Atmospheric corrosion of carbon steel in cold regions remains insufficiently understood, particularly under winter conditions characterized by low temperatures combined with high airborne sea salt deposition. This gap necessitates a mechanistically based predictive model. This study developed a concise mathematical model based on outdoor exposure tests to describe the atmospheric corrosion of carbon steel in cold regions, with a focus on oxygen transport. Despite the cold climate, the corrosion rates increased in winter owing to increased airborne sea salt levels and the formation of thicker aqueous salt solution layers. The corrosion rate increased proportionally with the fourth root of the solution-layer thickness. Numerical analyses conducted using the finite element method suggest that oxygen transport processes, including dissolution from the gas phase, influence the corrosion behavior under thin NaCl solution layers. The corrosion rates calculated from solution layer thicknesses using weekly, monthly, seasonal, and/or annual averages of airborne sea salt deposition and relative humidity showed good agreement with the measured values. These results demonstrate the applicability of the proposed simplified model for predicting the corrosion lifetime of carbon steel in cold regions, within the assumptions adopted in this study. Corrosion rate of steel increases with increasing solution layer thickness.Corrosion products of carbon steel are identified as gamma-, beta-, and alpha-FeOOH.Oxygen dissolution rate decreases as solution layer becomes thinner.Corrosion depth calculated from weekly to annual data agree with measured values.

This study investigates the influence of rust layer composition on cathodic reactions and hydrogen permeation behavior of SM490 carbon steel exposed to NaCl-containing environments. Rust layers were formed on the SM490 specimens by repeated wet/dry cycles using NaCl-containing droplets. Polarization measurements revealed that rust formation and growth enhanced the cathodic reaction. The reduction reaction of rust components, particularly FeOOH, was identified as one of the predominant contributors to this enhancement. Notably, the results indicated that, in addition to the reduction of rust components, the hydrogen evolution reaction could actively proceed on Fe3O4. Because hydrogen evolution occurs less readily on FeOOH, an increase in the Fe3O4 fraction within the rust layer should promote hydrogen evolution reactivity and consequently enhance hydrogen permeation. This correlation was confirmed by electrochemical hydrogen permeation tests. These findings provided new insights into the relationship between rust layer composition and hydrogen permeation behavior, thereby advancing mechanistic understanding of hydrogen entry processes in practical steels under atmospheric corrosion conditions. (c) 2026 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, https://creativecommons.org/licenses/ by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/ 1945-7111/ae6483]

Seawater electrolysis has attracted considerable attention as a promising technique for hydrogen production. Stainless steels are candidate materials for electrolyzer components owing to their favorable mechanical properties, cost-effectiveness, and corrosion resistance. Previous studies have shown that phosphate in the electrolyte can enhance the corrosion resistance of stainless steels by phosphorus incorporation into the passive film and the pH-buffering effect. In this study, the corrosion resistance of 430 ferritic stainless steel was evaluated in a phosphate-free environment, following the formation of phosphorus-containing passive films, to elucidate the individual contributions of these factors. Electrochemical measurements and cyclic corrosion testing demonstrated that phosphate treatment improved the corrosion resistance of 430 steel in phosphate-free environments. In addition, STEM-EDS analysis revealed that the passive film formed in the 0.5 M phosphate-containing electrolyte had two layers with chromium enrichment in the outer layer. These findings provide valuable insights into the design and operation of seawater electrolysis systems and highlight the importance of pre-treatment and passive film engineering.

A laser thermal processing method was developed to enhance the corrosion resistance of martensitic stainless steels, which typically offer high hardness but poor corrosion resistance. Specifically, type 420J2 martensitic stainless steel (420J2 SS) plates were subjected to laser irradiation, causing rapid heating and quenching that modified the microstructure. After processing, three distinct layers were observed from the surface downward: a remelted layer, a phase-transformed layer, and the substrate. The remelted layer was formed by localized melting from the laser heat, followed by rapid quenching and solidification. Corrosion-inducing inclusions within this layer melted into the matrix, and their reprecipitation was effectively suppressed. The microstructure of the remelted layer comprised fine needle-like features, along with martensitic and austenitic phases. Due to the dominance of the martensitic phase, the hardness of the remelted layer increased significantly. Consequently, the remelted surface exhibited improved corrosion resistance and hardness. In contrast, the microstructure and corrosion resistance of the inner layer remained similar to those of the untreated 420J2 SS, indicating that the laser thermal processing did not adversely affect the substrate. Overall, this method provides an effective means of improving both the corrosion resistance and surface hardness of martensitic stainless steels. Development of laser thermal processing method for corrosion resistance enhancementMartensitic stainless steel subjected to laser irradiationEffects of technique on corrosion resistance, microstructure, and hardness assessedLaser-thermal-processed stainless steel exhibited enhanced corrosion resistanceMethod promising for achieving high corrosion resistance and surface hardness

Water electrolysis is an effective method for producing hydrogen, which is a valuable alternative to fossil fuels. However, the presence of chloride ions can cause significant structural degradation, and electrolysis systems compatible with seawater must be developed. In this study, phosphate was added to an electrolyte simulating seawater electrolysis, and the effects on the corrosion resistance of ferritic (Type 430) and austenitic (Type 304 and 316) stainless steels were investigated using electrochemical techniques. The phosphate-induced changes in the passive films on the steels were examined using X-ray photoelectron spectroscopy (XPS). The phosphate in the electrolyte enhanced the pitting corrosion resistance of all the stainless steels. However, excessive phosphate concentrations promoted the partial dissolution of the passive film, particularly for the 430 steel. XPS analysis showed that phosphorus was incorporated into the passive film as phosphate for every type of steel in this study, which likely enhanced the pitting corrosion resistance. Cyclic polarization measurements of the 430 steel indicated that the pH-buffering action of the phosphate in the electrolyte suppressed pitting propagation. These findings provide fundamental insights into the role of phosphate additives in stabilizing stainless steels, which may contribute to the improved safety of seawater electrolysis systems.

Ni-based materials are promising candidates for anodic electrodes and electrolysis cell materials in seawater electrolysis systems for hydrogen production. However, their practical application is hindered by severe corrosion in chloride-rich electrolytes. Incorporating phosphate into the electrolyte has emerged as an effective strategy to enhance corrosion resistance, though the underlying mechanisms remain poorly understood. In this study, the corrosion inhibition mechanism of phosphate on Ni was systematically investigated. Polarization measurements in 0.5 M borate-0.5 M KCl electrolytes at pH 9.2 with varying phosphate concentrations revealed that Ni undergoes pitting corrosion with Cl-, which is significantly mitigated by phosphate addition. This enhancement is attributed to two factors: 1) structural modification of the passive film, and 2) suppression of pit propagation. STEM/EDS analysis shows phosphate incorporation into the Ni passive film, altering its structure from crystalline to amorphous, which correlates with the enhanced protective ability of the passive film. Furthermore, potentiostatic polarization reveals that phosphate addition inhibits pit propagation even after its initiation. This inhibition is likely due to the pH buffering effect of phosphate. These findings offer new mechanistic insights into phosphate-assisted corrosion resistance enhancement and provide a foundation for the design of corrosion-resistant nickel-based materials for highly concentrated chloride environments.

In recent years, the automotive industry has increasingly adopted multi-material structures to reduce vehicle weight and, consequently, lower CO₂ emissions. In the development of materials technology for multi-material vehicle bodies, effective measures must be implemented to prevent galvanic corrosion at material interfaces. However, there are many parts in a vehicle that are concerns about corrosion, and it is known that the corrosion environment differs greatly depending on the part. Numerical simulations utilizing computational science provide a highly effective approach for investigating corrosion behavior under a wide range of environmental conditions. In this study, galvanic corrosion between carbon steel and aluminum alloy was analyzed through quantitative numerical simulations based on the electrochemical properties of each material. The numerical simulation model was constructed under the assumption that corrosion would occur at the butt joint of carbon steel and aluminum alloy in a NaCl aqueous solution. COMSOL Multiphysics was utilized for the simulations. Electrochemical measurements were conducted to validate the numerical simulation model. The test electrodes were prepared by arranging carbon steel and aluminum alloy in parallel and embedding them in epoxy resin. NaCl solutions of varying concentrations were used as the test solution. Changes in galvanic current and potential over time were measured in a non-degassed environment at room temperature. After three hours of measurement, the surface corrosion morphology of the aluminum alloy and carbon steel was observed using a microscope. Galvanic current measurements revealed that the aluminum alloy acted as the anode, while carbon steel functioned as the cathode in the NaCl solution. The galvanic current increased with rising NaCl concentration, and the galvanic potential exhibited a tendency to shift toward more negative values. Surface observations after the corrosion test indicated the occurrence of pitting corrosion on the aluminum alloy, with the number of pits increasing in proportion to the NaCl concentration. In contrast, no significant corrosion was observed on carbon steel in high-concentration NaCl solutions, however, the amount of corrosion on carbon steel increased as the NaCl concentration decreased. A computational simulation was conducted, considering the expansion of the pitting corrosion area on the aluminum alloy with increasing NaCl concentration. The results demonstrated that the anode current on the aluminum alloy almost matched the cathode current generated on the carbon steel in highly concentrated NaCl solutions, but as the NaCl concentration decreased, the anode current on the aluminum alloy diminished, leading to anode dissolution (self-corrosion) on the carbon steel.

The Fukushima Daiichi Nuclear Power Plant experienced power loss in Units 1-4 due to the Great East Japan Earthquake on March 11, 2011, causing the shutdown of the fuel cooling systems for the reactor cores and spent fuel pools. Consequently, the reactor buildings suffered damage from core meltdowns and hydrogen explosions, releasing radioactive materials. During this incident, fuel debris accumulated within the reactor. Currently, efforts are made to minimize corrosion inside the reactor by maintaining low dissolved oxygen concentration underwater through degassing treatment of circulating water for fuel debris cooling and nitrogen injection into the Primary Containment Vessel (PCV) to prevent hydrogen explosions. However, future fuel debris retrieval operations may expose the PCV to the atmosphere temporarily, raising concerns about increased dissolved oxygen concentration, inducing acceleration of corrosion reaction. Hence, it is necessary to establish corrosion inhibition techniques resilient to radiolysis, as an alternative to nitrogen injection. Utilization of inert gas ultrafine bubbles (UFB) are anticipated to effectively reduce dissolved oxygen in water. Therefore, in this study, effects of the argon gas UFB introduction treatment on corrosion reaction of carbon steel in a simulated reactor environment (without radioactive material) were evaluated to explore methods for effectively inhibiting corrosion reactions during fuel debris retrieval. The round-shaped carbon steel (SM490A) specimen mounted in epoxy resin was used as corrosion-monitoring sensor electrode. Two types of electrodes were prepared: immediately after polishing and rust-formed. The latter electrode was prepared by accelerating corrosion reaction using a saltwater spray testing machine (STP-30, Suga Testing Machine, 50 gL-1 NaCl solution at 303K) for three days to induce the thick rust layer formation on the surface. A solution of artificial seawater (Aquamarine, Yashima Pure Chemicals, Japan) diluted 200-fold was employed for the simulated reactor environment. UFB introduction treatment was conducted for 10 liters of the solution using an ultrafine bubble generation machine (BUVITAS, NEXCO Engineering Kansai, Japan) with Ar gas flowing at a rate of 650 mL/min for 30 minutes. The aerated (open air condition) test solution without UFB introduction treatment was also prepared for comparison. To prevent overheating the solution during the treatment, an immersion cooler (BE201F, Yamato Scientific, Japan) was used. It was confirmed that the solution temperature could be kept below 25°C even after 30 minutes of the UFB-introduction treatment by cooling the solution to 5°C in advance. A glass water vessel with a cubic shape (15 cm × 15 cm × 15 cm) was used as electrochemical cell, and it was completely sealed inside a larger 20 L plastic container directly connected to the UFB-generation machine. A drainage trap was installed to prevent the backflow of oxygen from the atmosphere during the solution discharging process. Pairs of the electrodes were placed facing each other with distance of 1 cm. These steel electrodes for corrosion monitoring were fixed at the bottom of the cell. The measurements were conducted in the following manner. Before pouring the solution inside the cell, Ar gas purging for 30 minutes was performed in advance to eliminate the effect of residual oxygen in the container. The corrosion rate was evaluated by simplified electrochemical impedance spectroscopy using corrosion monitor (CT-7, Riken Densi, Japan). The dissolved oxygen (DO) concentration during UFB introduction treatment decreased rapidly from the beginning of the treatment and reached nearly 0 ppm within a few minutes. The corrosion rate of carbon steel without rust layer in with UFB condition remained stable and was kept lower value than that without UFB condition during testing period of 6 d. Thus, it was considered that the initial corrosion reaction of carbon steel in diluted seawater environment was effectively inhibited by the presence of UFB with lower DO concentration. In the case of the rust-formed specimen, the corrosion rates were nearly an order of magnitude higher than those of the specimens without rust layer. The corrosion rate tended to decrease in the UFB-introduced solution. On the other hand, it exhibited a clear increasing tendency under the UFB-free condition. These results clearly demonstrate that the introduction of Ar gas UFB effectively suppresses corrosion of carbon steel in diluted seawater, regardless of the presence of rust layer. It has been demonstrated that the introduction of ultrafine bubbles (UFB) using inert gas effectively reduces the dissolved oxygen (DO) concentration in the solution for longer period. In addition, , corrosion inhibition effect due to low DO condition was found to be available regardless of the presence of rust layer formed on carbon steel surface in advance.

Geological disposal is being considered as a disposal method for liquid waste resulting from nuclear power generation. In geological disposal, liquid waste and glass materials are melted together and solidified in an overpack (metal container), and a buffer material is compacted between the overpack and the bedrock. Carbon steel is as a candidate for overpack material and bentonite as a buffer materialin Japan. Overpack materials are required to be safe and reliable over a very long period, and their corrosion behavior and long-term corrosion resistance have been investigated before. However, most of these researches have been conducted in environments where oxygen is fully consumed, and the corrosion behavior and resistance during transient periods have not been fully clarified. In this study, the effects of bentonite properties on the corrosion behavior of carbon steel in compacted bentonite were investigated by electrochemical methods in order to clarify the corrosion behavior of carbon steel during the transient period. An iron wire grinded with sandpapers was used as the sample. The amount of bentonite was weighed to achieve the specified compacted density, and firstly approximately one-half of the amount was filled into a titanium compacted vessel. The iron wire was then set on the bentonite and the remaining bentonite was filled into the vessel using a hydraulic press. The properties of the bentonite were adjusted from 0.9 Mg/m3 to 1.8 Mg/m3 with the compaction density when packed. The titanium vessel was then immersed in a 0.1 mol/L NaCl solution and depressurized at room temperature. The solution was supplied to the cell through a filter on the side of the cell. Polarization and electrochemical impedance measurements were performed in a three-electrode system under open air. The test solution was 0.1 mol/L NaCl solution, a saturated KCl/Ag/AgCl electrode (SSE) was used as the reference electrode, and a platinum wire as the counter electrode. Polarization measurements were conducted by polarizing in the anodic and cathodic directions from the immersion potential at a scanning rate of 0.5 mV/sec. The polarization measurements of an iron wire were also performed in the same solution for comparison. Electrochemical impedance was measured at 24 and 48 hours after immersing the titanium vessel in the test solution. Polarization curve of iron wire in bentonite with a compressibility density of 1.5 Mg/m3 were compared with that in the solution. A clear diffusion-limiting current of oxygen was observed in the polarization curve of iron wire in the solution, whereas it was unclear for iron wire in bentonite. This result is attributed to the influence of water reduction reaction in a cathodic region. On the other hand, in the anodic region, the anodic current of iron in bentonite was about two orders of magnitude lower than that in the solution. This indicates that electrochemical measurements are possible in bentonite as well as in solution, but the influence of compacted bentonite is not small. Electrochemical impedance characteristic of the iron wire in bentonite with a compressive density of 1.6 Mg/m3 showed one time constant after 25 hours of immersion and two time constants after 48 hours, respectively.The diameter of the capacitive semicircles decreased with immersion time. The titanium vessel immersed for 48 hours was opened and the surface of the iron wire was visually observed. As a result, corrosion products were observed. These results suggest that the test solution penetrated into the compacted bentonite after at least 25 hours, and that corrosion progressed to form corrosion products on the electrode surface by 48 hours. Curve fitting was performed on the obtained impedance spectra, assuming the two equivalent circuits. As a result, it was confirmed that the polarization resistance at the electrode surface decreases with time.

Aluminum alloys are attracting attention as lightweight materials for automobiles, and the fact that intermetallic particles indispensable for improving strength of the alloys can be harmful to the corrosion resistance has become an issue. However, the initial corrosion behavior at the interface of intermetallic particles for aluminum alloys has been hardly examined using a comprehensive analysis method. In this study, a nano-micro scale multimodal analysis of Energy Dispersive X-ray Analysis (EDS), Electron Backscatter Diffraction (EBSD), and Kelvin Force Microscopy (KFM) was performed to clarify the initial corrosion behavior for AA6016. AA6016 specimens (20 × 20 × 1 mm) were subjected to SEM-EDS/EBSD measurements after mirror polishing, followed by continuous KFM measurements after ion milling. The specimens were then immersed in 100 mL of 0.5 wt% NaCl solution for 1 hour at a corrosion test. The surfaces of the specimens were cleaned with distilled water, dried and subjected to subsequent KFM measurements. The measurement of SEM-EDS was conducted again after the series of KFM measurements prior and post corrosion test. Fig. 1 shows SEM images of the typical specimen surface for AA6016 before corrosion tests. As shown in Fig. 1, two types of intermetallic particles: Al-Fe-Si and Mg-Si were identified by EDS measurements. The KFM measurements of the potential distribution on specimen surfaces confirmed that both intermetallic particles were noble in potential relative to the aluminum matrix. However, less significant difference in the potential was detected at the grain boundaries identified by the EBSD measurements. After the corrosion test, some trenches were formed at the boundaries of the Al-Fe-Si type particle, and the noble potential area expanded around the particle. As for the Mg-Si type particles, a dissolution of Mg was detected at the particle, and the noble potential region disappeared at the particles immediately after the corrosion test but reappeared with time. Furthermore, the formation of thick oxide films was implied after the corrosion test, especially at the boundaries of Al-Fe-Si type particle as well as at the Mg-Si type particle. Thus, the initial corrosion behaviors of AA6016 related to the compositional/electrochemical modifications at and around intermetallic particles were demonstrated by using the nano-micro scale multimodal (EDS-EBSD-KFM) analysis. Although the modifications of compositions (Mg and O) as well as noble potential distribution were restricted within the particle for Mg-Si type particle, they were expanded at the boundaries and around the particle for Al-Fe-Si type particle. The Al-Fe-Si type intermetallic particles would play more important role than those of Mg-Si type particles in promoting localized corrosion for AA6016. Figure 1

Previous research has shown that Fe-Mn-Cr-Ni-Si alloys offer excellent low -cycle fatigue resistance via reversible bidirectional transformation between face -centered cubic (FCC) gamma-austenite and hexagonal closed -packed (HCP) epsilon-martensite. The alloy shows superior low -cycle fatigue life and is used for seismic damping applications, but there have been concerns over their resistance to highly corrosive environments. In this study, Fe-15Mn-aCr-bNi-4Si alloys were prepared with different Cr and Ni concentrations to evaluate the effects on the fatigue and corrosion resistances: Z1 with (a, b) = (14, 10.1), Z2 with (a, b) = (12.5, 8.8), Z3 with (a, b) = (11, 7.5), Z4 with (a, b) = (9.5, 6.1), and Z5 with (a, b) = (8, 4.8). Z2 had the longest fatigue life. The alloy showed Gibbs free energy difference between gamma-austenite and epsilon-martensite phases close to the ideal of zero and the alpha '-martensitic transformation was suppressed well, which agreed with the design criteria for achieving bidirectional transformation -induced plasticity. The developed alloys showed superior corrosion resistance in seawater. Local pitting corrosion was observed that was attributed to the high Mn concentration of the alloys, although this was greatly mitigated by adjusting the Cr and Ni concentrations, especially with Z1 and Z2.

In recent years, Proton Exchange Membrane Fuel Cell (PEMFC) have attracted attention as an important technology for achieving carbon neutral society due to high energy efficiency without carbon dioxide emission during power generation. In order to cost reduction of PEMFC, the use of cheaper structural materials, such as stainless steels, that possess excellent processability, has been studied in substitution for titanium for bipolar plate (separator). The practical environment in PEMFC is severely corrosive: high temperature, acidic, and fluoride-containing solution. Many study has been conducted on the development of materials that have sufficient corrosion resistance and low contact resistance enabling long-term durability even in such severe environment. However, present evaluation method is insufficient for diagnosis the lifetime of bipolar plate. The new evaluation technique, that accurately simulates the environment inside PEMFC and appropriately accelerates the corrosion degradation reaction, make it easier to predict the lifetime of products and precisely evaluate the performance of newly developed materials. Therefore, the purpose of this study is development of novel corrosion evaluation method with high reproducibility, high and rapid diagnosis performance via applying various electrochemical measurements simulating PEMFC environment and for elucidation the corrosion behavior in practical environment. Firstly, several common corrosion resistance tests were performed in a specific solution (80 ℃, pH3, 10ppm Cl-, 3ppm F-) that simulated the internal environment of PEMFC. As a result, potentiostatic polarization (0.724 V vs. Ag/AgCl, 24 h) showed poor reproducibility because the corrosion behavior in later stage depends largely on the occurrence of localized corrosion initial stage of the test. In the case of potentiodynamic polarization(+1 mV/s sweep from open-circuit potential), risk of localized corrosion could be investigated, however, it was not suitable to predict long-term corrosion behavior. In the case of potentiodynamic cyclic polarization (cyclic voltammetry), it was found that the cathodic sweep, which never occur at in-sever PEMFC condition, inhibited the stable growth of the passivation film on the steel surface by reduction reaction, resulting in improperly accelerated excessive corrosion. Then, new testing method of the intermittent polarization, which was a combination of potentiostatic polarization and open-circuit condition, was performed. It has the advantage in flexibility of testing conditions such as polarization potential and duty cycle in accordance with to the degree the corrosion acceleration. In fact, it was found that the localized corrosion caused by initial stage of potentiostatic polarization step was repassivated during the following open circuit step. Therefore, the reproducibility was improved and the passive film growth was properly evaluated by intermittent polarization test, therefore. It was expected to be promising technique to diagnose the long-term durability of PEMFC bipolar plate. In order to demonstrate the effectiveness of the intermittent potentiostatic polarization method, a variety of type 304 stainless steel specimens with different corrosion resistance were used. The detailed results will be presented on the relevant session.

The utility of hyperspectral measurement was assessed as a means of predicting the corrosion risk of steel materials based on surface information. Carbon steels exposed to outdoor conditions in Choshi and Miyakojima were used as the test specimens. Exposure tests were conducted every six months for a duration of two and a half years. Corrosion loss was calculated by comparing the weight of specimens before exposure test and after removing corrosion products from the surface. Hyperspectral measurements were conducted on these specimens, with corrosion products identified through SAM (Spectral Angle Mapper) analysis. alpha-FeOOH, beta-FeOOH, gamma-FeOOH, and Fe3O4 were employed as reference data for SAM analysis. In both Choshi and Miyakojima test sites, gamma-FeOOH was predominantly detected on the specimens after ordinary exposure tests, whereas Fe3O4 was prevalent on the specimens exposed to sheltered environments. The correlation between the proportion of each corrosion product identified through SAM analysis and the amount of corrosion change for one year was explored. alpha-FeOOH exhibited a positive correlation with the amount of corrosion change, whereas the amount of corrosion change tended to decrease with an increase in beta-FeOOH fraction.

This study investigates the effect of P addition on the corrosion resistance of steels before and after rust formation. Electrochemical measurements and surface analysis of P-containing steels (Fe-0.5 mass% P, Fe-1.0 mass% P, and Fe-1.5 mass% P) were conducted to analyze the contribution of P to their initial corrosion resistance before rust formation. The results showed that the initial corrosion resistance of the steel worsened with increasing P content. According to the surface analysis conducted by SEM/EDS, more P segregations at the grain boundaries occurred with higher P content. Polarization measurements indicated that these P segregations became initiation sites for localized corrosion, resulting in a decrease in the initial corrosion resistance. Although the initial corrosion resistance was worse with higher P content, the long-term corrosion resistance showed the inverse trend, improving with increasing P content. Atmospheric exposure tests at Miyakojima and surface analysis of the rust layers showed that P was incorporated into the rust layer, and it promoted the protective ability against corrosion.

Passivity-maintaining current of high-strength (Si-Mn) steel in boric acid-borate buffer at pH 6.5 was larger than that of ferrite/pearlite (F/P) steel. Charge transfer resistance of the Si-Mn steel surface passivated at 1.10 V vs SHE in pH 6.5 buffer was smaller than that of F/P steel surface. Nano-analyses using AES and STEM revealed the details of the structure and components of the passive film on the Si-Mn steel. The inferiorities of passivity of Si-Mn steel were suggested to be due to the alloying components and metallurgical structure to increase the strength of the steel.

In the field of materials engineering, physical properties such as mechanical properties are considered as the most important properties, however, chemical properties such as corrosion resistance are also critical properties that cannot be ignored. In particular, Japan is island country where metallic materials are frequently used in coastal areas. Therefore, the problems of failures and accidents caused by corrosion reaction of structural materials are more serious than in other countries. Recently, laser powder bed fusion (LPBF) process which is a type of additive manufacturing (AM), has been attracting attention as an advanced processing tool for fabrication of metallic materials in various industrial fields. The direct-forming process by AM with the applicability of intricate-structured materials is strong advantage for manufacturing value-added and relatively small products such as medical implants or aerospace assemblies. Our previous study demonstrated the excellent corrosion resistance of LPBF-processed type 316L austenitic stainless steel, focusing on its crystallographic planes and grain boundaries [1]. Therefore, in this study, we investigated the corrosion resistance of LPBF-processed type 420J2 martensitic stainless steel. Martensitic stainless steels have the lowest corrosion resistance among all kinds of stainless-steel types such as precipitation hardening, ferritic, austenitic, and duplex, because of minimum chromium content without other corrosion-resistant elements, and high content of carbon. The primary advantage of martensitic stainless steel is superior hardness, which can be achieved quenching heat treatment. In other words, corrosion resistance and hardness are conflicting properties for stainless steels, and it is difficult to realize coexistence. In this study, the electrochemical and the non-electrochemical corrosion tests were performed to evaluate the corrosion behavior of LPBF-processed and commercial 420J2 stainless steels. The microstructural characterization and hardness tests were also performed. Commercial 420J2 stainless steel powder was used as the primary material for specimen fabrication by LPBF in this study. The nominal composition of 420J2 stainless steel was Fe-12Cr-0.3C. The LPBF process was performed in an argon atmosphere using a 3D printer. To examine the evolved texture and planes of interest of the specimens, the z-axis was defined as the build direction, and the x- and y-axes were defined as the laser scanning directions. In this study, we fabricated cubic specimens with dimensions of 11 mm × 11 mm × 11 mm. The specimens were cut mechanically to expose their yz-, xz-, and xy-planes. No post-processing was performed on the LPBF specimens. Anodic polarization measurement (linear sweep voltammetry) was performed using a potentiostat (HABF-501G, Hokuto Denko, Japan) connected to a function generator (HB-111, Hokuto Denko, Japan) with an analog cable. A saturated calomel electrode (SCE) and platinum electrode were used as the reference and counter electrodes, respectively. The specimens were fixed in a polytetrafluoroethylene holder with an O-ring. The exposed area contacting the electrolyte was 0.35 cm2 (6.7 mm in diameter). After immersing the specimens in a simulated body fluid (physiological saline: 0.9 mass% NaCl aqueous solution, aerated) at 310K, their open circuit potentials (OCPs) were recorded for 10 min. Then, a gradient anodic potential was applied at a constant sweep rate of 1 mVs-1 from the initial potential of −50 mV from the OCP. The measurement was stopped when the current density limit of 1 mAcm-2 was recorded. Figure shows the polarization curves of LPBF-processed and commercial 420J2 stainless steel specimens in physiological saline. The pitting potentials of LPBF specimens were significantly higher than those of commercial specimens. These experimental results indicate that localized corrosion resistance was effectively improved as like 316L austenitic stainless steel in the previous study [1]. The results of the hardness test showed that LPBF specimens was 53.50 ± 0.25 HRC, regardless of the measuring position. The hardness of the commercial specimens varied from 54 to 27 HRC depending on the tempering treatment conditions after the primal quenching. Thus, LPBF is found to be an ideal process that can simultaneously enhance corrosion resistance and hardness of martensitic stainless steels. The experimental results of another corrosion resistance evaluation and inclusion extraction will be presented in the session. Reference: [1] Tsutsumi Y et al. Additive Manufacturing 45 (2021) 102066. Figure 1

The monitoring of invasion/permeation hydrogen on entry/exit surfaces of cathodically charged SUS316 columnar crystals was conducted with a scanning Kelvin probe force microscope (SKPFM) under atmospheric pressure. Columnar crystal specimens covered with oxide films on their surfaces under room conditions were prepared for cathodic charging tests and subsequent SKPFM measurements. The invaded hydrogen on the entry surface was detected at the 8 -ferrite phases for 7 d after charging, and the segregation of invaded hydrogen at the boundaries between the 8 -ferrite and austenite matrix was prolonged for >10 d after charging. The permeated hydrogen on the exit surface was detected at the 8 -ferrite phases for 3 d after charging, but was not substantial at some of the 8 -ferrite phases regardless of the charging. Segregation of permeated hydrogen at the boundaries between the 8 -ferrite and some of the intermetallic precipitates was prolonged for 7 d after charging. The behaviors of invaded/permeated hydrogen based on heterogeneous microstructures are discussed to improve understanding of the hydrogen embrittlement mechanism in weld metals.