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The hot salt stress corrosion cracking (HSSCC) behavior of the selectively laser melted (SLM) Ti-6Al-4V alloy in the as-built and heat-treated conditions was studied at 300 and 400 °C. The HSSCC resistance of very susceptible as-built alloy can be significantly increased through annealing at 750 °C, losing only a small amount of ultimate tensile strength (UTS). The improvement in the HSSCC resistance is brought out by a transformation of martensite into lamellar α + β. The absence of equiaxed α found in the wrought alloy seems to help in increasing the HSSCC resistance of SLM Ti-6Al-4V alloy at high strength level.

The influence of 3.5 wt pct NaCl on the work-hardening response of AA 7004 is investigated through subjecting the tensile data to Kock-Mecking analysis. Due to the exposure to chloride, the decay in the work-hardening rate (−dθ/dσ) increases by ~28 pct when the alloy has GP zones and decreases by ~22 pct when η′/η precipitates are present. In contrast, no significant variation in ultimate tensile strength and elongation values are observed for the alloy, although it suffered embrittlement which results in tearing topography surfaces. Accordingly, −dθ/dσ seems to be a more reliable parameter to assess the environmentally assisted cracking (EAC) susceptibility of the less EAC susceptible AA 7004.

The role of aging conditions vis-a-vis the precipitate state on the hydrogen embrittlement behavior of IN 718 has been studied. Slow strain rate tests showed that the overaged condition was most susceptible to hydrogen embrittlement followed by the peak-aged and solution-annealed conditions. Hydrogen permeation studies carried out using Devanathan–Stachurski cell showed that the hydrogen diffusivity values varied in the order: overaged condition

The influence of chemistry apart from the pH of buffers on the electrochemical corrosion behaviours of mild steel (MS) and 7004 aluminium alloy (AA) was studied using acetate, citrate, and citrate-...

Gas tungsten arc welding is extensively used in the fabrication of 18Ni 250 maraging steel motor casings. In this work, the role of flux addition on the environmentally assisted cracking (EAC) and electrochemical corrosion behaviors of these welds in their peak-aged condition is presented. Studies were performed in order to bring out the potential influence of long-term exposure of maraging steel welds to neutral 0.1 M NaCl (mildly corrosive) and acidified 0.1 M NaCl (a relatively more corrosive) environments to understand the EAC behavior of the welds. The study showed that both the fluxed and unfluxed weld metals suffered 21% to 23% loss in ductility in neutral 0.1 M NaCl solution from that of obtained in air. The EAC of the welds in the neutral chloride solution was due to anodic dissolution, while in the acidified chloride solution it was found to be predominantly due to hydrogen embrittlement. The effect of flux in refining the weld microstructure was reflected in: (a) preventing intergranular mode of hydrogen embrittlement of the unfluxed welds and transforming the mode to transgranular cracking, (b) lowering a wide scatter in electrochemical corrosion rate, and (c) reducing the selective attack in the interdendritic regions of the unfluxed welds. Such a transition from intergranular to transgranular cracking might affect long-term service behavior of the welds to corrosion and EAC, though it is not reflected in the short-term tests.

This paper aims to produce Al casting over SS pipe by introducing Cd, Cu, Ag and brass interlayer and to study their effect on the interface bonding between aluminium (Al) and stainless steel (SS). Castings were produced by novel thermocol moulding method and their mechanical properties and metallurgical characteristics were studied. The shear punch test was conducted to evaluate the bond strength of Al–SS bimetallic castings (BmC). Without interlayer, castings showed bond strength of 5.21 MPa, whereas Cd and Cu interlayered BmC showed bond strength of 91.2 MPa and 112.45 MPa, respectively. Ag and brass interlayered BmC showed less effect on bond strength. Interlayers with low melting point and high solubility showed good metallurgical bonding than the interlayers with the high melting point. Eutectic formation of Cd with Al and diffusion of Cu into Al led to the formation of a better metallurgical bond. Hence, they might be used as a potential candidate material for applications like lightweight, hot corrosive fluid and chemical transports.

In the present study, the corrosion behavior of sputter-deposited amorphous Al-40Cr-xMo alloy coatings in 0.5 M H2SO4 + 2 ppm NaF solution at 70 °C (± 2 °C) was investigated for a possible application as bipolar plates (BPPs) for proton exchange membrane fuel cells (PEMFCs). Particular attention was given to understand the effect of Mo concentration on the passive films and, thereby, on the corrosion behavior of the sputter-deposited alloy coatings. X-ray diffraction (XRD), scanning electron microscopy, X-ray photoelectron spectroscopy (XPS), and electrochemical techniques were carried out to characterize the Al-40Cr-xMo coated specimens. XRD plots of the Al-40Cr-xMo coated specimens indicated the deposits to be amorphous in nature. SEM images showed the coatings were dense and uniform without the presence of any coating defects. XPS analysis indicated a depletion of Mo at concentrations above 12 at. pct. Potentiodynamic polarization test results indicated that the addition of Mo at approximately 12 at. pct is effective in enhancing the corrosion resistance by decreasing the passive current density. However, further addition of Mo raises the anodic current density, thereby lowering the protective ability of the passive film.

Insufficient corrosion resistance, electrical conductivity and wettability of bipolar plates are some of the important issues affecting the performance of hydrogen fuel cells. To address these issues, an amorphous Al–Cr–Mo–N coating is deposited on type 316L stainless steel using direct current (DC) magnetron sputtering. The electrochemical corrosion behaviour is investigated under simulated fuel cell anode (H2-purging) and cathode (air-purging) environment consisting of 0.5 M H2SO4 + 2 ppm NaF at 70 ± 2 °C. The corrosion current density is reduced to 0.02 μA cm−2 comparable to the commercially used Ta/TaN coatings. The polarization resistance increases by two orders of magnitude and the interfacial contact resistance (ICR) reduces significantly due to the application of the coating. Further, the coating shows better water management due to high hydrophobicity than the bare stainless steel.

This paper reports an attempt to understand the influence of temper conditions on the hydrogen embrittlement (HE) behaviour of AA 7010. A novel heat treatment technique reported in this paper exhibited crack growth rates on par with over aged temper in 3.5 wt.% NaCl under freely corroding conditions (@OCP). Copper enrichment on the grain boundary precipitates curtailed the hydrogen evolution and enhanced the alloy resistance to hydrogen embrittlement (@OCP) for both the temper conditions.

The role of noble element enrichment as cathodic sites for hydrogen evolution during the anodic polarisation of magnesium was evaluated by direct quantification of indium and gold enrichment using Particle-Induced X-ray Emission (PIXE). Magnesium alloyed with gold was shown to have a higher enrichment efficiency than magnesium alloyed with indium. However, the anodically induced ‘cathodic activation’ efficiency of magnesium alloyed with indium was higher. The estimated hydrogen evolution occurring at sites of enriched indium or gold was seen to be very small compared to the measured overall increased hydrogen evolution upon the respective alloys during anodic polarisation.

Anodically dissolved magnesium displays “cathodic activation” when compared to magnesium in an as-ground condition. The dark corrosion film that forms upon Mg by dissolution in 0.1 M NaCl is primar...

Automobile coating system consists of a metallic underlayer followed by a phosphate coating and, lastly, multilayer organic coating. In this work, the effect of the underlying metallic coatings, namely, a Mg-Al-Zn alloy coating (Magizinc) and a conventional galvanized Zn coating on the phosphate coatings formed thereon, and its corrosion performance was investigated. The corrosion resistance offered by the phosphate coating formed on the Magizinc coating was higher than the phosphate coating on the galvanized Zn coating (a reference coating employed in the study) in NaCl solution, as revealed by potentiodynamic polarization, electrochemical impedance spectroscopy, and salt-fog tests. In-depth characterization of the phosphate coatings was carried out using scanning electron microscopy and glow discharge optical emission spectroscopy. It was revealed that the phosphate crystals formed on the Magizinc coating were more fine-grained, compact, and crack-free as compared to that formed on the galvanized coating and contained Mg aiding 4-10 times increase in the corrosion resistance as determined by the electrochemical studies. However, it only improved marginally against the appearance of red rust in a salt-fog test over the unphosphated Magizinc coating. The phosphate coating on Magizinc marginally improved the adhesion of an epoxy primer coating applied on the phosphated Magizinc coating and significantly (>3.5 times longer exposure) retarded the deterioration of the epoxy primer coating in the salt-fog environment in comparison with the similar studies carried out on the phosphated conventional galvanized zinc coating. Notably, phosphating the Magizinc coating caused a ten times reduction in the H pickup compared to that in the galvanized coating under identical phosphating conditions, suggesting the former coating lowered the propensity for hydrogen embrittlement in the steel.

This study deals with microstructural influence on the H permeation behavior of Dual-Phase (DP) and Complex Phase (CP) steels using electrochemical permeation studies. The H diffusion coefficients in DP steels (DP800: 1.65 × 10−10 m2·s−1, DP1000: 1.58 × 10−10 m2·s−1) are half of that found in CP steels (3.07 × 10−10 m2·s−1).The banded microstructure along the specimen thickness and higher C content of the DP led to high H diffusivity of DP steels. The lower total H concentration along with a higher fraction of H was present in the stronger traps in CP steels suggest a better HE resistance of this steel. The H distribution in the specimens was non-uniform, with a higher H concentration speculated near the charging surface.

This study explores a methodology to raise environmentally assisted cracking resistance of alclad Al–Zn–Mg–Cu (AA7075) alloy, used in aircraft structures, stiffened with polymer matrix composite asymmetric patch in order to suppress growth of fatigue cracks. Fracture mechanics studies of adhesively bonded center-pre-cracked Al alloy specimens having stiffeners were carried out in 3.5 wt% sodium chloride environment to understand the effect of cerium chloride inhibitor on the threshold stress intensity for stress corrosion cracking as well as the second-stage (steady-state) crack growth rate. It was observed that in the peak- and the two-step-aged tempers, the crack growth rate of the alloy reduced by two and a half, and four times, respectively, while the threshold stress intensity increased by 14–15% due to the addition of 1000 ppm of this inhibitor. The study offers a method to enhance significantly life of aging aircrafts stiffened with polymer matrix composite.

Role of graded microstructure in a centrifugally cast A356-SiC particle (SiCp) composite alloy disk on passivity and localized corrosion behavior in 3.5 wt.% NaCl environment has been investigated. The area fraction of SiCp has been found to decrease from the outer periphery to the core of the disk, while the apparent size and area fraction of the intermetallics and Mg-rich phase increase from the periphery to the core. As a consequence, the corrosion resistance of the as-cast disk varies with the distance from the periphery in the order 45

Epoxy resins despite having excellent adhesion to various substrates, good corrosion and chemical resistance are less suitable for high-performance applications, as they are poor in ductility. In the present study, carboxyl terminated polyethylene glycol adipiate (CTPA) and carboxyl terminated polyethylene glycol succinate (CTPS) modifiers were synthesised and then used to modify bisphenol-A epoxy resin to raise the latter’s toughness. The study showed that in both the cases, 5 wt.% addition gave rise to the highest toughness as well as corrosion protection against salt fog and synthetic sea water exposure. However, between the two modifications the epoxy resin modified with 5 wt.% CTPS gave a better performance and so was pigmented with a mixture of zinc phosphate and red iron oxide to obtain the paint. The study showed that the epoxy resin modified with 5 wt.% CTPS + 30 vol.% pigments gave rise to the highest toughness (flexibility), adhesion and corrosion resistance when applied on to mild steel panels.

In order to improve both the hardness and corrosion resistance properties of AISI 430F stainless steel, plasma nitriding (PN) and nitrocarburising processes were carried out at different temperatur...

The present study was focused on the development of environmentally friendly graphene-based anti-corrosive coatings and understanding the effect of these coatings on the electrochemical corrosion behavior of copper. Through effective functionalization of graphene (

Hydrogen evolution rates upon magnesium were explored in 0.1 M sodium bicarbonate (NaHCO3) solution, with pH 8.4. Although the negative difference effect (NDE) was observed on Mg in 0.1 M NaHCO3, the hydrogen evolution rates showed a peculiar behavior in this electrolyte, where the hydrogen evolution rates first increased in the proximity of the open circuit potential and then decreased, with increasing anodic potential. This transition of the slope of hydrogen evolution rates versus applied anodic potential varying from positive to negative was revealed for the first time on Mg. In NaHCO3, hydrogen evolution rates decreased with time, even in the NDE region where hydrogen evolution rates increased with anodic potential. It is posited that dissolution of the air-formed surface film and evolution of a hydroxide-free magnesium carbonate film on Mg were responsible for the observed hydrogen phenomena. These findings have significant implications in clarifying the source of superfluous hydrogen upon anodically polarized Mg. (C) The Author(s) 2018. Published by ECS.

The study investigates the effect of manganese addition in the hot-dip galvanized zinc coating on steel. The Zn Mn hot-dip coatings were prepared under ambient conditions on a steel sheet using a custom-made galvanization simulator. The Zn Mn hot-dip coatings were always covered with oxides formed during the withdrawal, giving patchy coloration. A novel approach is demonstrated to prevent patchy coloration due to these oxides and produce alloy coated surfaces with homogeneous coloration. The microstructures of the alloy coated sheets were investigated to identify the effect of the Mn addition. The microstructure changes in the alloy coating due to increasing Mn addition were correlated to the composition, hardness of the individual intermetallic layers, and the basal texture. Compared to the hot-dip Zn coating, Mn addition decreased the corrosion rate (icorr from potentiodynamic polarization test) in the alloy coating by 65%. The Mn addition also controlled the degree of the white rust formation and prolonged the life of the alloy coated steels in accelerated salt spray test (> 300 h of exposure, ASTM B117 ). The bend test results show that Mn addition does not affect the deformation tolerance of the hot-dip alloy coatings.

With a size effect of the fillers, the two-way corrosion protection mechanism is manifested in the composite coating system containing graphene oxide stabilized polystyrene containers encapsulating 8-hydroxyquinoline (8-HQ@PS/GO) corrosion inhibitor. The active protection is induced by the surface damage, in response to which self-healing is observed in the coating by the release of encapsulated corrosion inhibitor. The release kinetics study of the 8-HQ@PS/GO inhibitor showed a gradual and continuous supply at the damage site. A strong size effect of the 8-HQ@PS/GO on the passive protection and self-healing properties of the composite coating is demonstrated. Compared to neat epoxy coatings, hardness and elastic modulus values in Epoxy-8-HQ@PS/GO composite coatings increased by 35% and 54%, respectively. The compact arrangement of 8-HQ@PS/GO in the composite coating matrix also lowers the surface electrical resistivity by four orders of magnitude compared to the neat epoxy coating, facilitating electrostatic charge dispersal, i.e., a potential coating for the antistatic application.

In view of their excellent combination of strength and toughness, maraging steels (18Ni 250 grade) are widely used for the fabrication of large sized solid rocket motor casings. Gas tungsten arc welding is commonly employed to fabricate these thin walled metallic casings, as the technique is not only simple but also provides the desired mechanical properties. However, sometimes, radiographic examination of welds reveals typical unacceptable indications requiring weld repair. As a consequence, there is a significant drop in weld efficiency and productivity. In this work, the nature and the cause of the occurrence of these defects have been investigated and an attempt is made to overcome the problem. It has been found that weld has a tendency to form typical Ca and Al oxide inclusions leading to the observed defects. The use of calcium fluoride flux has been found to produce a defect free weld with visible effect on weld bead finish. The flux promotes the separation of inclusions, refines the grain size and leads to significant improvement in mechanical properties of the weldment.

The work identifies parameters and proposes mechanisms that are crucial in making smart containers with encapsulating agents by Pickering emulsion route for anti-corrosion coating and advanced materials applications. We demonstrate the unmodified graphene oxide (GO) particles as potential stabilizers for oil-in-water emulsions not requiring any modification to the properties of the emulsion phases. The model system, GO stabilized styrene in water, was used to explore the mechanistic details of the stable emulsion formation and subsequent formation of polystyrene/graphene oxide (PS/GO) containers after emulsion polymerization. The results indicate a clear size effect during packing of GO at oil-water interface and smaller GO sheets produced homogenous emulsion without any residual water volume. The emulsion droplet size distribution changes from bimodal to monomodal with the increase in GO concentration and it reverses with oil–water phase ratio. For the first time, a correlation was developed between these variables and the size, polydispersity index (PDI), morphology, conversion, and yield of the containers produced by emulsion polymerization. The parametric controls required for preparing PS/GO containers with narrow size distribution (PDI = 0.23), smaller size and various morphologies are discussed, with a probable mechanism. All these studies were carried out in presence of a corrosion inhibitor, 8-Hydroxyquinoline (8-HQ), to be encapsulated in the container. The containers were produced with 75% encapsulation efficiency.

Polyvinyl alcohol (PVA) complexed with different weight percent ratios of Nickel Bromide (NiBr2) salt were prepared by using solution cast technique. X-ray diffraction analysis confirmed the complexation of the salt with the polymer. Differential scanning calorimetry was used to determine the glass transition and melting temperatures of pure PVA and PVA:NiBr2 complexed films. Electrical conductivity was measured using ac impedance analyzer in the frequency and temperature range 1 Hz–1 MHz and 303–373 K respectively. It was observed that the magnitude of electrical conductivity increases with NiBr2 salt concentration as well as temperature. Frequency dependence electrical conductivity of the complexed polymer electrolyte films follows the Jonscher’s equation. The dielectric behavior was analyzed using dielectric permittivity $$\left( {{\varepsilon ^\prime}} \right)$$ and loss tangent $$\left( {\tan \delta } \right)$$ of the samples. Relaxation time was determined from the variation of loss tangent with frequency at different temperatures. The modulus spectra indicated the non-Debye nature of the material.

The role of arsenate ions and cathodic charging on the environmentally assisted cracking (EAC) behaviour of a Mg-Mn alloy in 0.6 M NaCl saturated with Mg(OH)(2) was investigated. The stability of the surface film and the repassivation kinetics have been found' to play a critical role in the EAC tendency of the alloy than hydrogen-assisted mechanisms. Cathodic charging was found to be beneficial in lowering the EAC tendency of the alloy. (C) 2016 Elsevier Ltd. All rights reserved.

Pure magnesium reveals a previously unreported combination of enhanced anodic kinetics and unaltered cathodic kinetics in 0.1 M sodium chloride in the presence of 1 mM indium chloride. The corrosion potential of magnesium is lower than the electrode potential for indium/indium (+3) ions equilibrium causing deposition of indium on the magnesium surface. X-ray photoelectron spectroscopy confirms the presence of indium species on the magnesium surface. Despite the 1 millimolar indium chloride + 0.1 M sodium chloride solution exhibiting an acidic pH close to 4, the hydrogen evolution kinetics on magnesium remains close to that of the 0.1 M sodium chloride of near neutral pH indicating an inhibitive effect of indium on hydrogen evolution. On the contrary, the presence of 1 millimolar indium ions in 0.1 M sodium chloride has increased the anodic dissolution of Mg. The former is expected to result in higher magnesium anode efficiencies and the latter, high anode current output. Thus, the electrochemical characteristics exhibited by pure magnesium in 1 millimolar indium chloride +0.1 M sodium chloride solution have implications on magnesium as an anode, which is relevant in the context of higher performance and stable aqueous primary magnesium batteries., P. Gore acknowledges ‘Central Surface Analytical Facility of IIT Bombay’ for characterisation studies. PG would like to thank Dr Sutar for discussions regarding the interpretation of XPS data. NB is supported by Woodside Energy. S. Fajardo expresses his gratitude to the State Research Agency (Ministry of Economy, Industry and Competitiveness of Spain), the Spanish National Research Council (CSIC) and the European Regional Development Fund (ERDF) for the financial support under the Project MAT2015-74420-JIN (AEI/FEDER/UE).

By treating each current transient as a signal arising from single discrete pit events, the pit sizes of the locations associated with recurring and co-occurring pit events could be underestimated or overestimated. In the present study, the error introduced in the estimated pit sizes by this hypothesis and other factors such as the hydrogen evolution occurring within pits and pit geometry were investigated in-situ by mapping the pit locations. Further, with the insights derived, a criterion was proposed to identify the transients associated with recursive pit events that eventually developed into stable pits.

An attempt has been made to develop AA 7010 having significantly high strength and resistance to environmentally assisted cracking in 3.5 wt.% NaCl. By tailoring heat treatment based on the secondary aging concept, strength levels equaling peak aged condition and environmentally assisted cracking resistance exceeding conventional over aged condition has been obtained. Coarser and discontinuous network of grain boundary precipitates with higher Cu enrichment than even over aged alloy have led to this beneficial effect. (C) 2016 Elsevier Ltd. All rights reserved.

We report the formation of encapsulated graphene oxide/polystyrene (GO/PS) containers via Pickering emulsion polymerization of oil-in-water emulsions stabilized by GO sheets. Without further functionalization, the as-synthesized GO sheets were used for emulsion stabilization to minimize the process step. A one-step process was developed to encapsulate 8-Hydroxyquinoline (8-HQ) in GO/PS containers. FT-IR study confirmed the encapsulation. The inhibiting effect of 8-HQ was found to affect the kinetics of the emulsion polymerization and shift the polymerization temperature to a higher value. A detailed study of stability, size, and morphology of encapsulated containers (PS/GO) was carried out as a function of GO-to-oil ratio and oil volume fraction in the formulation, to optimize the process parameters. A possible mechanism for the formation of GO/PS containers at various GO-to-oil ratios was discussed. The corrosion inhibition performance of 8-HQ-encapsulated containers was studied in an epoxy coating. These encapsulated containers induced long-term anticorrosive property to the coating. The use of cost-effective materials and a scalable method used in this work can be utilized to prepare environmentally friendly coatings for large-area applications.

Prior studies on synthesis of nanocrystalline elements have discussed the effect of ball milling on lattice parameter, crystallite size, and micro-strain. For elemental milled powders, the anisotropic peak broadening does not change with increasing milling time. However, the effect of alloying addition on the anisotropic behavior of ball milled nanocrystalline powders remains an unexplored area. Here we report the effect of chromium and aluminum addition on the anisotropic behavior of iron in nanocrystalline Fe-20Cr-5Al (wt%) alloy powders synthesized by ball milling. The experimental results show that the anisotropic behavior of iron changes towards isotropic with milling. This change was also correlated to the theoretically calculated anisotropic factor from the change in elastic constant of iron due to milling. Addition of alloying elements exhibited a monotonic rise in the lattice parameter with crystallite size, which was attributed to the excess grain boundary interfacial energy and excess free volume at grain boundaries. Transmission electron microscopy image confirmed the crystallite size and nature of dislocation obtained using modified Williamson-Hall method. (C) 2016 Elsevier B.V. All rights reserved.

Gas tungsten arc welding is widely employed to fabricate large sized motor casings made of 18Ni 250 maraging steel. However, small transverse cracks have been found to appear at the interface of the dark etched heat affected zone (HAZ) of such weldments after about a year of welding, prior to aging treatment. In order to understand the delayed cracking phenomena vis-a-vis the influence of multiple weld passes, thermal cycles were obtained at different locations of the heat affected zone. The results suggest the formation of two more HAZs designated as HAZ III and HAZ IV in addition to those two currently reported in the literature. The two regions namely, HAZ III and HAZ IV, respectively correspond to peak aging and under aging conditions of the alloy. In order to examine the effect of these thermal cycles on the environmentally assisted cracking of the weldment, thermal simulations corresponding to various HAZs were applied on to specimens with Gleeble. They were examined for their susceptibility to EAC in neutral and acidified 0.1 M NaCl solutions. The study reveals that it is only the HAZ IV, lying at the interface of the dark etched HAZ, corresponding to under aging condition, is responsible for the weld failure.

A popular hardfacing alloy consisting of high chromium white iron (HCWI) was deposited on low carbon steel using shielded metal arc welding. The wear-resistant alloys are not only used in applications requiring wear resistance but also used in applications involving fluids of a range of pH values. It is well known that chromium containing alloys have good corrosion resistance due to the presence of passive film. In this study for understanding the film formation on carbides and eutectic austenite matrix, HCWI alloy were exposed to potentials (0.157 V vs. SCE and 0.758 V vs. SCE) derived from potentiodynamic studies in highly alkaline solution of pH 14, simulating Bayer refining process in alumina industry. Electrochemical impedance spectroscopy (EIS) technique was used to analyse the impedance of films. The thickness and the composition of film properties at various potentials were investigated by time-of-flight secondary ion mass spectrometry (ToF-SIMS). At potential 0.157 V versus SCE, chromium carbide had a thick layer of oxide film consisting of mainly FeO with minor amounts of CrO. The eutectic austenite matrix consisted of a much denser and slightly thinner film of FeO. It is proposed in this study that for the formation of an oxide film on carbides, the carbides would have to dissociate on the surface. This dissociation of carbides may likely render the hardfacing alloy less effective in wear-resistant applications. The results were explained through superimposing the Pourbaix diagram of chromium carbides and iron (Fe).

In recent times, plasma sprayed powder-solution precursor hybrid composite thermal barrier coatings have been developed to harness the dual benefits of both conventional atmospheric plasma spraying (APS) and solution precursor plasma spraying (SPPS) processes. In this study, hot corrosion behavior of plasma sprayed powder-solution precursor composite (PSP-SPC) YSZ TBCs in molten salt mixtures of 90 wt.% Na2SO4 + 5 wt.% V2O5 + 5 wt.% NaCl and 50 wt.% Na2SO4 + 50 wt.% V2O5 at 900 °C was investigated. The employed coating showed a bimodal microstructure comprising coarse splats derived from the powder feedstock as in the APS process and fine splats resulting from the solution precursor as typical of SPPS process. The PSP-SPC coatings showed a significantly higher resistance to spallation than APS, SPPS and EB-PVD coatings in both the salt environments. These coatings showed shorter life in vanadate environment compared to that of the chloride environments as the former promotes the formation of monoclinic ZrO2 and YVO4 phases more than the latter.

This article reports an attempt to develop high-strength aluminum alloys of 7xxx series resistant to intergranular stress corrosion cracking (SCC). A novel aging technique reported in this work exhibited improved strength levels (as high as similar to 100 MPa to that of conventional overaged temper for AA 7050) with significant resistance to SCC measured even at a low strain rate (10(-7) s(-1)) in 3.5 wt pct NaCl. The novel aging heat treatment produced a microstructure that is finer and dense enough in the matrix to impart strength, whereas it is enriched with Cu on the grain boundaries to impart SCC resistance. A detailed explanation for the enhanced strength and SCC resistance is discussed. (C) The Minerals, Metals & Materials Society and ASM International 2018

Hot salt stress corrosion cracking (HSSCC) behaviour of alloy IMI 834 was studied to examine the effect of long-term exposure and the role of hydrogen. Studies were conducted by exposing the salt-coated specimens at 300, 350 and 400 °C, at different stress levels for 1000 h, and in the unstressed condition; and the role of hydrogen in HSSCC was studied by using X-ray diffraction. Although the specimens passed 1000 h of hot salt exposure test at stress levels higher than their yield strength, at these temperatures, the post-exposure tensile tests showed a significant loss in ductility and the presence of micro cracks. The study shows the presence of hydrides, which are responsible for HSSCC. Keywords: Titanium alloy, Stress corrosion cracking, Hydrogen embrittlement, Scanning electron microscope, X-ray diffraction

2205 duplex stainless steel despite possessing good corrosion resistance suffers from low hardness. With a view to improve hardness and corrosion resistance, plasma nitriding and nitrocarburizing processes were performed at 350, 400, 450 and 500 degrees C for 4h. Specimens treated at 400 degrees C by plasma nitrocarburizing exhibits the highest corrosion resistant with a significant raise in surface hardness. At higher temperatures, specimens treated by these processes show poor corrosion resistance. This study highlights the difference in plasma nitriding behavior of the ferrite and austenite phases and not so different behavior of these phases when subjected to plasma nitrocarburizing. (C) 2015 Elsevier Ltd. All rights reserved.

A286 (AISI 660), an austenitic precipitation hardening stainless steel has good strength and corrosion resistance, but its low hardness restricts their use in applications involving abrasive wear. In order to improve the hardness without loss of corrosion resistance properties of A286 stainless steel, plasma nitrocarburizing process was carried out at temperatures ranging from 350 degrees C to 500 degrees C for 4 and 8 h. The gas mixture composition employed during the process was 78% nitrogen, 20% hydrogen and 2% acetylene gas. Plasma was generated using a pulse DC power supply having a frequency of 30 kHz. After plasma nitrocarburizing, a dual layer was formed consisting of nitrogen and carbon in the top layer followed by carbon in the bottom layer. Compared with the untreated A286 stainless steel specimens, the microhardness values measured from the plasma nitrocarburized specimens treated at 400 and 450 degrees C increased by a factor of two and three respectively. The electrochemical corrosion behavior of the plasma nitrocarburized A286 specimens has been studied in 3.5 wt.% NaCl solution. The results show that specimens treated at 400 degrees C for 4 h has better pitting corrosion resistance and higher surface hardness compared with that of the untreated A286 stainless steel. This is mainly attributed to the presence of carbon and nitrogen in the modified layer existing as a solid solution. (C) 2015 Elsevier B.V. All rights reserved.

The hot salt stress corrosion cracking behavior of Ti-6A1-2Sn-4Zr-2Mo-0.1Si (Ti-6242S) alloy was studied in the temperature range from 523.15 K to 673.15 K (250 °C to 400 °C). The alloy showed marginal susceptibility at 573.15 K (300 °C), and the extent susceptibility found to increase significantly at higher test temperatures. The specimens did not fail in long-term (1000 hours) hot salt constant load exposure tests carried out at 623.15 K and 673.15 K (350 °C and 400 °C), even at the stress levels more than the 80 pct of their ultimate tensile strength. However, the salt exposure in both stressed and unstressed conditions found to significantly impair the room-temperature ductility. The study shows that pitting and formation of slip step were the precursor events for SCC initiation; and the cracks were found to grow in transgranular manner in the primary-α phase and discontinuous-faceted manner in the transformed β colony. Furthermore, the XRD analysis of hot salt-exposed specimens revealed the presence of titanium hydride phase, which could be responsible for the embrittlement.

In order to understand the possible role of magnesia stabilised zirconia (MSZ) on hot corrosion in graded thermal barrier coatings, plain and 25 and 50 wt% MSZ dispersed NiCrAlY coatings, developed using air plasma spray technique was examined by exposing them to Na2SO4 + 50 wt% V2O5 and Na2SO4 + 10 wt% NaCl environments at 800 A degrees C. The study shows that MSZ dispersion significantly increases the hot corrosion tendency of NiCrAlY coatings in both the environments and that the MSZ phase suffers destabilization into monoclinic ZrO2 in both the environments suggesting that it cannot be used for producing graded bond coats. Mg ions leach out from MSZ phase forming Mg3V2O8 and MgCl2 on exposure to vanadates and chlorides respectively. The study further demonstrates that chloride is more detrimental to the coatings than vanadate.

The electrochemical studies using micro-cell technique gave new understanding of electrochemical behaviour of nickel base alloys in solution annealed and welded conditions. The welding simulated regions depicted varied micro structural features. In case of tungsten inert gas (TIG) weldments, the weld fusion zone (WFZ) showed least corrosion resistance among all other regions. For laser beam (LB) weldments it was the heat-affected zone (HAZ) that showed comparatively high stable anodic current density. The high heat input of TIG welding resulted in slower heat dissipation hence increased carbide precipitation and segregation in WFZ resulting in high stable anodic current density. (C) 2014 Elsevier Ltd. All rights reserved.

The influence of thermomechanical treatment on the stress corrosion cracking behavior of AA7075 aluminum alloy forgings was examined in 3.5% NaCl solution by varying the extent of thermomechanical working imparted to each of the conditions. The results show that inadequate working during billet processing resulted in inferior corrosion and mechanical properties. However, more working with intermediate pre-heating stages also led to precipitation of coarse particles resulting in lowering of mechanical properties marginally and a significant reduction in the general/pitting corrosion resistance. The results obtained in the present study indicate that optimum working with controlled pre-heating levels is needed during forging to achieve the desired properties. It is also demonstrated that AA7075 in the over aged condition does not show any environmental cracking susceptibility in spite of the microstructural variations in terms of size and volume fraction of the precipitates. However, the above microstructural variations definitely affected the pitting corrosion and mechanical properties significantly and hence a strict control over the working and pre-heating stages during billet processing is suggested.

The hot salt stress corrosion cracking (HSSCC) behaviour of a near alpha titanium alloy, IMI 834, was studied using slow strain rate test method in the temperature range 250-450 degrees C. The alloy was found to be susceptible to HSSCC at 300 degrees C and above; and the susceptibility increased with an increase in the test temperature. The secondary cracks grew exponentially with temperature. HSSC cracks initiated at alpha/beta interfaces and grew across the primary alpha grain and along the alpha/beta interfaces. (C) 2014 Elsevier Ltd. All rights reserved.