Electrochemical study on effect of Au,Ag, Pd and Pt on corrosion behaviour of Fe3Al in molten NaCl–KCl

The corrosion behaviour of Fe₃Al intermetallic alloyed with 1 at-%Ag, Au, Pt and Pd in NaCl–KCl (1∶1M) at 700°C, typical of waste gasification environments, has been evaluated using polarisation curves, linear polarisation resistance and electrochemical impedance spectroscopy measurements and compared with an Ni based alloy, namely Inconel 600. Results have shown that, for short testing times, the addition of noble elements increased the corrosion rate for base Fe₃Al intermetallic alloy, but it was decreased for long testing times by forming a protective corrosion product layer. Additionally, base Fe₃Al intermetallic alloy had higher corrosion rates than Inconel 600.     

The effect of Sn on autoclave corrosion performance and corrosion mechanisms in Zr–Sn–Nb alloys

The desire to improve the corrosion resistance of Zr cladding material for high burn-up has resulted in a general trend among fuel manufacturers to develop alloys with reduced levels of Sn. While commonly accepted, the reason for the improved corrosion performance observed for low-tin zirconium alloys in high-temperature aqueous environments remains unclear. High-energy synchrotron X-ray diffraction was used to characterize the oxides formed by autoclave exposure on Zr–Sn–Nb alloys with tin concentration ranging from 0.01 to 0.92 wt.%. The alloys studied included the commercial alloy ZIRLO® (ZIRLO® is a registered trademark of Westinghouse Electric Company LLC in the USA and may be registered in other countries throughout the world. All rights reserved. Unauthorized use is strictly prohibited.) and two variants of ZIRLO with significantly lower tin levels, referred to here as A-0.6Sn and A-0.0Sn. The nature of the oxide grown on tube samples from each alloy was investigated via cross-sectional scanning electron microscopy. Atom probe analysis of ZIRLO demonstrated that the tin present in the alloy passes into the oxide as it forms, with no significant difference in the Sn/Zr ratio between the two. Synchrotron X-ray diffraction measurements on the oxides formed on each alloy revealed that the monoclinic and tetragonal oxide phases display highly compressive in-plane residual stresses with the magnitudes dependent on the phase and alloy. The amount of tetragonal phase present and, more importantly, the level of tetragonal-to-monoclinic phase transformation both decrease with decreasing tin levels, suggesting that tin is a tetragonal oxide phase stabilizing element. It is proposed that in Zr–Nb–Sn alloys with low Sn, the tetragonal phase is mainly stabilized by very small grain size and therefore remains stable throughout the corrosion process. In contrast, alloys with higher tin levels can in addition grow larger, stress stabilized, tetragonal grains that become unstable as the corrosion front continues to grow further inwards and stresses in the existing oxide relax.     

Experimental and computational study on the effect of yttrium on the phase stability of sputtered Cr–Al–Y–N hard coatings

The effect of Y incorporation into cubic Cr–Al–N (B1) was studied using ab initio calculations, X-ray diffraction and energy-dispersive X-ray analysis of sputtered quaternary nitride films. The data obtained indicate that the Y incorporation shifts the critical Al content, where the hexagonal (B4) structure is stable, to lower values. The calculated critical Al contents of x ≈ 0.75 for Cr_1?xAl_xN and x ≈ 0.625 for Cr_1?x?yAl_xY_yN with y = 0.125 are consistent with experimentally obtained values of x = 0.69 for Cr_1?xAl_xN and x = 0.68 and 0.61 for Cr_1?x?yAl_xY_yN with y = 0.02 and 0.06, respectively. This may be understood based on the electronic structure. Both Cr and Al can randomly be substituted by Y. The substitution of Cr by Y increases the phase stability due to depletion of non-bonding (anti-bonding) states, while the substitution of Al by Y decreases the phase stability mainly due to lattice strain.     

New understanding of the effect of hydrostatic pressure on the corrosion of Ni–Cr–Mo–V high strength steel

Corrosion of Ni–Cr–Mo–V high strength steel at different hydrostatic pressures is investigated by scanning electron microscopy (SEM) and finite element analysis (FEA). The results indicate that corrosion pits of Ni–Cr–Mo–V high strength steel originate from inclusions in the steel and high hydrostatic pressures accelerate pit growth rate parallel to steel and the coalescence rate of neighbouring pits, which lead to the fast formation of uniform corrosion. Corrosion of Ni–Cr–Mo–V high strength steel under high hydrostatic pressure is the interaction result between electrochemical corrosion and elastic stress.     

Effect of Mo on corrosion behavior of Ni20Cr–xMo alloys in air with NaCl–KCl–CaCl2 vapor at 570°C

The effect of Mo on the corrosion behavior of Ni20Cr–xMo alloys in an oxidizing chlorine-containing atmosphere using air mixed with the salt-vapor mixture of NaCl–KCl–CaCl₂ at 570°C was investigated. The results revealed that the corrosion performance of the Ni20Cr alloys in the oxidizing chlorine atmosphere was improved by Mo addition of up to 3 wt%. The Mo-free alloy formed a potassium chromate during corrosion as a result of the reaction between the Cr₂O₃ scale and KCl vapor. The chromate formation increased the chlorine potential at the scale surface and induced the breakdown of the protective Cr₂O₃ scale, resulting in internal chromium chloride precipitates and a Cr-depleted zone. In contrast, the presence of Mo resulted in the formation of a NiO scale, which did not react with the salt vapors and, therefore, prevented the formation of chromates. The beneficial effect of Mo on the high-temperature chlorination of Ni–Cr alloys in salt-vapor-containing atmospheres was ascribed to the suppression of chlorine generation due to NiO scale formation.     

Effect of citrate additive on the electrodeposition and corrosion behaviour of Zn–Co alloy

The electrodeposition of Zn–15Co alloy on a stainless steel substrate from a citrate bath has been investigated as a function of pH and citrate concentration in the electroplating bath. UV–vis results and speciation diagrams of zinc and cobalt in aqueous citrate solution were used to select deposition baths with different dominant Zn–Co citrate complexes. Voltammetry and potentiostatic methods were used to study the codeposition mechanism. The corrosion behaviour of electrodeposits of Zn–15Co alloys was studied by means of electrochemical experiments in a solution of 3.5% NaCl using potentiodynamic polarisation and open circuit potential. The results showed that the corrosion resistance of the deposits was highly influenced by the presence of citrate in the electroplating bath.     

Influence of Ca ions and temperature on the corrosion behavior of WC–Co hardmetals in alkaline solutions

The effect of temperature and Ca ions on the corrosion behavior of hardmetals was investigated in 0.1 M NaOH and 0.05 M Ca(OH)₂ alkaline electrolytes using impedance spectroscopy, potentiodynamic polarization and surface analytical techniques. It was found that calcium containing alkaline solutions efficiently decrease the anodic currents up to 5 times by forming a calcium containing deposit on the top of the WC–Co hardmetal surface, which remains stable even at higher temperatures (40 °C and 60 °C). This positive influence of Ca ions is predominant under polarization in the range from 0 to +0.85 V (Ag/AgCl) but is not apparent under OCP conditions. In NaOH, however, the corrosion resistance strongly decreases at higher temperatures as compared with the room temperature. At the slightly elevated temperature in 0.1 M NaOH the Co binder phase loses its passivity and is almost completely washed out of the compound material. A WC skeleton remains on the surface and hence the ductility in the hardmetal is lost. In the end, the material could completely fail under such operating conditions. Also the Ni alloyed binder loses its strong passivation ability at the elevated temperatures.     

The Oxidation Behaviour of Alloys Based on the Ni–Co–Al–Ti–Cr System

The isothermal oxidation behaviour of a series of quinary Ni–Co–Al–Ti–Cr alloys were studied at 800 °C. Alloys with higher Cr concentrations exhibited lower mass gain after 100-h exposure, as did the alloys richest in Ni and Al for a given Cr concentration. Extensive internal oxidation and nitridation was also observed in all alloys, except those containing the highest concentrations of Ni and Al. All alloys studied generated continuous chromium oxide layers, beneath which alumina particles were observed. Compositional analysis of the subscales identified shallower Cr concentration gradients in alloys containing equiatomic levels of Ni and Co, suggesting increased availability of Cr in the alloy. Thermodynamic calculations confirmed that these alloys contained higher concentrations of Cr in their γ matrices as a result of a combination of both the elemental partitioning behaviour and the increased mole fraction of γ′ precipitates forming in the alloy.     

Study of the corrosion product films formed on the surface of Mg–xZn alloys in NaCl solution

The corrosion product films formed on the surface of Mg–2Zn and Mg–5Zn alloys in NaCl solution were investigated by electrochemical measurement, scanning electron microscopy (SEM) observation and X-ray photoelectron spectroscopy (XPS) analysis. It is found that a compact corrosion product film is formed in the initial stage of immersion, and then the film gradually degrades due to dissolution reaction. The product film formed on Mg–2Zn alloy presents better protection property than that on Mg–5Zn alloy, which can be attributed to the different chemical composition and microstructure of the both alloys.     

Influence of TiC additions on the corrosion behaviour of WC–Co hardmetals in alkaline solution

In this work, the effect of TiC additions on the corrosion behaviour of WC–10 wt.% Co hardmetals was investigated in a 1 mol/L NaOH solution. The results showed that the addition of TiC could make the open circuit potential (OCP) in the test solution more positive than the base alloy, lead to nobler free-corrosion potential value and decrease corrosion current density. The XRD results showed that the existence of TiC can inhibit the corrosion of carbide in hardmetals. Moreover, XPS and TEM results demonstrated that titanium dioxide formed in the surface films of the TiC content specimens in the NaOH solution.     

Study of the effect of Ti and Ge in the microstructure of Nb–24Ti–18Si–5Ge in situ composite

The effects of Ti and Ge on the microstructure and hardness of the as cast and heat treated Nb-24Ti–18Si–5Ge (at.%) alloy (ZF3) were studied. There was macrosegregation of Si. The phases present in the as cast alloy (ZF3-AC) were the (Nb,Ti)_ss, and the (Nb,Ti)₃(Si,Ge), β(Nb,Ti)₅(Si,Ge)₃ and hexagonal (Ti,Nb)₅(Si,Ge)₃ silicides, with the latter forming a eutectic with the solid solution. The same phases were present after heat treatment at 1200 °C for 100 h (ZH3-HT12) but only the (Nb,Ti)_ss, and the (Nb,Ti)₃(Si,Ge) and (Nb,Ti)₅(Si,Ge)₃ silicides were present after 100 h at 1500 °C (ZF3-HT15) where TiO₂ was also formed. Alloying with Ti did not stabilise the (Nb,Ti)_ss + (Nb,Ti)₃Si eutectic. The formation of the eutectic in ZF3-AC was strongly influenced by the partitioning behaviour of Ti in the solidifying melt that was enhanced by the presence of Ge. There were Ti rich areas in the (Nb,Ti)_ss and the (Nb,Ti)₃(Si,Ge) silicide only in ZF3-AC. The solubility of Ge in (Nb,Ti)₃(Si,Ge) increased after heat treatment at 1500 °C. The transformation of β(Nb,Ti)₅(Si,Ge)₃ to α(Nb,Ti)₅(Si,Ge)₃ progressed from ZF3-HT12 to ZF3-HT15 but equilibrium was not reached in ZF3-HT15. The synergy of Ti with Ge resulted to a strong hardening effect and a remarkable retention of the hardness. Alloying with Ti led to a reduction of the hardness of Nb₅Si₃ and to an increase of the hardness of Nb₃Si. The synergy of Ti with Ge resulted to a strong hardening effect for the (Nb,Ti)_ss.     

A Comparative Study on the Shape Evolution of the TiC Particles in Ti–C,Ti–Al–C,and Ti–Si–C Systems after HF Treatment

Protection of Metals and Physical Chemistry of Surfaces - This study aims to compare the shape evolution of the titanium carbide (TiC) particles in different systems after immersion in hydrofluoric...     

Growth morphology and crystallography of precipitate in a new high-temperature structural intermetallic Ti–40Al–10Fe

Based on TiAl alloys bearing high levels of Nb, which are prospective high-temperature intermetallics, a new dual-phase alloy (Ti–40Al–10Fe (at%)) with precipitate growth direction close to the slipping plane normal of the matrix was developed. Transmission electron microscopy revealed that the annealed alloy was composed of equiaxed grains of τ₂-TiAl₂Fe and γ-TiAl phases. Aging treatment at 1000 °C produced polygon-shaped γ-TiAl precipitate from the τ₂-TiAl₂Fe matrix. The precipitation reaction of the face-centered cubic structure of τ₂-TiAl₂Fe into the ordered face-centered tetragonal structure of γ-TiAl phase was systematically investigated in terms of crystallographic features, including mutual orientation relationship, growth direction, habit plane, side facets, and interfacial dislocation structure. The observed growth direction <973> was close to the normal of the slip plane {111}, with a discrepancy angle of ∼20°. Thus, the observed two-phase microstructure might significantly improve strengthening. It could also be quantitatively controlled by selection of precipitate and matrix with phase structures and lattice parameters that meet a certain requirement.     

First-principles study on the lattice stability of elemental Co,Rh,and Ir in the ⅧB group

Lattice constants, total energies, and densities of state of transition metals Co, Rh, and Ir in the VIIIB group with different crystalline structures were calculated via generalized gradient approximation (GGA) of the total energy plane wave pseudopotentiai method in first-principles. The lattice stabilities of Rh and Ir are △Gbcc-hcp △Gfcc-hcp 0, agreeing well with those of the projector augmented wave method in first-principles and the CALPHAD method in spite of elemental Co. Analyses of the electronic smlctures to lattice stability show that crystalline Rh and Ir with fcc structures have the obvious characteristic of a stable phase, agreeing with the results of total energy calculations. Analyses of atomic populations show that the transition rate of electrons from the s state to the p or d state for hop, fcc, and bcc crystals of Co and Rh increases with the elemental period number to form a stronger cohesion, a higher cohesive energy, or a more stable lattice between atoms in heavier metals.     

A computational study of the temperature dependence of interface and surface energies in WC–Co cemented carbides

Interfaces and surfaces often play a vital role for the properties of polycrystalline materials, such as cemented carbides, and the study of these planar defects is, therefore, of great importance. Cemented carbides (or hardmetals) is a unique class of materials where hard carbide (WC) grains, usually micrometer sized, are embedded in a more ductile metal binder phase (usually Co) in order to combine superb strength with high hardness, making them ideal as tool material in e.g. metal machining. In the manufacturing and industrial usage of cemented carbides temperatures reach high levels, especially in the former case where the material is sintered at temperatures where the binder phase is a liquid.This is a computational study of the temperature dependence of interface and surface energies in WC–Co cemented carbides upto and above the melting temperature of Co. We make use of an analytical bond order potential (ABOP) fitted to density functional theory (DFT) data in order to make the free energy calculations feasible. A variety of free energy methods are used: including quasi harmonic approximation, temperature and thermodynamic integration, and calculation of liquid surface tension and work of adhesion for phase boundaries. We present the temperature dependent interface and surface energies for some typical cases, data which should be useful as a supplement to other studies limited to 0 K.     

The effect of precipitate size on the yield strength-pitting corrosion correlation in Al–Cu–Mg alloys

The effect of precipitate size and state on the combinations of yield strength and corrosion pit susceptibility exhibited by a model aluminum alloy Al–2.5Cu–1.5 Mg (wt.%) has been measured for the first time. For aging times less than 2 h at 200 °C, it is shown that the fine nanoscale precipitates that form strongly affect the yield stress of the alloy but do not contribute to electrochemical heterogeneity that leads to corrosion susceptibility. This illustrates that a precipitate size range exists where the usually observed inverse correlation between strength and corrosion resistance can be broken. At longer aging times, when the precipitates become larger, a three orders of magnitude increase in the pitting rate is observed between aging times of 2 h and 9 h. The step change in pitting rate corresponds with a critical precipitate size lying between ～3 nm and 8 nm and it is hypothesized that this size corresponds to a critical size above which it is not possible for the formation of a continuous protective passive oxide film to occur.     

Hot corrosion behavior and mechanism of FGH96 P/M superalloy in molten NaCl–Na_2SO_4 salts

Hot corrosion behavior of FGH96 powder metallurgy(P/M) superalloy in 25 % NaCl + 75 % Na_2SO_4 molten salts at 650, 700, and 750 ℃ was investigated in this paper. The methods of mass loss measurement, X-ray diffraction(XRD), scanning electron microscopy(SEM),and energy-dispersive spectroscopy(EDS) were used here.The experimental results show that hot corrosion kinetics follows a square power law at 650 ℃, while a linear one at 700 and 750 ℃. The corrosion layer is detected to be composed of Cr_2O_3, NiO, and Ni2S_3 at each temperature.The cross-sectional morphologies and corresponding elemental maps indicate that the corrosion layer is a stratified structure of oxide and sulfide. The results and analyses confirm that the hot corrosion mechanism of FGH96 P/M superalloy is a cooperate process of oxidation and sulfidation. Furthermore, the relatively higher concentrations of Cr, Co, and Ti provide better corrosion resistance to the attack of S and Cl~-.     

A comparative study of the interfacial reaction between electroless Ni–P coatings and molten tin

A comparative study of the reaction characteristics between molten tin and both as-plated and heat-treated Ni–P coatings was carried out, with a specific focus on the stability of the Ni₃P intermetallic layer and its effects on the subsequent reaction. It was found that a continuous layer of Ni₃P may be formed on both types of Ni–P during the interfacial reaction, despite the fact that heat-treated Ni–P is a two-phase mixture of Ni₃P and Ni. The Ni₃P formed on the heat-treated Ni–P was thinner than that on as-plated Ni–P. A mass conservation analysis of P revealed that no or limited P was lost into the molten tin when the Ni₃P layer was thin, whereas a significant loss of P took place as the Ni₃P thickness increased. It is proposed that the Ni₃P phase is stable and may not undergo chemical decomposition during the interfacial reaction. The loss of P to the molten tin observed in the present study is most likely due to the crumbling of Ni₃P particles into the liquid phase, as a result of the enhanced mass transport due to use of thin copper wire substrates rather than a planar surface. Finally, the results show that the Ni₃P phase cannot act as an effective barrier layer to the attack of molten tin toward the substrate. Defects in the Ni₃P were found to allow localized penetration of molten tin.     

Erosion and Abrasion Resistance,Mechanical Properties,and Structure of the TiN,Ti–Cr–Al–N and Cr–Al–Ti–N Coatings Deposited by CFUBMS

Protection of Metals and Physical Chemistry of Surfaces - The results of structural studies and mechanical, abrasion, and erosion tests of single-layer and multi-layer TiN,...