Dissolution Current Density Research Articles

Anodic dissolution and passivation of manganese monosilicide in fluoride-containing sulfuric acid solutions

The purpose of this study was to investigate the anode resistance of manganese monosilicide MnSi in fluoride-containing sulfuric acid solutions and the concentration effect of sodium fluoride on the anodic dissolution and passivation of the silicide. The study was carried out on a single-crystal MnSi sample in 0.5 M H2SO4 + (0.0025−0.05) M NaF solutions. The study presents micrographs and elemental composition of the electrode surface after anodic polarization from E corrosion to E = 3.2 V in 0.5 M H2SO4 and 0.5 M H2SO4 + 0.05 M NaF solutions. A stronger etching of the electrode surface was observed in the presence of fluoride ions; elemental analysis showed an increase in the oxygen content in certain areas of the silicide surface associated with the formation of manganese and silicon oxides and their partial removal at high polarization values. The kinetic regularities of the MnSi-electrode anodic dissolution were studied by the methods of polarization, capacitance, and impedance measurements. It was established that the addition of fluoride ions leads to weaker barrier properties of the silicon dioxide surface film, which determines the high silicide resistance in a fluoride-free medium. The order of the reaction was calculated for the MnSi anodic dissolution for NaF depending on the potential. In the region of low anodic potentials (from Ecor to E ≈ –0.2 V), the reaction order ranged from 1.8 to 1.1, which was due to the high influence of silicon in the composition of the silicide and its oxidation products. With an increase in the polarization value (up to E = 0.9 V), the reaction order decreased to 0.5. An increase in the contribution of manganese ionization and oxidation reactions to the kinetics of the anodic dissolution of the silicide was observed. The silicide passivation in a fluoride-containing electrolytewas characterized by higher values of the dissolution current density (10–4−10–3 A/cm2) as compared to a fluoride-free electrolyte (10–6 A/cm2), the reaction order in region of the passive state was ~1.0. Passivation was due to the formation of MnO2 and SiO2 oxides on the surface. In the transpassivation region (E ≥ 2.0 V), there was a weak dependence of the current density on the concentration of fluoride ions. Oxygen release was observed on the surface of the electrode, and the formation of MnO4 – ions was recorded in the near-electrode layer. The article discusses mechanisms and kinetic regularities of anodic processes on an MnSi-electrode in sulfuric acid solution in the presence of fluoride ions

A Nickel Dissolution Process for Multilayer Electroforming to Achieve Ultrahigh Adhesion Strength.

Multilayer electroforming has a high potential to produce Ni/Ni layer structured metal walls with excellent material properties and a high thickness uniformity. However, Ni is easily oxidized in air, which fundamentally leads to a low adhesion strength between the Ni layers. Here, a novel in situ treatment is proposed for improving the adhesion performance between Ni layers. This treatment integrated the steps of electrochemical dissolution, surface protection, and electroforming. A study of the polarization behavior implied the electroformed Ni layer was dissolved efficiently in the NH2SO3H solution, beginning at a dissolution current density of 5 A·cm-2, which could remove the oxide film. A smooth substrate surface with a good surface hydrophilicity was obtained starting at 8 A·cm-2, helping to protect the activated substrate from being contaminated and oxidized. The experimental results showed that ultrahigh normal and shear adhesion strengths over 400 MPa between the Ni layers were achieved.

Examination of the passivity and passivity breakdown of OFP copper in simulated geological saline solution, effect of sulfate

The effect of sulfate on the passivity of oxygen free phosphorus (OFP) copper in simulated, anaerobic geological saline solutions was investigated by DC polarization and electrochemical impedance spectroscopy (EIS). The dominant point defect within the passive film is the cation vacancy as identified by Point Defect Model (PDM) coupled EIS optimization (PEO) and Mott-Schottky analysis. Kinetic parameters for the generation/annihilation of point defects were extracted by PEO. Sulfate is found to induce passivity breakdown via facilitating the ejection of cation vacancies and to catalyze the dissolution of the barrier layer leading to a thinning of the layer and a larger passive dissolution current density.

(Digital Presentation) Progress Towards the Key Factors Governing Pit Stability

The pit stability product—defined as the product of pit depth and dissolution current density (x·i)—was first introduced by Galvele as the criterion indicating the conditions that sustain a critically acidic solution within the pit. The one-dimensional (1D) electrode is the most commonly used experimental configuration for determining the pit solution chemistry and pit stability product. In our current works, a ‘sandwich’ like 1D pit electrode was developed, which enabled the in-situ and ex-situ visualization of pit depth. A methodology that avoids lacy cover formation was proposed to pre-dissolve a Type 316L stainless steel (SS) 1D electrode to specific depths. Based on the new methodology and mathematical models, it was found that 1D Fick’s law of diffusion could not be used to estimate the pit stability product under a salt film because electro-migration had a measurable contribution (i.e., 67%) to the dissolution current. Although the diffusion coefficient of metal cations decreased with an increasing concentration inside the pit, it could be replaced by a constant diffusion coefficient to estimate the pit stability product, defined as an equivalent diffusion coefficient in our work. Later, a more comprehensive mathematical model, which included hydrolysis reactions and activity, was built to predict the local chemistry within the pit cavity. Recently, a galvanostatic method was developed to estimate the critical pit stability product of Type 316L SS, which was between 0.8 and 0.9 A/m in 0.6 M NaCl at 25°C, findings that were also confirmed by mass transport modelling. Here, we will present the proposed improved theoretical framework and discuss the next steps in modelling the propagation of pits in stainless steels.

A mathematical model describing the surface evolution of Mg anode during discharge of aqueous Mg-air battery

A numerical model is developed, aiming to understand and describe anode dissolution behavior and respective interface effects at an Mg-0.1Ca anode during discharge within an aqueous magnesium-air battery (MAB). In this model, the negative difference effect (NDE) that impairs the performance (e.g. utilization efficiency and capacity) of the anode is considered by adding a linear semi-empirical equation to calculate the total dissolution current density responsible for the dissolution of the Mg-0.1Ca anode. The proposed model is capable of predicting the effect of Mg(OH)2 deposit on the discharge process by consideration of surface coverage, layer thickness, and deposit porosity. In addition, the model can track the pH changes within electrolyte nearby the anode surface during discharge processes. The simulation results representing the development of local pH and the thickness of the deposited layer are validated by corresponding experimental measurements and are in good agreement with the experimental findings.

The galvanic effect of pyrite enhanced (bio)leaching of enargite (Cu3AsS4)

Enargite is a representative arsenic-bearing copper resource, and pyrite is often found co-existed with it. Understanding the effect of pyrite on enargite (bio)leaching is of vital importance for the recovery of copper from enargite. The galvanic interaction between enargite and pyrite in bioleaching and electrochemical systems was investigated. Bioleaching experiment showed that the addition of pyrite significantly promoted the enargite leaching rate due to the galvanic effect. However, the pyrite promotion effect on the enargite leaching rate decreased due to the oxidation of pyrite and the surface area of pyrite reduced in the bioleaching process. The dissolution rate of enargite without pyrite was controlled by diffusion through a product film, suggesting a passivation layer formed around the enargite surface. The presence of pyrite changed the rate-limiting factor of enargite dissolution to the surface chemical control, indicating that the addition of pyrite eliminated the passive effect. Electrochemical studies showed the mixed potentials (EM) and dissolution current densities (Idiss) of enargite electrodes increased when the enargite was in contact with a pyrite electrode, and the Emix and Idis further increased with increasing the surface area of the pyrite electrode. The effect of the pyrite electrode surface area on the dissolution of enargite electrodes was quantified by the kinetics of the half-cell reactions occurring in the pyrite-enargite galvanic couple under mixed potentials with oxygen and ferric ions as oxidants. Electrochemical impedance spectroscopy studies demonstrated a significant decrease in the charge transfer resistance and passivation film resistance of the enargite electrode in contact with pyrite, indicating that the cathodic reduction controlled the dissolution of enargite electrode and galvanically coupled pyrite continued to act as a cathode to facilitate the oxidation of enargite by oxygen and ferric ions even after enargite was passivated.

Localized corrosion: Passive film breakdown vs. Pit growth stability, Part VI: Pit dissolution kinetics of different alloys and a model for pitting and repassivation potentials

The pitting resistance of SS316L, SS304, SS430, and pure Fe was studied in chloride-containing solution. Among these alloys, SS316L exhibits the highest pit initiation rate, but also the highest pitting potential. To explain this phenomenon, the maximum pit dissolution current density (idiss,max) of the alloys in saturated pit solution was measured using one-dimensional artificial pit electrodes. A lower idiss,max was found to correlate with higher pitting and repassivation potentials. Based on a recently proposed pitting framework, a conceptual model for pitting and repassivation potentials was established, based on which the effect of idiss,max on pitting resistance was discussed.

Investigation of the anodic behavior of w-based superalloy for electrochemical selective treatment

W-based superalloys are widely used as elements of drilling equipment, high-speed steel cutting tools, or penetrators for armor-piercing munitions. Used or broken superalloy products are valuable waste that can be recycled to recover valuable components. The most economically and technologically viable method for recycling superalloy scrap is a selective treatment with the dissolution of the binder metal and the production of non-oxidized tungsten powder. The aim of this work was to determine the possibility of anodic treatment of the VNZh90 superalloy scrap with the selective dissolution of the binder metal. The anodic behavior of the VNZh90 superalloy (5 % Ni, 5 % Fe, 90 % W) in HCl solutions with a concentration (wt %) of 9, 13, 17, and 30 was studied by voltammetry. It was shown that the anodic polarization curves of the alloy contained two dissolution peaks on a fresh surface (Fe and Ni components of the binder metal) with a further decrease in the current density. The effect of significant passivation of the VNZh90 alloy was revealed: repeated polarization curves in a 9 % HCL solution contained only the Ni dissolution peak with a 6-fold reduced current density. The passivation of the VNZh90 alloy was explained by the depletion of the surface due to the dissolution of the active Fe component and the Ni passivation due to the W dissolution during the formation of a superalloy. An increase in the HCl concentration did not reveal an activating effect. It was found that there was no activation effect when FeCl3 was added to the electrolyte. The introduction of NaCl showed a high activation effect, and the dissolution current density of the passivated Ni component increased by 1.69 times. The efficiency of selective dissolution of the binder metal of the highly passive VNZh90 alloy must be confirmed by the galvanostatic or volt-static method

Electrochemical Behavior of Salt Film and Its Role in Pit Growth

In a recently published series of papers for pitting corrosion,1-5 a new concept of maximum pit dissolution current density, i diss,max, was proposed to establish a unifying framework for pit growth stability. This framework provides an opportunity to revisit several key topics of this field and generates new insights to interpret the previous observations in a self-consistent way. In this work, the unifying framework is further expanded to establish a mathematical model describing the electrochemical behavior of salt film after it precipitates, i.e. the situation when i diss,max > i lim. It can be mathematically shown that the main function of a salt film during diffusion-controlled pit growth is to accommodate the extra potential of (E max - E sat) by regulating its thickness to adjust the actual potential at the pit surface to E sat, thus restricting the anodic dissolution current density at the diffusion-limited value.4 The salt film will respond to any changes of the applied potential, temperature, pit depth, and perforation radius of the pit cover. In addition, an analytical model is proposed for salt film growth based on ion transport, in which the film grows at the metal/film interface and dissolves at film/solution interface. Any disturbance causing a difference in growth and dissolution rates will trigger a self-regulating process that will, in turn, bring the pitting system back to steady state. Acknowledgments: This work was supported as part of the Center for Performance and Design of Nuclear Waste Forms and Containers, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0016584.Reference G. S. Frankel, T. Li, and J. R. Scully, Journal of the Electrochemical Society, 164 (4), C180-C181 (2017).T. Li, J. R. Scully, and G. S. Frankel, Journal of The Electrochemical Society, 165 (9), C484-C491 (2018).T. Li, J. R. Scully, and G. S. Frankel, Journal of The Electrochemical Society, 165 (11), C762-C770 (2018).T. Li, J. R. Scully, and G. S. Frankel, Journal of The Electrochemical Society, 166 (6), C115-C124 (2019).T. Li, J. R. Scully, and G. S. Frankel, Journal of The Electrochemical Society, 166 (11), C3341-C3354 (2019).

Explanation of the Electrochemical Behavior of Salt Film in Pit Growth

A unifying framework has recently been developed to describe the critical conditions for pit growth stability and salt film formation 1-3. It was shown that a salt film is not required for pit stabilization; it is just a consequence of a pit achieving diffusion-controlled growth. A salt film can form on the surface of both metastable and stable pits when the maximum pit dissolution current density, i diss,max, exceeds the diffusion-limited current density, i lim. In this work, the unifying framework is further expanded to establish a mathematical model describing the electrochemical behavior of salt film after it precipitates, i.e. the situation when i diss,max > i lim. It can be mathematically shown that the main function of a salt film during diffusion-controlled pit growth is to accommodate the extra potential of (E max - E sat) by regulating its thickness to adjust the actual potential at the pit surface to E sat and thus restricting the anodic dissolution current density at the diffusion-limited value 4. Therefore, the salt film will respond to any changes of the applied potential, temperature, pit depth, and perforation radius of the pit cover. This new model can explain the current peak observed during the downward potential scan of a 1D pit at the transition point from diffusion-controlled region to charge-transfer-controlled region. According to this new model, the current peak results from the supersaturation generated by thinning of the salt film during the downward scan. The model predicts that the amplitude of the current peak decreases with decreasing scan rate, which is in agreement with the experimental results from 1D artificial electrode, providing strong support for the validity of the new model. Acknowledgments: This work was supported as part of the Center for Performance and Design of Nuclear Waste Forms and Containers, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0016584. Reference G. S. Frankel, T. Li and J. R. Scully, Journal of the Electrochemical Society 164, C180-C181 (2017).T. Li, J. R. Scully and G. S. Frankel, Journal of The Electrochemical Society 165, C484-C491 (2018).T. Li, J. R. Scully and G. S. Frankel, Journal of The Electrochemical Society 165, C762-C770 (2018).T. Li, J. R. Scully and G. S. Frankel, Journal of The Electrochemical Society 166, C115-C124 (2019).

First-Principles Investigation of the Effect of Interstitial Carbon on Corrosion Resistance of Martensitic Medium-Carbon Steel

Martensitic carbon steels with fine grains are used for many industrial applications. Interstitial carbon provides high strength to martensitic structure. Although it has been reported that the interstitial carbon has the beneficial effect on the pitting corrosion resistance of steels,1,2 the role of interstitial carbon is still unclear. In this research, first-principles calculations were applied to the clarification of the effect of interstitial carbon on the anodic dissolution of martensite. The base specimen was AISI 1045 carbon steel, and the chemical composition of this steel (mass%) was 0.44%C, 0.20%Si, 0.85%Mn, 0.008%P, 0.002%S, <0.01%Ni, <0.01%Cr, <0.01%Mo, <0.01%Cu, <0.001%Ti, <0.002%Nb, 0.035%Al, 0.003%N, and 0.002%O. The specimens were heat-treated at 1123 K for 10 h in air and then, quenched in water. Since the surface of the specimens was decarburized during this heat-treatment, the low carbon martensitic structure was formed in the surface region of the specimens, and high carbon martensitic structure remained in the core area. Potentiodynamic polarization of the low and high carbon martensitic specimens was conducted using boric-borate buffer solution with 1 mM NaCl at pH 8.0. In addition to the electrochemical measurement, the first-principles calculations based on density functional theory (DFT) were carried out. For the calculation, the carbon concentration was varied from 0.40 at.% to 5.88 at.%. In the potentiodynamic polarization curves of the low and high carbon martensitic specimens in 1mM NaCl containing boric-borate buffer at pH 8.0, the region of active dissolution appeared on both specimens at about -0.5 V. The dissolution current density for high carbon martensitic specimen was lower than that of the low carbon martensitic one, suggesting that the interstitial carbon suppressed the active dissolution. Next, the effect of interstitial carbon atoms on the electronic structure of the martensitic structure was analyzed. It was clarified that the electronic density of states (DOS) of iron atoms at and near the Fermi level decreased by increasing carbon concentration. This result indicates that the interstitial carbon atom makes weak atomic bonding with the nearest iron atom, and stabilizes the electronic structure of martensite. This change in the energy state of iron atom was, therefore, attributed to the high corrosion resistance of martensitic carbon steels. References; M. Kadowaki, I. Muto, Y. Sugawara, T. Doi, K. Kawano, and N. Hara, J. Electrochem. Soc., 164, C962 (2017).M. Kadowaki, I. Muto, Y. Sugawara, T. Doi, K. Kawano, and N. Hara, J. Electrochem. Soc., 165, C711 (2018).

A New Framework for Pit Growth Stability and Its Experimental Validation

A new framework has been developed in recent works to describe pit growth stability 1-4. In this framework, the concept of maximum pit dissolution current density, i diss,max, was proposed to consider the competition between the dissolution and diffusion processes within a pit 2, 3, an issue that has not been satisfactorily addressed previously. The quantity i diss,max is the rate of dissolution in a pit in the absence of a salt film. According to this new framework, the pit stability and salt film formation can be defined by the critical conditions of i diss,max = i diff,crit and i diss,max = i lim, respectively, where i diff,crit and i lim are diffusion current densities related to maintain critical and saturated pit concentration 3. Unifying sets of principal parameters for pit stability and salt film formation were developed based on these critical conditions: (T crit, E crit, r crit) and (T sat, E sat, r sat), respectively 3. This talk described experiments using 1D artificial pit electrodes that validate this new framework for pit stability. According to the model prediction, the critical potential for salt film formation (E sat) should decrease with the log of pit depth. The results from experiments on 1D pits followed this relationship exactly, which provides strong support of the validity of the new model. Additionally, a critical pit concentration equal to 43% of the saturation concentration, C sat, was determined, which is much lower than the critical concentration determined by others. Furthermore, i diss,max values at fixed pit surface solution concentrations (100%C sat and 70%C sat) were determined using pits of varying depth. The Tafel slope was found to be about 116 mV in both environments. This is the first ever determination of pit dissolution kinetics in pit solutions of fixed concentration. This new framework provides an opportunity to revisit several key topics of this field and generates new insights to interpret previous observations in a self-consistent way, such as to understand the effects of alloy composition, bulk chloride concentration, ohmic potential drop of the solution, etc. Acknowledgments: This work was supported as part of the Center for Performance and Design of Nuclear Waste Forms and Containers, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # DE-SC0016584. Reference G. S. Frankel, T. Li and J. R. Scully, Journal of the Electrochemical Society 164, C180-C181 (2017).T. Li, J. R. Scully and G. S. Frankel, Journal of The Electrochemical Society 165, C484-C491 (2018).T. Li, J. R. Scully and G. S. Frankel, Journal of The Electrochemical Society 165, C762-C770 (2018).T. Li, J. R. Scully and G. S. Frankel, Journal of The Electrochemical Society 166, C115-C124 (2019).

Influence of Zn Content on Grain Boundary Precipitates on Stress Corrosion Cracking of Al–Zn–Mg Alloys under Environments Containing Chloride Solutions

A commercial AA7075 (Zn: 5.4 mass%) and experimental Al–Zn–Mg alloys with 8.5 or 10.5 mass% of Zn were used to investigate the influence of Zn in the alloys on the composition of grain boundary precipitates and the susceptibility to the stress corrosion cracking (SCC) of the alloys in NaCl solutions. The stress-strain curves of the alloys under the slow strain rate test (SSRT) demonstrated that the susceptibility of the Al–Zn–Mg alloys to SCC increased as the Zn content increased from 5.4 to 8.5 mass% in the alloys. The susceptibility of the Al–10.5Zn–Mg alloy to SCC was not evaluated because of its brittleness. A coarse mainly Zn-containing precipitate was observed at the grain boundary only in the Al–8.5Zn–Mg alloy by STEM/EDS analysis. Calculated Al–Zn–Mg–Cu–Cr phase diagram predicts that MgZn2 is appeared in the Al–8.5Zn–Mg alloy but not in the Al–5.4Zn–Mg alloy. Anodic polarization measurements of the alloys demonstrated that the dissolution current density increased and the pitting corrosion potential decreased as the Zn content in the alloys increased. The generation of active grain boundary precipitates, such as the mainly Zn-containing precipitates, appears to lead to a higher susceptibility to SCC in Al–Zn–Mg alloys.

Localized Corrosion: Passive Film Breakdown vs. Pit Growth Stability: Part IV. The Role of Salt Film in Pit Growth: A Mathematical Framework

In Part II of this series, a framework for pit stability established, and it was expanded in Part III to describe salt film formation in pits. It was shown that a salt film was not required for pit stabilization; it is just a consequence of a pit achieving diffusion-controlled growth. A salt film can form on the surface of both metastable and stable pits when the maximum pit dissolution current density, idiss,max, exceeds the diffusion-limited current density, ilim. Based on this clarifying framework, this paper shows mathematically that the main function of a salt film is to adjust the actual potential at pit surface by regulating its thickness, thus to restrict the anodic dissolution rate of pit surface metal at the value of diffusion-limited current density. As a result, the film thickness will respond to any changes in the applied potential, temperature, pit depth, ohmic potential drop in the solution and perforation radius of the pit cover. Additionally, the pit stability criteria that have been discussed previously in the literature are reinterpreted using the new framework, and they are unified by the critical temperatures, potentials and pit depths for pit stabilization and salt film formation proposed in Part III.

Localized Corrosion: Passive Film Breakdown vs Pit Growth Stability: Part V. Validation of a New Framework for Pit Growth Stability Using One-Dimensional Artificial Pit Electrodes

Experimental evidence is provided to support the validity of a new framework for pit growth stability established in previous papers in this series. Downward potential scans from the diffusion-controlled condition were performed on SS316L one-dimensional (1D) artificial pit electrodes in 0.6 M NaCl. The variation of the critical pit surface potential for salt film formation with pit depth was in agreement with the theoretical prediction of the new framework. Using pits of different depths, the maximum pit dissolution current density, idiss,max, was obtained as a function of maximum pit surface potential, Emax, at fixed pit surface metal cation concentrations, Csurf, equal to the saturation concentration, Csat, and 70%Csat. The apparent Tafel slope of the pit dissolution kinetics determined using the downward potential scan of a 1D pit in the charge-transfer-controlled region, where Csurf changes continuously during the scan, was smaller than the Tafel slope for idiss,max(Emax) at a fixed Csurf, indicating that Csurf is a key parameter for pit dissolution. The critical concentration for pit stability/repassivation was found to be approximately 43% of Csat. Charge-transfer-controlled 1D pit growth was shown to be a transient state, which will spontaneously transition to diffusion-controlled growth in accordance with the predictions of the new framework.

Effect of Interstitial Carbon on Anodic Polarization Behavior of Fe-33Mn-C Austenitic Steels

Interstitial carbon has been reported to successfully improve the corrosion resistance of austenitic stainless steels.1,2 Low-temperature carburizing treatments have been applied to introduce a substantial amount of interstitial carbon at the surface region of austenitic stainless steels. The carburizing treatments are known to dramatically improve the pitting corrosion resistance of austenitic stainless steels in chloride-containing environments. It has been proposed that the formation of a stable Cr2O3-rich passive film on the surface of the carburized stainless steel improves the corrosion resistance of these steels. 3 However, it has yet to be clarified whether interstitial carbon improves the corrosion resistance of non-chromium containing steels. With the aim of analyzing the effect of interstitial carbon on the corrosion behavior of steel, five Fe-33Mn-C steels, which were referred to as 0 C, 0.3 C, 0.6 C, 0.8 C, and 1.1 C steels according to their carbon content in mass%, were prepared in this study. The XRD patterns of the five steels indicated that the 0.3 C, 0.6 C, 0.8 C, and 1.1 C steels had a fully austenitic structure with no carbide precipitate, and the 0 C steel had a mechanical-grinding-induced ε-martensite in an austenite matrix. The increase in the lattice parameters of the 0.6 C, 0.8 C, and 1.1 C steels up to 0.78% over that of the 0.3 C steel calculated from the γ(111) peaks suggested that the added carbon was presented as interstitial carbon in the steels. Dot-like MnO and Mn-S-O inclusions with a diameter of less than 10 μm were evenly dispersed in the five steels. The 0.6 C, 0.8 C, and 1.1 C steels were passivated during the anodic polarization in 0.1 M Na2SO4 solution at pH 12.0, whereas the 0 C and 0.3 C steels actively dissolved. The anodic polarization measurements of the 0.3 C, 0.6 C, 0.8 C, and 1.1 C steels in 0.05 M Na2B4O7-NaOH buffer solution at pH 10.0 with 0.1 M Na2SO4 revealed that the dissolution current density of the steels decreased with higher amounts of interstitial carbon. The dissolution current density at 0.3 V vs. Ag/AgCl (3.33 M KCl) of the 1.1 C steel was reduced to about 1 × 10-2 A m-2, which was one hundredth that of the 0.3 C steel. A new finding demonstrated here is that a higher interstitial carbon content in the Fe-33Mn-C steels resulted in a stronger inhibition in the dissolution current density of the steels, resulting in an improvement in the corrosion resistance of the steels. The dissolution current density of the steels was not inhibited by CO3 2- ions, which is the expected dissolution product of the interstitial carbon, during the anodic polarization in Na2CO3-NaHCO3 buffer solution (0.1 M CO3 2-) at pH 10.0 with 0.1 M Na2SO4. It was confirmed that the decrease in the dissolution current density of the steels as a function of the interstitial carbon content measured in 0.05 M Na2B4O7-NaOH buffer solution at pH 10.0 with 0.1 M Na2SO4 was not the effect of the formation of CO3 2- ions in the solution. The XPS analysis of the 1.1 C steel detected the chemical shifts approximately 0.1 eV higher in the Fe 2p 3/2 electron binding energy and approximately 0.2 eV higher in the Mn 2p 3/2 electron binding energy compared with the peak positions of the Fe 2p 3/2 and Mn 2p 3/2 spectra measured for the 0 C steel. This can likely be attributed to the partial chemical bonding of interstitial carbon to iron and manganese, respectively. References. Y. Sun, Corros. Sci., 52, 2661 (2010).A. Chiba, S. Shibukawa, I. Muto, T. Doi, K. Kawano, Y. Sugawara, and N. Hara, J. Electrochem. Soc., 162, C270 (2015).A. H. Heuer, H. Kahn, F. Ernst, G. M. Michal, D. B. Hovis, R. J. rayne, F. J. Martin, and P. M. Natishan, Acta Mater., 60, 716 (2012).

The Electrochemistry of Copper Release from Austenitic Stainless Steels and Its Role in Localized Corrosion

In a recent paper, pitting corrosion around manganese sulfide (MnS) inclusions in stainless steel 303 (303) was investigated using in situ Atomic Force Microscopy (AFM), Scanning Kelvin Probe Force Microscopy (SKPFM) and Scanning Electron Microscopy (SEM).[1] In situ AFM experiments in 0.1M sodium chloride solution at voltages near the pitting potential combined with post immersion energy dispersive x-ray spectroscopy (EDS) found copper (Cu) deposition on MnS inclusions. SKPFM images before and after Cu deposition found that Cu ennobled the inclusion. That is, prior to immersion the SKPFM potential of MnS inclusions was more negative than the matrix, however, post immersion the potential was more positive. It was conclude that pit propagation near MnS inclusions occurs in five steps: 1) Preferential corrosion of the MnS inclusion, 2) Formation of a critical pitting solution at the dissolution site, 3) Oxidation of SS and the release of Cu(II), 4) MnS passivation and ennoblement by Cu deposition and 5) Accelerated oxidation of the SS owing to galvanic coupling with the ennobled MnS particle. In the current study, the role of Cu deposition during localized corrosion at MnS inclusions in stainless steel 303 was investigated by means of potentiodynamic polarization and RRDE (ring disk electrode) experiments. Potentiodynamic polarization curves were conducted as a function of alloy Cu content. Although the pitting potential in these alloys did not increase with Cu content, it was found that below a threshold concentration, a step decrease in the pitting potential was observed. For specimens above the threshold Cu concentration, SEM/EDS analysis after the experiments found intact MnS inclusions passivated by a layer of Cu. In comparison, no intact MnS inclusions were found on specimens below the threshold Cu concentration at the end of the experiment. These observations reinforce our previous findings that Cu passivation of MnS inclusions plays an important role during localized corrosion. To study the electrochemical release of Cu from 303 we use RRDE experiments. In these experiments a 303 disk in the RRDE was held at a potential that was in the passive region of the polarization curve but below the Cu oxidation potential at this pH. The Pt ring in the RRDE was held at a much more negative potential. As such, any Cu(II) released from the 303 disk would be reduced onto the ring. After the collection period, the ring was stripped using cyclic voltammetry and those results are presented in Figure 1. As can be seen in this figure, a large oxidation peak is observed at 0.25V SCE indicative of Cu oxidation. From the passive dissolution current density and ring current density we have used Levitch equation to calculate the concentration of Cu(II) in solution. It was found that the concentration of Cu in solution due to passive dissolution of 303 was on the order of 0.02mM during the RRDE experiment. In the static experiment, as was the case in our polarization experiments, the concentration would be lower, potentially by orders of magnitude. Therefore, the concentration of Cu(II) in solution necessary to passivate MnS inclusions is exceedingly small. R.S. Lillard, M.A. Kashfipour, W. Niu, "Pit Propagation at the Boundary between Manganese Sulfide Inclusions and Austenitic Stainless Steel 303 and the Role of Copper," Journal of the Electrochemical Society, 163 (8), C44051, 2016. Figure 1 Stripping Voltammetry results for 303 specimen containing 0.57wt% Cu in 0.1M NaCl solution (after 5 hours of rotating ring disk collection procedure and deaeration). Figure 1

The Effect of Interstitial Carbon on Pitting Corrosion Resistance of Martensitic Carbon Steels

Ultra-high strength steels with excellent ductility are required to reduce the weight of automobiles and other transportation vehicles. While martensite including high amount of interstitial carbon is favored for strengthening steels, the ductility is relatively low. To optimize the balance between ductility and strength, tempering is conducted. In the case of carbon steels, the precipitation of carbides occurs, and the amount of interstitial carbon decreases during tempering. Although tempering provides the balance between the strength and the ductility, it was reported that the pitting corrosion resistance tends to decrease by tempering.1 To achieve a balance between pitting corrosion resistance and mechanical properties, it is important to determine the quantitative effect of tempering on the pitting corrosion resistance. The specimens were AISI 1045 carbon steel (0.47%C, 0.19%Si, 0.85%Mn, 0.015%P, 0.015%S, 0.02%Ni, 0.18%Cr, 0.01%Mo, 0.01%Cu, <0.001%Ti, <0.002%Nb, 0.024%Al, 0.005%N, 0.002%O). The specimens were heat-treated at 1123 K for 3.6 ks and then quenched in water to form a full martensitic structure. To obtain a tempered martensitic structure, the as-quenched specimens were heated to 873 K and kept at this temperature for 0.1, 1, 10, 15, and 20 h in an Ar atmosphere. The metallographic inspection was performed to ascertain the characteristics of the microstructures of the as-quenched and tempered specimens by using FE-SEM. In tempered martensitic specimens, many small white dots were observed. This is evidence of the carbides formed by tempering. The sizes of the carbides increased with tempering time, meaning that the interstitial carbon decreased. The micro-scale anodic polarization measurements of the as-quenched and tempered specimens were conducted in a boric-borate buffer solution with 500 mM NaCl (pH 8.0) under naturally aerated conditions at 298 K. A small area without non-metallic inclusions was selected for the working electrode. The polarization measurements started at -0.1 V to eliminate the influence of active dissolution on pitting corrosion initiation. For 20 h tempered specimen, a stable pit was initiated immediately after immersion. The pitting potential of 20 h tempered specimen was thought to be less than -0.1 V. In the case of 15 h tempered specimen, the sharp increase in current density was observed at 0.05 V due to the pit initiation. In contrast, no pit was initiated on the as-quenched specimen and the specimens tempered for less than or equal to 10 h. It was found that the pitting corrosion resistance of the martensite tempered for less than or equal to 10 h was higher than that of the martensite tempered for more than or equal to 15 h. Pitting corrosion resistance of martensite decreased with tempering time. To clarify the role of the interstitial carbon on the pitting corrosion resistance, the macro-scale anodic polarization curves of as-quenched, 10 h tempered, and 20 h tempered specimens in a boric-borate buffer solution with 1 mM NaCl (pH 8.0) were measured. The polarization started at -1.0 V after a cathodic treatment (-1.2 V, 600s). For the macro-scale polarization, the region of active dissolution appeared on all specimens in the potential region from -0.7 to -0.6 V. The dissolution current density for 20 h tempered specimen was certainly higher than that of other two specimens. It is suggested that the higher pitting corrosion resistance of the as-quenched and short-time tempered specimens was due to the low dissolution rate of those specimens. It is proposed that the inhibition of the active dissolution rate of these specimens is because of the existence of interstitial carbon, and interstitial carbon improves the pitting corrosion resistance. Short-time tempering is one of the answers for striking an optimal balance between pitting corrosion resistance. References; 1. M. Kadowaki, I. Muto, Y. Sugawara, T. Doi, K. Kawano, and N. Hara, J. Electrochem. Soc., 164, C962 (2017).

Effects of microstructure transformation on mechanical properties, corrosion behaviors of Mg-Zn-Mn-Ca alloys in simulated body fluid.

Magnesium and its alloys have unique advantages to act as resorbable bone fixation materials, due to their moderate mechanical properties and biocompatibility, which are similar to those of human tissue. However, early resorption and insufficient mechanical strength are the main problems that hinder their application. Herein, the effects of microstructure transformation on the mechanical properties and corrosion performance of Mg-Zn-Mn-Ca were investigated with electrochemical and immersion measurements at 37 °C in a simulated body fluid (SBF). The results showed that the number density of Ca2Mg6Zn3/Mg2Ca precipitates was remarkably reduced and grain sizes were gradually increased as the temperature increased. The alloy that received the 420 °C/24 h treatment demonstrated the best mechanical properties and lowest corrosion rate (5.94 mm/a) as well as presented a compact and denser film than the others. The improvement in mechanical properties could be explained by the eutectic compounds and phases (Mg2Ca/Ca2Mg6Zn3) gradually dissolving into a matrix, which caused severely lattice distortion and facilitated structural re-arrangement of the increased Ca solute. Moreover, the difference in potential between the precipitates and the matrix is the main essence for micro-galvanic corrosion formation as well as accelerated the dissolution activity and current exchange density at the Mg/electrolyte interface. As a result, the best Mg alloys corrosion resistance must be matched with a moderate grain size and phase volume fractions.

Interstitial Carbon Enhanced Corrosion Resistance of Fe-33Mn-xC Austenitic Steels: Inhibition of Anodic Dissolution

Five Fe-33Mn-xC steels, referred to as 0 C, 0.3 C, 0.6 C, 0.8 C, and 1.1 C steels according to their carbon content in mass%, were prepared to clarify the effect of interstitial carbon on the dissolution behavior of steel. The 0.3 C, 0.6 C, 0.8 C, and 1.1 C steels indicated a fully austenitic structure with no carbide precipitate. The lattice parameters of the 0.6 C, 0.8 C, and 1.1 C steels calculated from the γ(111) and γ(200) diffraction peaks increased by up to around 0.8% over that of the 0.3 C steel, suggesting that the added carbon was present as interstitial carbon in the steels. The 0.6 C, 0.8 C, and 1.1 C steels were passivated during the anodic polarization measurements in 0.1 M Na2SO4 solution at pH 12.0, whereas the 0 C and 0.3 C steels actively dissolved. The anodic polarization measurements in a buffer solution at pH 10.0 demonstrated a lower dissolution current density for the 0.3 C, 0.6 C, 0.8 C, and 1.1 C steels with higher amounts of interstitial carbon. The dissolution current density at 0.3 V vs. Ag/AgCl (3.33 M KCl) of the 1.1 C steel was reduced to approximately 1 × 10−2 A m−2, which was one hundredth that of the 0.3 C steel. The dissolution current density of the steels was not inhibited by the presence of 0.1 M CO32− ions, which is an expected dissolution product of interstitial carbon, implying that the interstitial carbon improved the electrochemical property of the steels themselves. The work function of the 1.1 C steel, which showed improved corrosion resistance with interstitial carbon, was 0.12 eV lower than that of the 0 C steel. The peak positions of the Fe 2p3/2 and Mn 2p3/2 spectra of the 1.1 C steel indicated the binding energies were approximately 0.1 eV and 0.2 eV higher than those of the 0 C steel. This can likely be attributed to the partial chemical bonding of interstitial carbon to iron and manganese, respectively.