Improving the corrosion resistance of Mg–4.0Zn–0.2Ca alloy by micro-arc oxidation

In this paper, corrosion resistance of the Mg–4.0Zn–0.2Ca alloy was modified by micro-arc oxidation (MAO) process. The microstructure and phase constituents of MAO layer were characterized by SEM, XRD and X-ray photoelectron spectroscopy (XPS). The corrosion resistance of MAO treated Mg–4.0Zn–0.2Ca alloy in the simulated body fluid were characterized by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. The microstructure results indicated that a kind of ceramic film was composed by MgO and MgF₂ was formed on the surface of Mg–4.0Zn–0.2Ca alloy after MAO treatment. The electrochemical test reveals that the corrosion resistance of MAO treated samples increase 1 order of magnitude. The mechanical intensity test showed that the MAO treated samples has suitable mechanical properties.     

Microstructure and indentation mechanical properties of plasma sprayed nano-bimodal and conventional ZrO2–8wt%Y2O3 thermal barrier coatings

The nanostructured agglomerated feedstock used for plasma spraying was obtained by the nanoparticle reconstituting technique. Nanostructured and conventional ZrO₂–8wt%Y₂O₃ (8YSZ) thermal barrier coatings (TBCs) have been prepared by atmospheric plasma spraying (APS) on 45# steel substrates with the NiCrAlY as the bond-layer. The microstructure and phase composition of feedstocks and corresponding coatings were characterized. The top layer of nanostructured 8YSZ TBCs is denser and has fewer defects than that of conventional TBCs. The elastic modulus, micro-hardness and Vickers hardness of nanostructured 8YSZ TBCs exhibit bimodal distribution while the conventional 8YSZ exhibit mono-modal distribution. The elastic modulus and elastic recoverability were also obtained by the nanoindentation test. The results indicate that the elastic modulus of nanostructured 8YSZ coating is lower than that of conventional 8YSZ coating, but the nanostructured 8YSZ coating has higher elastic recoverability than that of the conventional 8YSZ coating. The prediction of the average elastic modulus was established by the mixture law and weibull distribution according to the fraction of phases with different molten characteristic.     

Effects of ceria nanoparticle concentrations on the morphology and corrosion resistance of cerium–silane hybrid coatings on electro-galvanized steel substrates

This work investigates the effect of the ceria nanoparticle concentration on the morphology and electrochemical behavior of cerium–silane hybrid coatings deposited on electro-galvanized steel substrates. The substrates were pre-treated with 3-glycidoxypropyl-trimethoxysilane and bisphenol A, modified with cerium ion-activated CeO₂ nanoparticles. The morphology of the coating before and after corrosion tests was examined using atomic force microscopy and scanning electron microscopy. The results indicate the formation of nanostructured surfaces with relatively uniform thicknesses and nanoparticle distribution. Microscopic observations explain the increased durability of the silane coating doped with the lowest content of activated ceria nanoparticles after short-term corrosion tests (456 h). The corrosion behavior of the sol–gel coatings was also investigated using natural salt spray tests, electrochemical impedance spectroscopy, and potentiodynamic polarization tests. The results show that the concentration of nanoparticles has a significant impact on the barrier properties of the silane films, which are improved for films with lower nanoparticle contents.     

High-temperature corrosion of electric-arc coatings sprayed from powder core wires based on the Fe–Cr–B–Al system

We study the kinetics of oxidation of electric-arc dispersion-hardened coatings of the Fe–Cr–B–Al system alloyed with 6% Ni, 3% W, 1% Mo, and 1% V by spraying from powder-core wires at 700 °C. The coatings 0.5 mm in thickness were applied to plates of low-carbon steel by using powder-core wires with a diameter of 1.8 mm developed at the Karpenko Physicomechanical Institute for the coefficient of filling of the charge equal to 20–27%. The specimens were tested for heat resistance at 700°C for 100 h. The variations of the structure of the coating in the course of its long-term high-temperature oxidation were studied in an EDX metallographic microscope with EVO-4XVP microanalytic system and by using the X-ray diffraction analysis. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 5, pp. 93–97, September–October, 2008.     

Fracture behaviour of a magnesium–aluminium alloy treated by selective laser surface melting treatment

Fracture behaviour of AZ91D magnesium alloy is dominated by the brittle fracture of the β-Mg₁₇Al₁₂ phase so its modification is required to improve the toughness of this alloy. The novel laser treatment named as Selective Laser Surface Melting (SLSM) is characterized by the microstructural modification of the β-Mg₁₇Al₁₂ phase without altering the α-Mg matrix. We have studied the effect of the selected microstructural modification induced by the laser treatment in the fracture behaviour of the alloy has been studied using in situ Scanning Electron Microscopy bending test. This test configuration allows the in situ observation of the crack progression and the record of the load–displacement curve. It has been observed that the microstructural modification introduced by SLSM causes an increase of 40% of the fracture toughness of the treated specimen. This phenomenon can be related with the transition from brittle to ductile fracture behaviour of the laser modified β-phase.     

On the microstructure and erosion–corrosion resistance of AlCrFeCoNiCu high-entropy alloy via annealing treatment

As-cast AlCrFeCoNiCu high-entropy alloy (HEA) was a solid solution of body-centered cubic (BCC) and face-centered cubic phases with a typical dendrite and interdendrite structures. The precipitated intermediate phases in 600°C annealed HEA are responsible for the improved hardness. After 1000°C annealing treatment, the crystalline was re-arranged because of the decomposition of BCC phases. The 1000°C annealed HEA has the best corrosion resistance due to its re-arranged crystalline structure. Meanwhile, the 600°C annealed HEA has the best erosion–corrosion resistance for its highest hardness and its wear rate is more than two times lower than that of AISI 304 stainless steel. Therefore, the HEA has a potential application in saline–sand slurry because of its superior corrosion and erosion–corrosion resistance.     

Study of the microstructure of plasma sprayed coatings obtained from Al2O3–13TiO2 nanostructured and conventional powders

The microstructure of coatings obtained from nanostructured or conventional Al₂O₃–13TiO₂ powders and deposited by plasma spraying technique on low-carbon steel was examined by transmission electron microscopy techniques. The dominating phase in both coatings was γ-Al₂O₃ phase. It has been observed that the grains of γ-Al₂O₃ grew in various shapes and sizes, that are particularly visible in the case of coating sprayed from nanostructured powder. The coatings obtained from the fully melted conventional powders exhibited a typical lamellar microstructure, into which the strips of TiO₂ phase were extended. The microstructure of coatings produced from agglomerates of nanostructured particles also revealed the regions consisting of partially melted α-Al₂O₃ powders surrounded by the net-like structure formed from fully melted oxides that improved the coating properties. Along with the observed morphology diversity some changes in the chemical composition on the cross sections of obtained coatings have been also noticed.     

Copper-oxide whisker growth on tin–copper alloy coatings caused by the corrosion of Cu6Sn5 intermetallics

This paper reports the effect of corrosion caused by high temperature and humidity on pure tin and tin–copper alloy coatings. A new phenomenon was observed; the development of copper-oxide whiskers on tin–copper alloys plated on copper substrates (1–5 % copper content stored at 105 °C/100 % relative humidity). The copper-oxide whiskers showed similar growth properties to tin whiskers. We have made a model to understand the development of copper-oxide whiskers. Localized corrosion of the tin coating reaches the Cu₆Sn₅ intermetallic layer, and copper oxide accumulates after the corrosion of Cu₆Sn₅. The dilating SnO _x compresses and extrudes out the copper oxide in a whisker form.     

Formation and sintering of 75 mol% alumina/25 mol% zirconia (2–3.5 mol% yttria) composite powder prepared by the hydrazine method

Al₂O₃/Y₂O₃-doped ZrO₂ composite powders with 25 mol% ZrO₂ have been prepared by the hydrazine method. As-prepared powders are the mixtures of AlO (OH) gel solid solutions and amorphous ZrO₂. The formation process leading to -Al₂O₃/t-ZrO₂ composite powders is investigated. Hot isostatic pressing has been performed for 1 h at 1500 °C under 196 MPa. Dense ZrO₂-toughened Al₂O₃ (ZTA) ceramics with homogeneous-dispersed ZrO₂ particles show excellent mechanical properties. The toughening mechanism is discussed.     

The effect of 3 wt.% Cu addition on the microstructure,tribological property and corrosion resistance of CoCrW alloys fabricated by selective laser melting

Microstructure, tribological property and corrosion resistance of orthopedic implant materials CoCrW-3?wt.% Cu fabricated by selective laser melting (SLM) process were systematically investigated with CoCrW as control. Equaxied γ-phase together with the inside {111}?<?112?>?type twin and platelet ε-phase was found in both the Cu-bearing and Cu-free alloys. Compared to the Cu-free alloy, the introduction of 3?wt.% Cu significantly increased the volume fraction of the ε-phase. In both alloys, the hardness of ε-phase zone was rather higher (~4 times) than that of γ-phase zone. The wear factor of 3?wt.% Cu-bearing alloy possessed smaller wear factor, although it had higher friction coefficient compared with Cu-free alloys. The ε-phase in the CoCr alloy would account for reducing both abrasive and fatigue wear. Moreover, the Cu-bearing alloy presented relatively higher corrosion potential E_corr and lower corrosion current density I_corr compared to the Cu-free alloy. Accordingly, 3?wt.% Cu addition plays a key role in enhancing the wear resistance and corrosion resistance of CoCrW alloys, which indicates that the SLM CoCrW-3Cu alloy is a promising personalized alternative for traditional biomedical implant materials.

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Microstructure and wear resistance enhancement of cast steel rolls by laser surface alloying NiCr–Cr3C2

The laser surface alloying technique was used to form wear resistant layers on 70MnV cast steel rolls with NiCr–Cr₃C₂ powders. The objective was to investigate the effects of the scanning speed on microstructure, phases, microhardness and wear resistance. Results indicate that the alloyed layers had dense, pore and crack free and homogeneous structures, as well as a metallurgical bonding with the substrates. With the increase of scanning speed, volume of retained austenite in the alloyed layer increased, microhardness and wear resistance increased and the microstructure refined. Wear results indicated that the wear resistance of the alloyed layer was enhanced by 7.8 times compared with that of the cast steel substrate. The improvement in wear resistance was attributed to the combined results of the grain refining effect, the solution strengthening effect, the tough γ-Fe matrix of the layer, the distribution of the hard Cr₇C₃, Fe₃C and martensite phases, and the good bonding between these hard phases and the matrix.     

The effects of surface treatment and stannate as an electrolyte additive on the corrosion and electrochemical performances of pure aluminum in an alkaline methanol–water solution

Pure aluminum electrodes were treated in alkaline stannate solutions, and the effects of some factors such as NaOH content and treating time were explored. The corrosion and electrochemical performances of the modified aluminum anodes in 4.0 M KOH methanol–water mixed solutions containing a methanol/water volume ratio of 7:3 (30% water) with and without stannate were investigated by means of hydrogen collection, polarization curve, galvanostatic discharge, scanning electron microscopy (SEM), and energy dispersive analysis of X-ray (EDAX). Metallic tin with high hydrogen evolution overpotential was deposited in aluminum surfaces by the modification treatments using stannate, resulting in the relatively low corrosion rate and markedly enhanced discharge performance of the modified aluminum anodes. In our experimental range the aluminum electrode treated in the solution with 0.1 M NaOH for 30 min showed lower corrosion rate and better discharge performance. The addition of Na₂SnO₃ in 4.0 M KOH methanol–water mixed solutions with 30% water inhibited the corrosion of the aluminum electrodes modified in the treating solution with 0.1 M NaOH for 30 min, resulting from the deposition of tin with high hydrogen evolution overpotential in aluminum surfaces. The deposition of metallic tin on the electrode surface and the existence of stannate in the electrolytes were responsible for the notable enhancement in the discharge performance of the modified aluminum anode.     

The formation of a hydroxyl bond and the effects thereof on bone-like apatite formation on a magnesia partially stabilized zirconia (MgO–PSZ) bioceramic following CO2 laser irradiation

For the purpose of improving the bioactivity of a magnesia partially stabilized zirconia (MgO-PSZ) and to explore a new technique for inducing OH group and apatite formation, a CO(2) laser has been used to modified the surface properties. The bioactivity of the CO(2) laser modified MgO-PSZ has been investigated in stimulated human fluids (SBF) with ion concentrations almost equal to those in human blood plasma. Some hydroxyl groups were found on the MgO-PSZ following CO(2) laser treatment with selected power densities. The surface melting on the MgO-PSZ induced by CO(2) laser processing provides the Zr(4+) ion and OH(-) ion, in turn, the incorporation of the Zr(4+) ion and the OH(-) ion creates the Zr-OH group on the surface. After 14 days of SBF soaking, the apatites formed on the MgO-PSZ with relatively high amount of hydroxyl groups generated by the CO(2) laser treatment, while no apatite was observed on the untreated with few hydroxyl groups. It exhibits that the Zr-OH groups on the MgO-PSZ surface is the functional groups to facilitate the apatite formation. The increased surface roughness provides more active sites, meantime, increased surface energy benefits to the adsorption and reaction on the surface.     

C–V,DLTS and conductance transient characterization of SiN<Subscript>x</Subscript>:H/InP interface improved by N<Subscript>2</Subscript> remote plasma cleaning of the InP surface

Electrical characterization of Al/SiN_x:H/InP structures shows that ECR nitrogen plasma cleaning of InP surfaces gives rise to a noticeable improvement in the interface quality, whereas insulator and semiconductor bulk properties are maintained at a level sufficient to be used as the gate dielectric in MIS devices. Nitrogen plasma exposure was carried out just before the SiN_x plasma deposition without vacuum breaking. To obtain interface state density and to detect deep levels in the semiconductor bulk, deep level transient spectroscopy (DLTS) measurements were carried out. We have also evaluated the insulator damage density, the so-called disorder-induced gap states (DIGS), by means of conductance transient analysis. Our results show that the plasma exposure in N₂ atmospheres is a valuable and simple surface conditioning method.     

Enhancement on the high-temperature joint reliability and corrosion resistance of Sn–0.3Ag–0.7Cu low-Ag solder contributed by Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Nanoparticles (0.12 wt%)

In this study, the evolution of interfacial microstructures and mechanical properties of the joints soldered with Sn–0.3Ag–0.7Cu (SAC0307) and SAC0307-0.12Al₂O₃ nanoparticles (NPs) aged at 150 °C for different hours (72–840 h) were investigated. It was found the joint soldered with SAC0307-0.12Al₂O₃ displayed a significantly enhanced high-temperature joint reliability, reflected in a higher shear force than that of the original. This enhancement in shear force primarily benefited from the refinement in solder microstructure contributed by Al₂O₃ NPs. As aging time reached 840 h, a controlled growth of interfaical IMC layer resulted from the pinning effect of Al₂O₃ NPs contributed to the increase in shear force. Theoretical analysis showed 0.12 wt% Al₂O₃ NPs effectively lowered the growth constant of total interfacial IMCs (D_T) from 3.19?×?10^?10 to 1.02?×?10^?10 cm² s^?1. Moreover, comparative studies on the corrosion resistances of SAC0307 and SAC0307-0.12Al₂O₃ were also conducted by electrochemical test and analyzed by electrochemical impedance spectroscopy (EIS). The results revealed SAC0307-0.12 Al₂O₃ solder displayed a stronger corrosion resistance (R_t; ~?3.1 kΩ cm² vs?~?7.1 kΩ cm²). This is also related with the tailored microstructure, which provides more grain boundaries for the initial nucleation of passive film.