Chloride-containing Environments Research Articles

Stainless steels as a solution for corrosion and erosion problems involving grains in agribusiness sector applications

Agricultural activities such as grain processing play a crucial role in Brazil's economy, contributing significantly to its gross domestic product (27.4% in 2021). The use of carbon steel equipment leads to considerable environmental and economic losses in agribusiness due to corrosion. This study explores the erosion-corrosion behavior of a dual-phase 11Cr and an 18Cr8Ni stainless steel (SS), compared to a mild (ASTM A36) and a high-strength carbon (AHSS) steel under conditions close to the agribusiness reality. The impact of corn particles as erosive component associated to pure water and chloride-containing environments was studied using a fog chamber, a topic until now unexplored in the literature. The methodology adopted is novel and explores the erosion-corrosion synergy in cumulative steps, aiming to shed light on the mechanisms of corrosion and wear that affect the materials in the agribusiness industry. The findings reveal that corn particles, despite their soft nature, can easily remove non-adherent corrosion products from the steel surface. In the deionized water medium, the impact of the soft particles can also shift the corrosion pattern of advanced high-strength steels from general corrosion to localized pitting damage reduzing the corrosion rates. The carbon and 11Cr steels showed the highest corrosion rates under the pure corrosion condition in both media. The erosion-corrosion process considerably reduced the corrosion rate, mainly in the deionized water medium for the carbon steels. An exception to this behavior was 18Cr8Ni steel, whose corrosion rates, however, were the lowest among the materials. Low-chromium stainless steel emerges as a cost-effective solution to erosion-corrosion challenges in the agribusiness segment.

Microstructure and Chlorine Ion Corrosion Performance in Bronze Earring Relics.

Chlorine ions play an important role in the corrosion of bronzeware. This study employs techniques such as XRD, OM, SEM, EBSD, and electrochemical testing to analyze the microstructure, crystal structure, chemical composition, and corrosion performance of bronze earrings unearthed at the Xindianzi site in Inner Mongolia. The results indicate the presence of work-hardened structures, including twinning and equiaxed crystals, on the earrings' surface. With an increase in chloride ion concentration in NaCl solutions from 10-3 mol/L to 1 mol/L, the corrosion current density of the bronze earrings increased from 2.372 × 10-7 A/cm2 to 9.051 × 10-7 A/cm2, demonstrating that the alloy's corrosion rate escalates with chloride ion concentration. A 3-day immersion test in 0.5% NaCl solution showed the formation of a passivation layer of metal oxides on the earrings' surface. These findings underscore the significance of the impact chloride ions have on the corrosion of copper alloys, suggesting that activating the alloy's reactive responses can accelerate the corrosion process and provide essential insights into the corrosion mechanisms of bronze artifacts in chloride-containing environments.

Laser Powder Bed Fusion Printing of CoCrFeMnNi High Entropy Alloy: Processing, Microstructure, and Mechanical Properties

Equiatomic CoCrFeMnNi high entropy alloy (HEA) powder was processed by laser powder bed fusion (LPBF) additive manufacturing (AM). The properties of the spherical pre-alloyed CoCrFeMnNi powder were characterized and its processability using LPBF AM was systematically investigated through the volumetric energy density (VED) based on the surface roughness, defects (micro-cracks and porosity) and densification. After optimization, LPBF processing at a VED of 104 J/mm3 achieved highly dense and crack-free vertical and horizontal test specimens with a porosity fraction lower than 0.01% and micro-pores having a mean size of, respectively, 25.9 μm and 13.4 μm, as determined from X-ray micro-computed tomography (μCT) inspection. Scanning electron microscope (SEM) analysis of the as-built (AB) CoCrFeMnNi processed at a VED of 104 J/mm3 showed a heterogeneous solidification microstructure, consisting of columnar grains with a cellular subgrain structure, and electron backscattered diffraction (EBSD) revealed a crystallographic texture mainly along the < 100 > direction. Post treatment with hot isostatic pressing (HIP) was effective in closing the remnant micro-pores in the bulk volume of the AB CoCrFeMnNi. Also, the cellular sub-grain structure in the AB CoCrFeMnNi completely disappeared after HIP and the resulting microstructure consisted of recrystallized equiaxed grains with annealing twins. The room temperature tensile response was anisotropic for AB CoCrFeMnNi with horizontally built specimens exhibiting higher strength and fracture strains (global and local) compared to vertically built ones; HIP reduced the anisotropy in the tensile properties and led to similar tensile strength with elongation values that were ~ 50% higher than in the AB condition. The HIPed CoCrFeMnNi also displayed higher Charpy impact toughness and absorbed energy at both room and liquid nitrogen temperatures compared to the AB material. Examination of the fracture surfaces after tensile and Charpy impact testing revealed ductile features with characteristic dimpled appearance and pointed to the important role of the remnant micro-pores on failure in the AB CoCrFeMnNi. Tribological assessments pointed to the superior low-stress abrasion resistance of AB and HIPed CoCrFeMnNi compared to 316L stainless steel (SS), which was included in this study to reinforce the analysis. SEM observations revealed that scratching and micro-fracture are the dominant wear mechanisms for the CoCrFeMnNi HEA, whereas ploughing and cutting parallel to the abrasive flow direction are the dominant mechanisms for 316L SS. To the authors’ knowledge, this study is the first to evaluate and report the low-stress abrasion resistance of any high entropy alloy. To understand the corrosion behavior, polarization curves of AB and HIPed CoCrFeMnNi were measured in 3.5 wt% NaCl and 1N H2SO4 solutions, and the results were compared to those of 316L SS. The findings indicate that AB and HIPed CoCrFeMnNi outperform 316L SS in a chloride-containing environment, but not in an acid-containing environment. Additionally, observations of hydrogen permeability revealed that AB CoCrFeMnNi permeates a lower volume of hydrogen atoms (by ~ 5 times) compared to 316L SS, despite its higher (by nearly 3 times) diffusion coefficient. Electrochemical hydrogen permeation data showed that the concentration of atomic hydrogen in the sub-surface of AB and HIPed CoCrFeMnNi was, respectively, about 32 and 26 times lower than in 316L SS. This study provides important material–structure–property data and indicates a promising outlook for LPBF of the CoCrFeMnNi HEA with high-performance.

Investigation of Iron Dissolution Mechanism in Acidic Solutions with and without Dissolved CO2—Part II: Time of Flight-Secondary Ion Mass Spectrometry 3D Mapping

Time-of-flight-secondary ion mass spectrometry (ToF-SIMS) 3D mapping and depth profiling were used to study the anodic iron dissolution mechanisms of mild steel in chloride-containing aqueous CO2 environments. The technique detected adsorbed hydroxide and chloride intermediates formed during the corrosion process, consistent with the proposed multipath reaction mechanism for anodic iron dissolution reaction. Despite the presence of aqueous carbonic species and their observed effect on the kinetics of iron dissolution, no additional adsorbed intermediates have been detected in aqueous CO2 environments, indicating that carbonic species do not directly participate in the iron dissolution reaction. ToF-SIMS 3D mapping results on characterization of the specimens immersed in a chloride-containing solution with and without CO2 suggest that one role of aqueous carbonic species CO2 could be to accelerate the adsorption of chloride ions and the formation of chloride intermediates.

Localized Corrosion Resistance of Nickel Alloy 718 in Chloride-Containing Environments

Resistant to aggressive reducing environments and combining excellent mechanical properties, the age-hardenable Alloy 718 is the most broadly used nickel alloy in the oil and gas industry. Nevertheless, its localized corrosion resistance in oxidizing halide-containing environments has yet to be thoroughly investigated, with conflicting results discussed in the literature. In this regard, Alloy 718 has a relatively low pitting resistance equivalent, limiting in practice its use, where localized corrosion is expected, particularly in seawater applications. This work quantified the localized corrosion resistance of the alloy in chloride-containing environments. The potentiodynamic-galvanostatic-potentiodynamic (PD-GS-PD) technique was used to determine the crevice corrosion repassivation potential (ER,CREV) in deaerated natural seawater at different temperatures for two Alloy 718 grades (i.e., aerospace, as per ASTM B670, and oil and gas, as per API 6ACRA, grades). Additionally, PD polarization testing was conducted for the API 6ACRA grade in acidified solutions with varying chloride concentrations to simulate pit/crevice-like conditions. In the aerospace heat treatment condition, Alloy 718 suffered crevice corrosion at temperatures as low as 10°C, while the API 6ACRA grade remained unaffected at the same testing temperature. Stable crevice corrosion occurred at 20°C and 50°C for both grades.

Evaluation of the Applicability of Voltammetric Modes in Scanning Electrochemical Microscopy for In Situ Corrosion Characterisation of Copper-Based Materials

Chemical imaging of corrosion processes involving copper species using scanning electrochemical microscopy has been hampered by the lack of soluble oxidation states for copper that can be achieved by amperometric conversion at the tip. Indeed, the only possibility is to reduce the corrosion products at the tip, thus modifying the chemical response of the electrode material and requiring subsequent redissolution of the copper deposits. Consequently, the limitations arising from the system prevented a full-scale quantification, requiring the development of new methodologies or the optimisation of those currently available, as we pursued with the present work. Therefore, the voltammetric behaviours of gold macro- and microelectrodes were evaluated with respect to the collection and redissolution of Cu2+ ions, with the aim of using them as sensing probes in scanning electrochemical microscopy (SECM) to investigate the activity of copper surfaces in acidic chloride-containing environments. Cyclic and square-wave voltammetric techniques were explored for copper collection and subsequent stripping on Au microelectrode tips in SECM with the objective to capture in situ image electrochemical reactivity distributions across copper surfaces undergoing corrosion.

Development and optimization of trivalent chromium electrodeposit on 304L stainless steel to improve corrosion resistance in chloride-containing environment

In this study, we developed and optimized a trivalent chromium coating electrodeposited on 304L stainless steel (SS) from a Cr-trivalent bath. The results reveal that the Cr coatings at all bath temperatures except for 80 °C showed clusters of polyhedral grains, however, the grain sizes decreased with an increase in bath temperature. Also, the coatings deposited at bath temperatures of 30, 50, and 60 °C experienced networks of cracks, which decreased in population density as temperature increased. However, the coatings deposited at bath temperatures of 70 and 80 °C were crack-free due to surface modification, confirmed by 3D profile results with an advanced power spectral density and a multi-Gaussian histogram analysis. The mechanical test results demonstrate that the adhesion and wear resistance of the Cr-coatings formed on the SS substrate significantly improved, with the optimal coefficient of friction of 0.18. Likewise, electrochemical behavior observations of the Cr coatings show that pitting resistance improved with the increase in bath temperature conditions, as shown in the pitting potential values which increased from 272.6 mV to 436.2 mV as bath temperature increases from 30 °C to 80 °C. From this study, it is proposed that the Cr-coatings deposited at a bath temperature of 80 °C presents the optimal coating performance concerning a combination of all the target qualities aimed, such as better tribological behavior and improved pitting resistance. Thus, enabling the establishment of an innovative method to overcome the conventional issues encountered in Cr electrodeposition of SSs.

Time-Dependent Corrosion Behavior of Aluminum Using Advanced Electrochemical and Characterization Techniques

The time-dependent corrosion behavior of pure aluminum (Al) in a chloride-containing environment was investigated using various electrochemical and characterization techniques for up to 336 h. Transmission electron microscopic and secondary ion mass spectroscopic analysis revealed the continuous dissolution of the surface film over the immersion time. In the meantime, the increasing passive oxide thickness resulted in the surface film resistance enhancement over the immersion time, as indicated by the electrochemical impedance spectroscopic analysis. The electrochemical noise measurements showed an increase in the corrosion kinetics with immersion time until 60 h because of the accelerated localized corrosion in the early stage of immersion. However, an inhibition in corrosion kinetics occurred after longer immersion times due to corrosion product deposition inside the pit.

Corrosion susceptibility of the nanophases of the Al-based amorphous-nanocrystalline coatings

The influence of the size and types of nanophases on the corrosion behavior for Al86Ni6Y4.5Co2La1.5 amorphous-nanocrystalline coatings in a chloride-containing environment were systematically studied. Results revealed that when the two coatings contained only α-Al nanophase with different size, it was the combined effects of the carrier density and the thickness for the passive film that determined the stability of the passive film. Moreover, with the presence of Al-TM and Al-RE nanophases, the origin of localized corrosion on the coating was ascribed to the weak passive film stability on Al-RE nanophase, which was preferentially dissolved by the interaction with chloride ions.

Corrosion inhibition of low-alloy carbon steel by gum Arabic and zinc acetate in neutral chloride-containing environment

In this work, we investigated the inhibitory properties of an eco-friendly composition based on polysaccharide (gum Arabic) and zinc acetate for corrosion resistance enhancement of low-alloy carbon steel in a neutral environment. Electrochemical results showed that the inhibitory efficiency of the composition increases with increasing its concentration and reaches its maximum at 2.0 g/l of each component. The inhibitory efficiency of the composition, calculated from impedance dependences after 24 hours immersion of steel in a 0.1% NaCl solution, was 98%. The protective effect of the composition is associated with the adsorption of zinc acetate and gum Arabic due to the presence of hydroxyl and carboxyl groups in them, and the ability of Zn2+ cations to interact with OH– anions with the formation of zinc hydroxides, which block cathode areas and slow down corrosion. The adsorption of zinc acetate and gum Arabic on the steel surface was described by the Langmuir isotherm most completely. Electrochemical impedance spectroscopy results indicate the formation of a protective film, which probably consists of Fe2+-Zn2+-gum Arabic-acetate complexes. The formation of a protective film on the steel surface in inhibited solutions was confirmed by scanning electron microscopy and energy dispersive X-ray analysis results.

Effect of wetting/drying cycling on flexural behavior of RC beams strengthened by prestressed CFRP plate: Experimental and analytical investigations

Using prestressed carbon fiber reinforced polymers (CFRPs) is a promising technique for strengthening reinforced concrete (RC) structures. This study aimed to investigate the prestress losses and flexural behavior of strengthened RC beams under wetting/drying cycling (WDC) in a chloride-containing environment. Two prestress levels (20% and 40%) and two WDC durations (90 and 180 cycles) were considered. The time-dependent prestress loss under WDC was first monitored by fiber bragg grating (FBG) for 90 days, and the composition of the prestress loss was theoretically analyzed. A four-point bending test was conducted on the strengthened beams after the WDC exposure, and the intermediate crack (IC) debonding strain and the ultimate moment of the strengthened beams were analytically evaluated. The results show that although WDC exposure aggravated the time-dependent prestress losses, the maximum time-dependent prestress losses after 90 WDCs were only 1.75%, indicating the effectiveness of the end anchorage system. After WDC exposure, the failure mode of the strengthened beams shifted from CFRP rupture to IC debonding, accompanied by the reduction of cracking and the ultimate load, CFRP utilization, and the increase of the maximum crack width. Besides, the model of Lu et al. and Said and Wu could be applied to the IC debonding strain prediction after the WDC. When the environmental reduction coefficient was incorporated, ACI 440.2 R and CECS-146 resulted in a precise IC debonding strain. The proposed method could accurately predict the ultimate moment of the prestressed FRP-strengthened RC beams with different failure modes after WDC.

Constructing passivation film on plain carbon steel similar to that on 316L stainless steel

A high corrosion-resistant passivation film with cerium-molybdenum-chromium-nickel ions has been synthesized on plain carbon steel rebar through a chemical transformation method which significantly lowered the pitting susceptibility of the rebar in the chloride-containing concrete environment. The results indicated that the prepared film has a re-passivation self-healing property in a concrete environment with a concentration of 0.1 M NaCl, which is similar to the behavior of 316L stainless steel. The charge transfer resistance increased markedly with the extension of immersion time, reaching 3.70 × 106 Ω cm2 in about 1500 h of immersion (about 10 times of the initial Rt value). The high corrosion resistance mechanism of the film was discussed here in depth.

Estimation of Durability of HC-550 Floor Slabs Based on Electrochemical Tests of Corrosion Rate of Reinforcement Strings in Concrete

The consequences of the loss of the load-bearing capacity due to the corrosion of prestressing steel can be much more dangerous than in the case of reinforced concrete structures, since failure can occur quite rapidly and without warning. A very important issue, therefore, is to determine the factors affecting the durability of prestressed structures exposed to aggressive agents, especially chloride ions. The aim of this study was to verify the protective properties of concrete of prefabricated HC-type prestressed concrete slabs in order to evaluate the possibility of their application in the ceilings of multilevel garages. In this paper, the corrosion rate of rebar steel in HC-550 floor slabs at a width of 1200 mm was estimated with nondestructive electrochemical methods: linear polarization and impedance spectroscopy. The general and mechanical properties of concrete prepared according to a formula in a laboratory and analogous concrete cut directly from the analyzed floor slabs were also studied. The porosity of concrete from these slabs was determined using X-ray-computed tomography for pore-related characterization. The values of the diffusion coefficient of chloride ions determined in previous works and the previously proposed model for the overexposure of the durability of floor slabs in chloride-containing environments were used to determine the durability of these slabs. Based on the empirical correlations adopted from the literature presenting the relationship of durability/adhesion over time and the corrosion parameters studied, a safe service life was determined at the nominal class of concrete equal to Δtcor.red = 30.48 years. In addition, in the case of discontinuities in the concrete structure, there may be a dangerous reduction in the time of corrosion initiation and a subsequent reduction in service life due to the loss of the adhesion of strut strands for up to 10.68 years of service life.

MATHEMATICAL MODELING OF THE CORROSION BEHAVIOR OF AUSTENITIC STEELS IN CHLORIDE-CONTAINING ENVIRONMENTS DURING THE OPERATION OF PLATE HEAT EXCHANGERS

Purpose. It consists in the development of mathematical models that describe the dependence of the critical pitting temperatures of AISI 304, 08X18N10, AISI 321, 12X18N10T steels in model circulating waters with pH 4...8 and chloride concentration from 350 to 600 mg/l.&#x0D; Research methods. The developed mathematical models are based on linear square regression and a neural network of direct signal propagation for a reduced set of features.&#x0D; Results. It was established that the critical pitting temperatures of the studied austenitic chrome-nickel steels increase with an increase in the pH of the circulating water, the number of oxides up to 3.95 μm in size, the average distance between titanium nitrides, the Cr content and a decrease in the concentration of chlorides in the circulating water, the average distance between oxides, and the austenite average grain diameter.&#x0D; Scientific novelty. Based on the established relationships between the critical pitting temperatures of corrosion-resistant steels AISI 304, 08Х18Н10, AISI 321, 12Х18Н10Т, their chemical composition within the standard and structural heterogeneity, the mechanisms of their influence on the pitting resistance of these structural materials in circulating chloride-containing waters have been developed. It was established that metastable pitting is formed in a solid solution of austenite of steels around oxides with a size of 1,98...3,95 microns and repassivates before reaching critical dimensions of about 5 microns, which contributes to the growth of their pitting resistance in reversible chloride-containing environments.&#x0D; Practical value. The developed mathematical models are proposed to be used for the selection of optimal melts of austenitic chrome-nickel steels for the production of heat exchangers and prediction of their pitting resistance during their operation in circulating waters. The processes contributing to the perforation of heat transfer elements of heat exchangers during their operation have been identified.

Binary Aluminum Alloys from 1-ethyl-3-methylimidazolium-based Ionic Liquids for Cathodic Corrosion Protection

Aluminum cannot provide continuous cathodic corrosion protection under ambient conditions due to the formation of an insulating oxide layer and therefore it should be alloyed. Binary aluminum alloys with Cr, Zn and Sn from AlCl3/1-ethyl-3-methylimidazolium chloride ([EMIm]Cl) containing CrCl2, ZnCl2 or SnCl2 have been deposited and their morphology and composition were investigated using SEM/EDS. The corrosion behavior of alloys with 2–4 wt% Cr, Zn or Sn was investigated using potentiodynamic polarization in 3.5 wt% NaCl solution, neutral salt spray test (NSS) and environmental exposure (EE). Pure aluminum provides excellent corrosion protection of steel in a chloride-containing environment, but not under ambient conditions. AlCr alloys show poor corrosion protection while AlZn alloys provide excellent corrosion protection in the NSS test and superior cathodic protection in the EE test compared to aluminum. AlSn alloys are highly active at even low tin contents and dissolve rapidly in chloride-containing electrolytes. However, a slightly improved cathodic protection in the EE test compared to pure aluminum has been observed. The results prove the necessity of alloying aluminum to achieve effective cathodic corrosion protection under mild atmospheric conditions.

Effect of micro-alloying element P on the pitting behavior of copper

The pitting behavior of copper in aqueous chloride-containing environments was investigated by the doping of the P-element and the data was interpreted using a point defect model (PDM). Cyclic polarization tests investigated the passivation breakdown behavior of copper and phosphorus-doped copper, with phosphorus-doped copper exhibiting a higher pitting potential. And the cumulative distribution of breakdown potentials for both copper and phosphorus-doped copper is close to normal, demonstrating the accuracy of the PDM predictions. The scanning electron microscope observed the microscopic morphology of the passivated film surface and the XPS analyzed the composition of the passivated film.

Corrosion of Low Carbon Steel in Chloride Containing Environment

This research aims to investigate the corrosion of low-carbon steel in a chloride-containing environment. The investigation started by preparing and studying the surface condition of the specimens. The mechanical properties, microstructures and chemical compositions were also comparatively investigated between standard and corroded specimens. The results showed that the surface of the corroded samples was rough and peeling, different from the standard specimens, which were relatively smooth and non-corrosive. The average ultimate tensile strength and average yield strength of normal specimens were higher than those of corroded specimens at 5.39% and 2.32%, respectively. The corrosion products developed on the surface of corroded specimens caused a high-stress concentration area. The specimen is more prone to cracks and breaks when subjected to tensile stress. The surface of the normal specimen was composed of iron, oxygen, carbon, and silicon. In contrast, the surface of the corroded specimen contained the same elements as the normal specimen except chlorine which was detected. The film layer of the corroded specimen was cracked in contrast with the normal specimen, where the film still covered the metal substrate. The chemical composition analysis found that the corroded specimen's film layer contained chlorine, which is not detected in the film layer of the normal specimen. This is because the film layer of hot-rolled steel remains porous even passing the hot-rolling process. These pores allow chloride ions to diffuse and react with the film layer and the metal substrate. This makes the film layer of the corroded specimen thicker and initiates cracks. When stable alpha-iron-oxyhydroxide is formed on the steel surface, it can help to suppress corrosion.

Comparative study of corrosion behaviors of die cast LA42 and AZ91 alloys

In our previous work, die cast LA42 (Mg–4La–2.5Al–0.3Mn, wt.%) alloy with an excellent combination of thermal-conducting performance and mechanical properties was developed. This study, taking commercial die cast AZ91 (Mg-9Al-1 Zn, wt.%) alloy as a comparison, investigates the microstructure and corrosion behavior of the LA42 alloy in chloride-containing environment. The findings revealed that the microstructure of die cast LA42 alloy displays an Al-depleted α-Mg dendrite and a network shape eutectic phase comprising of eutectic α-Mg and lamellar Al11La3. The LA42 alloy exhibited slightly inferior corrosion resistance and deeper corrosion pits than the AZ91 alloy, which were related to the interphase attack and the presence of a layer of corrosion products containing more magnesium hydroxide of the LA42 alloy. The corrosion behavior of the LA42 alloy was characterized by an interphase attack on the periphery of Al-depleted α-Mg dendrite adjacent to Al11La3, which primarily ascribed to the relatively lower potential of the Al-depleted α-Mg phase and the interfacial reactivity between it and the adjacent Al11La3, whereas the AZ91 alloy showed a trait of preferential corrosion of the interior of α-Mg dendrite for its lowest potential among different phases. This comprehensive research can provide a guidance for future alloy design of Mg-Al-La alloys and their industrial applications.

(Corrosion Division Morris Cohen Graduate Student Award) Microscopic Electrochemical Properties of Carbon Steels and Metallurgical Approach for High Corrosion Resistance

Carbon steels are widely used in the production of automobiles, bridges, plates, and so on because of their superior mechanical properties and low cost. Carbon steels contain various types of microstructures such as ferrite, pearlite, and martensite, and desired mechanical properties can be obtained by combining these microstructures. While carbon steels exhibit excellent mechanical properties, their pitting corrosion resistance is relatively low in chloride-containing environments. Although the steels are successfully protected from corrosion by coating and/or painting, localized corrosion is readily initiated at cut edges and in coating defect areas in atmospheric environments. To prolong their service life and improve their reliability, it is necessary to elucidate the initiation mechanism of pitting corrosion on carbon steels. While it is widely known that the microstructures of carbon steels affect their corrosion resistance, the detailed mechanism is still unclear. The objective of this research is to clarify the relationship between the microstructure and pitting corrosion resistance of carbon steels in chloride-containing near-neutral pH environments.The pitting corrosion resistance of the typical microstructures of carbon steels was investigated using a micro-electrochemical polarization technique. Various microstructures (ferrite, pearlite, tempered martensite, decarburized martensite, as-quenched martensite etc.) were obtained by conducting different heat-treatments, and micro-scale electrodes (ca. 100 μm × 100 μm) were prepared with only one microstructure included. Small areas without any non-metallic inclusions were chosen as the micro-scale electrodes to eliminate the effect of pit initiation at the inclusion and evaluate the intrinsic corrosion behavior of the microstructure itself. Then, potentiodynamic anodic polarization measurements were carried out in NaCl-containing boric-borate buffers at pH 8.0. It was determined that the pitting corrosion resistance depends on the type of microstructure, with the as-quenched martensite showing the highest pitting corrosion resistance. The as-quenched martensite includes a high amount of interstitial C in the Fe lattice, and it was also clarified that the anodic dissolution of steels was clearly suppressed with increasing interstitial C content. During pitting corrosion, the local breakdown of the passive film is induced by chloride ions, which results in the initiation of active dissolution. It was considered that the low dissolution rate provided by interstitial C results in the superior pitting corrosion resistance of as-quenched martensite.To clarify the reason for the superior active dissolution resistance obtained by interstitial C, first-principles calculations were conducted. At first, the electronic structure of martensite was analyzed using bulk-model supercells with bct Fe crystal structures which included interstitial C. According to first-principles calculations, the electronic density of states (DOS) at and around the Fermi level decreased due to the presence of interstitial C. In addition, a valence electron transfer occurred from Fe to C atoms, resulting in a decrease in the number of valence electrons of Fe. Because the presence of valence electrons at and around the Fermi level affects the kinetics of electron (charge) transfer in oxidation/reduction reactions, including the active dissolution reaction, it was considered possible that the presence of interstitial C suppressed the electronic reactivity of Fe. Furthermore, first-principles calculations were also carried out for slab-model supercells consisting of a metal/vacuum interface, and the effect of interstitial C on the work function of martensite was analyzed. The result of the calculations indicated the work function of martensite linearly increased with increasing interstitial C content. Since the work function and ionization energy of metals are the same, the electrochemical properties of metals, such as dissolution resistance, are considered to be principally associated with their work function. In other words, it is understood that metals with a high work function are associated with a higher dissolution resistance. According to the above calculation results, the change in the electronic structure of Fe and the increase in the work function provided by interstitial C are key factors contributing to the superior dissolution resistance of martensite.

Development of a Framework for Modeling Protection Mechanisms Provided By Multi-Layered Organic Coatings Applied to Aerospace Al Alloys Via Finite Element Analysis

Aluminum alloys, such as AA2024 and AA7050, are widely used in aerospace applications due to their high strength-to-weight ratio. However, because of the heterogeneous microstructure of these alloys, they are susceptible to microgalvanic corrosion and pitting in chloride-containing environments. The corrosion susceptibility is further increased when these alloys are in contact with more noble alloys, such as in locations near fasteners, where the galvanic interaction leads to accelerated corrosion attack of the less noble Al alloy. To protect these structures against corrosion attack, organic coatings are commonly used. Typical coating systems are usually composed of three layers: a conversion coating, a primer, and a topcoat. When the topcoat is breached, the primer can provide active corrosion protection by the release of inhibitors embedded in the primer. Depending on the type of inhibitor, it can provide chemical protection by the formation of a passive film on the surface of the exposed metallic area and/or provide cathodic protection by the preferential dissolution of sacrificial anodes.The efficacy of the corrosion protection provided by the multilayered coating system to Al alloys in aircraft components exposed to atmospheric corrosion is affected by a large number of variables. The dynamic environment in which the component is exposed to (e.g., relative humidity, temperature, salt loading density), the geometry of the component, the chemistry of the primer, and the type of alloy are examples of important parameters that impact the corrosion and protection mechanisms. To unravel the impact of each one of these variables and/or the synergistic effects between them, a large volume of experimental work is needed, making the prediction of coating failure, corrosion damage, and, ultimately, component lifetime, remarkably challenging. In this context, the development of computational models that can simulate corrosion and corrosion protection mechanisms is attractive, as they can be used as tool to effectively assess the large parameter space and increase efficiency of experimental work.This work focuses on developing a framework for modeling the protection mechanisms offered by chromate-based and Mg-rich primers via finite element analysis. Chemical and electrochemical protection mechanisms were modeled taking into consideration leaching of inhibitor from the primer, mass transport and the impact of the inhibitors in the electrochemistry of the Al alloys. The impact of environmental, geometric, and coating parameters on the effectiveness of the protection mechanisms was assessed. Ultimately, the framework developed will provide a better understanding of protection mechanisms of the organic coating systems and serve as a framework to study new coating systems.