respectively.     

The passivation behaviour of copper-containing high-performance ferritic stainless steels in the cathode environment of polymer electrolyte membrane fuel cells

The passivation behaviour of copper-containing high-performance ferritic stainless steels in the cathode environment of polymer electrolyte membrane fuel cells was investigated using electrochemical tests and XPS surface analyses. The addition of copper to the alloy deteriorated the corrosion resistance at a passive region of 0.6 V_SCE. With an increase of the copper content, the passivity of the alloy degraded due to an increase of the formation of Cr-containing inclusions, and a decrease of the Cr oxide and hydroxide in the passive film.     

Electrochemical corrosion behaviors of amorphous Co<Subscript>69</Subscript>Fe<Subscript>4.5</Subscript>Ni<Subscript>1.5</Subscript>Si<Subscript>10</Subscript>B<Subscript>15</Subscript> alloy

The corrosion behavior of an amorphous Co₆₉Fe_4.5Ni_1.5Si₁₀B₁₅ alloy ribbon was examined as a function of solution temperature (15 °C to 55 °C) and pH (3 to 11). The results of potentiodynamic polarization tests in H2SO4 solution, NaCl solution, and HCl + NaOH solution at various levels of pH showed that the corrosion resistance for the alloy ribbon significantly deteriorated with increasing temperature and decreasing pH for given conditions. The Co₆₉Fe_4.5Ni_1.5Si₁₀B₁₅ alloy was actively dissolved in solutions at pH 3 to 9 but passivated in a solution at pH 11. By comparison of the corrosion behaviors of Co₆₉Fe_4.5(Nb,Cr,Ni)_1.5Si₁₀B₁₅ alloys in the solution at pH 11, Ni was considered to contribute less in improving the corrosion resistance of the alloy than did Cr and Nb.     

Phase Diagram of the Quaternary Al(NO<Subscript>3</Subscript>)<Subscript>3</Subscript>-Cu(NO<Subscript>3</Subscript>)<Subscript>2</Subscript>-Zn(NO<Subscript>3</Subscript>)<Subscript>2</Subscript>-H<Subscript>2</Subscript>O System

The phase diagram of the H₂O-Zn(NO₃)₂-Al(NO₃)₃-Cu(NO₃)₂ quaternary system at 30 °C has been established by using the conductivity measurements. The solid-liquid equilibria of the H₂O-Zn(NO₃)₂-Al(NO₃)₃, H₂O-Zn(NO₃)₂-Cu(NO₃)₂, H₂O-Al(NO₃)₃-Cu(NO₃)₂ ternary systems and two isoplethic sections were determined experimentally. The solid phases in equilibrium with the saturated solution are the tri- and hemipentahydrate of copper nitrate, the hexahydrate α and β of the zinc nitrate and the nonahydrate of aluminum nitrate. The copper and zinc nitrates are relatively soluble in opposition to the aluminum nitrate which presents some important precipitation domains.     

Cr doping effect in B-site of La<Subscript>0.75</Subscript>Sr<Subscript>0.25</Subscript>MnO<Subscript>3</Subscript> on its phase stability and performance as an SOFC anode

La_0.75Sr_0.25Cr _y Mn_1−y O₃ (LSCM) (y = 0.0–0.6) composite oxides were synthesized by a complexing process of combining ethylene diamine tetraacetic acid (EDTA) and citrate. X-ray diffraction (XRD), temperature-programmed reduction, electrical conductivity, I–V polarization, and impedance spectroscopy were conducted to investigate the Cr doping effect of La_0.75Sr_0.25MnO₃ on its phase stability and electrochemical performance as a solid-oxide fuel cell (SOFC) anode. The chemical and structural stabilities of the oxides increased steadily with increasing Cr doping concentration, while the electrical conductivity decreased on the contrary. At y ≥ 0.4, the basic perovskite structure under the anode operating condition was sustained. a cell with 0.5-mm-thick scandia-stabilized zirconia electrolyte and La_0.75Sr_0.25Cr _y Mn_1−y O₃ anode delivered a power density of ∼15 mW·cm⁻² at 850°C.     

Oxidation Behavior of NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript> Spinel-Coated Interconnects in Wet Air

Corrosion of boilers and heat exchangers is accelerated in the presence of vanadium, sodium, and sulfur from low-grade fuels. Several iron- and nickel-based alloys were immersed in 60 mol% V₂O₅–40Na₂SO₄ salt for 1000 h in order to investigate their degradation behavior at 600 °C in air. Materials performance was analyzed by means of substrate recession rate and metallographic characterization. Their corrosion mechanism is characterized by the formation of a sulfide/oxide layer adjacent to the metal, the dissolution of scale oxides in the molten deposit, and their precipitation near the outer surface of the deposit. High Ni- and Cr-containing alloys show the lowest metal loss rates. Al addition was detrimental due to low-melting eutectic AlVO₄–V₂O₅ formation. Fe–Cr-based alloys showed the highest metal loss rates. In such alloys, high Cr additions (above 20%) did not improve the performance due to the negative synergetic effect by simultaneous dissolution of Fe₂O₃ and Cr₂O₃. The predominant salt composition at the corrosion front varied from vanadate rich to sulfate rich during the exposure. This change in the attacking salt makes it difficult to find a protective material for mixed sulfate–vanadate-induced corrosion.     

Improvement on the pitting corrosion resistance of 304 stainless steel via duplex passivation treatment

A remarkable improvement in the pitting corrosion resistance of 304 stainless steel was attempted using a novel duplex passivation treatment method. First, chemical passivation in nitric acid followed electrochemical passivation via potential polarization of step cycling in sodium nitrate electrolyte. Compared with traditional chemical passivation, breakdown potential was increased from 0.31 V_SCE to positive than 0.9 V_SCE at 70°C in a solution bearing 0.6 M [Cl^?] concentration. The critical pitting temperature was enhanced from 21.5°C to above 70°C in a solution with 6 M [Cl^?] concentration. Impedance analysis and X‐ray photoelectron spectroscopy results show that a more compact passive film with a higher ratio of chromium oxide on iron oxide was achieved by electrochemical passivation compared with chemical passivation. Morphology observation suggested that the potential polarization of step cycling slightly increased the dissolution of inclusions after being subjected to chemical passivation. The probable reason for the improvement on pitting resistance is discussed in detail based on inclusion dissolution and the protectiveness in passive film.     

Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>-Deposit-Induced Corrosion of Mo-Containing Alloys

Disk alloys used in advanced gas turbine engines often contain significant amounts of Mo (2 wt% or greater), which is known to cause corrosion under Type I hot corrosion conditions (at temperatures around 900 °C) due to alloy-induced acidic fluxing. The corrosion resistance of several model and commercial Ni-based disk alloys with different amounts of Mo with and without Na₂SO₄ deposit was examined at 700 °C in air and in SO₂-containing atmospheres. When coated with Na₂SO₄ those alloys with 2 wt% or more Mo showed degradation products similar to those observed previously in Mo-containing alloys, which undergo alloy-induced acidic fluxing Type I hot corrosion even though the temperatures used in the present study were in the Type II hot corrosion range. Extensive degradation was observed even after exposure in air. The reason for the observed degradation is the formation of sodium molybdate. Transient molybdenum oxide reacts with the sodium sulfate deposit to form sodium molybdate which is molten at the temperature of study, i.e., 700 °C, and results in a highly acidic melt at the salt alloy interface. This provides a negative solubility gradient for the oxides of the alloying elements, which results in continuous fluxing of otherwise protective oxides.     

Influences of Cr/Ni equivalent ratios of filler wires on pitting corrosion and ductility-dip cracking of AISI 316L weld metals

To study the pitting corrosion of AISI 316L weld metals according to the chromium/nickel equivalent ratio (Cr_eq/Ni_eq ratio), three filler wires were newly designed for the flux-cored arc welding process. The weld metal with delta-ferrite at less than 3 vol.%, was observed for ductility-dip cracking (DDC) in the reheated region after multi-pass welding. The tensile strength and yield strength increased with increasing Cr_eq/Ni_eq ratio. The result of anodic polarization tests in a 0.1 M NaCl solution at the room temperature (25) for 45 min, revealed that the base metal and weld metals have a similar corrosion potential of −0.34 V_SCE. The weld metal with the highest content of Cr had the highest pitting potential (0.39 V_SCE) and the passivation range (0.64 V_SCE) was higher than the base metal (0.21 V_SCE and 0.46 V_SCE, respectively). Adding 0.001 M Na₂S to the 0.1M NaCl solution, the corrosion occurred more severely by H2S. The corrosion potentials of the base metal and three weld metals decreased to −1.0 V_SCE. DDC caused the decrease of the pitting potential by inducing a locally intense corrosion attack around the crack openings.     

Phase Equilibria in the System Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-CaO-CoO and Gibbs Energy of Formation of Ca<Subscript>3</Subscript>CoAl<Subscript>4</Subscript>O<Subscript>10</Subscript>

An isothermal section of the system Al₂O₃-CaO-CoO at 1500 K has been established by equilibrating 22 samples of different compositions at high temperature and phase identification by optical and scanning electron microscopy, X-ray diffraction, and energy dispersive spectroscopy after quenching to room temperature. Only one quaternary oxide, Ca₃CoAl₄O₁₀, was identified inside the ternary triangle. Based on the phase relations, a solid-state electrochemical cell was designed to measure the Gibbs energy of formation of Ca₃CoAl₄O₁₀ in the temperature range from 1150 to 1500 K. Calcia-stabilized zirconia was used as the solid electrolyte and a mixture of Co + CoO as the reference electrode. The cell can be represented as: From the emf of the cell, the standard Gibbs energy change for the Ca₃CoAl₄O₁₀ formation reaction, CoO + 3/5CaAl₂O₄ + 1/5Ca₁₂Al₁₄O₃₃ → Ca₃CoAl₄O₁₀, is obtained as a function of temperature: /J mol⁻¹ (±50) = −2673 + 0.289 (T/K). The standard Gibbs energy of formation of Ca₃CoAl₄O₁₀ from its component binary oxides, Al₂O₃, CaO, and CoO is derived as a function of temperature. The standard entropy and enthalpy of formation of Ca₃CoAl₄O₁₀ at 298.15 K are evaluated. Chemical potential diagrams for the system Al₂O₃-CaO-CoO at 1500 K are presented based on the results of this study and auxiliary information from the literature.     

Polarisationsmessungen an niedriglegierten Stählen in konzentrieten Nitratlösungen

Polarisation measurements on low alloy steels in concentrated nitrate solutions Current density potential curves were determined by potentiostatic measurements involving constant potentials and stepwise potential change for six low alloy steels in Ca(NO₃)₂ and NaNO₃ solutions. The instationary curves vary markedly with the polarisation technique and do not characterize the anodic behaviour of the materials. For Ca(NO₃)₂ solutions the polarisation measurements do not show any correlation with stress corrosion cracking behaviour. Similar results were also obtained in NaNO₃, solutions. Contrary to the situation in Ca(NO₃)₂ solutions, a potential range for resistance against intergranular corrosion types exists at about U_H = 0,5 V. This range is characterized by relatively high stationary current densities. For materials without this range of resistance, the stationary current densities are significantly smaller. The electrochemical properties of the system steel/nitrate solution are extremely complicated. The free corrosion potential varies between the potential ranges for passive and transpassive corrosion. For anodic polarisation, the potential ranges of susceptibility as well as resistance against intergranular corrosion types follow each other.     

Effect of yttria(Y<Subscript>2</Subscript>O<Subscript>3</Subscript>) coating for high temperature oxidation resistance of 9Cr-1Mo steel

This paper investigates the effect of yttria (Y₂O₃) coating on high temperature oxidation behaviour of low alloy 9 Cr–1Mo steel. The superficial coating is Y₂O₃ was prepared for experimental investigation. The isothermal corrosion study of uncoated and coated specimens was carried out in air oxidation environment at 973 K for 8 h. The corrosion rate and reaction kinetics were studied and the post corroded scales were characterized in SEM, EDS and XRD. The results clearly indicate that Y₂O₃ coated specimen improves the high temperature oxidation resistance than uncoated specimens. The improvement of oxidation resistance in presence of Y₂O₃ coating can be attributed to the changed mechanism of scale growth from outer cation migration to inner anion migration and enhancement of scale adhesion with the substrate. Further, enhancement of scale adhesion with the substrate in case of Y₂O₃ coating also improves the oxidation resistance. The detail mechanism of the oxidation of Y₂O₃ coated and uncoated specimen is further discussed in this paper.     

Effects of hydrogen on the anodic behavior of Alloy 690 at 60 °C

Polarization and electrochemical impedance spectroscopy (EIS) measurements, Mott-Schottky (M-S) analysis and X-ray photoelectron spectroscopy (XPS) were used to investigate the effects of hydrogen on the anodic behavior of a one-dimensionally (1D) 25% cold worked (CW) Alloy 690 thermal treated (TT) in a boric acid and sodium sulphate solution at 60 °C. The pre-hydrogen-charged specimen exhibited a higher anodic current than that of the uncharged specimen below the transpassive potential. The charged hydrogen can be trapped in the metal. Electrochemical impedance spectroscopy (EIS) showed that the resistance capacitance loop of the hydrogen-charged specimen was significantly smaller than that of the uncharged specimen. Mott-Schottky analyses indicated that the passive film formed on Alloy 690 at −0.2 V_SCE was an n-type semiconductor, with a p-n hetero-junction at 0.2 V_SCE. Charged hydrogen increased the carrier density and the thickness of the passive film both at −0.2 V_SCE and 0.2 V_SCE. The Ni/Cr ratio in the surface film decreased after hydrogen charging, indicating that charged hydrogen could enhance the oxide film growth by increasing the OH⁻ (O²⁻) concentrations through its reaction with vacancies.     

Mechanism for the degradation of MmNi<Subscript>3.9</Subscript>Co<Subscript>0.6</Subscript>Mn<Subscript>0.3</Subscript>Al<Subscript>0.2</Subscript> electrode and effects of additives on electrode degradation for Ni-MH secondary batteries

Electrode degradation can affect the lifetime and safety of Ni-MH secondary batteries. This study examined the factors responsible for the degradation of metal hydride (MH) electrodes. The charge-discharge characteristics and cycle life of an MmNi_3.9Co_0.6Mn_0.3Al_0.2 (Mm: misch metal) type MH electrode were examined in a cell with a KOH electrolyte. After the charge-discharge cycles, the surface morphology of the electrodes was analyzed to monitor the extent of degradation. Electrochemical impedance spectroscopy provided information on the conductivity of the electrode. X-ray photon spectroscopy (XPS) was used to quantify the degradation of the electrode in terms of its composition. The MH electrodes degraded with cycling. This phenomenon was more prominent at higher C-rates and temperatures. The electrode degradation was attributed to the loss of active material from the current collector by the repeated absorption and desorption of hydrogen and the formation of an Al₂O₃ oxide layer on the electrode surface with cycling. In addition, the effects of the addition of Co nano and Y₂O₃ powder on the degradation of the MmNi_3.9Co_0.6Mn_0.3Al_0.2 electrode were examined. The addition of the Y₂O₃ and Co nano powder significantly improved the performance of the MH electrode by increasing the cycle life and initial activation rate.     

AgNO<Subscript>3</Subscript>-LiNO<Subscript>3</Subscript>-RbNO<Subscript>3</Subscript> phase diagram

The ternary system of silver, lithium, and rubidium nitrates has been studied. Five vertical sections were established: AgNO₃-Li_0.5Rb_0.5NO₃; Li_0.5Rb_0.5NO₃-Ag_0.5Rb_0.5NO₃; 20 mol% AgNO₃; 80 mol% AgNO₃; and the section 5 mol% LiNO₃. Ten invariant points were found. A schematic representation of ternary equilibria is given. The three binary systems are also reported.     

Effects of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Nano-Particles on Corrosion Performance of Plasma Electrolytic Oxidation Coatings Formed on 6061 Aluminum Alloy

Corrosion resistance improvement of plasma electrolyte oxidation coatings on 6061 aluminum alloy in silicate electrolyte containing Al₂O₃ nano-particles was studied, with particular emphasis on the microstructure, coating growth, and corrosion behavior in 3.5 wt.% NaCl solution. The microstructure of coatings, their thickness, and phase composition were characterized using scanning electron microscopy and x-ray diffraction. All characterization data showed that the maximum coating thickness and lowest amount of porosity were obtained in a low concentration of KOH, a high concentration of Na₂SiO₃, and moderate concentration of Al₂O₃ nano-particles in the electrolyte. This combination describes the optimum plasma electrolytic oxidation electrolyte, which has the best conductivity and oxidizing state, as well as the highest incorporation of electrolyte components in the coating growth process. On the other hand, incorporation and co-deposition of Al₂O₃ nano-particles were more pronounced than SiO₃ ^2? ions in some level of molar concentration, which is due to the higher impact of electron discharge force on the adsorption of Al₂O₃ nano-particles. The electrochemical results showed that the best protective behavior was obtained in the sample having a coat with the lowest porosity and highest thickness.     

Synthesis,Structural, and Adsorption Characteristics of γ-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>/SiO<Subscript>2</Subscript> Composite

The γ-Fe₂O₃/SiO₂ composite is synthesized by coprecipitation of the magnetic carrier γ-Fe₂O₃ (specific surface area S = 17 m²/g, pore volume V = 0.51 cm³/g) and silicon dioxide from a solution of sodium-liquid glass. The influence of the synthesis conditions (SiO₂ content, temperature, introduction of electrolyte into reaction mixture) on the structural and adsorption characteristics of the resulting composites is studied. Coprecipitation in the presence of electrolyte (5% NaCl) makes it possible to obtain the most highly porous composites. At SiO₂ content from 20 to 50%, S is from 70 to 150 m²/g, V is from 0.74 to 0.89 cm³/g. These composites have a large adsorption capacity for test substances: a main dye, methylene blue, and an enzyme, cytochrome C. The capacity of these composites on dye (15–40 mg/g) is approximately 10–30 times larger that of the initial iron oxide (1.2 mg/g), and on enzyme (130–280 mg/g) 3–6 times greater than the capacity of iron oxide (45 mg/g). Such composites can be used as magnetic sorbents to for purification, concentration or immobilization of water-soluble organic substances and biopolymers.     

Effect of mixed rare earth oxides and CaCO<Subscript>3</Subscript> modification on the microstructure of an <Emphasis Type="Italic">in-situ</Emphasis> Mg<Subscript>2</Subscript>Si/Al-Si composite

The effects of mixed rare earth oxides and CaCO₃ on the microstructure of an in-situ Mg₂Si/Al-Si hypereutectic alloy composite were investigated by optical microscope, scanning electron microscope, and energy dispersive spectrum analysis. The results showed that the morphology of the primary Mg₂Si phase particles changed from irregular or crosses to polygonal shape, their sizes decreased from 75 μm to about 25 μm, and the compound of both the oxide and CaCO₃ was better than either the single mixed rare earth oxides or CaCO₃.     

Thermal and structural role of additives in P<Subscript>2</Subscript>O<Subscript>5</Subscript>-ZnO-Sb<Subscript>2</Subscript>O<Subscript>3</Subscript> glass for low expansion sealing materials

A glass based on the P₂O₅-ZnO-Sb₂O₃ ternary system was modified with various additives, such as RO (R=Ca, Ba and Mn), B₂O₃ and V₂O₅, for low temperature and low expansion sealing materials. The glass transition temperature (T_g) and coefficient of thermal expansion (CTE) were monitored and reduced with additive compositions of RO. Additional incorporation of B₂O₃ at the expense of RO also showed a similar result. Significant improvement was observed when the glass was modified with V₂O_5. A glass showing T_g<350 °C along with CTE<65×10⁻⁷/°C was found, suggesting a high potential for low temperature sealing materials especially for display applications. The role of the quaternary element within the glass is discussed, along with the structural effect using Raman spectroscopy.     

The effect of boron oxide on the crystallization behavior of MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript> spinel phase during the cooling of the CaO-SiO<Subscript>2</Subscript>-10 mass.% MgO-30 mass.%Al<Subscript>2</Subscript>O<Subscript>3</Subscript> systems

The microstructural characteristics of the CaO-SiO₂-B₂O₃-10 mass.% MgO-30 mass.% Al₂O₃ systems solidified during slow cooling from 1600 °C were investigated using SEM-EDS and a thermochemical computation package. The effect of boron oxide on the crystallization behavior of the spinel in the aluminosilicate system was observed because boron oxide is believed to become a potential flux to reduce the melting point of the liquid oxides. The primary crystalline phase was spinel, mainly MgAl₂O₄, irrespective of the boron content. The liquidus temperature T _L continuously decreased as the boron oxide content increased, indicating that the boron oxide decreased the activity of the MgAl₂O₄ spinel phase in liquid melts at high temperatures. The size of the spinel crystals increased as the temperature range for the solid + liquid coexisting region, viz. the mushy zone, increased. In the present systems, because the T _L continuously decreased with the increase in the boron oxide content, the viscosity of the liquid oxide may have affected the crystallization behavior of the spinel during cooling. Based on these results, an injection of a small amount of B₂O₃ flux into molten steel containing liquid aluminosilicate inclusions is not recommended because large spinel crystals can originate from the changes in the thermophysical properties of the liquid inclusions due to the incorporation of boron oxide into the aluminosilicate networks.