Electrochemical Dissolution Behavior Research Articles

Magnetic field effects on electrochemical dissolution behavior and surface quality of electrochemical machining of Ti-48Al-2Cr-2Nb alloy

Magnetic field effects on electrochemical dissolution behavior and surface quality of electrochemical machining of Ti-48Al-2Cr-2Nb alloy

Electrochemical dissolution behavior of Ti6Al4V alloy: Effect of microstructure and processing method

We report the electrochemical dissolution behavior of Ti-based alloys prepared by two different processing methods including laser solid foming (LSF) additive manufacturing and forging in a 15 wt % NaCl solution. The corrosion resistance of the forged Ti6Al4V alloy is slightly better than that of the LSFed state, and is ascribed to the dominant galvanic effect of fine α/β lamellae accelerating the corrosion dissolution rate of the LSFed sample. The size, morphology, and content of the α/β phase and Al segregation affect the anodic dissolution behavior by promoting/inhibiting the galvanic effect. The galvanic effect of duplex Ti-based alloys is the key factor determining the anodic dissolution performance and dissolved surface quality. The distinction in electrochemical dissolution behavior on Ti6Al4V alloy produced by LSF and forging provides a mechanism support on their difference of electrochemical machining machinability. • Effect of processing method on electrochemical dissolution performance of Ti6Al4V were revealed. • Fine basketwave microstructure has inferior corrosion resistance but better roughness than coarse equiaxed α phase. • Galvanic effect determines the corrosion resistance and dissolved surface quality of Ti6Al4V alloy.

Mitigating the Effects of Stray-Current Attack on Non-Machined Surfaces in Electrochemical Machining Through Gas-Shielding in C6H5K3O7 Solution

Electrochemical machining (ECM) is a promising method for processing the leading and trailing edges of the blades that are composed of nickel-based superalloys (Inconel 718), but often leads to unwanted stray corrosion on non-machined surfaces. This study investigates the use of a novel C6H5K3O7 solution to fabricate the leading and trailing edges of the blade that is free of stray corrosion. First, the basic electrochemical dissolution behavior of Inconel 718 in the C6H5K3O7 solution is analyzed in comparison with that in NaNO3 solution by means of a potentiodynamic polarization. The results showed that Inconel 718 in the C6H5K3O7 solution has a lower breakdown potential and the structure of its passive film was looser than that in the NaNO3 solution. Second, the current efficiency of Inconel 718 in the C6H5K3O7 solution exhibited non-linear dissolution behavior, and the material removal rate was much lower than that in the NaNO3 solution. In-situ observations indicated that a stable and continuous insulating gaseous layer was induced surrounding the anode surface through C6H5K3O7 solution to suppress the stray-current attack on the non-machined surfaces. Third, corresponding models and simulations of ECM process in the C6H5K3O7 solution were formulated to examine the effects of gaseous layer on stray corrosion. Fourth, the effects of the parameters of pulse processing on stray corrosion on the non-machined surfaces was analyzed. Experimental results showed that the self-induced gaseous layer could suppress stray machining by insulation effect, and a pulsed current with a short-pulse duration and a high-pulse frequencies was helpful for reducing the zones of stray corrosion. Precise structure of the leading/trailing edges of the twisted blade free of stray corrosion were successfully fabricated in the C6H5K3O7 solution. In comparison with that manufactured in the NaNO3 solution, the zones of stray corrosion was reduced by 8.4 times under the same processing parameters. The results here verify the feasibility of C6H5K3O7 as an electrolyte to reduce the stray-current attack on non-machined surfaces.

Investigation of electrochemical dissolution behavior of Ni(γ)/Ni3Al(γ') and Co(γ)/Co3Al(γ') superalloys in NaNO3 solution

The electrochemical dissolution behavior of Ni(γ)/Ni3Al(γ'), Co(γ)/Co3Al(γ') in NaNO3 electrolyte solution was investigated. Experimental results showed that Al, W, and Ni ions in the γ' phase destroy the dense oxide film formed by Cr ions in the γ phase. The different dissolution rates of the γ' and γ phases led to selective corrosion on the surface of Ni(γ)/Ni3Al(γ'). Cobalt ions reacted with the dense Cr2O3 passivation film to form porous CoCr2O4, and the corrosion resistance of the Co(γ)/Co3Al(γ') passivation film worsened due to bromination by Br− ions. Qualitative models for the ECM of the Ni(γ)/Ni3Al(γ') and Co(γ)/Co3Al(γ') are proposed.

Study of the electrochemical dissolution behavior of Nitinol shape memory alloy in different electrolytes for micro-ECM process

Nickel-titanium alloy (Nitinol) is an excellent shape memory alloy (SMA) for microelectro-mechanical systems (MEMS) particularly in biomedical applications owing to its three excellent features shape memory effect (SME), superelasticity, and biocompatibility. The temperature-dependent material transformation properties of Nitinol SMAs make conventional machining difficult. Micro-ECM, a non-conventional machining process for conductive materials regardless of their strength and hardness, has the potential to fabricate microfeatures on Nitinol. This study presents the investigation on the electrochemical dissolution behavior of Nitinol in different electrolytes for micro-ECM. The influence of electrolytes on the nature of dissolution of Nitinol was studied by fabricating microchannels in three levels of parameters containing applied voltage and electrolyte concentration. The first three electrolytes were all aqueous neutral electrolytes, i.e., NaCl, NaNO3, and NaBr. The aqueous NaNO3 was successful in fabricating microchannels at all levels of process parameters. However, aqueous electrolytes form relatively a huge amount of sludge on the machined surface reducing both dissolution efficiency and machining accuracy. Thus, ethylene glycol-based NaNO3 was used to fabricate microchannels with lower depth overcut (DOC), width overcut (WOC), and length overcut (LOC) with respect to aqueous NaNO3 electrolyte. The potentiodynamic polarization (PDP) tests of Nitinol through cyclic voltammetry (CV) shows passive dissolution in aqueous electrolyte and active dissolution in non-aqueous electrolyte.

The Anisotropic Electrochemical Machinability of Laser Cladding Deposited Ti6Al4V Alloy in NaCl Solution

The hybrid manufacturing method of laser cladding deposition (LCD) additive manufacturing and electrochemical machining (ECM) is a promising approach to advanced manufacturing technology for difficult machined materials. The anisotropic electrochemical performance of LCD-produced Ti6Al4V alloy was studied in 15 wt.% NaCl solution by polarization curve measurements and ECM tests. The horizontal-plane (X0Y plane) exhibits a more stable passive film in both static electrolyte and low current density ECM processes than the vertical-plane (X0Z plane). Additionally, the horizontal-plane exhibits a higher material removal rate and more consistent dissolved surface roughness in comparison with the vertical-plane during the high current density ECM process. The microstructure of the LCD-produced Ti6Al4V alloy on the horizontal-plane consisted of equiaxed-like prior-β grains and slightly finer α-laths but was composed by columnar prior-β grains and coarser α-laths on the vertical-plane. These differences in the microstructural characteristics produce the distinctions observed in the electrochemical dissolution behavior and electrochemical machinability on the horizontal- and vertical-planes.

Investigation of Electrochemical Dissolution Behavior of Near-α TA15 Titanium Alloy in NaCl Solution with Low-Frequency Pulse Current

TA15 material is a typical near-α titanium alloy and widely used for the aircraft key load bearing components. Electrochemical machining (ECM) is a cost-effective method to machine difficult-to-cut TA15. Due to the high chemical reactivity of titanium, titanium alloy is prone to passivation, which increases the difficulty of ECM, especially for some common ECM methods with low-frequency pulse currents. To investigate the change of surface characteristics of titanium alloy in pulse ECM, the dissolution behavior of TA15 in NaCl solution under the low frequency pulse current was analyzed by scanning electronic microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and electrochemical impedance spectroscopy (EIS). The results showed that even after removal of initial air-formed oxide film, TA15 would be passivated to form a new passive film during long pulse-off time. Under short pulse-on time, the passive film formed in pulse-off time could not be effectively removed in one pulse-on time, and the passivation effect would be accumulated, resulting in poor machined surface with humps and pits. When the pulse-on time was long enough to completely remove the newly formed passive film, TA15 was dissolved normally to obtain excellent surface with clear crystal structure.

Study of the influence of the preparation method on the electrochemical dissolution behavior of Inconel 718

Study of the influence of the preparation method on the electrochemical dissolution behavior of Inconel 718

Effect of pulse frequency and duty cycle on electrochemical dissolution behavior of multi-tip array tool electrode for reusability in the ECDM process

Effect of pulse frequency and duty cycle on electrochemical dissolution behavior of multi-tip array tool electrode for reusability in the ECDM process

Cathode shape prediction for uniform electrochemical dissolution of array tools for ECDM applications

ABSTRACT The effect of various cathode shapes, i.e., flat-plate, conical, and spherical-shaped cathodes, on the electrochemical dissolution (ECD) behavior of the 3 × 3 area-array tool electrode is presented. The allowed variation in all tip heights is less than 30 μm. Initially, the finite element method (FEM) was used to predicts the reduction in size and heights of the 3 × 3 tips. The spherical shape cathode exhibited uniform current density distribution around the tips, resulting in the consistent dissolution for a longer time than other cathode shapes. Detailed experiments were also performed to predict the dissolution behavior. Experimental results confirmed a similar dissolution behavior. The flat-plate electrode was not suitable in the ECD of array tool electrodes as the difference between the outermost and central tip heights was more than 130 µm, while the difference was less than 30 µm with a spherical-shaped cathode. Electrochemical discharge machining (ECDM) was carried out using the area-array electrodes made by the spherical and flat plate cathodes. The energy channelized by the uniform tips during the ECDM resulted in uniform material removal beneath all the tips, resulting in consistent crater formation. In contrast, a single crater was formed when the non-uniform tips were used.

РАСТВОРЕНИЕ НЕРЖАВЕЮЩЕЙ СТАЛИ В РАСТВОРЕ ХЛОРИДА НАТРИЯ ПРИ ПОЛЯРИЗАЦИИ НЕСТАЦИОНАРНЫМИ ТОКАМИ

Stainless steel is in great demand due to its mechanical strength, heat resistance, and resistance to corrosive environments. This article presents the result of a study of the electrochemical dissolution behavior of a stainless steel electrode (12X18H10T) at polarization by 50 Hz alternating current in a neutral medium (NaCl). Preliminary experiments have shown that the main processes do not take place when two stainless steel electrodes are polarized with an alternating current. It was observed that by the polarization of the “stainless steel – titanium” pair electrodes with alternating current, the alloy is intensively dissolved with the formation of iron (II) and chromium (III) ions. This is due to the "valve" properties of the oxide layer formed on the surface of the titanium electrode. A change in the value of the current density of the titanium and steel electrodes significantly affects the process of electrochemical dissolution of the alloy. At high current densities, the dissolution rate of the alloy decreases due to the deterioration of the current correcting properties of titanium. With an increase in the electrolyte concentration, the current efficiency is reduced as salt passivation occurs. A maximum value of the current efficiency of dissolution of stainless steel was observed at a current frequency of 50 Hz. High frequencies of the alternating current do not provide an adjustable duration of the anodic half-cycle for the oxidation reaction due to the frequent change of halfcycles of the alternating current. It was observed that increasing the temperature of the electrolyte reduces the current efficiency of the electrochemical dissolution of stainless steel electrodes. The effects of main electrochemical parameters on the electrolysis process have been investigated, and the optimal conditions of the alloy dissolution were established (іТі = 60 kA/m2 , іSS = 800 A/m2 , [NaCl] = 2.0 M, t = 30, ν = 50 Hz.).

Effect of electrolyte solutions on the electrochemical dissolution behavior of additively manufactured Hastelloy X superalloy via laser solid forming

The electrolyte solution significantly affects electrochemical machining. This study investigates the electrochemical dissolution behavior of a laser solid formed Hastelloy X nickel-based superalloy in 15 wt% NaNO3, 7.5 wt% NaNO3 + 7.5 wt% NaCl, and 15 wt% NaCl solutions. The Cl- ions in the electrolyte solution accelerates the passive dissolution rate because the electrochemical reactions at the metal/film and film/solution interfaces are promoted. During the transpassive dissolution process, the Cl- ions in the electrolyte solution weaken the hindering effect of the transpassive film on the dissolution process, resulting in a faster transpassive dissolution rate and a higher current efficiency. Furthermore, the flatness of the dissolved surface in the chloride-containing solution is significantly higher than that in the chloride-free solution. Two models are proposed to illustrate the passive and transpassive dissolution behaviors. This study provides theoretical guidance for the selection of electrolyte solutions for the electrochemical machining of laser solid formed nickel-based superalloys.

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.

Morphological effects on the electrochemical dissolution behavior of forged and additive manufactured Ti-6Al-4V alloys in runway deicing fluid

The particular microstructure of Ti-6Al-4V fabricated by additive manufacturing (AM) makes it exhibit a different corrosion resistance compared with that of the conventional forged Ti-6Al-4V. Here we utilize the electrochemical impedance (EIS) and potentiodynamic polarization (Tafel) to evaluate the electrochemical dissolution behavior in runway deicing fluids (RDF) of three kinds of Ti-6Al-4V alloy, which are respectively fabricated by forging, electron beam melting (EBM), and selective laser melting (SLM). The microstructure and electrochemical results suggest that EBM samples have a better corrosion resistance than others', which is attributed to an appropriate proportion of α-lath width and the volume fraction of β phase. A higher relative proportion of TiO2 is formed on surface, resulting in the fine-lamellar structure with a higher density of grain boundaries. Besides, the corrosion resistance anisotropy of AMed Ti-6Al-4V samples is ascribed to the existence of β columnar gains. In addition, the acicular martensite α′ phase with metastable structure leads to the worst corrosion resistance of SLMed Ti-6Al-4V samples. This work enriches the corrosion resistance of Ti-6Al-4V alloy in strong electrolyte solutions, which helps to promote the service performance of aviation titanium alloys in harsh environments.

Electrochemical dissolution behavior of gold and its main coexistent sulfide minerals in acid thiocyanate solutions

Electrochemical tests were developed to investigate the electrochemical dissolution behavior of gold and its main coexistent sulfide minerals in acid thiocyanate solutions. The optimal leaching conditions for gold in acidic thiocyanate system were pH 2, 0.15 mol·L−1 thiocyanate and 0.2 g·L−1 Fe3+. Fe3+ addition to the acidic thiocyanate system promoted gold dissolution significantly, arsenopyrite dissolution was inhibited, chalcopyrite dissolution was increased, and the dissolution behavior of other associated minerals remained mostly unchanged. Thiocyanate made gold and associated mineral leaching easier. The galvanic corrosion effect of gold and its main coexistent sulfide minerals in an acidic thiocyanate-free system was that the chalcocite, arsenopyrite and pyrite acted as a cathode to reduce anodic gold dissolution; galena as an anode undergoes oxidation to inhibit anodic gold dissolution. There was almost no galvanic corrosion behavior between stibnite, yellow sphalerite and black sphalerite and gold. Thiocyanate addition changed the galvanic corrosion behavior of stibnite and yellow sphalerite in the thiocyanate system, which inhibited anodic gold dissolution. In the acidic thiocyanate system in the presence of ferric iron, the arsenopyrite promoted anodic gold dissolution, the chalcocite and gold were mostly free of galvanic corrosion, and the remaining minerals inhibited anodic gold dissolution.

Electrochemical dissolution behavior of S-04 high-strength stainless steel in NaNO3 aqueous solution

The excellent strength, toughness, and corrosion resistance of high-strength stainless steel has led to its extensive use in the chemical, nuclear, aerospace, and aviation industries. To shape this hard-to-cut material, electrochemical machining is a promising process. However, few reports have focused on the electrochemical dissolution behavior of high-strength stainless steel. Therefore, we focus herein on the electrochemical dissolution behavior of S-04 high-strength stainless steel in NaNO3 aqueous solution. The current efficiency indicates that current density can be divided into three regions (I, II, and III). Interestingly, a black machined surface appears in regions II and III, and the machined surface is shiny for region III. X-ray photoelectron spectroscopy indicates that the black machined surface is mainly due to the presence of Fe3O4. In addition, γ-Fe2O3 was also detected on the black machined surface in region III, whereas both γ-Fe2O3 and α-Fe2O3 were also detected on the black machined surface in regions I and II. Moreover, dissolution morphologies examined by SEM/EDX at different current densities and machining times showed that a good surface finish of the S-04 specimen can be obtained in region III. Finally, grooves with black surfaces in regions II and III were obtained by electrochemical milling.

Improved high-temperature cyclability of AlF3 modified spinel LiNi0.5Mn1.5O4 cathode for lithium-ion batteries

In this current work, high-voltage spinel LiNi0.5Mn1.5O4 (LNMO) decorated with AlF3 thin layer was successfully synthesized and investigated for the application in Li-ion batteries (LIBs). The electrochemical properties and dissolution behavior of transition metals of the AlF3-modified LNMO electrodes were investigated, especially at elevated temperature. The well-defined and homogeneous AlF3 coating on the surface of LNMO was conducted by a facile chemical modification approach, in which the thickness of AlF3 modification layer was adjusted by adjusting the AlF3 precursor concentrations. After the optimization of the AlF3 precursor concentration, the 1 wt% AlF3-modified LNMO electrode revealed the remarkably improved cyclability especially at elevated temperature. The 1 wt% AlF3-modified LNMO electrode displayed an impressive capacity retention up to 81.7% after charged/discharged at 0.2C at elevated temperature for 100 cycles, which was superior to that of the pristine LNMO electrode (70.1%). Its improved cyclability can be ascribed to the efficient suppression of electrolyte decomposition reaction and transition metal dissolution, thus signaling promising potential in high-voltage LIBs applications.

Surface morphology and electrochemical behaviour of Ti-48Al-2Cr-2Nb alloy in low-concentration salt solution

Electrochemical machining (ECM) is becoming increasingly important for the efficient machining of parts with a large machining area. This is an addition challenge for ECM because of the very high machining current. To overcome this difficulty, a direct and effective strategy is to adopt the machining mode that uses a low-concentration electrolyte with a low current density. The purpose of this study is to reveal the electrochemical behaviour and surface morphology in low-concentration electrolyte. The polarization behavior of Ti-48Al-2Cr-2Nb is measured by linear sweep voltammetry and cyclic voltammetry curves. The ηω-j curves demonstrate the special dissolution behaviour of Ti-48Al-2Cr-2Nb at low current densities. The surface morphology, surface quality, and dissolution mechanism are analysed in three low-concentration electrolytes at different current densities after the ECM dissolution experiments. The results demonstrate that Ti-48Al-2Cr-2Nb exhibits three unique dissolution morphologies in the three solutions, and we found that the γ-TiAl phase dissolves faster than the α2-Ti3Al phase. These results also show that 1% NaCl solution is more suitable for Ti-48Al-2Cr-2Nb in ECM compared with the other two solutions, considering its good surface quality, low breakdown potential, and high material removal rate. Later, the dissolution process of the sample in 1% NaCl solution at different corrosion times is revealed. Moreover, a dissolution model is proposed for the electrochemical dissolution behaviour of Ti-48Al-2Cr-2Nb in 1% NaCl solution.

Electrochemical dissolution behavior of heat treated laser solid formed Inconel718

Microstructure characteristic and its effects on electrochemical dissolution behavior of horizontal plane (HP) and vertical plane (VP) in the laser solid formed (LSF) Inconel718 alloy after double aging (DA) heat treatment (DA-LSF Inconel718) were investigated. Compared to the as-deposited one, DA-LSF Inconel718 shows more phases and presents electrochemical anisotropy between HP and VP including the polarization curves, current efficiency curves and surface roughness. It is caused by different electrochemical dissolution processes on HP and VP and proved that surface energy has no obvious effect. Three models were proposed to illustrate electrochemical dissolution behavior, surface formation mechanism and electrochemical anisotropy, respectively.

Investigation of electrochemical properties of electropolishing Co–Cr dental alloy

Electropolishing is a promising method for the post-treatment of the dental material Co–Cr alloy compared with the currently reported laser polishing by considering the damage-free surface and simplicity of the electropolishing method. The electrochemical properties of electropolishing the Co–Cr dental alloy is a critical issue in the process development. The electrochemical properties are first investigated by measuring the open circuit potential and the polarization curves and a limiting current density plateau region is identified, where the electropolishing is clarified. With the different applied potentials along the limiting current density plateau region, the electrochemical impedance spectroscopy spectra are studied to understand the mass transport limitation mechanism. The compact salt film model is confirmed to be responsible for the mass transport limitation in the electropolishing. Samples are electropolished with a constant applied potential, and a selective dissolution is achieved between the matrix phase and precipitate phase according to the electropolished surface topography. Moreover, the composition of the electropolished surface is analysed by the X-ray photoelectron spectroscopy spectra. Finally, a qualitative model is established to illustrate the electrochemical dissolution behaviour of the Co–Cr alloy electropolishing in the phosphoric acid-containing electrolyte.