Effect of Magnesium Content on the Corrosion Behaviors of Grf/Al Composite

Graphite-fiber-reinforced aluminum composites (Gr_f/Al) with different magnesium content were fabricated with the pressure infiltration method. Their corrosion behaviors were investigated by potentiodynamic polarization measurements, immersion tests, electrochemical impedance spectroscopy (EIS) analysis, and scanning electron microscopy (SEM). Both the corrosion potential (E _corr) and the pitting potential (E _pit) decreased with the increase of magnesium content, whereas the corrosion current density (i _corr) decreased sharply at first and then increased slightly. The i _corr of Gr_f/Al-3.2Mg was the lowest among the four composites with different magnesium contents, which indicated that Gr_f/Al-3.2Mg had the best corrosion resistance. EIS showed that the capacitive reactance of Gr_f/Al-8.5Mg was 1682 Ω × cm⁻², which was the worst, whereas that of Gr_f/Al-3.2Mg was 3498 Ω × cm⁻², which was the best. SEM results revealed that magnesium and silicon formed the Mg₂Si phase in Gr_f/Al-3.2Mg, which hindered the extension of corrosion crack and improved the corrosion resistance.     

Analysis of Electrochemical Current Noise From Metastable Pitting of SS304L in NaCl Solutions

Electrochemical noise emanating from a corrosion situation gives indication about the nature and form of corrosion. An attempt has been made to analyse electrochemical current noise signal generated under potentiostatic condition (for metastable pitting) for SS304L–NaCl system. To begin with polarisation plots of SS304L were obtained in various test solutions to precisely know pitting potential of SS304L. It is found that E_pit and i_pass increase on increasing chloride content. The electrochemical current noise was measured at potentials 20–30 mV below the pitting potentials. The current time record shows two types of current transients; (i) slow rise and rapid decay and (ii) rapid rise and slow decay. Power spectral density analysis of current noise shows that the power (A²/Hz) of the signal measured at metastable pitting range increases with increasing chloride. Sampling frequency has to be properly selected otherwise some of the spikes are not recorded and as a result, size and shape of few current transients is altered.     

An Investigation on High-Temperature Oxidation and Hot Corrosion Resistance Behavior of Coated TLP (Transient Liquid Phase)-Bonded IN738-LC

Despite all achievements to improve nickel-based superalloy, these classes of alloys are still prone to degradation via high-temperature oxidation and hot corrosion. Repairing damaged parts could decrease the life cycle, cost of equipment, and a transient liquid phase (TLP) bonding is a favorable method that has successfully been used for this purpose. One way to increase the lifetime of the repaired parts and the main body is to utilize protective coating. In the current study, aluminized coating was applied on IN738-LC which was first bonded by TLP process. Coating performance on the joint centerline compared to the other parts of the sample was investigated using a scanning electron microscope (SEM and FESEM) and X-ray diffraction method (XRD). The oxidation test result showed that coating provided less protection on the joint centerline due to coating’s chemical composition difference in this area: particularly Fe and Cr. XRD results showed that at the initial time of oxidation, all (α, γ, δ and θ)-Al₂O₃ were formed and by prolonged exposure were transformed to α-Al₂O₃. The hot corrosion test also proved that the joint centerline and the diffusion-affected zone were less resistant to the corrosion attack of 3Na₂SO₄?+?NaCl salts and severity of damage in these zones were clearly distinguished from microscopic images.     

Enhanced Corrosion Resistance of a Transient Liquid Phase Bonded Nickel-Based Superalloy

Electrochemical analysis of corrosion performance of a transient liquid phase (TLP) bonded nickel-based superalloy was performed. The TLP bonding process resulted in significant reduction in corrosion resistance due to the formation of non-equilibrium solidification reaction micro-constituents within the joint region. The corrosion resistance degradation is completely eliminated through a new application of composite interlayer that had been previously considered unusable for joining single-crystal superalloys. The effectiveness of the new approach becomes more pronounced as the severity of environment increases.     

Characterization and corrosion behavior of Al–Co–rare earth (Ce–La) amorphous alloy

A new Al-based amorphous alloy with excellent corrosion resistance and high thermal stability was produced in the Al–Co–RE (Ce–La) system. In this regard, two different amorphous-nanocrystalline ribbons: Al_87.6–Co_6.4–Ce_3.8–La_2.2 (M₁) and Al_82.3–Co_10.1–Ce_4.8–La_2.8 (M₂) were prepared using melt spinning. The results reveal the supercooled liquid region (ΔT_x) of 218 °C which shows that the M₂ alloy has higher thermal stability in comparison to the M₁ alloy. The M₂ ribbons present the superior corrosion resistance because of the formation of amorphous phase. The i_corr, E_corr, E_pit, and E_pit – E_corr values of fully amorphous Al–Co–Ce–La metallic ribbons have a better trend of anti-corrosion performance compared to other crystalline and amorphous aluminum alloys.     

Microstructural Evolution and Bonding Behavior during Transient Liquid-Phase Bonding of a Duplex Stainless Steel using two Different Ni-B-Based Filler Materials

Microstructural evolution and bonding behavior of transient liquid-phase (TLP) bonded joint for a duplex stainless steel using MBF-30 (Ni-4.5Si-3.2B [wt pct]) and MBF-50 (Ni-7.5Si-1.4B-18.5Cr [wt pct]) were investigated. Using MBF-30, the microstructure of the athermally solidified zone was dependent on B diffusion at 1333.15 K (1060 °C). Ni₃B and a supersaturated γ-Ni phase were observed in this zone. BN appeared in the bonding-affected zone. However, using MBF-50, the influences of base metal alloying elements, particularly N and Cr as well as Si in the filler material, on the bond microstructure development were more pronounced at 1448.15 K (1175 °C). BN and (Cr, Ni)₃Si phase were present in the bond centerline. The formation of BN precipitates in the bonding-affected zone was suppressed. A significant deviation in the isothermal solidification rate from the conventional TLP bonding diffusion models was observed in the joints prepared at 1448.15 K (1175 °C) using MBF-50.     

Local electrochemical studies after heat treatment of stainless steel: Role of induced metallurgical and surface modifications on pitting triggering

The morphology, the chemical composition, and the pitting corrosion resistance of a resulfurized stainless steel heated at 1000 °C for 2 hours were investigated at the microscale using ex-situ (field-emission-scanning electron microscope/electron dispersion spectrometer (FE-SEM/EDS) and secondary ion mass spectroscopy (SIMS)) and in-situ (electrochemical microcell technique and in-situ atomic force microscope (AFM) techniques. Although microcracks, which may have a deleterious effect, exist, the formation of a compound (Cr,Mn)₂(O,S)₃ instead of MnS is responsible for the better pitting corrosion resistance of sites containing an inclusion. Local electrochemical measurements indicate that no pitting was detected on these sites below 800 mV/SCE (saturated calomel electrode), whereas stable pitting was observed at around 350 mV/SCE before heating. Micropores were detected on the highly oxidized grains in which the ionic activity was found to be more marked than on the remaining surface (determined using in-situ AFM). Local electrochemical measurements revealed that the presence of such defects reduces significantly the corrosion resistance of the metallic alloy in NaCl-based media.     

Electrochemical Characterization of a Novel Mg70Al18Zn6Ca4Y2 Low Entropy Alloy in Different Aqueous Environments

In the present investigation, design and development of a novel Mg-based multicomponent low entropy alloy (Mg LEA-Mg₇₀Al₁₈Zn₆Ca₄Y₂) was carried out using Disintegrated Melt Deposition (DMD) technique. Various electrochemical techniques such as potentiodynamic polarization test (PDP) and electrochemical impedance spectroscopy (EIS) are used to investigate the electrochemical behaviour of the present Mg-LEA alloy at different molar concentrations in acidic (HCl), neutral (NaCl) and alkaline (NaOH, at different pH levels) solutions. The results show that, this alloy easily gets corroded with the increase of Cl⁻ ion concentration due to the breakdown of the Mg(OH)₂ passive layer in both acidic and neutral solutions. However, in the case of alkaline solution, the corrosion resistance of the alloy increases due to the formation of a stable Mg(OH)₂ layer along with AlMg₂Zn and Al₂Y phases, which is more stable than αMg. The ranking of Mg-LEA alloy’s corrosion rate is as HCl> NaCl> NaOH. The impedance measurements have correlated well with polarizations results and the data obtained according to the equivalent circuit provide insights between electrode and electrolyte interface. Through SEM analysis, pitting corrosion was observed in Mg-LEA alloy in acidic and neutral solutions and their chemical compositions were obtained using energy-dispersive X-ray spectroscopy (EDS).

     

Microstructural study of transient liquid phase bonded cast INCONEL 738LC superalloy

The microstructure of the transient liquid phase (TLP) bonded joint of a cast INCONEL 738LC superalloy, made with a commercial Ni-Cr-B filler alloy, Nicrobraz 150, was examined by analytical scanning and transmission electron microscopic techniques. Due to incomplete isothermal solidification at the bonding temperature, the residual liquid interlayer transformed to nonequilibrium eutectic microconstituents consisting of chromium-rich M₅B₃, nickel-rich M₂₃B₆, and nickel-based γ solid solution phases. Also, a significant volume fraction of complex fcc Cr-Mo-W rich carboborides was observed in the joint/base alloy interface region. This is contrary to the predictions of the currently available TLP models that predict a precipitate-free joint/base alloy interface. It is suggested that solid-state diffusion of boron prior to completion of equilibration process induced the formation of carbo-boride phase, which needs to be adequately considered to develop an optimum postbraze heattreatment process to produce a joint with optimum properties.     

Diffusion induced isothermal solidification during transient liquid phase bonding of cast IN718 superalloy

In transient liquid phase (TLP) bonding for commercial applications, one of the important key parameters is the holding time required for complete isothermal solidification t_IS, which is a prerequisite for obtaining a proper bond microstructure. The objective of the study is to analyse the isothermal solidification kinetics during TLP bonding of cast IN718 nickel based superalloy. Experiments for TLP bonding were carried out using a Ni–7Cr–4·5Si–3Fe–3·2B (wt-%) amorphous interlayer at several bonding temperatures (1273–1373 K). The time required to obtain TLP joints free from centreline eutectic microconstituents was experimentally determined. Considering the solidification behaviour of residual liquid, t_IS could be predicted by a mathematical solution of the time dependent diffusion equation based on Fick’s second law.

Dans la brasure en phase liquide transitoire (TLP) pour applications commerciales, l’un des paramètres clés importants est le temps de maintien requis pour la solidification isotherme complète (t_IS), qui est une condition requise pour l’obtention d’une microstructure adéquate du lien. L’objectif de cette étude est d’analyser la cinétique de la solidification isotherme lors de la brasure TLP du superalliage coulé à base de nickel, IN718. On a effectué les expériences de brasage de TLP en utilisant une couche de liaison amorphe de Ni–7Cr–4·5Si–3Fe–3·2B (% en poids) à plusieurs températures de brasage (1273–1373 K). On a déterminé expérimentalement le temps requis pour l’obtention de joints TLP libres de micro constituants eutectiques à la ligne centrale. En considérant le comportement de solidification du liquide résiduel, on pouvait prédire t_IS au moyen d’une solution mathématique de l’équation de diffusion dépendante du temps basée sur la seconde loi de Fick.     

Structure and mechanical and corrosion properties of new high-nitrogen Cr-Mn steels containing molybdenum

The structure, mechanical properties, and pitting corrosion of nickel-free high-nitrogen (0.8% N) austenitic 06Kh18AG19M2 and 07Kh16AG13M3 steels have been studied in various structural states obtained after hot deformation, quenching, and tempering at 300 and 500°C. Both steels are shown to be resistant to the ?? ?? ?? and ?? ?? ? martensite transformations irrespective of the decomposition of a ?? solid solution (06Kh18AG19M2 steel). Austenite of the steel with 19 wt % Mn shows lower resistance to recrystallization, which provides its higher plasticity (??₅) and fracture toughness at a lower strength as compared to the steel with 13 wt % Mn. Electrochemical studies of the steels tempered at 300 and 500°C show that they are in a stable passive state during tests in a 3.5% NaCl solution and have high pitting resistance up to a potential E _pf = 1.3?C1.4 V, which is higher than that in 12Kh18N10T steel. In the quenched state, the passive state is instable but pitting formation potentials E _pf retain their values. In all steels under study, pitting is shown to form predominantly along the grain boundaries of nonrecrystallized austenite. The lowest pitting resistance is demonstrated by the structure with a double grain boundary network that results from incomplete recrystallization at 1100°C and from the existence of initial and recrystallized austenite in the 07Kh16AG13M3 steel. To obtain a set of high mechanical and corrosion properties under given rolling conditions (1200?C1150°C), annealing of the steels at temperatures no less than 1150°C (for 1 h) with water quenching and tempering at 500°C for 2 h are recommended.     

Corrosion resistance of high-strength austenitic chromium–nickel–manganese steel containing nitrogen

The corrosion resistance of high-strength Cr–Ni–Mn austenitic steel containing nitrogen and copper is compared with that of Cr18Ni9 and Cr18Ni10N chromonickel steel by means of the Zive MP2 electrochemical system. The polarization curves and electrochemical characteristics of the alloys are determined in general, pitting, and intercrystallite corrosion by various media: aqueous solutions of NaCl (3%); FeCl₃ 6H₂O (100 g/L); H₂SO₄ (0.5 M); H₂SO₄ (0.5 M) + injected H₂S; and H₂SO₄ (0.5 M) + KSCN (0.01 M). The corrosion rates are calculated. The results indicate that all the steel samples are corrosion-resistant: they exhibit high resistance to intercrystallite corrosion and also to pitting and general corrosion in chloride-bearing media. No pitting corrosion is observed when Cr–Ni–Mn steel of balanced composition containing nitrogen (and especially steel containing both nitrogen and copper) is immersed in sea water, even when the steel’s nickel content is low. This steel outperforms traditional Cr18Ni9 steel in terms of strength and corrosion resistance, even in an acidic medium (0.5 M H₂SO₄).     

Effect of solid-solution treatment on corrosion and electrochemical behaviors of Mg-15Y alloy in 3.5 wt.% NaCl solution

The effect of solid-solution treatment on corrosion and electrochemical mechanisms of Mg-15Y alloy in 3.5 wt.% NaCl solution was investigated by electrochemical testing,immersion testing and SEM observation.The results indicated that the corrosion resistance of Mg-15Y sample gradually deteriorated with immersion time increasing,which was consistent with the observation of corrosion morphologies.The solid-solution treatment decreased the amounts of second phase Mg24Y5.The Ecorr and corrosion rate of as-cast samples were both lower than those of solid solution-treated samples,and both increased with increment of solid solution-treated time.The corrosion mechanism was proposed for the galvanic,pitting and filiform corrosion which varied with the immersion time and solid-solution treatment.     

Pitting corrosion behavior of Cu-P-RE weathering steels

The weathering steels are prone to pitting corrosion in an environment containing chloride ions.The pitting behavior of Cu-P-RE weathering steels and its effect on the corrosion resistance of steels were investigated by multifarious analytical techniques,such as field emission-scanning electron microscopy(FE-SEM),electron probe microanalysis(EPMA),scanning Kelvin probe force microscopy(SKPFM),electrochemical workstation and a series of immersion tests.The results show that the original stripshap...     

Wide Gap TLP Bonding a Single-Crystal Superalloy: Evolution of the L/S Interface Morphology and Formation of the Isolated Grain Boundaries

The microstructural evolution and bonding behavior of transient liquid phase (TLP) bonded joints for single-crystal superalloy samples with a 300-μm-wide gap have been investigated. The results show that at the initial stage, the interface grows with a cellular structure. With the increase of time, the length and numbers of the cellular structures decrease. At the final stage, the interface evolves with a planar manner. The gradient of B ahead of the liquid/solid interface exists and becomes sharper from the initial to the final analyzed from the electron probe microanalysis (EPMA) results. Inevitably, isolated grain forms during the evolution of the interface and the growth rate of isothermal solidification zone (ISZ) changes through the whole bonding process, and these both deviate from the traditional TLP bonding models. The mechanisms of the formation of the isolated grain boundaries and the reasons for the deviation of the solidification rate are discussed. Also, some effective methods to avoid the formation of isolated grain boundaries in the ISZ are proposed.     

Combined influence of Ce(Ⅲ) and iodide ions for corrosion protection of AA 2024-T3 in acidic to neutral chloride-rich environments:Electrochemical and surface characterization studies

It has been reported that trivalent cerium salts are effective inhibitors for corrosion of AA2024-T3 in neutral to alkaline co rrosive enviro nments,but poor in acidic environments.In this study,work was done for extending corrosion resistance provided by Ce(NO₃)₃ to acidic pHs as low as 2.5 through the addition of iodide ions.To this end,potentiodynamic polarization was used to ascertain the optimum Ce(Ⅲ)concentration for the inhibition of AA2024-T3 corrosion in 3.5%NaCl s...     

Interface Development in Cu-Based Structures Transient Liquid Phase (TLP) Bonded with Thin Al Foil Intermediate Layers

Proper bonding and assembly techniques are essential for fabrication of functional metal-based microdevices. Transient liquid phase (TLP) bonding is a promising technique for making enclosed metallic microchannel devices. In this paper, we report results of TLP bonding of Cu-based structures at temperatures between 823 K and 883 K (550 °C and 610 °C) with thin elemental Al foils as intermediate boding layers. In situ X-ray diffraction was utilized to examine the structure of Cu/Al interface in real time, resulting in a proposed sequence of structural evolution of the Cu/Al/Cu TLP bonding interface region. Three different types of bonding interface structures, the “γ ₁ structure,” the “eutectoid structure” (“E structure”), and the “E/γ ₁/E structure,” were observed through electron microscopy, and related to the proposed sequence of interfacial structural evolution. Tensile fracture tests were conducted on TLP-bonded Cu/Al/Cu coupon assemblies. Hardness of the various phases within the bonding interface region was probed with instrumented nanoindentation. Results of mechanical testing were correlated to the structure of the bonding interface region. The present results provide an understanding of the structural evolution within the Cu/Al/Cu TLP bonding interface region, and offer guidance to future bonding of Cu-based microsystems.     

Dissolution of chromium-enriched inclusions and pitting corrosion of resulfurized stainless steels

The dissolution of MnS inclusions enriched in chromium (30 to 40 wt pct Cr) and the corrosion susceptibility of resulfurized stainless steel were studied in various NaCl- and NaClO₄-based solutions by X-ray photoelectron spectroscopy, secondary ion mass spectroscopy, scanning electron microscopy (SEM), and the electrochemical microcell technique. It has been shown that chromium-enriched inclusions do not undergo dissolution under free corrosion conditions. By contrast, electrochemical dissolution of inclusions occurs at high potential values (above 500 mV vs saturated calomel electrode) in all the solutions, followed by stable pitting. It has also been shown that some areas containing chromium-enriched inclusions exhibited stable pitting at low potentials (below 100 mV vs saturated calomel electrode) in the electrolytes with chloride ions. Field-emission SEM experiments have revealed that the matrix undergoes dissolution around these inclusions. An assumption is proposed for describing pitting corrosion mechanisms.