Electrochemical Dissolution Rate Research Articles

Comparison of electrochemical dissolution properties of Hastelloy X in NaCl and NaNO3 solutions

The electrochemical dissolution properties of Hastelloy X are of great significance to its electrochemical machining (ECM). This study focuses on the electrochemical dissolution properties of Hastelloy X in NaCl and NaNO3 solutions. The open circuit potential, polarization curve, electrochemical impedance spectroscopy (EIS) and current efficiency were investigated, and the dissolution mechanisms were revealed. The results revealed that the breakdown potential and electrochemical impedance in NaCl solution were smaller than that in NaNO3 solution, confirming that oxide film was broken more easily in NaCl solution. Local pits and many electrochemical products were observed on the surface, which were formed by electrochemical dissolution, and the main composition of products was the hydroxide and oxide. With the increase of current densities, the current efficiency increased in NaCl solution, while decreased in NaNO3 solution. The current efficiency values and surface roughness demonstrated that at high current density, the rate of electrochemical dissolution was higher in NaCl solution, but better surface quality can be obtained in NaNO3 solution. The research can provide valuable theoretical guidance for the ECM of aerospace parts related to Hastelloy X.

Study on anodic dissolution behavior and surface evolution of laser-drilled Ni-based superalloys during electrochemical post-processing

Laser drilling is always accompanied by recast layers, micro-cracks, and heat-affected zones, which limit its application in engineering. Due to its unique advantages, such as no heat-affected zones, no residual stresses, and no dependence on the mechanical properties of the materials, electrochemical dissolution has been adopted to remove the laser drilling-induced surface defects. The electrochemical dissolution characteristics and surface evolution of the laser-drilled Ni-based superalloys were studied. The polarization curves and electrochemical impedance spectroscopy (EIS) of the laser-drilled materials and the substrate were measured, indicating that the laser-drilled surface had a smaller corrosion resistance than the original substrate. X-ray photoelectron spectroscopy (XPS) results showed that the laser-drilled surface had a much higher oxygen content than the substrate. Additionally, electric current efficiency curves were obtained to study the anodic dissolution behavior of Ni-based superalloys. An electric current density in the 5–40 A/cm2 range was preferred to enhance the electrochemical dissolution rate. Moreover, the effects of electric current densities and post-processing time on the surface roughness and the recast layer thickness of the laser-processed Ni-based superalloys were investigated using a NaCl and NaNO3 solution electrolyte. The evolution of the surface quality and recast layer thickness were clarified. Furthermore, an anodic dissolution model of the laser-processed surface was established to illustrate the electrochemical post-processing mechanisms. Results revealed that electrochemical dissolution could completely remove the laser drilling-induced surface defects in 20 s using a 12.5 wt.% NaNO3 solution electrolyte and a 40 A/cm2 electric current density. Finally, an electrochemical post-processed surface with surface roughness (Ra) of 1.7 µm and without a recast layer was achieved. Micro holes without recast layers and Ra of 0.69 µm were obtained with high efficiency using laser processing and electrochemical post-processing.

Electrochemical Kinetics Study of Ultrasound-Assisted Chalcopyrite Oxidation

Ultrasound-assisted chalcopyrite leaching has been reported previously, showing a noticeable improvement in leaching kinetics. However, the effect of ultrasound on the chalcopyrite oxidation kinetics from an electrochemical perspective has not been addressed. This study examines the ultrasonic enhancement of chalcopyrite oxidation kinetics in sulfuric acid solution from both conventional leaching and electrochemistry aspects. Electrochemical techniques, including linear sweep voltammetry (LSV) and chronoamperometry (CA), were used to illustrate the kinetics of chalcopyrite ultrasound-assisted leaching. Tafel analysis by LSV showed that 20% amplitude ultrasound power had increased the chalcopyrite electrochemical dissolution rate by about 20% in both Fe3+-free and 10 mM Fe3+-containing 0.5 M sulfuric acid solution. The CA tests indicated a drastic increase in the Fe3+ reduction reaction when ultrasound was applied (20% amplitude). At 0.5 V, the Fe3+ to Fe2+ reduction current density at 30 min drastically increased from − 65.54 without ultrasound to − 1165.84 µA cm−2 with ultrasound.Graphical

Obtaining High Surface Quality in Electrochemical Machining of TC17 Titanium Alloy and Inconel 718 with High Current Densities in NaNO3 Solution

Electrochemical machining (ECM) is a very promising technology for the processing of titanium alloys (such as TC17) due to its many inherent advantages. Halogen electrolytes are commonly used in ECM of such alloys, but this often results in unwanted stray corrosion and poor surface quality. In this study, a processing mode of titanium alloy in NaNO3 solution with high current density is proposed. Inconel 718 is used for comparison with TC17. The passive characteristics of the sample are analyzed by polarization curves and electrochemical impedance spectroscopy. The results show that TC17 has a high breakdown potential and the structure of its passive film is more compact than Inconel 718. The corresponding schematic model is established to characterize the interface structure between the sample’s surface and passive film. The effects of current density on surface microstructure and surface quality are investigated. It is found that the α phase has a faster electrochemical dissolution rate than the β phase for TC17, and it can obtain a good surface quality (about Ra 0.69 μm) when the current density reaches 160 A cm−2. Finally, the corresponding dissolution model is established to characterize the electrochemical dissolution mechanism of the sample in NaNO3 solution at high current density.

Effect of microstructure on the electrochemical dissolution behaviour of Hastelloy® X superalloy processed by selective laser melting and heat treatments

As a proven and highly efficient processing technique, electrochemical machining (ECM) is used for machining difficult-to-cut Ni-based superalloys (such as Hastelloy® X [HX] fabricated via selective laser melting [SLM]) with complex shapes. However, the metallic surface is susceptible to passive film formation to reduce the dissolution rate of HX under low current density conditions during the ECM process. This study focused on optimising microstructures for SLM-fabricated HX via heat treatment to enhance its electrochemical dissolution behaviour. The rates of electrochemical dissolution with different heat treatments increased in the following order: as-received < solution treatment (ST) < ageing treatment (AT) < solution and ageing treatment (ST + AT) in a 10 wt% NaNO3 solution at 35 °C. These results are attributed to 1) a large number of two representative precipitates (Fe2Mo and M23C6) in the grain or on the grain boundary (GB), 2) the decreased low-angle GB density (1.71%) and 3) few compact products on the surface of the samples with ST + AT. The parameter I, which is related to surface product composition, was quantitatively characterised based on the electrochemical dissolution rate of the samples. The enhanced mechanism of electrochemical dissolution is proposed to offer new insights into Ni-based superalloys in ECM applications.

Electrochemical cleaning grinding (ECCG) of aluminum alloy 6061 with electroconductive resin-bonded diamond wheels

A new manufacturing process named Electrochemical Cleaning Grinding (ECCG) is introduced to help reduce wheel loading in grinding of the highly ductile metals which exhibit high adhesion tendency. The key component of the ECCG system is an electroconductive resin-bonded diamond wheel, and its abrasive layer is made of phenolic resin bond and core-shell structured Ni-P alloy coated diamond abrasives. The alloy coating over diamond grains not only helps to increase the bond strength between the resin and the abrasives but improve the electrical conductivity properties of the grinding wheel. During the ECCG process, the abrasives on the wheel surface simultaneously act as micro cutting edges as well as microelectrodes where the anodic dissolution of the adherent metal materials takes place. This dissolution process is defined as “cleaning”. When the rate of accretion of adherent material is balanced by its electrochemical dissolution rate, the sharp abrasives are exposed to carry out efficient grinding, so that better grinding performance can be achieved. The grinding experiments of aluminum alloy 6061 (AA6061) were conducted to verify the effectiveness of the ECCG process. Compared with conventional grinding experiments, the most important changes brought in by the ECCG are the decrease in wheel loading degree and the improvement in machined surface quality. Additionally the electric energy consumption of the cleaning process is relative low and equal from 1.88 × 10−3 J/mm3 to 9.34 × 10-3 J/mm3.

A Review on Laser Assisted Jet Electrochemical Machining Process

Hybrid Machining Processes (HMP) consist of two different types of machining processes which are applied at the same time in the machining zone. These processes are applied for machining of various advanced materials of complex shapes and sizes. One of the important HMP is Laser Assisted Jet Electrochemical Machining (LAJECM) where a laser beam of low power and a jet of electrolyte are coaxially focused through a nozzle on a particular machine zone. Thus it eliminates the stray machining effect of ECM process. Machining zone temperature as well as electrolyte current density is increased by laser. Thus electrochemical dissolution rate and MRR are increased. In this process, jet of electrolyte is used as tool which eliminates the complex tool design problem of stand-alone ECM process. Laser assistance in ECM process improves precision, accuracy, surface quality and also productivity. Various researchers have performed their research work on LAJECM. This article focuses on various aspects and implementations of LAJECM.

Fabrication of deep and small holes by synchronized laser and shaped tube electrochemical machining (Laser-STEM) hybrid process

The synchronized laser and shaped tube electrochemical machining (Laser-STEM), a hybrid machining process that combined the advantages of electrochemical machining (ECM) and laser beam machining (LBM), has been proposed and studied. The mechanisms of Laser-STEM were studied theoretically and experimentally. Mathematical model has been developed to study the effects of laser-induced thermal effects on the electrochemical dissolution rate. The influences of electrolyte concentration and laser power on the laser attenuation coefficient have been studied. Results showed that the side gap could be decreased by 62.7% and the feeding rate could be raised by 108% when utilizing Laser-STEM with a proper laser power for drilling small holes, compared with that without laser assistance. Moreover, the performance of Laser-STEM has been investigated, in terms of laser power and pulsed voltage. Experimental results showed that the machining efficiency increased with higher laser power, pulse voltage, and feeding rate, and the precision could be improved with the higher laser power and feeding rate and a smaller pulsed voltage. Finally, small holes with a diameter of 1.25 mm and free of recast layer have been fabricated on the aluminum alloy workpiece of 5 mm in thickness.

Effect of layer band and heterogeneity of microstructure on electrochemical dissolution of laser solid formed Ti-6Al-4V alloy

Laser solid forming (LSF) is a newly advanced additive manufacturing technique which offers a huge potential in efficiency and cost savings for fabricating the components of aerospace titanium alloys with high performance. In this research, the macrostructure/microstructure and electrochemical dissolution features of the Ti-6Al-4V alloy produced by LSF and forging were investigated. The LSFed Ti-6Al-4V that included as-deposited and annealing heat treatment (HT) conditions contains three periodically distributed microstructural morphologies: fine microstructure zone (FMZ), coarse microstructure zone (CMZ), and layer band. All zones of the different microstructural morphology in an entire deposit are constituents of α-laths and a small amount remained in the β phase. An alternating structure of coarse and fine microstructures was observed along the deposition direction in the deposit. Layer bands are generally spaced above the CMZ. The CMZ is a constituent of a slightly coarser basket weave structure than that of the FMZ. However, the layer bands present the microstructure of colony α. Additionally, the α-laths are slightly coarsened by HT, and this simultaneously results in a more homogeneous microstructure compared with the as-deposited microstructure. Moreover, the forged Ti-6Al-4V possesses the most homogeneous microstructure and the largest size of the α phase. Simultaneously, a variety of dissolved morphology features were observed in the three types of samples after potentiodynamic polarization tests. The order of the surface macrosmooth level of these samples is forged &amp;gt; HT &amp;gt; as-deposited. For LSFed Ti-6Al-4V, the FMZs and the layer bands present a faster electrochemical dissolution rate than that of the CMZs. Consequently, the uneven surface is revealed by the distinct dissolution rate of different microstructural zones at the same LSFed sample. The forged Ti-6Al-4V alloys present a uniformly dissolved behavior, which is attributed to the homogeneity of the microstructure. The elimination of the layer bands and the homogenization of the microstructure of the LSFed Ti-6Al-4V are of great significance for improving the surface quality during electrochemical machining.

Mechanistic Analysis of Anodic Dissolution of Zr in Acidic Fluoride Media

Anodic dissolution of Zr in 10 mM HF was investigated using potentiodynamic polarization and electrochemical impedance spectroscopy. The surface state of the electrode was analyzed using X-ray photon spectroscopy, and the surface morphology was characterized using atomic force microscopy. EIS data acquired at multiple dc potentials were subjected to mechanistic analysis. Reaction mechanism analysis approach reveals that at least four intermediates are required to describe the observed results. The intermediates are likely to be Zr sub-oxides, oxyfluorides and ZrO2. The proposed mechanism successfully predicts the major features observed in polarization and impedance spectra. At low overpotentials, the fractional surface coverage of Zr3+ species is higher than that of Zr4+ species, and the electrochemical dissolution rate is higher than the chemical dissolution rate. As the overpotential increases, the surface is covered with Zr4+ species and chemical dissolution rate becomes comparable to electrochemical dissolution rate. Although the surface is covered with Zr4+ species at higher overpotentials, significant chemical and electrochemical dissolution processes continue to occur and hence Zr is not protected in acidic fluoride media under anodic conditions.

Electrochemical dissolution of magnetite electroplated coatings on titanium

As a part of understanding of iron and steel passivity, the dissolution rate of magnetite has been studied in anaerobic borate buffer at 25 °C between −0.5 and −0.75 V/SHE. The dissolution rates have been estimated from weight loss measurements. Linear weight losses in time have been obtained corresponding to constant dissolution rates. Nevertheless the rates depended on potential according to a Butler-Volmer law. Another Butler-Volmer law has been obtained for the steady-state cathodic current density flowing through the magnetite layer and ending by the proton reduction step. For both, the quantitative Butler-Volmer parameters have been estimated from the experiments. The cathodic feature of the electrochemical dissolution rate for magnetite was confirmed but the electron balance analysis has shown that this dissolution did not involved electrons as mentioned in the literature. One possible mechanism is proposed in which the kinetics of the proton reduction step tunes the outer voltage drop at the magnetite-solution interface and this voltage drop steers the kinetics of the dissolution rate.

Electrochemical dissolution of Тс–Ru alloys in nitric acid solutions

Electrochemical dissolution of Тс–19 wt % Ru, Тс–45 wt % Ru, and Тс–70 wt % Ru in 1 to 6 M HNO3 solutions in the amperostatic mode was studied. In the solutions formed from anodic dissolution of Тс–Ru alloys, Ru is present in the form of Ru(IV), and Тс, in the form of Тс(VII). A kinetic study of the electrochemical dissolution of the Тс–70 wt % Ru alloy in the examined interval of HNO3 concentrations showed that the alloy dissolved congruently and the Тс(VII) and Ru(IV) accumulation in the solution linearly depended on time. Anodic dissolution of Tc–Ru alloys with lower Ru content was also characterized by a linear increase in the Тс(VII) concentration in the solution with time. On the other hand, formation of poorly soluble hydrated Ru(IV) oxide on the electrode surface was observed simultaneously with accumulation of soluble Ru(IV) species in solution. The rate of electrochemical dissolution of the Тс–70 wt % Ru alloy linearly increased with increasing HNO3 concentration. The rates of electrochemical dissolution of Tc, determined for alloys with lower Ru content, were independent of the HNO3 concentration in the electrolyte. Oxidation of water with the evolution of oxygen on the surface of Tc–Ru alloys was observed simultaneously with the anodic dissolution of Tc and Ru. This process leads to a decrease in the current efficiency of the Tc and Ru dissolution. Examination of the corrosion damage of the working electrode surfaces by scanning electron microscopy shows that the electrochemical dissolution of Tc–Ru alloys leads to uniform corrosion of their surface.

Simulation of electrochemical machining using the boundary element method with no saturation

The simulation of electrochemical machining (ECM) is based on determining the surface shape at each point in time. The change in the shape of the surface depends on the rate of the electrochemical dissolution of the metal (conducting material), which is assumed to be proportional to the electric field strength on the boundary of the workpiece. The potential of the electric field is a harmonic function outside the two domains—the tool electrode and the workpiece. Constant potentials are specified on the boundaries of the tool electrode and the workpiece. A scheme with no saturation in which the strength of the electric field created by the potential difference on the boundary of the workpiece is proposed. The scheme converges exponentially in the number of grid elements on the workpiece boundary. Given the rate of electrochemical dissolution, the workpiece boundary, which depends on time, is found. The numerical solutions are compared with exact solutions, examples of the ECM simulation are discussed, and the results are compared with those obtained by other numerical methods and the ones obtained using ECM machines.

Electrolyte composition and removal mechanism of Cu electrochemical mechanical polishing

The optimization of electrolytes and the material removal mechanisms for Cu electrochemical mechanical planarization (ECMP) at different pH values including 5-methyl-1H-benzotriazole (TTA), hydroxyethylidenediphosphoric acid (HEDP), and tribasic ammonium citrate (TAC) were investigated by electrochemical techniques, X-ray photoelectron spectrometer (XPS) analysis, nano-scratch tests, AFM measurements, and polishing of Cu-coated blanket wafers. The experimental results show that the planarization efficiency and the surface quality after ECMP obtained in alkali-based solutions are superior to that in acidic-based solutions, especially at pH=8. The optimal electrolyte compositions (mass fraction) are 6% HEDP, 0.3% TTA and 3% TAC at pH=8. The main factor affecting the thickness of the oxide layer formed during ECMP process is the applied potential. The soft layer formation is a major mechanism for electrochemical enhanced mechanical abrasion. The surface topography evolution before and after electrochemical polishing (ECP) illustrates the mechanism of mechanical abrasion accelerating electrochemical dissolution, that is, the residual stress caused by the mechanical wear enhances the electrochemical dissolution rate. This understanding is beneficial for optimization of ECMP processes.

Effect of NH4F concentration in electrolyte on the fabrication of TiO2 nanotube arrays prepared by anodisation

Highly-ordered TiO2 nanotube arrays (TNTs) were prepared by anodisation in ethylene electrolytes containing 0.2–0.7 wt% NH4F. The dimension, chemical composition and optical characterisation of prepared TNTs were determined by the field-emission scanning electron microscopy, X-ray photoelectron spectroscopy and photoluminescence spectroscopy, respectively. Both the length and inner diameter of the prepared TNTs were increased with increasing NH4F concentration of electrolyte because of the faster electrochemical dissolution rate of TiO2. However, dimension of TNTs approached stable lengths and inner diameters as the NH4F concentration was higher than 0.3 wt%. The presence of Ti3+ state was observed for TNTs prepared with increasing NH4F concentration because of the charge compensation of the substitution of fluoride ions for oxygen ions. TNTs prepared in the electrolytes containing higher NH4F concentrations exhibited superior performance on the photocatalytic decomposition of Acid Red 4 possibly because more fluorides incorporated with TNTs to inhibit the recombination of electron–hole pairs.

Electrochemistry of Active Chromium: Part 1—Anomalous Corrosion and Products of Chromium Dissolution in Deaerated Sulfuric Acid

Abstract Chromium corroding in deaerated aqueous solution of sulfuric acid (H2SO4; pH 1 to 3) produces Cr(II) and Cr(III) ions simultaneously in the ratio 7:1, as well as H2. The corrosion potentials of electrochemically activated chromium are determined by the electrochemical processes as expected according to the Wagner-Traud model. However, the real rates of chromium corrosion determined by collecting evolved hydrogen, spectrophotometric determination of the accumulated Cr ions in the solution, or by weight-loss measurements are higher than the electrochemical dissolution rate by up to 12 times for pH 1.0. The effect is smaller for higher pH. This was due to the simultaneous “anomalous” (or chemical) dissolution process of the direct chemical reaction of Cr with H2O molecules, as proposed some time ago by Kolotyrkin and coworkers. Since “anomalous” dissolution cannot be detected by electrochemical means, it has been pointed out that in the presence of “anomalous” dissolution processes during metal corrosion, electrochemical corrosion rate measurements should be taken only as approximate, while the level of approximation should be determined by some other direct corrosion rate measurement method.

Parameter Sensitivity Analysis of Pit Initiation at Single Sulfide Inclusions in Stainless Steel

Sensitivity analysis methods were used in conjunction with a mathematical model for corrosion pit initiation in the vicinity of MnS inclusions in stainless steel to investigate the relationship between physicochemical parameters and the potential and concentration distributions. The finite difference method with central differences was used to calculate sensitivities. The mathematical model of pit initiation included 20 species plus the potential and 13 physicochemical parameters including rate constants for chemical and electrochemical surface reactions and equilibrium constants for homogeneous reactions. It was found that the potential and concentration profiles are most sensitive to the Tafel slope of the rate of electrochemical dissolution of sulfur-containing inclusions and least sensitive to changes in the equilibrium coefficients of the homogeneous reactions. The rate constant for the electrochemical reaction for dissolution of sulfide inclusions was also found to be significant. The procedure provides a first step toward selecting the most important parameters, designing critical experiments, and selecting the hypothesis that best fits experimental data. © 2004 The Electrochemical Society. All rights reserved.

Electrochemical dissolution of simulated "white inclusions" in nitric acid. Linear voltammetry study with Mo, Re, Ru metals and their alloys electrodes

Summary The process of electrochemical dissolution of pure Mo, Re and Ru metals, as well as their alloys 52.5 at. Mo – 47.5 at. Re and 90 at. Mo – 10 at. Ru in 0.5-6.0M HNO3 was studied using the linear voltammetry technique. The dissolution potentials of the pure metals were found to increase from 0.39 to 0.48V/SCE (Mo), from 0.7 to 0.81V/SCE (Re) and from 1.15 to 1.25V/SCE (Ru) with the augmentation of HNO3 concentration in the electrolyte. The dissolution potentials of the alloys 52.5 at. Mo + 47.5 at. Re and 90 at. Mo + 10 at. Ru were found to change from 0.49 to 0.58 (Mo-Re) and from 0.92 to 1.09 (Mo-Ru) when HNO3 concentration in the electrolyte increased from 0.5 to 6.0M. The secondary passivation of Mo-containing electrodes was observed in the solutions 2.0-6.0M HNO3 at the potentials from 1.4 to 1.9V/SCE, decreasing their dissolution rate in this range of potentials. The maximum electrochemical dissolution rate constants of pure Mo, Re, Ru metals and Mo-Re, Mo-Ru alloys have been estimated using the data of linear voltammetry. The obtained values (1.85-2.20g/cm2 h for Mo-Re and 0.58-0.64g/cm2 h for Mo-Ru alloys electrochemical dissolution) are found to be thousand times greater, than reported for the process of chemical dissolution of corresponding alloys in boiling HNO3.

A Quantitative Description of the Anodic Dissolution of Nickel in Water–Organic Media1

The effect of aprotic solvent additives on the anodic behavior of nickel in aqueous sodium chloride solutions is studied. A three-parameter equation correlating the physicochemical properties of the medium (conductivity, polarity, and polarizability) with the electrochemical dissolution rate of the metal is formulated. A substantial role of conductivity agrees with the diffusion control of the processes under investigation.

RELATION BETWEEN THE DISSOLUTION REACTIVITY OF POWDER SUBSTANCES AND EFFICIENCY OF THEIR ELECTROPLATING

ABSTRACT The importance of the passivation layer on Fe powder surfaces formed by oxides, hydroxides, and/or salts and water molecules is pointed out for both electrochemical oxidative dissolution of iron powder and electrodeposition of Ni layer on it. This layer behaves as ohmic contact resistance. The layer thickness affects the surface reactivity of the powder and is influenced by the way of preactivation the powder prior to the electrochemical process. A reliable voltammetric method reflecting the surface reactivity was found, theoretically treated, and experimentally verified by measuring the rate of electrochemical dissolution of Fe powder. The efficiency of Ni electrodeposition on this powder was determined in a series of independent electrolyses. The correlation between both later mentioned parameters was satisfying for small powder particle fractions as well as for activation procedures which include chemical step.