The influence of aluminum (Al) addition content on the high-temperature chlorine corrosion behavior of Fe-Ni-Cr-Mo alloy (254SMo) alloy modified by Al additions was investigated in a simulated waste incinerator environment containing N2-2.6 vol%, CO2-1.3 vol%, and O2-2,700 vppm HCl at 700 degrees C. The results demonstrate that as the Al content increases, the microstructure transforms from a single-phase austenite to an intermetallic compound phase structure comprising (Fe,Ni) and FeAlCr, accompanied by grain refinement. The corrosion kinetics curves of 254SMo alloys with 0 at%, 5 at%, 10 at%, and 15 at% Al addition after 80 h of corrosion all follow the parabolic law. The 254SMo:5Al alloy exhibited the highest corrosion weight gain of 1.3280 mg/cm2, while the 254SMo:15Al alloy showed the lowest weight gain of 0.1116 mg/cm2. The chlorine corrosion product layer exhibits a tri-layer structure of "FexOy/Cr2O3/Al2O3", where the density of the inner Al2O3 layer increases with higher Al content, effectively inhibiting the chlorine cycle during the "active oxidation" process. The 254SMo:5Al alloy demonstrated poor corrosion resistance due to the discontinuous Al2O3 layer, leading to localized oxide formation. In contrast, the 254SMo:15Al alloy forms a continuous Cr2O3/Al2O3 composite oxide layer, significantly enhancing the inhibition efficiency against Cl-diffusion and achieving optimal high temperature chlorine corrosion resistance.
Corrosion in aquatic environments causes significant economic losses and structural degradation. This study models the corrosion rate of S235 carbon steel using machine learning (ML) under real-world aquatic conditions. A field dataset from 46 locations along the Ghent-Terneuzen canal was used, encompassing exposure and environmental parameters such as temperature, pH, total dissolved oxygen (HDO%), chlorophyll concentration, oxidation-reduction potential (ORP), total dissolved solids, chloride concentration, specific conductivity, depth, and salinity. Six ML algorithms, including Light Gradient Boosting Machine (LightGBM), Gradient Boosting Regressor, Random Forest, Extreme Gradient Boosting, Neural Network, and Categorical Boosting were benchmarked before and after feature selection. This work demonstrates that environmental feature selection provides substantially greater predictive improvement than model architecture choice: feature selection enhanced all algorithms from poor (R2 <= 0.14) to strong performance (R2 = 0.70 to 0.80), reduced intermodel variation by 64%, and decreased prediction error by 48% (RMSE) and 74% (MSE). LightGBM achieved the best performance (MSE = 0.003, R2 = 0.80). Unexpectedly, feature importance analysis identified that salinityand depth, traditionally considered critical factors, showed minimal predictive influence, while exposure duration, pH, HDO%, temperature, chlorophyll concentration, and ORP-dominated corrosion behaviour. These findings emphasize the critical role of environmental parameters and feature selection over model complexity, supporting more efficient corrosion monitoring and management in marine and aquatic environments.
Aluminum (Al) alloys, especially the 5xxx and 7xxx series, are widely used in various applications due to their excellent mechanical properties achieved through alloying. Exploring the effect of alloying elements on the corrosion resistance of Al alloys is an important step in the development of high-performance Al materials. In this work, density functional theory (DFT) calculations were utilized to examine the role of alloying elements on galvanic corrosion behavior in 5xxx and 7xxx series Al alloys, and the impact of aggressive ions on the protective properties of their passive films. According to the DFT calculation results, the potential difference between the Al matrix and Mg2Al3 shows an initial decrease followed by an increase as the concentration of Mg increases in 5xxx series alloys (Al-Mg). For 7xxx series alloys (Al-Zn-MgCu), increasing Cu content enhances the susceptibility to galvanic corrosion. Additionally, competitive adsorption diagrams of various ions on the passive film of the alloyed Al in chloride-containing solutions were plotted. The Si-additive in the film promotes the adsorption of hydroxide ions (OH-) on the passive films, thus effectively blocking the penetration of harmful Cl- ions. However, when passive films are modified by Mg, Zn, or Cu additives, the film degradation is accelerated. This study explains the mechanisms of microgalvanic corrosion and ion adsorption on the passive film of Al alloys, thereby providing the theoretical basis for designing corrosion-resistant alloys.
Protective epoxy coatings on oil and gas pipelines can effectively mitigate corrosion. However, their performance deteriorates under the impact of externally applied potentials or in the presence of physical defects such as scratches or pores. To better understand the impact and associated mechanism, this work studied the cathodic delamination, anodic corrosion resistance, and associated mechanisms of two coatings on pipeline steel in a weakly alkaline environment: a solvent-free epoxy glass fiber-reinforced plastic coating (FRP) and a modified variant coating (MFRP). Their performances were evaluated in the presence of pre-existing defects of drilled holes and crevices at the coating/steel interface. Under cathodic conditions, the FRP coating exhibited a near-parabolic power-law increase in delamination length with time, indicating an ion-migration-controlled delamination process, whereas the MFRP coating showed negligible delamination. When the MFRP coating with a drilled hole defect acted as the anode, substantial iron substrate dissolved to primarily form Fe3+, leading to a weak acidic pH. In contrast, both oxidation and mass transport within the crevice defect were substantially inhibited, leading to a reduced iron concentration (primarily Fe2+) and an elevated electrolyte pH. Multiphysics simulations revealed the spatial distribution of current densities across the exposed defects and reproduced the similar qualitative trend observed experimentally in terms of dissolved iron concentration. These findings elucidate the mechanisms by which the coatings mitigate corrosion at different defective coating/metal interfaces under potentiostatic conditions.
The influence of 1,000 ppmm of acetic acid on the corrosion mechanisms of mild steel with up to 10 mbar (10,000 ppmv) of H2S with and without CO2 at atmospheric pressure was investigated using electrochemical and surface analysis techniques. The results showed that anodic reaction retardation caused by acetic acid in CO2 corrosion at low temperatures was not observed when H2S was present in an H2S/CO2 environment. The potentiodynamic sweeps for H2S/CO2 corrosion showed a retardation of the corrosion rate, which is thought to be caused by a thin mackinawite layer that is known to form in these conditions.
This study investigates the corrosion behavior of 316L stainless steel in KCl-LiCl molten salt using a natural circulation loop. The loop was operated for 144 h with a maximum temperature of 600°C (hot leg) and a minimum temperature of 450°C (cold leg). At the end of the test, the salt was allowed to solidify within the loop, and the loop was sectioned into small coupons to examine the corrosion behavior at various locations. The salt chemistry was analyzed at corresponding positions to correlate with the observed corrosion damage. Material characterization of extracted coupons revealed distinct corrosion behaviors between the hot and cold legs. In the hot leg, intergranular corrosion was the predominant mechanism, with preferential dissolution of Fe, Cr, and Mn. Additionally, deposition of Cr-rich oxide was observed in the hot leg. In contrast, the cold leg exhibited metallic deposits on the surface. These findings highlight the influence of the temperature gradient and salt chemistry on corrosion processes in molten salts. The underlying corrosion mechanisms are discussed, focusing on the role of thermal gradients and salt composition in modulating corrosion rates and morphology. The results of this work have implications for materials selection and system design in high-temperature molten-salt loops.
This work focuses on the development of an in situ sensitization remediation heat treatment in Al-Mg alloy structural components retrieved from operating vessels, with emphasis on the influence of microstructure, residual stresses, and operational history on the desensitization kinetics. Optimal conditions vary based on the above-mentioned parameters for different vessels. Optimum conditions were identified as 280 degrees C for 2 h for vessel I and 320 degrees C for 2 h for vessel II in the laboratory scale. The degree of sensitization (DOS) was able to be reduced from 54 mg/cm2 to 61 mg/cm2 and 97.60 mg/cm2 to below 10 mg/cm2. The minimum time required to achieve a measurable DOS reduction was 40 min for vessel I (partial mitigation), while achieving DOS lower than 20 mg/cm2 required at least 70 min for laboratory scale samples; however, in the case of vessel II, 2 h was the minimum time required. The experimental results demonstrated that indeed the initial microstructure, the microstrain state, and the operational history critically influence the response of the remediation treatment. Metallography revealed disruption and coarsening of the intergranular beta phase network with minimal grain growth consistent with diffusion-controlled transformation. Discontinuous precipitation reaction was detected in an intermediately sensitized specimen remediated to 21 mg/cm2, representing the first such observation in the Al-Mg system. It is stressed that large specimens from real vessels and not tiny lab-scale specimens that are usually tested require considerably longer treatments, confirming that effective remediation demands tailored treatment conditions based on the individual structural and service characteristics.
This article examines the hypothesis that magnesium hydride (MgH2) forms during the corrosion of magnesium and its alloys. A brief literature review presents the existing analytic evidence of MgH2 in the corrosion products of magnesium, along with pertinent information related to the chemical characteristics of MgH2. Anodic polarization tests were conducted on high-purity magnesium in unbuffered 0.1 M sodium chloride solutions. A substantial amount of H2 gas evolved from the corrosion products when dissolved in chromic acid solution, indicating the presence of a reducing substance in the corrosion products. Optical microscopy showed the release of thin, plate-shaped metal particles. A numerical estimate indicated that the mass of the disintegrated metal particles was negligible in comparison to the overall mass loss of the electrode. The corrosion was localized, and the attacks displayed a layered appearance with a preferential orientation, most likely parallel to the basal (0001) plane of Mg. The complete charge balance for galvanostatic anodic polarization tests at 10 mA/cm2 was established by quantifying (1) the external current, (2) the volume of H2 gas being evolved during the polarization test, (3) the volume of H2 gas being evolved during the dissolution of the corrosion products in chromic acid, (4) the quantity of Cr(III) ions formed through the reduction of chromic acid during the dissolution, and (5) the metal mass loss. The charge balance showed that, on average, 27% of the cathodic reaction products, quantified as charge, remained as a reducing residue in the corrosion products after the tests. The composition of the reducing substance was primarily examined within the framework of a hypothesis regarding MgH2, which recent research has indicated is partially protected from hydrolysis by a layer of reaction products. Other plausible metastable substances were also discussed.
To address activation and durability of Al-based sacrificial anodes in carbon capture, utilization, and storage (CCUS) weakly acidic chloriderich environments, this study investigated electrochemical performance and corrosion behavior of Al-Zn-In-Sn-Mg alloys. Using 1060 industrial aluminum, six alloy groups with varying Zn, Mg, In, and Sn contents were designed, characterized via microstructure analysis, electrochemical tests, current efficiency measurements, and three-dimensional surface characterization. Alloy 6 exhibited optimal performance with uniformly distributed secondary phases (Zn-rich particles, Mg-Zn intermetallics, and In-Sn activation phases) and refined dendrites. It demonstrated corrosion potential of -1.129 VSCE, corrosion current density of 2.64 & times; 10-6 A/cm2, current efficiency of 92.13%, and theoretical capacity of 2,882.98 A & centerdot;h/kg, with uniform corrosion morphology and no surface product adhesion. Superior performance resulted from Zn-Mg-In-Sn synergy: high Zn/Mg enhanced activation and matrix strengthening, while balanced In/Sn promoted uniform dissolution. The corrosion process involved three stages: secondary phase dissolution, pit initiation/propagation, and transition to uniform corrosion. Optimizing Zn/Mg contents and In/Sn ratio significantly improved current efficiency and stability, providing theoretical/experimental foundations for highperformance Al-based sacrificial anodes in CCUS weakly acidic environments.
Corrosion fatigue crack growth (CFCG) of thermally aged Z3CN20-09M cast austenitic stainless steel (CASS) was studied in simulated pressurized water reactor (PWR) primary water. Specimens were aged at 400 degrees C (6,000 h and 16,000 h) and 450 degrees C (6,000 h) and tested under varied loading conditions. The PWR primary water increased crack growth rates by typically up to 15 times compared to air, due to synergy between cyclic loading, corrosion, and the duplex microstructure, yet with weaker frequency and stress intensity dependence than conventional austenitic steels. Thermal aging had a limited influence (acceleration factor < 4), indicating that ferrite decomposition does not substantially affect the crack-tip oxide film (inner spinel/outer magnetite bilayer) or its rupture behavior. The study further establishes that all measured corrosion fatigue crack growth rates consistently fall below the ASME Code Case N-809 design curve, validating its continued applicability as a conservative safety benchmark for aged CASS components in PWR environments.
It has been known for over a century that carbon steel corrodes in syngas at moderate temperatures due to reaction with carbon monoxide to form gaseous iron pentacarbonyl, Fe(CO)5. However, commercially produced syngas also contains various minor contaminant species, and their impact on corrosion remained unclear. In this work, experiments have been performed for carbon steel in syngas flowing through a reaction chamber, using proton transfer reaction mass spectroscopy to measure the carbonyl concentration in the effluent gas. We have investigated the effects of temperature and contaminants (H2S, H2O, CO2, and O2). For comparison, we have also used online ultrasonic wall thickness monitoring of piping in an operating industrial plant. Results show that in clean syngas containing 21 bar CO and flowing at up to approximately 5 m/s, the corrosion rate peaks at approximately 200°C, but remains <0.1 mm/y. The corrosion process is very strongly catalyzed by ppb levels of H2S, much less strongly activated by H2O and CO2, and can be inhibited by ppm levels of oxygen. Both in the laboratory and field work, corrosion rates up to approximately 0.2 mm/y have been measured. The peak in the temperature dependence of corrosion rate is caused by a transition from kinetic control at lower temperatures to thermodynamic control at higher temperatures, but the position of the peak is dependent on the flow rate and other details of the experimental setup. The carbonyl corrosion reaction has been modeled using the Langmuir-Hinshelwood (LH) formalism for interactions at the gas/solid interface.
Abstract France Oxides and microstructure from welds of safety injection pipeline have been studied by Transmission Electron Microscopy in order to provide a better understanding of the SCC mechanism. In particular, presence or absence of oxygen tracer and/or pollutants and sensitization and deformation of the material have been investigated. For oxides on the internal walls of base metal at the weld roots, results show a duplex structure typical of a PWR environment. It consists of mostly Fe-rich spinels in the external oxide layer and Cr-rich spinels in the internal oxide layer. Both oxide types also contain Ni. In addition, a Ni enrichment in the base metal at the interface with oxides is also noted, typical of a PWR environment. Oxides at the crack tips are mainly (Cr, Fe, Ni)-rich spinels with local amorphous signatures. No oxygen tracer nor pollutants were detected as well as no peculiar sensitization signature (no Cr depletion along grain boundaries, no Cr-rich carbide). Deformation features are characteristic of a strain-hardened stainless steel. Further investigations are still ongoing on different welds to better understand SCC mechanism on auxiliary lines’ welds.
This study investigates the role of environmental exposure on creep crack growth in AISI 316H stainless steel, a material widely used in 500 ppm H2O, 300 ppm CH4, and 100 ppm H2) revealed enhanced intergranular cracking at stress intensity factors above 13 MPapm, accompanied by grain boundary cavitation and crack branching. To distinguish the individual contributions of creep, oxidation, and carburization, comparative tests were conducted in inert argon and hydrogenated steam environments. The inert atmosphere was used to isolate the mechanical effects of creep alone, while the hydrogenated steam environment allowed oxidation to be studied independently of carbon ingress. Both conditions resulted in significantly reduced crack growth compared to AGR exposure, indicating that carburization plays a critical role in accelerating damage. Electron probe microanalysis suggests elevated carbon concentrations along cavitated grain boundaries, suggesting that carbon segregation contributes to embrittlement and facilitates crack propagation under creep conditions. These findings provide insights into the environmental degradation mechanisms affecting reactor-grade stainless steels. Importantly, the insights gained from this study are directly relevant to high-temperature gas-cooled reactors, where similar alloy-environment interactions may influence long-term structural integrity under service conditions.
Outer diameter stress corrosion cracking (ODSCC) is an active and significant degradation mode that affects steam generator (SG) tubes in pressurized water reactors. This degradation mode is mostly due to the local and continuous accumulation of deleterious pollutants in confined areas, known as heat transfer crevices, in steam generators. Understanding the role of these pollutants, in particular lead and sulfur, in the degradation of SG tubes is a key gap in the safe long-term operation of nuclear power plants (NPP). This study focuses on Alloy 690 (Ni-30Cr11Fe) thermally treated (TT) SG tubes, mainly used in French NPP. Reverse U-bend specimens made of Alloy 690TT were exposed for 7,000 h at a temperature of 320 degrees C to simulate crevice environments. These environments included the presence of sludge, liquid, and wet steam phases, polluted with S only or S combined with Pb. Postexposure characterization of specimens extracted from the steam phase, in the presence of S and Pb, revealed intergranular stress corrosion cracking. At the nanoscale, traces of S and Pb were detected along the cracks, especially at the crack tip and at the oxide/metal interface.
Phosphate additions to Pb-caustic water chemistry were investigated as a potential mitigation strategy for lead-assisted stress corrosion cracking (PbSCC) of Alloy 690TT at approximately 310 degrees C. Autoclave tests were performed for 500 h using C-ring specimens strained in the plastic regime and exposed to four caustic environments (pH_high-T similar to 10) containing different PbO and phosphate concentrations. Two surface conditions were examined: as-received and OPS-polished. Results showed that 1,700 ppm phosphate provided only a minor benefit in reducing PbSCC severity, while 3,000 ppm phosphate mitigated cracking for as-received surfaces but promoted localized attack on polished surfaces. Advanced microstructural characterization (scanning electron microscope, focused ion beam-transmission electron microscopy, and scanning transmission electron microscopy with energy-dispersive x-ray) revealed similar crack-tip oxide compositions across all environments, with Pb incorporated in a (Cr,Fe)_3O_4 spinel phase at approximately 1 wt% to 3 wt%. These findings show that phosphate reduces PbSCC kinetics by partially sequestering Pb from solution rather than altering crack-tip chemistry. Although mitigation was incomplete, the results provide insight into inhibitor performance under concentrated crevice-like conditions and highlight the complexity of controlling PbSCC. These findings inform inhibitor selection and water chemistry strategies aimed at improving long-term PWR steam generator integrity.
Anodization of 304 stainless steel in ammonium-fluoride-based electrolytes produced thick (similar to 3 mu m) Fe-Cr-O-F oxide films whose chemistry and corrosion behavior were examined before and after postanodization heat treatment. XPS and XRD revealed mixed-valence Fe2+/Fe3+ oxides with Cr3+ incorporation in the as-formed layer, which was highly hydrated and fluoride-rich. Heat treatment at 450 degrees C induced defluorination, dehydration, and crystallization into a dense spinel of Fe3O4-Fe2O3. Electrochemical testing in borated water (2 ppm Li, 1,000 ppm B) and 3.5 wt% NaCl showed that corrosion response depends strongly on oxide composition and formation voltage. The heattreated oxide exhibited the lower current density (similar to 0.08 mu A/cm(2)) and higher resistance (similar to 7 & times; 105 Omega & centerdot;cm(2)) in borated water, whereas in chloride solution, performance varied nonmonotonically with anodizing voltage due to competing effects of porosity, cracking, and localized breakdown resistance. These results demonstrate that anodization markedly alters the Fe-Cr oxide chemistry, and corrosion behavior is governed by fluoride content, hydration, and film integrity.
Additive manufacturing (AM) of nickel-based alloys offers design flexibility and reduced lead times for critical components in pressurized water reactors. However, its influence on stress corrosion cracking (SCC) behavior remains poorly understood. This work investigates the microstructural evolution and SCC initiation susceptibility of laser powder bed fusion (LPBF) Alloy 625, with emphasis on the role of hot isostatic pressing (HIP). The as-built LPBF microstructure revealed a columnar dendritic structure with pronounced interdendritic segregation of Nb, Mo, and Ti. HIP treatment effectively homogenized the microstructure, resulting in equiaxed grains containing Nb-enriched grain boundary carbides. Cold work introduced fractured Nb-rich grain boundary carbides and nanoscale cavities, providing a mechanical baseline for distinguishing purely deformation-induced damage from environmentally assisted cracking. Accelerated oxidation tests of HIP-AM samples in hydrogenated steam at 480 degrees C revealed selective degradation of Nb-rich carbides, forming voids along grain boundaries. Slow strain rate tensile (SSRT) tests were conducted on 20% cold-rolled HIP-AM specimens in simulated PWR water (342 degrees C, 30 cc/kg H2). Minimal intergranular SCC (IGSCC) was observed at 4.8% strain, whereas clear IGSCC initiation occurred at 26% strain, indicating high but finite SCC resistance under combined mechanical and environmental loading. These findings highlight that while HIP improves microstructural homogeneity, it introduces grain boundary features susceptible to oxidation-assisted degradation. The results provide critical insights for optimizing postprocessing strategies and assessing the long-term integrity of AM Alloy 625 in high-temperature reactor water.
As the deployment of small modular nuclear reactors (SMRs) in Canada approaches, waste management considerations become increasingly critical. This study investigates the potential for galvanic degradation between the graphite outer casing of spent TRi-structural ISOtropic (TRISO) fuel and candidate container alloys during the interim dry storage phase. Four alloys, A516 Gr. 70 carbon steel, 405, 430, and 304L stainless steels, were evaluated to assess the influence of Cr and Ni content on galvanic corrosion interactions in chloride-containing environments. Electrochemical testing was conducted to investigate the galvanic corrosion properties of all four alloys coupled to graphite, in increasing salinity (0.001 M to 6.14 M NaCl). Potentiodynamic testing revealed that galvanic corrosion of A516 is governed by the concentration of dissolved oxygen, with corrosion rate peaking at 0.6 M NaCl and decreasing due to oxygen depletion with increasing Cl ions. Immersion testing highlighted microgalvanic coupling between ferrite and cementite phases in A516, although absent in oxygen-limited conditions. All stainless steel alloys demonstrated comparable corrosion rates up to 0.6 M NaCl, yet in 6.14 M NaCl, 405 and 430 exhibited a significant increase in corrosion rate due to a shift in pitting potential equating to the galvanic potential. 304L maintains a low corrosion rate at all concentrations of NaCl, highlighting the influence of increased Cr content and Ni additions on passive film stability.
