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Friction stir welding (FSW) is an effective welding technique to realize the joining of dissimilar aluminum alloys. The microstructural heterogeneities induced by FSW across the joints could have curial implication for the corrosion performance of the joints. In this research, the microstructure and localized corrosion behavior of shoulder interface zone (SIZ), vortex zone (VZ), bottom zone (BZ) and bottom interface zone (BIZ) in the stir zone (SZ) of dissimilar FSW AA2024/AA7075 joint was systematically investigated through detailed microstructural characterization and relevant corrosion tests. The results indicated that plentiful of Cu-rich constituent particles are formed on AA2024 side and the areas near the interface on both sides, and corrosion originates from these regions. Grain size has little influence on corrosion behavior of the SZ, while the local regions with higher stored energy are more sensitive and liable to corrosion. The sequence of mixing degree of materials in the four regions of the SZ is: BZ > VZ > SIZ > BIZ, which is in contrast to the order of corrosion rate. Galvanic corrosion is detected in the SIZ and BIZ, and sufficient mixing of materials significantly weakens the galvanic corrosion, resulting in higher corrosion resistance in the BZ.

The evolution of mechanical properties, localized corrosion resistance of a high purity Al-Zn-Mg-Cu alloy during non-isothermal aging (NIA) was investigated by hardness test, electrical conductivity test, tensile test, intergranular corrosion test, exfoliation corrosion test, slow strain rate tensile test and electrochemical test, and the mechanism has been discussed based on microstructure examination by optical microscopy, electron back scattered diffraction, scanning electron microscopy and scanning transmission electron microscopy. The NIA treatment includes a heating stage from 40 °C to 180 °C with a rate of 20 °C/h and a cooling stage from 180 °C to 40 °C with a rate of 10 °C/h. The results show that the hardness and strength increase rapidly during the heating stage of NIA since the increasing temperature favors the nucleation and the growth of strengthening precipitates and promotes the transformation of Guinier-Preston (GPI) zones to η ′ phase. During the cooling stage, the sizes of η ′ phase increase with a little change in the number density, leading to a further slight increase of the hardness and strength. As NIA proceeds, the corroded morphology in the alloy changes from a layering feature to a wavy feature, the maximum corrosion depth decreases, and the reason has been analyzed based on the microstructural and microchemical feature of precipitates at grain boundaries and subgrain boundaries.

Dynamic electrochemical impedance spectroscopy (dEIS), an extension of conventional EIS, was used for rapid measurements of initiation of localised corrosion on 3005 aluminium alloys with different Mn (1.08–1.39 wt. %) and Cu (0.15–0.22 wt. %) content by evaluating the onset and evolution of a Faradaic process observed in 0.5 M Na 2 SO 4 +0.1 M NaCl. Differences between samples were detected with highest sensitivity from the charge transfer resistance during chronoamperometry at ca. 120 mV above the corrosion potential. These differences were caused by differences in pitting initiation, and correlated well with filament density from a filiform corrosion (FFC) test of samples coated with a 20 μm epoxy topcoat. The observation can be explained by the role of pitting in FFC initiation and the similarity in FFC propagation with pit growth. The results suggest that dEIS is an efficient method for screening alloys for their FFC susceptibility.

Pitting corrosion is a prevalent and highly detrimental form of localized corrosion, which can severely compromise the local load-bearing capacity of metallic materials and, in extreme cases, trigger structural failure. In response to the pronounced susceptibility of Q235 galvanized steel plates to localized pitting under the extreme service conditions of the South China Sea—characterized by high temperature, high salinity, high humidity, and coupled chemical corrosive effects—this study conducts a systematic investigation combining experimental characterization and numerical simulation. First, a novel accelerated pitting corrosion apparatus was designed and developed, and chloride ion cyclic corrosion (CICC) tests were performed on Q235 galvanized steel plates. The morphology and temporal evolution of pitting damage were comprehensively characterized. Subsequently, based on a coupled Cellular Automata (CA) and Finite Element Analysis (FEA) framework, a corrosion evolution model termed CAFE (Cellular Automata-Finite Element) was established. This model elucidates the initiation, growth, and corrosion product evolution of pitting pits under varying temperature and salinity conditions and further quantifies the spatial distributions of stress and temperature fields in the vicinity of pitting sites. Finally, experimental results were employed to validate the rationality and effectiveness of the proposed electro-thermo-mechanical-chemical (ETMC) multi-field coupling model. The results demonstrate that temperature and salinity are the dominant environmental parameters governing the evolution of localized pitting corrosion rates. A strong agreement between numerical predictions and experimental observations is achieved in both qualitative trends and quantitative metrics. Notably, the model reveals that under elevated current-driving conditions, localized plastic deformation plays a critical role in promoting pit propagation and accelerating the pitting corrosion process.

Biogas contributes to environmental protection by reducing greenhouse gas emissions and promoting the recycling of organic waste. Its utilization plays a crucial role in addressing the challenges of climate change and sustainability. However, the deterioration of process plants involved in biogas production due to corrosion has a critical impact on the safety and durability of their operations. In order to maintain the safety of structures in terms of service life with respect to corrosion, it is essential to develop effective corrosion engineering control methods. Electrochemical techniques have become a useful tool by which to evaluate corrosion resistance. However, these techniques may require microscopic analysis of the material surface and the analysis may be influenced by subjective factors. To solve this drawback, this work proposes the use of SVM models to predict the corrosion status of the material used in biogas production with no need to perform microscopic analysis after the electrochemical test. The obtained results of sensitivity and specificity equal to 0.94 and 0.97, respectively, revealed the utility of the proposed stochastic models to assure the corrosion state of the equipment involved in biogas production. SVM-based models are an effective alternative for accurately evaluating material durability and comparing the corrosion resistance of different materials in biogas environments. This approach facilitates the selection of the most suitable material to achieve greater durability and long-term performance. Synopsis: The results show that the proposed model is a useful tool to predict the behaviour of stainless steel against corrosion according to the environmental conditions to which the material is exposed in biogas production.

Prompted by the unexpected observation of the pitting of the weldments of a highly corrosion- and pitting-resistant duplex stainless steel, SAF2507, in chloride solutions with nitride addition, the pitting and corrosion resistance of SAF2507 and its weldments were investigated in chloride solutions with and without different levels of nitrite. The Incoloy 825 and 316L austenitic stainless steels were included for the purpose of developing a comparative appreciation. The microstructures of the weldments were characterised, and 316L showed a profound influence of nitrite addition in inhibiting pitting, while ‘meta-stable’ pitting transients that were clearly visible in the chloride solution without nitrite were absent when nitrite was added. Both the parent metal and the weldment of SAF2507 had similar pitting potential (Ep) in 0.1 M NaCl without nitrite, which was the highest Ep among the three alloys tested. Additions of nitrite at low concentrations had an inhibitive effect on pitting, whereas higher nitrite contents had a deleterious effect on pitting resistance. On the other hand, Incoloy 825 showed a trend of Ep ennoblement with an increasing nitrite content of 0.1 M NaCl, and the weldment underwent greater ennoblement. Moreover, 316L showed a trend similar to Incoloy 825; however, the Ep ennoblements were significantly more pronounced for both the weldment and the base metal of 316L.

The microstructure and corrosion processes of a friction stir welded (FSW) 7075-T6 aluminum alloy joint, before and after anodizing surface treatment, have been characterized by advanced techniques, and the feasibility of anodizing treatment as a corrosion mitigation method has been evaluated. The results showed that different zones of the FSW joint had distinctly different microstructure and consequently different corrosion behavior in NaCl solution. Stable localized corrosion occurred in the transitional regions between the thermo-mechanically affected zone (TMAZ) and heat affected zone (HAZ), and was characterized by intergranular corrosion. The intergranular corrosion was ascribed to the galvanic coupling effect between Cu-rich grain boundary precipitates and the precipitates free zones (PFZs). Although anodizing and the subsequent sealing treatments could greatly improve the corrosion resistance of the base metal, the TMAZ/HAZ transition regions still showed much higher corrosion susceptibility than other regions. The high corrosion susceptibility of the FSW joint after anodizing treatment is not ascribed to the difference of the anodic oxide film in the regions, but the heterogeneous microstructure of the alloy beneath the anodic film. The present paper has shown that the stable localized corrosion in the FSW joint is intrinsically stemmed from the welding process itself and traditional mitigation method such as anodizing treatment cannot solve the problem; more effective corrosion mitigation methods are still awaited.

This work demonstrates an approach towards the understanding of multi-scale and open-circuit localised electrochemical processes of AA2024-T3 in the presence and absence of an environmentally friendly rare-earth inhibitor; cerium diphenyl phosphate (Ce(dpp)3). At high temporal resolution, a wire bean electrode (WBE) made from 100 identical AA2024-T3 wires revealed sudden increases in galvanic anodic and cathodic activities immediately after dosing of 50 and 100 ppm of the inhibitor and an overall suppression of macro-scale activities by increasing the inhibitor concentration to 200 ppm, suggesting it as a fast-screening tool for inhibitors and measuring inhibition efficiency. At high spatial resolutions, scanning probe electrochemical techniques confirmed local activation of corroding microstructures on individual AA2024-T3 wires similarly by dosing the inhibitor up to 100 ppm. In agreement with WBE findings, the effective shutdown of both anodic and cathodic activities occurred after increasing the inhibitor concentration to 200 ppm confirming the optimal concentration of the Ce(dpp)3 and the mixed mode inhibition mechanism of this selected inhibitor on AA2024-T3.

Conventional ultrafine-grains can generate high strength in Mg alloys, but significant tradeoff of corrosion resistance due to inclusion of a large number of non-equilibrium grain boundaries. Herein, an ultrafine-grain structure consisting of dense ultrafine twins is prepared, yielding a high strength up to 469 MPa and decreasing the corrosion rate by one order of magnitude. Generally, the formation of dense ultrafine twins in Mg alloys is rather difficult, but a carefully designed multi-directional compression treatment effectively stimulates twinning nucleation within twins and refines grain size down to 300 nm after 12-passes compressions. Grain-refinement by low-energy twins not only circumvents the detrimental effects of non-equilibrium grain boundaries on corrosion resistance, but also alters both the morphology and distribution of precipitates. Consequently, micro-galvanic corrosion tendency decreases, and severe localized corrosion is suppressed completely. This technique has a high commercial viability as it can be readily implemented in industrial production. Conventional ultrafine grains can generate high-strength Mg alloys, but non-equilibrium grain boundaries deteriorates their corrosion resistance. Here, the authors present ultrafine grained Mg alloys with dense twins that display high strength and reduced corrosion rate by one order of magnitude.

Coupled multielectrode array sensors (CMAS) have been used for real-time monitoring of corrosion, particularly localized corrosion. The internal anodic current on the most anodic electrode in a CMAS was evaluated for aluminum and carbon steel in simulated seawater and dilute hydrochloric acid (HCl) solutions. The Tafel extrapolation method was used to estimate the internal current on the most anodic electrode and the average corrosion current for all electrodes. It was demonstrated that if a metal corrodes dominantly in the form of localized corrosion or uneven general corrosion, the CMAS effectively measures the localized or uneven general corrosion rate. However, if the metal corrodes dominantly in the form of uniform corrosion, the CMAS only measures the uneven portion of the corrosion current.