Transpassive Potential Research Articles

Titanium Alloy Ti-6Al-4V Electrochemical Dissolution Behavior in NaNO3 and NaCl Solutions at Low Current Density

Jet electrochemical micromilling (JEMM) exhibits significant potential for high-efficiency and high-quality machining of titanium alloy microstructures. However, during the JEMM process, the machined surface of the workpiece inevitably experiences stray current attacks at low current levels. Due to the formation of a dense passive film on the surface of the titanium alloy under electrochemical action, stray corrosion occurs on the machined surface. Hence, the electrochemical dissolution behavior of titanium alloys at low current densities directly impacts both machining efficiency and quality. This study first analyzed the effects of electrolyte composition and current density on the transpassive potential, breakdown of the passive film, current efficiency, and the dissolved surface on Ti-6Al-4V. The transpassive potential and electrochemical impedance of Ti-6Al-4V were found to be lower in NaCl solution than in NaNO3 solution. In addition, lower current densities enabled higher current efficiency and resulted in a more uniform and flat dissolution surface. Subsequent experiments used these two solutions for JEMM of complex-shaped microstructures on Ti-6Al-4V. The findings demonstrated that, compared to the NaNO3 solution, the use of NaCl solution increases the material removal rate by approximately 30%, enhances the aspect ratio by about 26%, and reduces surface roughness by roughly 58%. This indicates that employing NaCl solution can lead to superior machining efficiency and quality.

Electrochemical Properties and Jet Electrochemical Micromilling of (TiB+TiC)/Ti6Al4V Composites in NaCl+NaNO3 Mixed Electrolyte.

Difficult-to-cut titanium matrix composites (TiB+TiC)/Ti6Al4V have extensive application prospects in the fields of biomedical and aerospace metal microcomponents due to their excellent mechanical properties. Jet electrochemical micromilling (JEMM) technology is an ideal method for machining microstructures that leverages the principle of electrochemical anodic dissolution. However, the matrix Ti6Al4V is susceptible to passivation during electrochemical milling, and the inclusion of high-strength TiB whiskers and TiC particles as reinforcing phases further increases the machining difficulty of (TiB+TiC)/Ti6Al4V. In this study, a novel approach using NaCl+NaNO3 mixed electrolyte for the JEMM of (TiB+TiC)/Ti6Al4V was adopted. Electrochemical behaviors were measured in NaCl and NaCl+NaNO3 electrolytes. In the mixed electrolyte, a higher transpassive potential was required to break down the passive film, which led to better corrosion resistance of (TiB+TiC)/Ti6Al4V, and the exposed reinforcing phases on the dissolved surface were significantly reduced. The results of the JEMM machining indicate that, compared to NaCl electrolyte, using mixed electrolyte effectively mitigates stray corrosion at the edges of micro-grooves and markedly improves the uniformity of both groove depth and width dimensions. Additionally, the surface quality was noticeably improved, with a reduction in Ra from 2.84 μm to 1.03 μm and in Rq from 3.41 μm to 1.40 μm.

On corrosion and passivity of Inconel alloy 625 in HNO3 solution

Exploring the passivity and electrochemical behavior of Inconel alloy 625 in nitric acid solution could open up possibilities for potential applications, either in bulk form or as claddings. Hence, this research investigates corrosion behavior and passivity of Inconel alloy 625 in nitric acid solutions of varying concentrations (0.01–1.00 mol/L) using a range of analytical techniques, including potentiodynamic polarization testing, chronoamperometry measurement, Mott-Schottky (MS) analysis, X-ray diffraction (XRD) approaches, and point defect modeling (PDM). Results demonstrate that corrosion current density does not necessarily decrease with increasing electrolyte concentration (Please check it out). Additionally, the alloy exhibits higher passive film transpassive potentials in more concentrated solutions. Potentiostatic polarization tests reveal an increase in steady-state current densities attributed to the passive films formed in more concentrated solutions. Lower electrolyte concentrations lead to the formation of more intact bilayered passive films. In this context, the formation of passive films is elucidated by examining the generation and annihilation of point defects. The semiconducting behavior of passive layers is further examined through XRD patterns and electrochemical properties. According to PDM, the diffusivity of point defects within the passive films and their respective donor densities can be closely correlated.

Passivation and Corrosion Behavior of Modified S13Cr Stainless Steel in Ultra-high Temperature Geothermal Fluid

This research aims to investigate the passivation and corrosion behaviour of modified S13Cr stainless steel (SS) in ultra-high temperature geothermal fluids. In this study, S13Cr SS before and after modified were both immersed in a simulated geothermal fluid environment with a temperature of 210°C and CO2 pressure of 3 MPa for 120 h. The results show that the modified S13Cr SS had smaller grain size and lower reverse austenite content, and exhibited higher transpassive potential and lower passive current density in the ultra-high temperature environment. After 120 h of immersion, the passivation film of the modified 13Cr SS was completely dissolved, and a corrosion product film mainly composed of FeCO3 and FeCr2O4 formed with localized corrosion occurring. Moreover, a Ni-rich barrier layer formed at the interface between the inner layer of the product film and the substrate, which hindered the penetration of the corrosive medium. Additionally, the residual MoO2 in the product film played a stabilizing role. Overall, the corrosion resistance of the modified S13Cr SS in ultra-high temperature geothermal fluids is improved.

Extending the Passive Region of CrFeNi‐Based High Entropy Alloys

AbstractThis study provides principles for designing new corrosion resistant high entropy alloys. The theoretical framework is a percolation model developed by Newman and Sieradzki that predicts the ability of an alloy to passivate, i.e., to form a protective surface oxide, based on its composition. Here, their model is applied to more complex materials than previously, namely amorphous CrFeNiTa and CrFeNiW alloys. Furthermore, the model describes a more complex passivation process: reforming the oxide layer above the transpassive potential of Cr. The model is used to predict the lowest concentration of Ta or W required to extend the passive region, yielding 11–14 at% Ta and 14–17 at% W. For CrFeNiTa, experiments reveal a threshold value of 13–15 at% Ta, which agrees with the prediction. For CrFeNiW, the experimentally determined threshold value is 37–45 at% W, far above the predicted value. Further investigations explore why the percolation model fails to describe the CrFeNiW system; key factors are the higher nobility and the pH sensitivity of W. These results demonstrate some limitations of the percolation model and offer complementary passivation criteria, while providing a design route for combining the properties of the 3d transition metal and refractory metal groups.

Electrochemical and passivation behavior of a corrosion-resistant WC-Ni(W) cemented carbide in synthetic mine water

Two different grades, WC-20 vol.% Ni and WC-20 vol.% Co cemented carbides, respectively were systematically investigated concerning their microstructure, binder composition, and corrosion behavior. SEM-EBSD analysis verified that both grades have similar WC grain sizes (0.9–1.1 μm). AES analysis confirmed that the binder phase of the respective grade is an alloy of Ni-W and Co-W and that the concentration of W in the Ni- and Co-binder is 21 and 10 at. %, respectively.In synthetic mine water (SMW), the EIS behavior of WC-Ni(W) at the open circuit potential (OCP) conditions was studied for different exposure periods (up to 120 h). The EIS data fitting estimates low capacitance and high charge transfer resistance (Rct) values, which indicate that the passive film formed on WC-Ni(W) is thin and exhibits high corrosion resistance. At the OCP and potentiostatic-passive conditions, SEM investigations confirm the uncorroded microstructure of the WC-Ni(W). The AR-XPS studies confirmed the formation of an extremely thin (0.25 nm) WO3 passive film is responsible for the high corrosion resistance of WC-Ni(W), at OCP conditions. However, above the transpassive potential, the microstructure instability of WC-Ni(W) was observed, i.e., corroded morphology of both WC grains and Ni(W) binder. The electrochemical parameters, Rct, corrosion current density, and charge density values, confirmed that the WC-Ni(W) is a far better alternative than the WC-Co(W) for application in SMW.

Optimizing the interfacial potential distribution to mitigate high transient potential induced dissolution on C/Ti coated metal bipolar plates used in PEMFCs

Developing a conductive and corrosion-resistant coating on metal bipolar plates is essential to mitigate the degradation induced by the high cathodic transient potentials of proton exchange membrane fuel cells. In this work, a multilayer nanoscale C/Ti coating is prepared on SS316L (C/Ti/SS). The C/Ti/SS achieves a highly prominent ICR of 1.59 mΩ cm2 and corrosion rate of 2 × 10–8 A/cm2, remarkably better than the targets. Analysis of coating microstructure and composition suggests that the significantly enhanced corrosion resistance is attributed to the diffusional interfaces which optimize the potential distribution across the coating and improve the transpassive potential of C/Ti/SS.

Anodic behavior of nickel in sub-molten KOH and its relevance for the production of electroactive nickel oxides

This study investigated the electrochemical behavior of pure nickel in sub-molten KOH at 200 °C and the structure of surface films formed at different anodic polarization potentials. Potentiodynamic polarization measurements showed an active-to-passive transition followed by transpassivity. Unlike aqueous alkaline solutions, surface films created in active, far passive, and transpassive regions are few microns thick. Grazing-incidence x-ray diffraction (GI-XRD), Raman, and x-ray photoelectron spectroscopy (XPS) measurements revealed that a thick and defective layer of NiO forms during polarization in the active region. Polarization into the passive regime reduces the thickness and number of defects of NiO and promotes passivity. With the formation of +3 valent nickel at far passive region, a thick, flaky adlayer of potassium-intercalated γ-NiOOH is produced. At high transpassive potentials, the flake-like morphology deteriorated with the high rate of oxygen evolution reaction (OER). The micrometer thick electroactive anodic surface films formed during active, far passive, and transpassive region became a few micrometers thick because of the high dissolution rate of Ni and the high viscosity of the sub-molten KOH.

Enhanced corrosion resistance by engineering crystallography on metals

Nanometer-thick passive films, which impart superior corrosion resistance to metals, are degraded in long-term service; they are also susceptible to chloride-induced localized attack. Here we show, by engineering crystallographic configurations upon metal matrices adjacent to their passive films, we obtain great enhancement of corrosion resistance of FeCr15Ni15 single crystal in sulphuric acid, with activation time up to two orders of magnitude longer than that of the non-engineered counterparts. Meanwhile, engineering crystallography decreases the passive current density and shifts the pitting potential to noble values. Applying anodic polarizations under a transpassivation potential, we make the metal matrices underneath the transpassive films highly uneven with {111}-terminated configurations, which is responsible for the enhancement of corrosion resistance. The transpassivation strategy also works in the commercial stainless steels where both grain interior and grain boundaries are rebuilt into the low-energy configurations. Our results demonstrate a technological implication in the pretreatment process of anti-corrosion engineering.

Investigating the Corrosion Behavior of Corrosion-Resistant Alloys in Solutions Containing Dilute Fluoride Ions

The ability of fluoride ions, in trace concentrations, to promote film instability and transpassive dissolution was shown for several corrosion-resistant alloys. While the addition of fluoride ion was found to decrease the transpassive dissolution potential by approximately 0.1 V, the decrease was particularly pronounced in mildly acidic media (pH 5). Results suggest that the decreased potential range of stability for the heavily-relied-upon Cr(III)-rich oxide film is a concern in the presence of oxidizing impurities, which may push the corrosion potentials toward the transpassive region. The decreased potential range of passive film stability was observed using both dynamic and static polarization experiments and was supported by solution and surface analyses.

Pitting behavior of friction stir repair-welded 304L stainless steel in 3.5% NaCl solution at room temperature: role of grain and defect structures

Stainless steel (SS) canisters used for storing spent nuclear waste are prone to chloride-induced stress corrosion cracking. Friction stir welding (FSW), being a low heat input process, can be used for repair welding of these canisters. In this study, an artificial crack was introduced on the 304L SS coupons by an electric discharge machining process. The artificial crack was repair-welded by FSW by maintaining the tool temperature a constant at two levels (725 and 825 °C). Friction stirring significantly reduced the grain size of the stirred zone (SZ) from about 47 to 2–4 μm. The fraction of low-angle grain boundaries increased in the SZ from 2 to 37–43%. On the other hand, the fraction of special grain boundaries (Σ3 and Σ9) that was ~ 50% in the base metal reduced to < 10% in the SZ. During friction stirring, the oxide layer of artificial crack was broken and aligned to form a spiral defect called lazy-S structure. All these microstructural changes affected the corrosion behavior of the FSW specimens when tested in 3.5% NaCl at room temperature using cyclic polarization, chronoamperometry, and electrochemical impedance spectroscopy. The FSW specimens showed lower polarization resistance and lower transpassive potential than those of the base metal specimens. However, the pitting protection potentials of the FSW specimens were higher than that of the base metal. The pitting behavior of the FSW specimens was influenced more by the preferential attack on the lazy-S region than by passive film breakdown. The flat band potentials of the passive film formed on the FSW specimens were more positive than that of base metal. The charge carrier density of passive film formed on the FSW specimens was higher than that of the base metal. The higher fraction of low-angle grain boundaries present in the FSW specimens could supply more number of misorientation dislocations at the metal/film interface which could form anion vacancies by a climb process leading to formation of oxide at these locations without stressing the substrate. Therefore, low-angle boundaries are considered helpful for formation of stress-free passive films that will be highly stable and enhance both pitting and stress corrosion cracking resistance.

On the stability of the passive Ti-6Al-4V film of friction stir welds with stainless steel: Effect of not native metal species

Mechanical and microstructural characteristics of Ti-6Al-4V/stainless steel (SS) dissimilar friction stir welds have been well described in the literature; however, little is known about the electrochemical properties of such welds in terms of the passive film stability of TiO2 grown in oxidizing media. To clarify this issue, potentiodynamic polarization, cyclic voltammetry, and electrochemical impedance measurements were carried out in the Ti-6Al-4V alloy to analyze the effects of Fe and Cr contamination from the underlying SS on the passive film stability of TiO2 in concentrated (6.0 and 11.5 mol L–1) HNO3 solutions. Lower transpassive potentials were observed for samples in the stirred zone (SZ) than those of the base metal (BM) due to Fe and Cr contamination. Anodic charges obtained during cyclic voltammetry using 6.0 mol L–1 HNO3 showed that the grown passive film was completely dissolved and regrown after consecutive scans, except for the BM sample. According to the results of X-ray photoelectron spectroscopy (XPS) analyses an Al depleted layer was obtained for the latter condition, which confirms the stability of TiO2 film. The increasing values of charge transfer resistance obtained through electrochemical impedance measurements at distinct potentials in the passive region also supported the stability of the TiO2 film grown in 6.0 mol L–1 HNO3 solution. The formation of a medium frequency inductive loop and low frequency constant phase element are related to the dissolution of the oxide film and adsorption of NO3 species into the formed passive film, respectively, as evidenced by XPS analyses.

Effects of Mn on the electrochemical corrosion and passivation behavior of CoFeNiMnCr high-entropy alloy system in H2SO4 solution

The present work investigates the influence of Mn on the electrochemical corrosion and passivation behaviors of CoFeNiMnCr high-entropy alloys in a 0.1 M H2SO4 solution. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) results showed that the addition of Mn caused a marked difference in the active dissolution regions and active-passive transition regions. It was deduced from EIS and the potential decay results that the alloyed Mn suppressed the passivation processes and increased the chemical dissolution rate of the passive films. By contrast, the alloyed Mn did not affect the protective properties of the passive films of the alloys. Surface analysis indicated that Mn addition had little influence on the composition and thickness of the passive films. Thus the passivation current densities and transpassive potentials of the CoFeNiMnCr high-entropy alloy remained almost unchanged with the addition of Mn.

Magnetic Field Effect on Anodic Dissolution Behavior of Iron in Sodium Perchlorate Solution

It has been reported that magnetic field affects the anodic dissolution and passivation behavior of iron in acidic, neutral, slightly alkaline solutions. Perchlorate solution has been used in the mechanistic study of anodic dissolution mechanism of iron. In this study, the effects of 0.4T magnetic field on the electrochemical behavior and corrosion morphologies of iron in sodium perchlorate solution were studied by optical microscope (OM), scanning electron microscope (SEM) and several electrochemical methods such as potentiodynamic polarization curve and potentiostatic polarization. Magnetic field was placed horizontally and parallel to the working electrode. The anodic polarization curves show that magnetic field did not have strong effect on anodic current at the potential before the transpassivation potential. In the transpassivation range, magnetic field increased the anodic current, increased the slope of the i/E curve. After the potentiodynamic polarization curve measurements up to 2.0V(SCE), severe electrode thinning was observed under both 0T and 0.4T. Macroscopically uniform dissolution was observed on the electrode after measurements under 0T, while locally accelerated thinning was found on the iron electrode after the measurements under 0.4T, showing the accelerating effect of magnetic field on the anodic dissolution at the edges of the electrode along the horizontal direction, as the result of uneven distribution of the actual magnetic field flux density. Potentiostatic polarization measurements show that magnetic field decreased the anodic current density in the passive range, where the electrode was not significantly affected by magnetic field due to low current density. The accelerating or inhibiting effect of magnetic field can be interpreted based on the magnetohydynamic driving force and the magnetic field gradient driving force theory. Figure 1

Molybdenum surface enrichment and release during transpassive dissolution of Ni-based alloys

The role of alloyed Mo during transpassive dissolution of four commercially available Ni-based alloys in neutral chloride solution was investigated by atomic emission spectroelectrochemistry. Time-resolved dissolution rates of Ni, Cr, Mo, and Fe were obtained as a function of applied potential. Mo enrichment occurred at the transpassive potentials and redissolved when the potential returned to the passive domain. These results suggest a mechanism of Mo enrichment and release that could play a significant role in repassivation in initially neutral electrolytes such as occurs during crevice corrosion. It is proposed that Mo precipitation and redissolution are driven by local pH changes.

The effect of the inflammatory species hypochlorous acid on the corrosion and surface damage of Ti‐6Al‐4V and CoCrMo alloys

Performance of metallic orthopedic alloys is dependent on the local biological conditions within the joint. Inflammation within a joint can lead to severe conditions at the surface of the implant and may represent a driving force of corrosion which is not well understood. In this study, the corrosion response of CoCrMo and Ti-6Al-4V alloys was studied in hypochlorous acid (HOCl) saline solutions, which served as a simulated inflammatory environment. Anodic polarization and electrochemical impedance spectroscopy were used to evaluate the electrochemical behavior of these alloys in a range of hypochlorous acid solutions. The open circuit potential of CoCrMo rose to over 600 mV (vs. Ag/AgCl) in 30 mM HOCl solutions while the corresponding corrosion current increased three orders of magnitude over controls. Visual analysis of CoCrMo disks after immersion in 50 mM HOCl for 5 days revealed extensive surface damage. This study shows that HOCl can polarize CoCrMo surfaces above the transpassive potential without an external power source. Ultimately, long-term exposure to HOCl within an inflamed joint may significantly affect patient outcomes through accelerated generation of corrosion products. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 3185-3194, 2018.

Electrochemical Behavior of Alloy 825 at High-Temperature Pressurized Water–Chloride Solutions (30°C to 280°C)

Variations in the transpassive potential (Et) and open circuit potential (OCP) of Alloy 825 were investigated at high-temperature pressurized water–Cl solutions. Using an experimental design strategy, the effects of temperature (T) (30°C to 280°C), Cl− concentration (10 ppb to 1,000 ppb), and dissolved oxygen (DO) concentration (0 ppb to 200 ppb) on the electrochemical responses (Et and OCP) of the alloy were analyzed by conducting only a short series of experiments. The optimum values of each factor was obtained from the designed matrix of the experiments and the constructed 3D response surface plots based on the most positive values of the Et and OCP. The optimum experimental conditions in which the most positive Et can be recorded was found as follows: T ≈ 30°C (significant), [Cl−] ≈ 10 ppb (significant), and [DO] ≈ 200 ppb (less significant). The most positive OCP values were recorded when the analyzed factors were as follows: T ≈ 120°C (significant), [DO] ≈ 200 ppb, and [Cl−] ≈ 10 ppb (less significant). Finally, the microscopic surface analysis of the alloy coupons revealed the great impact of temperature on the oxide film formation on Alloy 825.

Improving Corrosion Resistance of 316L Austenitic Stainless Steel Using ZrO2 Sol-Gel Coating in Nitric Acid Solution

In this study, thin-film coating of zirconium oxide (ZrO2) was prepared by sol-gel method and subsequent heat treatment process. The sol was prepared by controlled hydrolysis of zirconium tetrapropoxide using acetic acid and ethanol/acetylacetone mixture as catalyst and chelating agent, respectively, and finally deposited onto the 316L austenitic stainless steel (316L SS) using dip coating method in order to improve its corrosion resistance in nitric acid medium. The composition, structure, and morphology of the coated surface were investigated by x-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The obtained results from XRD and FTIR state the formation of tetragonal and monoclinic ZrO2 phase. Also, the obtained results from surface morphology investigation by SEM and AFM indicate the formation of smooth, homogeneous and uniform coatings on the steel substrate. Then, the corrosion behavior of stainless steel was investigated in a 1 and 10 M nitric acid solutions using electrochemical impedance spectroscopy and linear polarization test. The obtained results from these tests for ZrO2-coated specimens indicated a considerable improvement in the corrosion resistance of 316L stainless steel by an increase in corrosion potential and transpassive potential, and a decrease in passive current density and corrosion current density. The decrease in passive current density in both the concentration of solutions was two orders of magnitude from bare to coated specimens.

Mechanism of Electrochemical Dissolution of Nickel Grown by Carbonyl Method

A laboratory-grown nickel sample was used to investigate the galvanostatic dissolution under industrial electroplating conditions. The sample contained multiple layers corresponding to changes in the Ni(CO)4 decomposition temperature ranging from 200 to 280°C. Cyclic voltammetry curves were used to determine the active dissolution, passive and transpassive potentials regions of the sample. Chronopotentiometric curves recorded during galvanostatic dissolution displayed oscillations indicating that the dissolution mechanism involved the breakdown of the passive layer and repassivation of the surface. Scanning electron microscopy (SEM) and white light interference microscopy (WLIM) were used to observe changes in morphology and roughness of the surface. The SEM images revealed the formation of pits with lacy covers at the surface during initial stages of the dissolution. At increased dissolution times, a transition from the lacy covered pits to open pits was observed. The images showed that preferential dissolution occurs on Ni layers grown at higher Ni(CO)4 decomposition temperatures. At longer dissolution times, the preferential dissolution also takes place at the boundaries between nickel laminae, which corresponds to the decomposition of Ni(CO)4 at different temperatures.

Effect of temperature, chloride and dissolved oxygen concentration on the open circuit and transpassive potential values of 316L stainless steel at high-temperature pressurized water

The electrochemical performance of 316L stainless steel was investigated in high-temperature pressurized NaCl solutions. Variations of the transpassive potential (Et) and open circuit potential (OCP) of steel were analyzed using an experimental design strategy versus three factors namely temperature (T) (30°C–350°C), chloride [Cl−] concentration (10ppb–1000ppb) and dissolved oxygen (DO) concentration (0ppb–200ppb) by conducting a short series of experiments. The 3D response surface plots of the electrochemical responses (Et and OCP) versus each factor were constructed and the optimum value of each factor was obtained from the designed matrix of the experiments and the 3D response surface plots based on the most positive Et and OCP values. The optimum conditions in which the most positive Et of steel can be achieved were found as follow: T≈30°C, [Cl−]≈10ppb and [DO]≈200ppb. The most positive OCP values however, were observed when the values of the analyzed factors were as follow: T≈140°C, [DO]≈200ppb and [Cl−]≈10ppb. Finally, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM) were used for further analysis of the steel samples exposed to high-temperature NaCl solutions under different experimental conditions.