Active Metal Dissolution Research Articles

Crevice Corrosion Behavior of Alloy 690 in High-Temperature Aerated Chloride Solution.

Crevice corrosion behavior of Alloy 690 in high-temperature aerated chloride solution was studied using a self-designed crevice device. The SEM, EDS, XRD, and XPS analyses results indicated that the oxide films outside the crevice consisted of Ni-Cr oxides containing a small amount of hydroxides, and the oxide films on crevice mouth consisted of a (Ni,Fe)(Fe,Cr)2O4 spinel oxides outer layer and a Cr(OH)3 inner layer, and the oxide films inside the crevice consisted of a α-CrOOH outer layer and a Cr(OH)3 inner layer. When crevice corrosion occurred, the hydrolysis of Cr3+ led to the formation of Cr(OH)3 inside the crevice, and caused the pH value of crevice solution to decrease, and Cl− migrated from outside the crevice into inside the crevice due to electrical neutrality principle and accumulation. When the water chemistry inside the crevice reached the critical value of active dissolution of metal, the active dissolution of metal inside the crevice occurred. In addition, (Ni,Fe)(Fe,Cr)2O4 spinel oxides on the crevice mouth were formed by the deposition of metal ions migrated from inside the crevice. The mechanism of crevice corrosion and the formation mechanism of oxide films at different regions were also discussed.

Electrodissolution Processes and Their Application in Manufacturing for Shaping and Finishing of Advanced Materials

Controlled material removal by anodic dissolution at high rates forms the basis of non-conventional metal shaping and finishing processes such as Electrochemical Machining (ECM), Electrochemical Micromachining (EMM), and Electropolishing (EP). These processes are employed in the manufacturing and processing industries for a variety of applications ranging from the machining of complex shaped components for heavy industries to the fabrication of microstructures in thin films and foils and microfinishing of superconducting radiofrequency cavities (1-3). Application of these processes for the production of reliable, reproducible, and high-yielding components require an understanding of the parameters that influence the anodic dissolution processes.ECM, EMM, and EP can be understood and theoretically modeled based on the same principles provided the scaling effects related to each process are considered (2). In the first part of this presentation, the high rate anodic dissolution behavior of different materials including refractory materials are described which helps in establishing the basic concepts that govern ECM/EMM/EP processes. Electrochemical reactions taking place at high anodic potentials depend on the metal-electrolyte combination and the operating conditions. These reactions include active metal dissolution, passivity, oxygen evolution, and the phenomena of passive film breakdown leading to the transpassive metal dissolution. In passivating electrolytes (containing oxidizing anions), oxygen evolution and metal dissolution take place simultaneously. The current density dependence of meat dissolution efficiency in such electrolytes provides minimized stray cutting in ECM. Mass-transport, current distribution, and surface film properties play a significant role in the metal removal rate and surface finish. Whereas the formation of a salt film on the dissolving anode in ECM electrolytes is well established, the identification of the rate-controlling species during electropolishing of metals in concentrated acid solutions remains a controversial issue.The second part of the presentation deals with the applications of these processes in the manufacturing of advanced products for the aerospace, biomedical, electronics, and photovoltaics industries. Different ECM techniques such as die-sinking, combined tool machining, EC drilling, and EC deburring are described that are used in the manufacturing of complicated shaped parts such as turbine blades. Maskless EMM methods such as Jet EMM, micro-drilling, and wire EMM are applied to fabricate precision microstructures. For process enhancement, assisted techniques such as laser, abrasive, vibration, and ultrasonic are incorporated with maskless EMM. For through mask electrochemical micromachining (TMEMM), an understanding of mass transport in a cavity and shape evolution modeling is critical. The use of laser patterned oxide film mask for Ti patterning is presented as an interesting application of TMEMM (4). A few case-studies on electropolishing are described which include the fabrication of STM probes and Nitinol stents, and microfinishing of print bands. In the microprocessor industry, a significant effort was invested in the development of EP and electrochemical mechanical planarization (ECMP) approaches as a no-contact or low-contact planarization processes for the integration of ULK/copper interconnects. The development work, including processing details and planarization efficiency of EP and ECMP, are reviewed. A critical evaluation of the current status of EP/ECMP vis a vis the chemical mechanical planarization (CMP) process is presented.References J. McGeough, Principles of Electrochemical Machining, Chapman and Hall, Wiley, London, 1974.M. Datta, Electrodissolution Processes: Fundamentals and Applications, CRC Press/Taylor and Francis, Boca Raton, 2020.E. J. Taylor and M. Inman, Electrochemical Surface Finishing, The Electrochemical Society Interface, Fall 2014, p47.P.-F. Chauvy, P. Hoffmann, D. Landolt, Electrochemical Micromachining of Titanium Through a Laser Patterned Oxide Film, Electrochem. Solid-State Lett., 4(5), C31 (2001).

An overlooked issue for high-voltage Li-ion batteries: Suppressing the intercalation of anions into conductive carbon

Summary High-voltage Li-ion batteries have been extensively studied to increase energy density of batteries. However, their cycling stability has remained poor, despite various strategies being proposed to overcome the issues of high-potential cathodes, such as electrolyte oxidation and transition metal dissolution. Herein, we report anion intercalation into the cathode conductive carbon as an overlooked yet critical issue. We propose a concentrated sulfolane (SL)-based electrolyte that prevents anion intercalation via two mechanisms: (1) by offering a high activation barrier to intercalation via its strong anion-Li+ interaction and (2) by forming a sulfur-containing, anion-blocking SL-derived interphase. This electrolyte, used with graphitized acetylene black, which is oxidatively stable but usually susceptible to anion intercalation, enables the stable operation of a Li2CoPO4F/graphite full cell (cut-off voltage = 5.2 V), with 93% capacity retention after 1,000 cycles and an average Coulombic efficiency of ≥99.9%. This is a pivotal strategy to enhance the reversibility of >5 V Li-ion batteries on a commercial level.

Microstructural Aspects of Corrosion

The effect of metal corrosion on the third-order microscopic properties, i.e., the root-mean-square deviation and internal residual stresses of the first-order, was studied. The corrosion process was studied on steel specimens under conditions of superheated steam (250°C) during 500 h. The X-ray diffraction method was taken as the basic research method. The process of corrosion failure was concluded to have two reproducible periods: the period of active metal dissolution and latent passivation period, followed by the corrosion initiation phase. The metal passivity occurs due to the compressive stresses.

Electrodeposition of corrosion-resistant Cr–P and Cr–P–W coatings from solutions based on compounds of trivalent chromium

Corrosion-resistant Cr–P and Cr–P–W coatings were obtained by electrodeposition from aqueous and aqueous–organic (DMF/water) solutions. The inclusion of phosphorus in the deposits formed on the cathode was confirmed using the EDX and XPS techniques. X-ray diffraction data revealed that co-deposition of phosphorus with chromium resulted in amorphization of the obtained coatings. It was shown that the incorporation of phosphorus into the coatings leads to the disappearance of the region of potentials corresponding to active metal dissolution as compared to Cr coatings. As a result, the coatings demonstrate higher corrosion resistance. Cracking of Cr–P coatings leads to a decrease in their protective ability with respect to steel. The impossibility of obtaining thick Cr–P layers by electrodeposition from aqueous solutions limits the sphere of their possible application. This problem was solved by electrodeposition of Cr–P–W coatings from aqueous–organic media. At coating thicknesses above 10 μm, fully penetrating pores in Cr–P–W coatings practically disappeared. The addition of tungsten to the composition of cathodic deposits resulted in an additional increase in their corrosion resistance.

Corrosion Inhibition of Aluminum Alloy AA6063-T5 by Vanadates: Microstructure Characterization and Corrosion Analysis

Corrosion inhibition of aluminum alloy AA6063-T5 by vanadates (NaVO3) in 0.05 M NaCl solution has been investigated by electrochemical and weight loss measurements, and associated with microstructure and Volta potential data. X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy analyses confirmed the presence of micrometer-sized Fe-rich Al4.01MnSi0.74, Al1.69Mg4Zn2.31, and FeAl3 intermetallic phases (IMPs) and nanometer-sized CuAl2, ZnAl2, and Mg2Si precipitates in the microstructure. Scanning Kelvin probe force microscopy measurements showed Volta potential differences of up to 600 mV between the microstructure constituents indicating a high susceptibility to micro-galvanic corrosion, with interphase boundary regions exhibiting the highest propensity to corrosion. Most IMPs had cathodic character whereas some nanometer-sized Mg-rich particles exhibited anodic nature, with large Volta potential gradients within interphase regions of large cathodic particles. Electrochemical potentiodynamic polarization measurements indicated that the vanadates provided mixed corrosion inhibition effects, mitigating both oxygen reduction, occurring on cathodic IMPs, and anodic metal dissolution reaction, occurring on anodic sites, such as Mg2Si and interphase boundary regions. Electrochemical measurements indicated that the sodium metavanadate inhibitor blocks active metal dissolution, giving high inhibition efficiency (>95%) during the initial exposure, whereas long-term weight loss measurements showed that the efficacy decreases after prolonged exposure.

Microstructural, mechanical and electrochemical behaviour of a 7017 Al–Zn–Mg alloy of different tempers

The aim of the investigation is to assess the microstructural features and associated physical, mechanical and electrochemical properties of a 7017 Al–Zn–Mg alloy of various tempers. A 7017 Al–Zn–Mg alloy was subjected to different ageing schedules to produce under-(T4), peak-(T6), over-(T7) and highly over-aged tempers. Optical microscopy, hardness measurement, electrical conductivity measurement, tensile testing and SEM fractographs, differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and electrochemical polarization studies have been used to characterize the alloy tempers. Hardness measurement and tensile testing showed the characteristic age hardening phenomenon of aluminium alloys. Optical and TEM micrographs have revealed the variation in size of matrix strengthening η′ (MgZn2) and also the size and distribution of grain boundary η (MgZn2) precipitate with ageing time. DSC thermograms exhibiting exothermic and endothermic peaks indicated the characteristic solid state reaction sequence of the 7017 alloy. Potentiodynamic polarization study of the 7017 alloy of various tempers in 3.5wt.% NaCl solution at near neutral pH showed typical active metal dissolution behaviour, but at pH12 an active–passive–transpassive transition behaviour has been observed.

Hydrogen generation by tin corrosion in lactic acid solution promoted by sodium perchlorate

A method to produce high purity hydrogen using the corrosion of tin metal in lactic acid solutions is studied. The addition of sodium perchlorate has been also investigated for promoting the tin-lactic acid reaction. The data reveal that the rate of hydrogen production increases with increasing lactic acid concentration. The presence of perchlorate ions in lactic acid solution enhances the active dissolution of tin metal and tends to breakdown the passive film and promoting the hydrogen generation rate. Polarization measurements show that the breakdown potential (Epit) decreases with increase in sodium perchlorate concentration. An increase in temperature accelerates the rate of solubility of passive layer on the tin surface. Moreover, a synergistic effect of sodium perchlorate in combination with increasing the solution temperature is key in promoting the hydrogen generation rate. Results obtained from hydrogen and polarization measurements are in good agreement. These measurements are complemented with SEM, EDX and XRD examinations of the electrode surface.

Flow microcapillary plasma mass spectrometry-based investigation of new Al–Cr–Fe complex metallic alloy passivation

Al-Cr-Fe complex metallic alloys are new intermetallic phases with low surface energy, low friction, and high corrosion resistance down to very low pH values (0-2). Flow microcapillary plasma mass spectrometry under potentiostatic control was used to characterize the dynamic aspect of passivation of an Al-Cr-Fe gamma phase in acidic electrolytes, allowing a better insight on the parameters inducing chemical stability at the oxyhydroxide-solution interface. In sulfuric acid pH 0, low element dissolution rates (in the µg cm(-2) range after 60 min) evidenced the passive state of the Al-Cr-Fe gamma phase with a preferential over-stoichiometric dissolution of Al and Fe cations. Longer air-aging was found to be beneficial for stabilizing the passive film. In chloride-containing electrolytes, ten times higher Al dissolution rates were detected at open-circuit potential (OCP), indicating that the spontaneously formed passive film becomes unstable. However, electrochemical polarization at low passive potentials induces electrical field generated oxide film modification, increasing chemical stability at the oxyhydroxide-solution interface. In the high potential passive region, localized attack is initiated with subsequent active metal dissolution.

Statistical atomic-topographic model of the active dissolution of metals with point lattice defects. The effect of point defects (vacancies and impurity atoms) on the metal dissolution rate

A physicochemical model is put forward for the dissolution (corrosion) of a metal with point lattice defects, i.e., vacancies and impurity atoms with corrosion stability radically different from that of the base metal. The model takes into account the substantial difference in the dissolution rates of the base metal atoms from positions with different numbers of neighboring atoms, which leads to the formation of characteristic nanofragments of the atomic surface relief. These fragments determine the dissolution rate. The point defects substantially affect the dissolution rate of the base metal exclusively due to their active involvement in the formation of the atomic relief. The equations describing this model allow the polarization curves of active dissolution of the base metal to be calculated as a function of the defect concentration.

Nickel-plating for active metal dissolution resistance in molten fluoride salts

Ni electroplating of Incoloy-800H was investigated with the goal of mitigating Cr dissolution from this alloy into molten 46.5%LiF–11.5%NaF–42%KF eutectic salt, commonly referred to as FLiNaK. Tests were conducted in graphite crucibles at a molten salt temperature of 850 °C. The crucible material graphite accelerates the corrosion process due to the large activity difference between the graphite and the alloy. For the purposes of providing a baseline for this study, un-plated Incoloy-800H and a nearly pure Ni-alloy, Ni-201 were also tested. Results indicate that Ni-plating has the potential to significantly improve the corrosion resistance of Incoloy-800H in molten fluoride salts. Diffusion of Cr from the alloy through the Ni-plating does occur and if the Ni-plating is thin enough this Cr eventually dissolves into the molten salt. The post-corrosion test microstructure of the Ni-plating, particularly void formation was also observed to depend on the plating thickness. Diffusion anneals in a helium environment of Ni-plated Incoloy-800H and an Fe–Ni–Cr model alloy were also investigated to understand Cr diffusion through the Ni-plating. Further enhancements in the efficacy of the Ni-plating as a protective barrier against Cr dissolution from the alloy into molten fluoride salts can be achieved by thermally forming a Cr 2O 3 barrier film on the surface of the alloy prior to Ni electroplating.

Effect of atomic hydrogen on the anodic dissolution of iron in a weakly acidic sulfate electrolyte

Using a membrane electrode, atomic hydrogen is shown to decelerate the dissolution of iron in sulfate and sulfate-citrate electrolytes (pH 5.5) in a potential range of the active metal dissolution and accelerates the process at the prepassivation potentials. Impedance spectra of iron at a controlled degree of surface coverage with hydrogen atoms are recorded. Rate constants of elementary stages of the anodic process are calculated and the reaction scheme of the iron dissolution in sulfate environments is made more accurate.

Modelling of anodic dissolution of pure aluminium in sodium chloride

A mathematical model for simulating a passive aluminium (Al) surface with a pit in which active electrochemical metal dissolution occurs has been developed. The model includes hydrolysis products of Al and the species obtained as a result of homogeneous reactions between chloride and Al 3+ ions and Al hydrolysis products. The model does not assume the equilibrium state in solution: all terms in homogeneous reactions are treated explicitly using kinetic constants taken from the literature. The validity of assuming reaction equilibrium has been addressed. Solution potential values and species concentrations are predicted for different dissolution current densities. The acidity in the pit is explained by the hydrolysis of Al 3+; an analytical expression for the pH values at the pit bottom for a given dissolution current density is presented. The model is applied to a real capillary geometry used in electrochemical microcell experiments. It was found that for r cap/ r pit < 100, where r cap and r pit are the capillary end and pit radii, respectively, the insulating capillary wall affects the species concentrations and the solution potential. Moreover, for r cap/ r pit < 20, the shape of the capillary, which might not be cylindrical, should be taken into account.

The effect of atomic hydrogen on the anodic dissolution of iron in a sulfate electrolyte studied with impedance spectroscopy

Impedance spectra of an iron membrane electrode are recorded in a range of the active metal dissolution potentials in an acidic sulfate solution. The rate constants of the elementary adsorption stages of iron dissolution are found to decrease with an increase in the amount of atomic hydrogen. This results in a decrease in the metal surface coverage with intermediate particles of the anodic process.

Reaction model for kinetic of cobalt dissolution in carbonate/bicarbonate media

Cobalt dissolution in a carbonate/bicarbonate media was studied in order to elucidate the kinetic of the metal corrosion processes. Steady-state polarization and electrochemical impedance measurements were carried out during the active metal dissolution at different electrode rotation speeds. A reaction mechanism is presented in order to explain the metal dissolution: cobalt electrodissolution starts with the formation of intermediate (CoHCO 3) ads on the electrode surface, followed by the simultaneous formation of a Co ( CO 3 ) 2 2 - complex and the formation of a CoO film. Mass transport is characterised by diffusion of the Co ( CO 3 ) 2 2 - complex away from the electrode. The reaction model was validated by an accurate simulation of the experimental steady-state polarisation and impedance measurements.

Localized corrosion of Alloy C22 nuclear waste canister material under limiting conditions

Localized corrosion behavior of Alloy C22 in simulated Yucca Mountain (YM) repository environments was studied at the highest achievable but realistic temperatures under boiling and dripping scenarios. Temperatures measured in concentrated boiling solutions of KCl and NaNO 3 were found to be stable at 140 °C, although transient boiling before dryout was observed at temperatures as high as 160 °C, as the electrolyte became progressively more concentrated. Experiments that simulated a dripping scenario with simulated J13 well water confirmed the existence of concentrated solutions stable at 142 ± 3 °C under controlled drip conditions leading to pit initiation in Alloy C22 after only a few hours. The polarization experiments conducted at 140 °C in a solution with 0.5 mol L −1 chloride concentration showed that the critical potential for localized corrosion was 250 mV (versus Ag/AgCl). Potentiostatic tests confirmed that active metal dissolution occurred only in the crevice region at this potential. The crevice corrosion of Alloy C22 required an incubation time to develop a critical crevice solution, and it was triggered by severe local chemistry (enrichment of Cl − and H +) aided by the high temperature.

Corrosion potential of 304 stainless steel in sulfuric acid

The potentiodynamic study of the electrochemical behavior of austenitic 304 stainless steel in deaerated aqueous sulfuric acid of pH 1 revealed that the steel achieved a stable corrosion potential of ca. - 0.350 V (SCE) independent of whether the electrode had previously been cathodically "activated" or anodically passivated. It was also shown that the experimentally observed anodic peak was not the usually obtained anodic passivation peak, as is the case with a number of metal, but an artifact due to the anodic oxidation of hydrogen absorbed during the previously employed cathodic polarization and hydrogen evolution, intended to activate the initially passive surface, or even hydrogen absorbed on the open circuit potential. It was shown that this potential establishes and electrochemical corrosion potential of the Wagner-Traud type due to the evolution of cathodic hydrogen on a passivated steel surface and anodic metal dissolution through the passive layer. It was impossible to activate 304 stainless steel in sulfuric acid of pH 1 by cathodic polarization, and the usually observed anodic peak obtained under these conditions should not be considered as an active metal dissolution process and a passivation anodic peak, but rather as an artifact due to the electrochemical oxidation of the in the steel absorbed hydrogen.

Pit Initiation at Single Sulfide Inclusions in Stainless Steel: II. Detection of Local pH, Sulfide, and Thiosulfate

Small wires of W, Ag, and Pt were placed within an electrochemical microcell above single sulfide inclusions to detect the pH, sulfide, and thiosulfate, respectively. In chloride-free electrolytes the onset of sulfide dissolution was accompanied by a decrease in pH of 0.5 units to 4.2, a value that is not low enough to initiate active metal dissolution. In chloride-containing electrolytes, metastable pitting events were found to cause rapid temporary periods during which the pH decreased to values as low as zero. However, the results suggest that sustained pH decreases occur only with onset of stable pitting. Sulfide species were only detected during chemical dissolution in a pH 2 electrolyte at the rest potential and not as a result of anodic dissolution at neutral pH. A Pt wire along with an iodide/triiodide couple was used to detect thiosulfate as a result of anodic dissolution. The results support the mechanism of electrochemical dissolution of sulfide inclusions to produce thiosulfate at neutral pH. © 2002 The Electrochemical Society. All rights reserved.

In-situ atomic-scale studies of the mechanisms and dynamics of metal dissolution by high-speed STM

The fundamental atomic-scale processes during active metal dissolution—the local removal/addition of atoms at atomic kinks in the steps on the crystal surface—were directly studied by novel instrumental methods based on in-situ scanning tunneling microscopy (STM), using the dissolution of Cu(100) in 0.01 M HCl solution as an example. Direct observation of these rapid dynamic processes with a novel high-speed electrochemical STM (Video-STM), capable of acquiring up to 25 atomic resolution images per second, reveals that metal dissolution proceeds at a single type of structurally well-defined kinks. The reactivity of these kinks is tentatively explained by the coordination of Cu surface atoms and Cl adsorbates at these sites. The kinks nucleate predominantly at outer terraces corners and can condense into larger facets, resulting in an apparent ‘collective’ local dissolution or growth of terraces. Quantitative data on the kink dynamics were obtained by an alternative approach (TOW-STM) and show pronounced local dissolution/redeposition fluctuations at the individual kinks even at the onset of Cu dissolution with average kink propagation and reaction rates at kinks in the range 10 3 and 10 5 atoms s −1, respectively.

The electrochemical behavior and characterization of the anodic oxide film formed on titanium in NaOH solutions

Titanium dissolution and passivation were studied in NaOH aqueous solution using open-circuit potential, potentiodynamic and potentiostatic techniques. Potentiodynamic data showed that the active-passive transition involves active metal dissolution followed by formation of a poorly conducting passive oxide film that passivates the electrode. The critical current density varied with pH as d log jm/d pH=-0.098 in the pH range 11.00–14.00, while the passivation potential is changed according to the following two features: at pH 10.55–13.00, dEm/d pH=-0.06 V; and at pH 13.50–14.00, dEm/d pH=-0.40 V. The apparent activation energy, E*, was calculated from the slope of the Arrhenius plot and was found to be 12.6 kJ mol–1. Current-time transients showed that the growth of titanium oxide passive film is a diffusion-controlled process. XPS measurements indicated that the passive oxide film consists mainly of TiO2 and a mixture of suboxides of Ti2O3 and TiO.