Grain Boundary Dissolution Research Articles

Illustrating the influence of anodic/cathodic second-phases on dissolution mode via Mg–Ca–Sn alloy anodes for primary Mg-air cells

Two second-phases with an opposite potential difference to the α-Mg matrix may affect the discharge mechanism of magnesium (Mg) anodes for primary Mg-air cells through different micro-galvanic behaviors. Herein, using Mg–Ca-xSn alloys anode with Mg2Ca and CaMgSn phases, we investigated their influence on the dissolution mode of Mg-anode during discharge state via in-situ scanning vibrating electrode technique (SVET) technology. Firstly, the peak surface positive potential of Mg–1Ca alloy only containing Mg2Ca phase and Mg–1Ca–1Sn alloy with CaMgSn/Mg2Ca phase increases multiplied by 10 during the discharge process compared to that of OCP, exhibiting accelerated anodic substrate dissolution. Moreover, Mg2Ca-phases can be attacked firstly while promoting the discharge of the α-Mg matrix surrounding them, leading to the preferential dissolution of grain boundaries and “chunk effects (CE)” occurrence. More CaMgSn/α-Mg couples are activated to form dispersed micro-galvanic cell islands, which also boosts the dissolution of the α-Mg matrix but reduces the area of high-intensity dissolution reaction due to CaMgSn acting as a cathode. However, excessive CaMgSn-phase generally fails to discharge effectively to reduce anode efficiency, although it can improve the self-peeling capacity of the film layer through physical detachment; in addition, with Sn content increasing to 3 wt%, the open circuit voltage of Mg–1Ca–3Sn significantly declines. Mg–1Ca-0.2Sn exhibits the highest cell voltage and anodic efficiency by introducing a tiny amount of dispersed short rod-like CaMgSn, due to the adjusted dissolution mode, enhanced electrochemical activity and inhibited CE. Finally, our study provides a universal mechanism for fabricating Mg-anode with high performance via phase composition modulation.

Desensitization of AA5083 alloy via pulsed laser irradiation and its microstructural mechanism

In the present work, a laser surface desensitization (LSD) method was carried out on sensitized AA5083 alloy with the underlying microstructural evolution examined. The enhanced corrosion resistance after LSD process was confirmed by nitric acid mass loss test (NAMLT), immersion test and electrochemical measurements, which is mainly ascribed to the modification of grain boundary chemistry. Accompanied with the dissolution of grain boundary β (Al3Mg2) phase during LSD process, the grain boundaries could be characterized by Mg segregation or nano-sized Mg-rich clusters, which may also be absent from chemical uniformity depending on grain boundary misorientation.

Quasi-In Situ Localized Corrosion of an Additively Manufactured FeCo Alloy in 5 Wt Pct NaCl Solution

FeCo alloys are important materials used in pumps and motors in the offshore oil and gas drilling industry. These alloys are subjected to marine environments with a high NaCl concentration, therefore, corrosion and catastrophic failure are anticipated. So, the surface dissolution of additively manufactured FeCo samples is investigated in a quasi-in situ manner, in particular, the pitting corrosion in 5.0 wt pct NaCl solution. The local dissolution of the same sample region is monitored after 24, 72, and 168 hours. Here, the formation of rectangular and circular pits of ultra-fine dimensions (less than 0.5 µm) is observed with increasing immersion time. In addition, the formation of a corrosion-inhibiting surface layer is detected on the sample surface. Surface dissolution leads to a change in the surface structure, however, no change in grain shape or grain size is noticed. The surface topography after local dissolution is correlated to the grain orientation. Quasi-in situ analysis shows the preferential dissolution of high-angle grain boundaries (HAGBs) leading to a change in the fraction of HAGBs and low-angle grain boundaries fraction (LAGBs). For the FeCo sample, a potentiodynamic polarisation test reveals a corrosion potential (Ecorr) of − 0.475 V referred to the standard hydrogen electrode (SHE) and a corrosion exchange current density (icorr) of 0.0848 A/m2. Furthermore, quasi-in situ experiments showed that grains oriented along certain crystallographic directions are corroding more compared to other grains leading to a significant decrease in the local surface height. Grains with a plane normal close to the langle {1}00rangle direction reveal lower surface dissolution and higher corrosion resistance, whereas planes normal close to the langle {11}0rangle direction and the langle {111}rangle direction exhibit a higher surface dissolution.

Stress corrosion cracking failure analysis of AISI 1018 carbon steel reinforcing bars in carbonated and chloride contaminated environment

Herein, the premature failure analysis of AISI 1018 carbon steel reinforcing bars induced by stress corrosion cracking (SCC) and its pit-to-crack transition mechanism are presented. Service conditions under the influence of carbonation process and chloride contaminated environment are mimic to emulate pH drop and localized attack, respectively. The crack growth rate (vcrack) was found to be more pH than chloride dependent, developing narrower and deeper cracks in high alkaline environment (pH 12.6), whereas short cracks were found in carbonated solution (pH 9.1). The formation of siderite (FeCO3) and further dissolution lead to ferric oxyhydroxide build up, enhancing the anodic kinetics by the local iron acid hydrolysis, increasing the dissolution rate, and promoting crack propagation after the stress intensity factor to develop SCC (KISCC) was reached. Under carbonated conditions the SCC susceptibility increased over the alkaline, considerably reducing the mechanical performance. During the electrode straining, the recorded potential lied within the SCC risk zone and above the iron oxidation potential, allowing for the selective dissolution of grain boundaries that favors intergranular SCC through the ferrite/pearlite interface. Microcrack propagation found on pearlitic colonies followed the Miller-Smith mechanism, promoting major transgranular cracking.

A study of reaction between palladium, palladium silver alloy and silicon carbide ceramics at high temperature

The reactions between palladium (Pd), palladium-silver alloy (Pd-Ag) and silicon carbide (SiC) from 1200 °C to 1400 °C have been studied to investigate the impact of liquid phases on reaction products formation. The liquid phases were generated in Pd/SiC reactions at 1400 °C and in Pd-Ag/SiC reactions above 1300 °C. An increase in the amount of liquid associated with higher temperatures or Ag presence strongly affected the reaction mechanism and was responsible for the inhomogeneous dissolution of SiC grains and grain boundaries by Pd-rich phase at reaction front. This study helps understand Pd’s role on Ag release through SiC layer in Tri-structural isotropic (TRISO) fuel particles.

Electrochemical micro-couple mechanism based on Si and Mn segregations revealing the initiation of grain boundary dissolution for Q235 carbon steel

The purpose of this work was to reveal the initiation mechanism of grain boundary dissolution (GBD) for carbon steels in acidic electrolytes containing NO2–. For Q235 carbon steel in a HNO3-NaNO2 solution with pH 4, the electrochemical methods of potentiodynamic polarization and scanning Kelvin probe force microscope (SKPFM) were applied to explore electrochemical behavior, and the surface methods of scanning electron microscope (SEM), electron back-scatter diffraction (EBSD) and scanning tunneling microscope (STM) were applied to explore surface microstructure and composition. Q235 carbon steel presented the electrochemical behavior of activation-passivation-transpassivation in pH 4 HNO3-NaNO2 solution, GBD occurred on steel surface during anodic polarization. GBD initiation, which showed a selective characteristic, was found at the grain boundaries of relatively high Si and Mn contents (high Si-Mn grain boundary) when Q235 carbon steel was polarized to active–passive transition potential. The grain boundary segregations of Si and Mn played a critical role in the initiation of GBD, and an electrochemical micro-couple (EMC) mechanism based on the Si and Mn segregations was proposed to reveal GBD initiation. EMC potential difference between high Si-Mn grain boundary and grain interior was greater than that between low Si-Mn grain boundary and grain interior, resulting in that GBD initiated at high Si-Mn grain boundary. The detailed EMC mechanism of GBD initiation was discussed in this work.

Epidote dissolution–precipitation during viscous granular flow: a micro-chemical and isotope study

Abstract. Deformation of polymineralic aggregates can be accommodated by viscous granular flow, a process mediated by the interplay among intracrystalline plasticity and dissolution–precipitation, each active in specific minerals under given P–T conditions. Some rock-forming minerals like quartz and feldspars have been intensively studied in terms of deformation processes. Instead, the deformation behavior of epidote and its role during viscous granular flow is not well investigated, although this mineral is ubiquitous in granitic rocks deforming under greenschist-facies conditions. In this contribution, we provide microstructural and geochemical evidence for the occurrence of dissolution–precipitation of epidote during deformation of an epidote–quartz vein. The main part of the vein is deformed, producing a fold, which is visible due to relicts of primary-growth layering inside the vein. The deformation mechanisms active during deformation include dynamic recrystallization of quartz by subgrain rotation recrystallization, producing grain size reduction in the primary vein quartz. Recrystallization occurs contemporaneously with dissolution and (re)precipitation of epidote and quartz grain boundary sliding, leading to a combined process described as viscous granular flow. The combination of grain boundary sliding and dissolution locally and repeatedly produces creep cavities. These represent not only loci for nucleation of new epidote grains at the expense of dissolved ones, but they also allow fluid-mediated transport of elements. The same trace element patterns between old epidote relicts and newly formed grains, with much narrower variability in the latter, indicate a process of chemical homogenization. The nature of the fluid that mediates deformation is investigated using Pb–Sr isotope data of epidote, which suggest that deformation is assisted by internally recycled fluids with the addition of a syn-kinematic external fluid component.

Improving creep property of additively manufactured Inconel 718 through specifically-designed post heat treatments

The application of Inconel 718 (IN718) alloy parts at elevated-temperature under loading leads to creep failure. To date, the creep performance of additively manufactured IN718 is usually inferior to the conventionally fabricated counterparts. In this work, post heat treatment schemes were designed and applied to the LPBF-built IN718 to achieve enhanced creep performance. The results have revealed an effective heat treatment approach that can significantly improve the creep performance of LPBF IN718 to be comparable to conventional counterparts. Systematic microstructure investigations, as well as theoretical calculations, indicated that the improved creep properties originated from significant modifications on grain structures, Laves phase and secondary precipitates. Specifically, the change from columnar to equiaxed grain structure through post heat treatment reduced the grain boundary areas that were susceptible to cavitation. Meanwhile, the effective dissolution of grain boundary Laves phase reduced the number of sites for cavity nucleation and improved the creep lifetime. Moreover, the dissolution of Nb into the matrix promoted the formation of the main strengthening γ'' precipitates and thus enhanced the creep resistance. The findings in the current work provide the heat treatment strategy for the design of additively manufactured superalloys towards better creep performance.

CMAS corrosion and thermal cycling fatigue resistance of alternative thermal barrier coating materials and architectures: A comparative evaluation

The corrosion of ceramic thermal barrier coatings (TBCs) by molten silicate deposits, usually known as “CMAS” from their main constituents (CaO-MgO-Al2O3-SiO2), is an issue of increasing concern in modern gas turbines as the turbine inlet temperatures are increased to enhance thermodynamic efficiency. Because conventional ZrO2-7wt%Y2O3 (7YSZ) dissolves quite readily in a CMAS melt, many alternative materials have been proposed, but there are not many comparative studies among them. Multi-layer architectures featuring a tougher 7YSZ bottom layer and a more brittle, but more CMAS corrosion-resistant top layer have also been proposed; therefore, a comparison among these architectures is also in order. In this paper we studied comparatively the resistance to CMAS corrosion and to thermal cycling fatigue (an essential pre-requisite for any TBC system) of Gd2Zr2O7, ZrO2–55wt%Y2O3 and Gd/Yb/Y co-doped ZrO2, both in the form of single, dense-vertically cracked (DVC) layers deposited by plasma spraying onto an MCrAlY bond coat, and as top layers with a bottom layer of either porous or DVC 7YSZ. It was found that Gd2Zr2O7 resists CMAS corrosion, without any grain-boundary dissolution, slightly better than does ZrO2–55wt%Y2O3. They both develop a solid Gd- or Y-apatite layer (respectively) at the interface with the CMAS melt, driven by the rather large difference in optical basicity between these compounds and CMAS itself, but the Y-apatite layer is less continuous and, therefore, a bit less protective. Gd/Yb/Y co-doped ZrO2, instead, suffers as much grain-boundary dissolution in contact with molten CMAS as does 7YSZ. A Gd2Zr2O7/porous 7YSZ system would therefore exhibit simultaneously high resistance to CMAS dissolution and to thermal cycling fatigue, although there is a risk that the CMAS melt might infiltrate the segmentation macro-cracks and the microcracks of the Gd2Zr2O7 layer and undermine the porous 7YSZ bottom layer.

Transmission Electron Microscopy Study on the Precipitation Behaviors of Laser-Welded Ferritic Stainless Steels and Their Implications on Intergranular Corrosion Resistance

The intergranular corrosion susceptibility of ferritic stainless-steel weldments is strongly dependent on chromium carbide precipitation phenomena. Hence, stabilization is widely used to mitigate the aforementioned precipitation. In contrast, stabilization has proved ineffective to fully prevent intergranular corrosion due to segregation of unreacted chromium during solid-state heat-treatments. To analyze the precipitation behavior of 17 wt.-% chromium ferritic stainless steels during laser welding, sheets of unstabilized and titanium-stabilized ferritic stainless steels were welded in a butt joint configuration and characterized with special consideration of precipitation behavior by means of transmission electron microscopy. While unstabilized ferritic stainless steels exhibit pronounced chromium precipitate formation at grain boundaries, titanium-stabilization leads to titanium precipitates without adjacent chromium segregation. However, corrosion tests reveal three distinctive corrosion mechanisms within the investigated ferritic stainless steels based on their inherent precipitation behaviors. In light of the precipitation formation, it is evident that immersion in sulfuric acid media leads to the dissolution of either grain boundaries or the grain boundary vicinity. As a result, the residual mechanical strength of the joint is substantially degraded.

Effects of critical defects on stress corrosion cracking of Al–Zn–Mg–Cu–Zr alloy

The effects of critical defects on stress corrosion cracking (SCC) of Al–7.82Zn–1.99Mg–2.41Cu–0.12Zr alloy under under-ageing (UA), peak-ageing (PA) double-ageing (DA) and retrogression and reageing (RRA) were investigated. It is found that anodic dissolution of grain boundary precipitates (GBPs) for UA sample generates critical defects due to low Cu content of GBPs, whereas the dissolution of Al, Mg and Zn elements within the grain interior is responsible for the formation of critical defects for PA, DA and RRA samples. The high distribution continuity of GBPs promotes the continuous propagation of stress corrosion crack along grain boundary, leading to intergranular SCC for UA sample. Coarsened GBPs which are distributed discontinuously could hinder anodic dissolution along the grain boundary. As a result, the cracks propagate along the grain interior, causing transgranular SCC for DA sample. And large size and low density of matrix precipitates (MPTs) decease SCC susceptibility. Al7Cu2Fe particles promote the dissolution of the surrounding matrix, inducing local stress concentration, and thus leading to the generation of stress corrosion cracks.

Current-driving intergranular corrosion performance regeneration below the precipitates solvus temperature in Al–Mg alloy

Heat treatment is an effective method to improve the intergranular corrosion resistance of the sensitized Al–Mg alloy due to dissolution of the grain boundary precipitates above the solvus temperature of β-phase. The grain boundary precipitates will grow and coarsening below the solvus temperature. In this study, the in-situ intergranular corrosion performance regeneration of the sensitized Al–Mg alloy can be realized by low-density electro-pulsing treatment below the solvus temperature of β-phase. Our findings show that the dissolution of grain boundary precipitates by electro-pulsing treatment is accelerated at relatively low temperature in comparison to traditional heat treatment. The athermal effect produced by the interaction between atoms and electrons on the dissolution of grain boundary precipitates is the main reason for the improved corrosion resistance below the solvus temperature of β-phase.

CMAS corrosion of YSZ thermal barrier coatings obtained by different thermal spray processes

Degradation of yttria-stabilized zirconia (YSZ) layers by molten CaO-MgO-Al2O3-SiO2 (CMAS)-based deposits is an important failure mode of thermal barrier coating (TBC) systems in modern gas turbines. The present work aimed to understand how the chemical purity and microstructure of plasma-sprayed YSZ layers affect their response to CMAS corrosion. To this end, isothermal corrosion tests (1 h at 1250 °C) were performed on four different kinds of YSZ coatings: atmospheric plasma-sprayed (APS) layers obtained from standard- and high-purity feedstock powders, a dense – vertically cracked (DVC) layer, and a suspension plasma sprayed (SPS) one. Characterization of corroded and non-corroded samples by FEG-SEM, EBSD and micro-Raman spectroscopy techniques reveals that, whilst all YSZ samples suffered grain-boundary corrosion by molten CMAS, its extent could vary considerably. High chemical purity limits the extent of grain-boundary dissolution by molten CMAS, whereas high porosity and/or fine crystalline grain structure lead to more severe degradation.

Effects of pre-strain on sensitization and interganular corrosion for 304 stainless steel

The degree of sensitization (DOS) and the susceptibility to interganular corrosion (IGC) were investigated for pre-strained 304 stainless steel (SS) following thermally aged at 750 °C for one hour. The maximum pre-strain applied to the steel is 20%.The microstructures of 304 SS subjected to pre-strain and thermal aging were observed. The volume fraction of strain-induced α'-martensite and the vickers hardness were measured, respectively. The double loop electrochemical potentiokinetic reactivation (DL-EPR) tests were used to characterize the DOS as a function of various pre-strain levels. The tests in copper‑copper sulfate‑sulfuric acid (16%) solution were performed to evaluate the susceptibility to IGC. The morphologies of the cracks caused by IGC were observed. The results show, although pre-strained 304SS had been thermally aged at 750 °C for one hour, strain-induced α'-martensite and shear bands in steel are most retained and increase with pre-strain, causing the strengthening effect and leading to the increase in hardness. The DOS and the susceptibility to IGC also increase with pre-strain. The higher the value of the DOS is, the more serious IGC becomes. Obvious intergranular cracks as well as the dissolution of grain boundaries and the grains began to appear in the steel with 5% pre-strain, and the attack had been deteriorated with the increasing pre-strain. When the pre-strain is reached 20%, the longest crack is about 250 μm and its width is about 38 μm, indicating that the severe IGC had occurred. No obvious reversion of martensite to austenite and desensitization occurred for 304 SS withstood the maximum pre-strain up to 20% and thermally aged at 750 °C for one hour.

Influence of the Structural-Phase State of Chromium Steels on Their Corrosion in Lead Melts

We study the influence of the phase-structural state of austenitic (Fe–18Cr) and ferritic (Fe–11Cr) steels on their corrosion behavior in lead melts with low concentrations of dissolved oxygen ( C[O]Pb = 2 ·10–6 –10–7 wt.%). The tests are performed at 550°С. It is shown that the ferritic steel is more corrosion resistant than the austenitic steel. Corrosion is accompanied by the dissolution of grain boundaries and penetration of melts into the bulk of the material with simultaneous dissolution of the alloying elements (Cr, Ni, and Mn).

Electron beam melting of Inconel 718: effects of processing and post-processing

ABSTRACTThis study reports the effect of process temperature on microstructure evolution of electron beam melted Inconel 718. Samples fabricated at 915°C had fine grain boundary δ (∼200 nm) along with coarse intragranular δ spanning through the length of the grains. On the other hand, samples fabricated at 990°C, only had grain boundary δ along with secondary carbides. During hot isostatic pressing, the distribution of carbides governs the grain growth vs. lack of it. The samples fabricated at 990°C having grain boundary carbides had no grain growth owing to the pinning effect of carbides. In contrast, the sample processed at 915°C had significant grain growth owing to dissolution of grain boundary δ phase and absence of grain boundary carbides.

Austenitic Reversion of Cryo-rolled Ti-Stabilized Austenitic Stainless Steel: High-Resolution EBSD Investigation

In this study, AISI 321 austenitic stainless steel (ASS) was cryo-rolled and subsequently annealed at 650 and 800 °C to reverse BCC α′-martensite to FCC γ-austenite. The texture evolution associated with the reversion at the selected temperatures was investigated using high-resolution EBSD. After the reversion, TiC precipitates were observed to be more stable in 650 °C-annealed specimens than those reversed at 800 °C. {110} texture was mainly developed in specimens subjected to both annealing temperatures. However, specimens reversed at 650 °C have stronger texture than those annealed at 800 °C, even at the higher annealing time. The strong intensity of {110} texture component is attributed to the ability of AISI 321 ASS to memorize the crystallographic orientation of the deformed austenite, a phenomenon termed texture memory. The development of weaker texture in 800 °C-annealed specimens is attributed to the residual strain relief in grains, dissolution of grain boundary precipitates, and an increase in atomic migration along the grain boundaries. Based on the observed features of the reversed austenite grains and estimation from an existing model, it is suspected that the austenite reversion at 650 and 800 °C undergone diffusional and martensitic shear reversion, respectively.

Metallurgical Analysis of Cracked AA7075 Aluminum Alloy Component Used in Control System of a Satellite Launch Vehicle

High-strength aluminum alloy AA7075 forgings are widely used in various control system components of liquid engines of satellite launch vehicles. In one such application, a forged component was found to crack under storage. Multiple cracks were noticed on the surface of the component. Fractographic features indicated intergranular mode of failure. Optical microscopic examination revealed dendritic coring in the material and crack propagation to be along the grain boundaries. Anodic dissolution of grain boundaries was also noticed. Based on detailed metallurgical investigations, it was concluded that the failure was due to ‘stress corrosion cracking.’

Crack Chemistry Control of Intergranular SCC in Sensitized Al-Mg

Fracture mechanics experiments and occluded crack chemistry modeling validate the mechanism for intergranular stress corrosion cracking (IGSCC) of sensitized Al-Mg alloys: dissolution of discontinuous grain boundary β (Al3Mg2) precipitates activates Al-Mg (α) solid solution dissolution to acidify the crack solution for crack tip hydrogen embrittlement. Slow-rising displacement experiments with precracked specimens establish the strong effect of applied potential on IGSCC kinetics for sensitized AA5083-H131 (S-L orientation, 22 mg/cm2) in neutral NaCl solution. Anodic polarization increases growth rates through enhanced α dissolution for crack acidification and H uptake, whereas cathodic polarization below the pH-sensitive α breakdown potential reduces growth rates by limiting Al dissolution, and thus crack acidification and the overpotential for H production (ηH). Polarization below the β breakdown potential eliminates IGSCC by precluding β dissolution and crack acidification. Cathodic polarization could ...

Surface Corrosion of Nanoscaled Hydroxyapatite During an In Vivo Experiment.

Hydroxyapatite (HA) is widely used as a bioactive ceramics as it forms a chemical bond with bone. However, the drawback to using this material is its inferior mechanical properties. In this research, surface corrosion and disintegration of nanoscaled HA in a dog were studied, and the mechanism by which phase-pure HA dissolved in vivo was investigated. Biological properties of HA in vivo are affected by the grain-boundary dissolution followed by a surface corrosion and microstructural disintegration. This kind of dissolution process, apparently evidenced at the grain boundary, causes particle generation, which indicates that both long-term bone in-growth and mechanical properties can dramatically deteriorate. Implant dissolution by osteoclasts in vivo is also observed on the surface of hydroxyapatite. Implant surface showed an aggressive corrosion by an osteoclast resorption. Severe and deeper dissolution underwent close to osteoclast resulting in formation of smaller and more round particle shape.