Potentiodynamic Polarization Tests Research Articles

Synergistic Effects of Zn-Rich Layered Double Hydroxides on the Corrosion Resistance of PVDF-Based Coatings in Marine Environments

In this study, zinc–aluminum layered double hydroxide (ZLDH) and its calcined counterpart (CZLDH) were synthesized and incorporated into a poly(vinylidene fluoride) (PVDF) matrix to develop high-performance anti-corrosion coatings for mild steel substrates. The structural integrity, morphology, and dispersion of the LDH fillers were analyzed using FTIR, XRD, Raman spectroscopy, and SEM/EDS, while coating performance was evaluated through water contact angle (WCA), adhesion tests, and electrochemical techniques. Comparative electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests in a 3.5% NaCl solution revealed that the ZLDH/PVDF coating exhibited superior corrosion resistance and long-term stability compared to CZLDH/PVDF and pristine PVDF coatings. The intact lamellar structure of ZLDH promoted excellent dispersion within the polymer matrix, enhancing interfacial adhesion, reducing porosity, and effectively blocking chloride ion penetration. Conversely, calcination disrupted the lamellar structure of ZLDH, reducing its compatibility and adhesion performance within the PVDF matrix. This study demonstrates the critical role of ZLDH’s structural integrity in achieving enhanced adhesion, barrier properties, and corrosion protection, offering an effective anti-corrosion coating for marine applications.

Rare‐Earth 4‐Hydroxyphenylacetate Complexes: Synthesis, Structural Characterization, and Corrosion Inhibition Properties

Four structural types of rare‐earth aqua 4‐hydroxyphenylacetate complexes {[Ce(L)3(H2O)2]·H2O}n (a), {[RE2(L)6(H2O)]·4H2O]}n (RE = Nd (b), Gd (c)), {[RE2(L)6(H2O)]·3H2O]}n (RE = Dy (d), Y (e)), {[RE2(L)6(H2O)]·5H2O]}n (RE = Er (f), Yb (g)), (L = 4‐hydroxyphenylacetate) have been synthesized by salt metathesis reactions of corresponding rare‐earth metal chlorides or nitrates and sodium 4‐hydroxyphenylacetate (NaL) in a water/ethanol solvent system. The compounds are targeted for structural interest and to determine if the 4‐hydroxy (4‐OH) substituent would enhance the anti‐corrosion properties of rare‐earth phenylacetates. Single crystal X‐ray diffraction (XRD) determined the crystal structures of all compounds except the complex of Gd (c), which was shown to be isomorphous with the Nd complex (b) by the powder XRD pattern. All compounds were obtained as one‐dimensional polymeric structures. The structure of a differs in being a monometallic repeating unit and having two coordinated waters whilst the others have binuclear repeating units that resulted from the alternating coordination of one water molecule along the chain. Weight loss and potentiodynamic polarization tests reveal that {[Gd2(L)6(H2O)]·4H2O}n has the best corrosion inhibition properties for mild steel in 0.01 M NaCl. Furthermore, all a–g complexes are more effective corrosion inhibitors than the aqua and 2,2’‐bipyridine (bpy) complexes of unsubstituted phenylacetate indicating that 4‐OH substitution of phenylacetate enhances anti‐corrosion properties.

Effect of copper content on the microstructure and electrochemical corrosion behavior of laser cladding 316L stainless steel coating

Abstract To evaluate the effect of copper (Cu) content on the microstructure and corrosion resistance of austenite stainless steel, 316L stainless steel coatings with varying Cu contents were prepared by laser cladding. The phase composition, microstructure, and electrochemical corrosion behavior of the coatings were studied in detail. The results indicated that γ-Fe was the dominant phase in the 316L stainless steel coating, and ε-Cu appeared after the addition of Cu. The incorporation of Cu was beneficial for refining the typical dendrite structures. Potentiodynamic polarization tests in 0.5 M H2SO4 solution revealed that as the Cu content increased, the corrosion potential of the coating became more positive and corrosion current density decreased, demonstrating that the addition of Cu could improve the corrosion resistance of the coating. Similar conclusions were also obtained from long-term electrochemical impedance spectroscopy tests. By characterizing the Mott-Schottky curve and element valence of the passivation film on the Cu-containing 316L stainless steel coating, it was found that Cu addition could reduce the point defect density and improve the stability of the passivation film. Moreover, Cu could also promote the enrichment of oxidized-state Cr and Cu/Cu2O in the passivation film, making it more protective.

A New Approach to Titanium-Hydroxyapatite-Bio Composite: Pressure-Assisted Coating on the Antibacterial and Electrochemical Properties of Ti6Al4V

This study aims to coat Ti6Al4V alloy with Ti-xHA (x = 2.5−10 wt%) mixture to improve its surface properties. A new approach using a powder metallurgical pressure-assisted sintering method was applied to the coating process. The in situ sintering and coating process was performed at 950°C for 45 min in a vacuum atmosphere of 10–4 mbar. A pressure of 50 MPa was applied during the sintering process. Staphylococcus aureus (ATCC® 29213TM) and Escherichia coli (ATCC® 25922TM) cultures were used to determine the antibacterial activity of the sintered and coated samples. The electrochemical properties of the samples were studied by Tafel extrapolation and potentiodynamic polarisation tests. The results showed that the coating layer containing 7.5 wt% of hydroxyapatite (HA) increased the antibacterial property against gram-positive and gram-negative bacterial cultures. Furthermore, it was determined that the icorr value of the material decreased and the corrosion resistance improved with an increasing HA ratio. In addition, no active-passive oxidation zone formation was observed up to 2000 mV in the HA-added samples.

The role of MgO nanoparticles on the corrosion resistance of hot-dip Al-Zn coating.

This study investigates the impact of MgO nanoparticles (0, 0.1, 0.5, and 1wt%) on the corrosion behavior of hot-dipped galvalume (Zn-55Al-1.6Si) coating. The corrosion behavior of coatings was assessed using a salt spray, electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization tests in air-saturated 3.5wt% NaCl solution. The surface of the coatings was examined using field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). The results revealed that MgO nanoparticles were mainly located in zinc-rich interdendritic areas and filled the micro-holes within the coating. By creating a protective shield and separating the coating from the corrosive agents, these nanoparticles enhanced the corrosion resistance of coatings. Notably, the galvalume coating with the highest MgO nanoparticle content (1wt%) exhibited the best corrosion resistance.

CORROSION BEHAVIOUR OF ADVANCED COMPOSITES CONTAINING SURFACE MODIFIED SIC AS REINFORCEMENT

The interface structure, micro-voids, pits, delamination, intermetallic compounds and internal stresses are the utmost critical factors influencing the corrosion performance of composite materials making them unsuitable for use in structural applications. One useful technique to control such adverse effect in composites is surface modification of the reinforcement. The present investigation involved the implementation of chemical techniques, particularly electroless plating (EP), to modify the surface of silicon carbide (SiC) particles by depositing a layer of nickel phosphorous (Ni-P) onto them. These plated SiC particles were subsequently employed in the production of Al/Ni-pSiC composites using the powder metallurgy (PM) method, with different weight percentages of plated SiC content ranging from 5% to 15%. The corrosion behaviour was assessed by potentio-dynamic polarization tests in 3.5% NaCl solution at room temperature. Results confirmed that AlNi-pSiC composites exhibit a greater resistance to pitting, in contrast to pure aluminium and Al/SiC composites without plating. The enhanced corrosion resistance in Al/Ni-pSiC composites can be attributed to strong interfacial bond, grain refinement, CTE difference, formation of Ni3Al, and reduced potential difference.

Electrodeposition of Al coating onto a 300M steel from AlCl3/acetamide deep eutectic solvents for corrosion protection

Aluminum coating was expected to be used for corrosion protection of 300M steel. Al coating was prepared using the galvanostatic electrodeposition from AlCl3/acetamide (1.3:1) system for the corrosion protection of 300M steel. This process was conducted at varying current density (2–8 mA·cm−2) and electrodeposition time (1 and 2 h). The chronopotentiometry revealed that the size of Al nucleus was proportional to the inverse of overpotential. Thus, the morphology of Al coating was characterized by scanning electron microscope and optical surface profiler. The results indicated a decrease in Al grain sizes and the roughness fluctuation with increasing current density. Moreover, the corrosion resistance of Al coatings was evaluated in a 3.5 wt% NaCl solution using the potentiodynamic polarization and electrochemical impedance spectroscopy test. It was illustrated that corrosion current density decreased by 57.8 % at 6 mA·cm−2 with the electrodeposition time increasing from 1 to 2 h, while the value decreased by 75.9 % with the current density raising from 5 to 6 mA·cm−2 for 2 h. Al coating electrodeposited at 6 mA cm−2 for 2 h represented the highest corrosion resistance with the charge transfer resistance of 5695 Ω·cm2, while the value of 300M steel was 914 Ω·cm2. Finally, X-ray diffraction pattern suggested that corroded coatings compositions included Al2O3, AlO(OH) and AlCl3·6H2O. The study was considered to provide a promising method using AlCl3/acetamide to electrodeposit Al coatings for protecting 300M steel from corrosion.

Corrosion inhibition potential of aqueous extract of Trifolium repens plant leaves on carbon steel in hydrochloric acid medium (TRPL)

This research explores the effectiveness of a liquid extract derived from the leaves of Trifolium repens as a corrosion inhibitor for carbon steel in a 1N hydrochloric acid environment. Corrosion poses significant challenges in industrial applications, and this study aims to provide an eco-friendly solution by leveraging plant extracts. The experiment demonstrated that the application of the Trifolium repens extract substantially reduced the weight loss of carbon steel, indicating its efficacy in mitigating corrosion. Inhibition efficiency was quantified through systematic weight loss measurements, revealing a clear inverse relationship between the concentration of the extract and the corrosion rate. As the concentration of the inhibitor increased, the rate of corrosion correspondingly decreased, underscoring the extract's potential for effective corrosion prevention. The mechanism of inhibition was further elucidated through electrochemical techniques. Potentiodynamic polarization (PDP) tests and alternating current impedance spectroscopy were employed to assess the electrochemical behaviour of carbon steel in the presence of the extract. These analyses confirmed the formation of a protective layer on the steel surface, which effectively blocks active corrosion sites, thereby enhancing the material's resistance to acidic attack. To gain deeper insights into the surface modifications caused by the inhibitor, the surface characteristics of the carbon steel samples were analysed using scanning electron microscopy (SEM). This technique allowed for the observation of differences in morphology between unprotected, corroded, and inhibited steel surfaces. Additionally, atomic force microscopy (AFM) was utilized to evaluate the roughness of the surface layer, providing quantitative data on the changes induced by the inhibition process. Overall, the findings indicate that the Trifolium repens extract not only acts as a potent corrosion inhibitor but also contributes to the formation of a protective barrier on carbon steel, thereby extending its lifespan in corrosive environments. This study highlights the potential for utilizing natural extracts as sustainable alternatives for corrosion prevention in industrial applications.

Advancing a Comprehensive Model for Carbon Steel Corrosion in Weak Acids: Validation Using Valeric and Acetic Acids

Acidic corrosion in industrial environments represents a serious threat that requires an active prevention and management strategy. In this context, weak acids can create a severe corrosion environment for metallic surfaces, sometimes exceeding the severity observed in strongly acidic solutions under similar conditions. While most of the research efforts of the last decades in the field of the predictive modeling of acidic corrosion have been focused on the specific case of sweet corrosion caused by carbonic acid, the goal of this work is to describe and validate a predictive model to be used as a more transversal tool for acidic corrosion. The model, called the Tafel–Piontelli model, leverages Tafel law to mechanistically describe the electrochemical behavior of carbon steel in acidic aqueous environments. Two different acids, acetic and valeric, were used to experimentally evaluate the performance of the model in weakly acidic solutions, varying the pH and the temperature conditions. Potentiodynamic polarization tests and mass loss tests were performed, allowing us to assess the kinetic parameters (the Tafel slope and the exchange current density of the cathodic and anodic reactions) and corrosion rates of the corrosion process. The promising results suggest that the Tafel–Piontelli model is able to adapt to different scenarios and its intrinsically theoretical nature allows us to extend its predictions outside the range of experimental conditions used to validate it.

Understanding Electrochemical Behavior and Localized Corrosion Susceptibility in Cold Sprayed Al-Mg Binary Deposits with Varying Mg Contents

This paper elucidates electrochemical behavior and localized corrosion susceptibility in cold sprayed Al-Mg binary deposits with varying Mg content. Cold spray (CS) deposition is a solid-state deposition process, being investigated as protective layers and repair applications. Nevertheless, a lack of understanding of corrosion mechanisms in CS deposits is prevalent due to its complex microstructure-driven mechanisms different from wrought counterparts. Analytical electrochemistry techniques, including potentiodynamic polarization and Mott-Schottky testing showed that corrosion resistance increased with increased Mg content in CS Al-Mg binary deposits. However, immersion tests (50 h) demonstrated that the effect of Mg content on localized corrosion damage was insignificant compared to that of prior particle boundaries that primarily governed localized corrosion initiation and propagation.

Enhancement in Corrosion Resistance of Low-Carbon Steel via Surface Modification

This research investigated the corrosion resistance of surface layers on low-carbon steel exposed to a chloride environment at room temperature. This study systematically evaluated the effects of varying pack compositions, coating temperatures, and application durations on the characteristics of the deposited coatings. The potentiodynamic polarization corrosion test was employed to assess the wet corrosion behavior of the specimens. Elemental compositions and microstructural features were analyzed using energy-dispersive X-ray spectroscopy (EDX) in conjunction with scanning electron microscopy (SEM), providing insights into phase distribution. The chromizing, titanizing, and chromotitanizing treatments were conducted at temperatures of 900 °C, 1000 °C, and 1100 °C, respectively, with varying coating times. X-ray diffraction analysis revealed a complex arrangement of elements and compounds within the coatings, including Cr, Ti, Cr1.9Ti, FeTi, Al2O3, Cr2O3, TiO2, Cr1.36Fe0.52, and (Ti0.86)3.58. The study found that as the deposition duration increased, the coating thickness increased, comprising a thin inner layer and a substantially thicker outer layer. This layered structure resulted from the outward diffusion of Fe atoms and the inward diffusion of Cr and Ti atoms. Electrochemical analysis in a 3.5% NaCl aqueous solution indicated a marked enhancement in the corrosion resistance of the coated specimens compared to their uncoated counterparts. The potentiodynamic polarization tests confirmed that the protective coatings significantly reduced the corrosion rate, with performance influenced by both the temperature and duration of the deposition process. These findings highlighted the potential of tailored coating techniques to improve the durability and performance of low-carbon steel in corrosive environments.

Impact of voltage variation on topography, hydrophobicity, and corrosion behavior of the CeO2-TiO2 nanocomposite coating on AM60B Mg alloy prepared by plasma electrolyte oxidation

Abstract A superhydrophobic oxide layer on AM60B magnesium alloy was created by using a plasma electrolytic oxidation (PEO) technique and subsequent surface modification. PEO was applied in an electrolyte with/without CeO2-TiO2 nanoparticles (NPs) by two different voltages of 350 and 450 V. Atomic force microscopy (AFM) and field emission scanning electron microscopy (FE-SEM) were used to evaluate the roughness and morphology of the surface. Water contact angles on the surface were measured to investigate the surface wettability. Potentiodynamic polarization tests in a 3.5 wt% NaCl solution were used to assess the corrosion resistance of the coating. Also, the FE-SEM images showed that increasing applied voltage, decreases the numbers and size of the pores on the surface. Results showed that applying PEO, increased the corrosion resistance of the bare AM60B because corrosion rate (C.R.) decreased from 1.523805 (bare AM60B) to 1.432107 mpy (PEO-450 V). Furthermore, using CeO2-TiO2 NPs effectively enhanced the corrosion resistance of the bare AM60B by decreasing the corrosion rate from 1.523805 to 1.286469 mpy.

Effect of Solution Annealing on Corrosion Resistance of Austenitic Stainless Steels in Chloride-Containing Environment

Abstract Austenitic stainless steels are highly corrosion-resistant in common oxidation environments. However, aggressive chloride-containing solutions can evoke local corrosion, which performs an important risk in the safe use of these materials. This research deals with the effect of the solution annealing (1050 °C, 15 min) on the electrochemical parameters of AISI 304 and AISI 316L stainless steels. Corrosion resistance of the solution-annealed specimens is evaluated and compared to the as received specimens by the potentiodynamic polarization test performed in 1M pH neutral NaCl solution at the 20 ± 3 °C temperature. The obtained results did not clearly confirm the positive effect of solution annealing on corrosion resistance in the given aggressive solution. Although the pitting potentials indicating higher pitting corrosion resistance increased, the kinetics of the corrosion process intensified.

Investigation of Cr2SiC Ceramic MAX Phase Coated Metallic Bipolar Plates in Ex-situ Conditions for Proton Exchange Membrane Fuel Cells

Essential for compact and lightweight proton exchange membrane fuel cell (PEMFC) stacks, metallic bipolar plates (MBPs) suffer from durability and conductivity issues due to surface corrosion and passivation. This study conducts ex-situ characterization of Cr2SiC ceramic MAX phase coatings on stainless steel (SS) 316 L substrates to evaluate their suitability for MBP application. The investigation encompasses coating structure, surface morphology, corrosion resistance, surface wettability, in-plane electrical conductivity, and interfacial contact resistance. X-ray diffraction and X-ray photoelectron spectroscopy confirm the presence of Cr2SiC coatings on SS316L, while energy-dispersive X-ray spectroscopy verifies uniform coverage and elemental weight percentage. Corrosion resistance is evaluated using potentiostatic and potentiodynamic polarization tests, showing excellent resistance with low corrosion current density (Icorr) at both 25 °C (3.29E-03 μA/cm2) and 80 °C (4.32E-02 μA/cm2), meeting US Department of Energy (DOE) technical targets. Potentiodynamic polarization reveals large corrosion potential and small Icorr values at both temperatures, outperforming uncoated samples. Electrochemical impedance spectroscopy post-accelerated corrosion tests show high charge transfer resistance at 25 °C (3.68E+05 Ω cm2) and 80 °C (3.02E+05 Ω cm2), indicating stability in acidic environments. Surface wettability analysis indicates low water affinity with a large contact angle (75°) and low surface free energy (28.92 mJ/m2) for the coated samples as compared to the uncoated samples. Electrical conductivity meets DOE targets with an in-plane conductivity of 4.59E+05 S/m and interfacial contact resistance of 8.04 mΩcm2 after 5-h accelerated corrosion tests at 80 °C. These results suggest that Cr2SiC coated SS316L exhibits excellent corrosion resistance, surface wettability, and electrical characteristics, making them viable for PEMFC applications.

Galvanic and Local Corrosion of Aluminum Alloy 6061-T6 Coupled to Non-Passivating and Passivating Alloys

Marine and aerospace structures sometimes combine lightweight aluminum alloys with dissimilar metals to optimize mechanical performance and reduce costs. Unfortunately, the exposure of such dissimilar couples in a harsh environment can cause severe corrosion damage to the aluminum structure due to its position in the galvanic series. Corrosion of Aluminum Alloy (AA) 6061-T6 coupled to non-passivating and passivating alloys was studied to elucidate galvanic effects on local corrosion. In this work, local and galvanic corrosion were quantified, the effects of cathode material on local electrolyte pH were explored, and a relationship between pH and local-corrosion of AA6061-T6 was established.Experimental studies quantified local and galvanic corrosion of AA6061-T6 coupled to 316 stainless steel, copper, titanium alloy Ti6Al4V, and 316 stainless steel coated with titanium nitride, chromium nitride, and a sol-gel nano-coating. Galvanic couples were misted with 3.15 wt.% sodium chloride solution and exposed in a controlled temperature-humidity chamber. Galvanic currents were measured during exposure, and the total mass loss of the aluminum alloy was determined to quantify the corrosion damage. Exposure tests showed that galvanic corrosion decreases over time and accounts for less than 15% of the total corrosion of AA6061-T6. Therefore, most aluminum corrosion damage was caused by local corrosion. In addition, immersion experiments of AA6061-T6 galvanic couples in gelled 3.15 wt.% NaCl solutions showed that galvanic coupling influences the evolution of electrolyte pH leading to severe acidification at the aluminum anode surface and alkalization around the cathode. The solution pH at the aluminum surface was decreased by galvanic action and severity of pH decrease depends on galvanic couple materials and design. To quantify the effect of acidity on local corrosion of AA6061-T6, potentiodynamic polarization tests were performed in aerated and deaerated 3.15 wt.% NaCl solution adjusted to different pH values. Anodic dissolution reactions and cathodic oxygen reduction reactions show significantly higher anodic and cathodic currents for more acidic solutions due to the instability of the passive oxide film of aluminum. This film is stable in near-neutral solutions and unstable in highly acidic solutions. As a result, the breakdown of the passive film increases the effective area contributing to anodic or cathodic currents resulting in enhanced local corrosion.The series of experimental results show that galvanic interaction leads to a decrease of pH at the aluminum anode, and therefore increasing local corrosion of the aluminum surface. The increase of local corrosion induced in a galvanic couple depends on the cathode material and underlines the importance of accounting for local corrosion and pH-dependent corrosion kinetics when predicting the galvanic compatibility of aluminum alloys.

Characterization of the amorphous metallic alloy Al85Ni10Sm5

Aluminum alloys are highly significant materials in the industry due to their excellent mechanical properties and corrosion resistance. These alloys are widely used in various applications in the automotive and aerospace industries. The addition of alloying elements such as nickel, copper, and magnesium allows for different combinations of properties, which makes the alloys more versatile. Among aluminum alloys, the amorphous alloys studied in this work have received increasing attention due to their unique properties. Additionally, there is a limited amount of research on these materials, unlike the numerous studies already conducted on conventional crystalline structure alloys. In this work, the metallic alloy with the atomic composition Al85Ni10Sm5 and an amorphous structure, produced by melt-spinning, was structurally, thermally, and electrochemically characterized. The structural and thermal characterization was performed using X-ray diffraction (XRD), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC) techniques to confirm its amorphous nature. The electrochemical characterization was carried out through open circuit and potentiodynamic polarization tests in an aqueous medium with 3.5% mass NaCl at room temperature. The results indicate that the fully amorphous alloy is of the glassy type, with Tg = 522 K, and exhibits low thermal stability (∆Tx = 16 K). The electrochemical tests show a corrosion potential of approximately -520 mV relative to the saturated calomel reference electrode, suggesting that the amorphous alloy is more corrosion-resistant than pure aluminum in its crystalline state.

Determination of E crit Using 1-D Pit Approach and Its Relationship to the Repassivation Potential of 3-D Pits

Localized corrosion is a major concern in industry, as it can compromise components made from corrosion-resistant alloys, ultimately leading to failures, reduction in the lifetime of equipment, downtime in production, and safety issues. Understanding the mechanisms underlying pitting corrosion is paramount to minimizing this form of attack and developing mitigation approaches. Cyclic potentiodynamic polarization (CPP) is a common electrochemical test used to evaluate the susceptibility of materials to pitting corrosion. An important parameter that can be extracted from this technique is the repassivation potential (E rp), which has been used as a measure of resistance to localized corrosion in long-term applications and also for assessing SCC susceptibility. Another interesting approach for evaluating the kinetics of localized corrosion is the 1-dimensional (1-D) pit test, which consists of a thin wire sample embedded in epoxy resin. This setup generates a real pit environment at the material surface as the wire dissolves and provides additional parameters that cannot be directly extracted from CPP curves.In this study, 1-D pit tests were used to evaluate the corrosion behavior of SS316L at different temperatures and to identify the critical conditions for repassivation. The potential was scanned downward from the mass-transport limit region at different potential scans rates () for pits with different depths to allow for precise determination of the repassivation potential and the critical concentration, C crit, associated with pit growth stability at each temperature. Tailored values of were needed because was found to affect the shape of the downward polarization curves of 1-D pits of varying depth. For small pit depths, a scan rate of 20 mV/s is suitable for measuring E rp, which is associated with a sharp decrease in current density caused by passive film formation at the bottom of the pit. For deeper pits, however, lower scan rates are needed to allow enough time for dilution of the electrolyte inside the pit cavity, providing an accurate measurement of E rp from the sharp drop in current.The critical pit bottom potential at repassivation, E crit, can be determined from E rp by correcting for the ohmic potential drop caused by current flow in the pit. Values of E crit measured using the same deviate from the expected logarithmic dependence of E crit on pit depth [1] for depths higher than 100 mm. However, if a tailored is used, i.e. scan rates that decrease with increasing pit depth, then accurate E crit values can be obtained as indicated by the logarithmic relationship over a 10x variation in pit depth. The E crit obtained from 1-D pit tests combined with the equations of mass transport for different pit geometries enable the prediction of E rp for hemispherical 3-D pits in bulk samples, which is approximately the pit geometry commonly seen after cyclic potentiodynamic polarization (CPP) tests. A good correlation was found between E crit from 1-D pit experiments and E crit from CPP curves. The effect of pit cover was also taken into consideration in the ohmic potential drop calculations for bulk 3-D pits. This methodology provides predictions for repassivation potential, enhances the understanding of the variability in E rp values obtained from the CPP technique, and improves the reliability of this parameter.This work was supported by the US Office of Naval Research under Grant No. N00014-23-1-2202.Reference Li, J.R. Scully, G.S. Frankel, Localized Corrosion: Passive Film Breakdown vs. Pit Growth Stability: Part III. A Unifying Set of Principal Parameters and Criteria for Pit Stabilization and Salt Film Formation, Journal of The Electrochemical Society, 165 (2018) C762-C770.

(Digital Presentation) Experimental Analysis of Pit Initiation at MnS Inclusion in Low-Alloy Steel Using in-Situ Raman Spectroscopy

Low-alloy steel is an indispensable material for industry and infrastructure and is widely used in many fields because it can save cost and elemental resources. However, one of the disadvantages of low-alloy steel is its limited corrosion resistance compared to that of high alloys. Specifically, a major weakness is its high susceptibility to pitting in an environment containing Cl- ions. MnS inclusions are well known as preferential pitting sites because of its poor stability against chemical dissolution and ease of deformation by rolling. A lot of studies have investigated the mechanisms of pit initiation at MnS so far. Some detail analyses revealed that S species such as S2O3 2-, HS- and elemental S were generated due to dissolution of MnS, and they were harmful to pit initiation and active dissolution. However, these studies did not focus on ‘when’ those S species were generated. In addition, most of these previous studies focused on pitting in high alloys such as stainless steel and Ni-based alloy, so the mechanism of pit initiation in low-alloy steel is not yet fully understood.This study aims to clarify the chronology between MnS dissolution, S species generation and pit initiation in low-alloy steel. The microscopic potentiodynamic polarization test was carried out in 20 mM NaCl added boric-borate buffer solution at pH 8.0. To analysis S species dissolved from MnS inclusions during the electrochemical test, in-situ observation using an optical microscope with high magnification and Raman spectroscopy were employed.The polarization curve started at cathodic region, and then the corrosion potential was observed. In the anodic region, active dissolution and passive regions appeared in the curve as the potential increased. Finally, surges in the current density, attributed to pit initiation, was observed. In-situ observation ascertained that pit initiated at MnS inclusion whose longer diameter was 20 µm. Interval analysis of Raman spectroscopy during the test detected elemental S about 0.1 V below the pitting potential.Analysis of the test solution after the polarization test also detected elemental S, although other S species such as S2O3 2- and HS- were not detected.To confirm the morphology of the pit, cross-sectional observation was carried out using FIB/SEM. The pit was found to grow hemispherically, and corrosion product composed by Fe, Mn, O, and S was formed on the inner wall of the pit.This study clarified that, MnS started to dissolve 0.1 V below the pitting potential. Then, elemental S was formed and concentrated in the vicinity of MnS inclusion. After elemental S concentrated enough to accelerate pit initiation, the active dissolution at the interface between MnS inclusion and steel matrix occurs rapidly to form pit. A part of elemental S deposited on the inner surface of the pit and form oxysulfide of Fe and Mn as corrosion product.

Study of the Microstructure and Mechanical Properties of Steel Grades for Ship Hull Construction.

This paper comprehensively examines three structural steel grades' microstructural features and mechanical properties, evaluating their suitability for shipbuilding applications. The steels analyzed include quench and tempered (Q and T) steel, thermomechanical controlled processed (TMCP) steel, and hot rolled (HR) steel. A microstructural characterization was performed using optical microscopy (OM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). The analysis was complemented by extensive mechanical testing including assessments of hardness, tensile, and Charpy impact tests across a range of temperatures. Additionally, corrosion behavior was evaluated using the potentiodynamic polarization test. The findings revealed that Q and T grade steel exhibited the most refined microstructure, characterized by a complex mixture of ferrite, tempered martensite, upper bainite, and Fe3C phases. In contrast, the TMCP grade steel demonstrated a balanced microstructure of polygonal ferrite and pearlite. Meanwhile, the HR grade steel contained polygonal ferrite and aligned pearlite. The tensile testing results demonstrated that the Q and T grade steel had superior hardness, yield strength (YS), and ultimate tensile strength (UTS), although it exhibited the lowest elongation % (El %). The TMCP grade steel met all ABS standards for marine steels, displaying optimal YS, UTS, and El %. Despite the superior YS of the HR grade steel, it did not meet the necessary criteria for UTS. Charpy impact tests revealed that the TMCP grade steel exhibited the highest impact energy absorption across a range of temperatures. As a result, the TMCP grade steel emerged as the optimal choice for ship construction, fulfilling all ABS requirements with a balanced combination of strength, ductility, and impact energy absorption. Additionally, the potentiodynamic polarization results revealed that the Q and T grade steel demonstrated the highest corrosion resistance. Following Q and T steel, the HR grade steel ranked second in corrosion resistance, with TMCP steel closely behind, showing only a slight difference.

Role of surface grain refinement in AZ31 Mg alloy by shot peening on surface energy, biomineralization and degradation behavior

Grain refinement of magnesium (Mg) alloys to improve their performance as potential candidates for degradable implant applications is a promising strategy in the field of materials engineering. Surface properties play an important role in promoting higher implant tissue interactions which dictate the healing rate of the fractured bone. In the present work, AZ31 Mg alloy was subjected to shot peening by using steel balls of 2 mm diameter. From the microstructural studies carried out at the cross section, fine grain structure was observed up to 50 μm depth from the surface. Grain refinement up to ∼1.5 μm was achieved at the surface of shot peened AZ31. X-ray diffraction analysis confirms the development of non-basal texture at the surface. Increased surface energy was measured by contact angle measurements for the shot peened AZ31. Higher hardness was measured from the surface in the thickness direction of the AZ31 after shot peening. Corrosion behavior assessed by potentiodynamic polarization tests indicated marginally increased corrosion resistance for shot peened AZ31. In vitro bioactivity studies carried out in simulated body fluids demonstrated higher mineral depositions and lower weight loss for the surface grain refined AZ31. The results demonstrate the potential of shot peening to promote higher biomineralization and to control the degradation in improving the performance of biodegradable AZ31 Mg alloy.