Electropolishing Of Metals Research Articles

Coupled Pulse/Pulse-Reverse Electrochemical Machining and Electrowinning for Metal Recovery, Elimination of Waste, and Minimization of Water Usage

Electrochemical machining (ECM) is well suited for machining “difficult to cut” materials and/or parts with complicated and intricate geometries difficult to produce by mechanical or electric-discharge fabrication methods. Conventional ECM processes typically operate with neutral salt electrolytes, in which the machined material is precipitated from the ECM electrolyte as hydrated metal oxide/hydroxide sludge. This sludge is generally transported off-site and disposed in a landfill. Depending on the alloying components in the machined metal, it may be necessary to designate the sludge as a hazardous material and landfill it according to much stricter requirements. Furthermore, the volume of sludge generated by ECM of many common alloys in pH-neutral solutions is typically more than 300 times greater than the amount of metallic material removed, requiring the use of a high-CAPEX and -OPEX sludge management equipment train. As recently noted, the generation and disposal of the sludge is a major impediment to the widespread use and implementation of ECM operations [1], especially for smaller fabrication operations seeking to add or expand ECM process capability.This talk will present work on development of “Recycling ECM” [(R)ECM], a novel, patented [2] form of ECM processing coupled with electrowinning (EW), where the machined material remains in soluble form, typically through the use of acidic or buffered-acid solutions, and is directly recovered from the electrolyte via EW without intermediate processing (see Figure 1, left). Significant challenges have been reported in literature in attempts to use direct current ECM in such electrolytes [3], including degraded workpiece surface finish and problematic deposition of dissolved metals on the ECM tooling. Faraday has previously demonstrated the ability of pulse current/pulse reverse current (PC/PRC) waveforms to electrochemically machine materials while achieving a high-quality surface finish [4,5], and our prior experience suggests that selection of a suitable ECM waveform should avoid the adventitious electrodeposition challenges observed in the literature. Figure 1 (right) presents a notional pulse-reverse waveform annotated with key timing and electric fiend intensity parameters, and enumerates some of the mechanisms of control afforded by the pulse-reverse processing paradigm. The enhanced control of the ECM process enabled by the PRC waveform enables operation with a variety of electrolytes, and also enables metal recovery via electrowinning from electrolytes which are not feasible to process with direct current approaches.In developing the (R)ECM process for a particular material, we first select an electrolyte in which the machined material is soluble, to avoid sludge formation, and from which electrodeposition of compact metallic films is feasible. The ECM and EW unit operations are coupled into a complete electrolyte circulation loop and their operating parameters adjusted to maintain soluble metal concentration(s) in a range where ECM is not adversely affected and EW is efficient and economical. Consequently, metals are recovered, sludge waste is avoided and water usage is minimized since losses are limited only to evaporation, adventitious electrolysis, and panel drag-out.After introducing the (R)ECM concept and motivation, this talk will summarize data gathered from operation of the (R)ECM process, wherein soluble metal concentration(s) were maintained in a pre-determined range by adjustment of ECM/EW operating parameters. Additionally, a brief overview will be provided of a β-scale electrowinning system Faraday developed and installed at the U.S. Army Benét Laboratories to enable (R)ECM operations there. This system is sized to recover up to 0.5 m3 per year of metal from an existing ECM operation, and will assist Benét Laboratories in their efforts toward meeting the U.S. Army’s “Vision for Net Zero” [6].The authors acknowledge financial support from U.S. Army Contract Nos. W15QKN-12-C-0010 and W15QKN-12-C-0116, and US EPA Contract No. EP-D-13-040.Figure 1. (Left) Schematic of a possible (R)ECM implementation for an alloy machined primarily to divalent metal cations. (Right) Schematic of pulse-reverse ECM waveform and process factors addressed by the various waveform components. [1] K.P Rajurkar, D. Zhu, J.A. McGeough, J. Kozak, A. De Silva “New Developments in Electro-Chemical Machining” Annals of the CIRP Vol 48(2) (1999). [2] U.S. Patent No. 9,938,632, granted 10 Apr 2018. U.S. Patent No. 10,214,832, granted 26 Feb 2019. [3] Wessel, “Electrochemical Machining of Gun Barrel Bores and Rifling,” Naval Ordnance Station, Louisville KY, September 1978. http://handle.dtic.mil/100.2/ADA072437. [4] C. Zhou, E.J. Taylor, J.J. Sun, L.E. Gebhart, R.P. Renz. “Electrochemical Machining Using Modulated Reverse Electric Fields.” U.S. Patent No. 6,402,931, 11 June 2002. [5] E.J. Taylor. “Sequential Electromachining and Electropolishing of Metals and the Like Using Modulated Electric Fields.” U.S. Patent No. 6,558,231, 6 May 2003. [6] “Vision for Net Zero” http://army-energy.hqda.pentagon.mil/programs/netzero.asp. Figure 1

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).

Development of an Electropolishing Process for Additively Manufactured 316L Parts Under Direct and Pulsed Potential Control

The initial purpose of additive manufacturing was mainly rapid prototyping, but all industrial sectors have adopted this technique that has increased in profitability and allows innovative shapes in almost all alloys. Advances in technology have led to AM components whose mechanical and structural properties are equivalent to machined parts. However, a major impediment to the extension of the process is due to the bad surface finish of the as built parts, characterized by a high roughness (Ra up to 40 µm) and the presence of un-melted particles and weld beads. These defects are incompatible with the planned applications (like inner channels for fluid flow), and also induce a degradation of the material functional properties such as an increased sensitivity to fatigue and corrosion. Electropolishing, an electrochemical method for planarization of metals based on their anodic dissolution in an appropriate electrolyte, is a solution to consider. During this work a specific electropolishing process dedicated to 316 L AM parts was developed with the originality of being under potentiostatic control [1,2]. First, a preliminary study done at laboratory scale in a three electrodes electrochemical cell, allowed us to determine the process parameters including the working potential range. Then, the same type of arrangement was subsequently up-scaled in a 3 L reactor in order to electrochemically polish bigger parts ( ≈40 cm2). The potential control is made possible thanks to the introduction of a reference electrode in the pilot and by the use of a high current/high voltage potentiostat able to monitor both direct and pulse potentials. When transposing the process at pilot scale, new difficulties are emerging, due to the large initial roughness of the parts. If a satisfactory roughness decrease is easily achievable in less than one hour, it goes with an inhomogeneous loss of dimension. This geometric deformation of the parts needs to be considered as least as much as roughness parameters. In this context, pulsed potential are emerging as an attractive way to performed electropolishing, by controlling the dissolution while getting interesting final surface properties and limited deformation [3]. Samples present a very bright finishing and low roughness values (Fig.1) In this study a special attention was paid to the quantification of the deformation on especially designed grooved test samples by 3D optical microscopy and laser scanning [4]. Acknowledgments : the authors would like to thank the IRT-M2P program AFTER ALM for its financial support. [1] Rotty C., Mandroyan A., Doche M.-L., Hihn J. Y., Surf. Coat. Technol., 307 Part A, 125–135 (2016) [2] Rotty C, Doche M.-L, Mandroyan A.,Hihn J-Y., Montavon G., Comparison of electropolishing behaviours of TSC, ALM and cast 316L stainless steel in H3PO4/H2SO4, Surfaces and Interfaces, 6, 170-176 (2017) [3] Taylor EJ., Sequential electromachining and electropolishing of metals and the like using modulated electric fields, US Patent 6,558,231 (2003) [4] C.M. Sulyma, P.C Goonetilleke, D.Roy, Journal of materials processing technology, 1189-1198 (2009) Figure 1

Tandem Pulse/Pulse-Reverse Electrochemical Machining and Electrowinning for Metal Recovery, Elimination of Waste, and Minimization of Water Usage

Electrochemical machining (ECM) is well suited for machining parts fabricated from “difficult to cut” materials and/or parts with complicated and intricate geometries. Conventional, direct-current (DC) ECM typically operates with neutral salt electrolytes, in which the machined material is precipitated from the machining electrolyte as hydrated metal oxide/hydroxide sludge. This sludge is generally transported off-site and disposed in a land-fill. Depending on the alloying components in the machined metal, the sludge must often be designated as a hazardous material. Furthermore, the volume of sludge generated by ECM of many common alloys in pH-neutral solutions is more than 300 times greater than the amount of metallic material removed. As recently noted, the generation and disposal of the sludge is a major impediment to the widespread use and implementation of ECM operations [[1]]. In this talk, Faraday will present recent work on development of “Recycling ECM” [(R)ECM], a novel, patent pending [[2]] form of ECM processing coupled with electrowinning (EW), where the machined material remains in soluble form, typically through the use of acidic or buffered-acid solutions, and is directly recovered from the electrolyte without intermediate processing. Literature reports significant challenges with direct current ECM in such electrolytes [[3]], including poor surface finish and adventitious metal deposition on the ECM tooling. Faraday has previously demonstrated the ability of pulse current/pulse reverse current (PC/PRC) waveforms to electrochemically machine materials while achieving a high quality surface finish [[4],[5]], and our prior experience also indicates that proper selection of an ECM waveform should simultaneously avoid the tool deposition observed in the literature. The accompanying figure presents a schematic of a pulse-reverse waveform and enumerates some of the mechanisms of control afforded by the pulse-reverse processing paradigm. The PRC waveforms enable control of the ECM process and consequently ECM with PRC is adaptable to a variety of electrolytes. As well, PC/PRC electrowinning enables metal recovery from electrolytes which are not feasible to process with direct current approaches. In developing the (R)ECM process for a particular material, we first select an electrolyte in which the machined material is soluble, to avoid sludge formation by precipitation, and from which we are able to deposit it in compact, metallic form. The ECM and EW unit operations are coupled into a complete electrolyte circulation loop and their operating parameters are adjusted to keep the soluble metal concentration in a range where the ECM process is not adversely affected and the EW operation is efficient and economical. Consequently, metals are recovered, sludge waste is avoided and water usage is minimized since losses are limited only to evaporation, adventitious electrolysis, and panel drag-out. In the presentation, after introducing the (R)ECM concept and its motivation, we will summarize the initial electrolyte selection studies for C18000, SAE4150, IN718 and SS316L materials. In addition, we will present data from integrated operation of the (R)ECM process, where the soluble metal concentration is maintained in a pre-determined range by suitable adjustments of the operating parameters of the ECM and/or EW unit operations. Finally, we will provide a brief overview of a β-scale electrowinning system Faraday has developed and installed at the U.S. Army Benét Laboratories to enable (R)ECM operations there. This system is sized to recover up to 0.5 m3 per year of metal from an existing ECM operation, and will assist Benét Laboratories in their efforts toward meeting the U.S. Army’s “Vision for Net Zero” [[6]]. The authors acknowledge the financial support of U.S. Army Contract Nos. W15QKN-12-C-0010 and W15QKN-12-C-0116, and US EPA Contract No. EP-D-13-040. References [[1]] K.P Rajurkar, D. Zhu, J.A. McGeough, J. Kozak, A. De Silva “New Developments in Electro-Chemical Machining” Annals of the CIRP Vol 48(2) (1999). [[2]] E.J. Taylor, M.E. Inman, B.T. Skinn, T.D. Hall, S.T. Snyder, S.C. Lucatero, E.L. Kathe. “Apparatus and Method for Recovery of Material Generated During Electrochemical Material Removal in Acidic Electrolytes.” U.S. Patent Application No. 2016/0230303 A1, 11 Aug 2016. [[3]] Wessel, “Electrochemical Machining of Gun Barrel Bores and Rifling,” Naval Ordnance Station, Louisville KY, September 1978. http://handle.dtic.mil/100.2/ADA072437. [[4]] C. Zhou, E.J. Taylor, J.J. Sun, L.E. Gebhart, R.P. Renz. “Electrochemical Machining Using Modulated Reverse Electric Fields.” U.S. Patent No. 6,402,931, 11 June 2002. [[5]] E.J. Taylor. “Sequential Electromachining and Electropolishing of Metals and the Like Using Modulated Electric Fields.” U.S. Patent No. 6,558,231, 6 May 2003. [[6]] “Vision for Net Zero” http://army-energy.hqda.pentagon.mil/programs/netzero.asp. Figure 1

Accelerated Electrochemical Machining Tool Design Via Multiphysics Modeling

Electrochemical machining (ECM) is a manufacturing technology that allows metal to be precisely removed by electrochemical oxidation and dissolution into an electrolyte solution. ECM is suited for machining parts fabricated from “difficult to cut” materials and/or parts with complicated and intricate geometries. In ECM, the workpiece is the anode and the tool is the cathode in an electrochemical cell; by relative movement of the shaped tool into the workpiece, the mirror image of the tool is “copied” or machined into the workpiece. Figure 1 shows a schematic and photograph of a shaped ECM tool (cathode) and the resulting part (anode) surface after machining. Compared to mechanical or thermal machining processes where metal is removed by cutting or electric discharge/laser machining, respectively, ECM does not suffer from tool wear or result in a thermally damaged surface layer on the workpiece. Consequently, ECM has strong utility as a manufacturing technology for fabrication of a wide variety of metallic parts and components, and includes machining, deburring, boring, radiusing and polishing processes. ECM provides particular value in that application is straightforward to high strength/tough and/or work-hardening materials such as high strength steel, chrome-copper alloy (C18200), nickel alloy (IN718), cobalt-chrome alloy (Stellite 25) and tantalum-tungsten alloy (Ta10W), since the material removal process involves no mechanical interaction between the tool and the part. A variety of commercial and military production applications are envisioned as well suited for ECM techniques. One notable difficulty with ECM, common to a variety of manufacturing operations, is an inability to predict a priori the tool and process parameters required in order to satisfy the final specifications of the fabricated part. In this talk, Faraday will present preliminary results from a Phase I SBIR program aiming to demonstrate the potential for a phenomena-based design platform to predict optimal ECM tool shape using commercially available multiphysics simulation software. This capability is anticipated to dramatically shorten the process/tooling development cycle, eliminating much or all of the iterative prototyping necessary in the absence of a predictive tool. The validation argument will be presented via comparison of simulation results with data from parallel ECM experiments. The initial validation work encompasses a small subset of experimental parameters: electrolyte salt (“active” NaCl vs “passive” NaNO3) and electrolyte flow/tool geometry (“cross flow” past a solid tool, and “through-flow” in a tubular tool). The initial feasibility demonstration of the tool design platform will include only ECM based on potentiostatic direct currents, constant tool advancement rates, and simulations including only primary current distributions. Reasonable agreement between the simulated and experimentally derived profiles is observed. Eventually, however, the goal is to extend the model to other modes of ECM processing, including: 1) pulse-current ECM (PECM), where the tool is withdrawn from the workpiece during pulse current off-times to flush the gap; and also 2) pulse/pulse-reverse ECM (P/PR ECM) in metal-solubilizing electrolytes, where the tool gap is maintained relatively constant. An approach to modeling the optimal inter electrode gap dimension during PECM has been reported by Rajurkar and collaborators [[1]]; to our knowledge no modeling has been performed on constant-gap pulse/pulse-reverse ECM in solubilizing electrolytes. Faraday has developed and patented novel approaches to ECM based on pulse reverse currents [[2],[3],[4]] that do not require complicated electrolytes and results in improved surface finishes and better process control; application of the predictive model to these approaches is anticipated to provide substantial gains in both manufacturing logistics and economics. The authors acknowledge the financial support of U.S. Army Contract No. W15QKN-16-C-0070. References [[1]] B. Wei, K.P. Rajurkar, S. Talpallikar. “Identification of Interelectrode Gap Sizes in Pulse Electrochemical Machining.” J. Electrochemical Society 144(11): 3913-19, 1997. [[2]] C. Zhou, E.J. Taylor, J. Sun, L. Gebhart, R. Renz. “Electrochemical Machining using Modulated Reverse Electric Fields.” U.S. Patent No. 6,402,931, issued 11 June 2002. [[3]] E.J. Taylor. “Sequential Electromachining and Electropolishing of Metals and the like using Modulated Electric Fields.” U.S. Patent No. 6,558,231, issued 6 May 2003. [[4]] E.J. Taylor, M. Inman. “Method and Apparatus for Pulsed Electrochemical Grinding.” U.S. Patent No. 9,403,228, issued 2 August 2016. Figure 1

Sustainable Electrochemical Machining for Metal Recovery, Elimination of Waste, and Minimization of Water Usage

Faraday is developing and installing a β-scale Recycling Electrochemical Machining ((R)ECM) system at the U.S. Army Benet Laboratory. The (R)ECM system is sized to recover up to 0.5 m3 per year of metal from an electrochemical machining operation. The metals of initial interest include: 1) beryllium-free nickel-copper alloy (C18000), 2) Cr-Mo alloy steel (SAE4150), 3) nickel alloy (IN718) and 4) tantalum tungsten (Ta10W) alloy. The (R)ECM system couples an electrochemical machining unit operation with an electrowinning (EW) unit operation for recovery of metals, elimination of waste, and minimization of water usage. Consequently, the RECM system is consistent with the U.S. Army’s “Vision for Net Zero”[1]. Electrochemical machining (ECM) is suited for machining parts fabricated from “difficult to cut” materials and/or parts with complicated and intricate geometries. Conventional, direct-current (DC) ECM typically operates with neutral salt electrolytes whereby the machined material is generally precipitated from the machining electrolyte as metal oxide/hydroxide sludge. This sludge is generally transported off-site and disposed in a land-fill. Depending on the alloying components in the machined metal, the sludge is often designated as a hazardous material. Furthermore, the rifling of a gun barrel by ECM generates ~350 gal of centrifuged sludge per ~250 in3 of material removed [2]. This represents greater than 300X volume multiplier of waste sludge relative to the amount of material removed. As recently noted, the generation and disposal of the sludge is a major impediment to the widespread use and implementation of ECM operations [3]. Faraday has previously demonstrated the ability of pulse current/pulse reverse current (PC/PRC) waveforms electrochemically machining materials while achieving a high quality surface finish (Figure 1) [4],[5]. Smooth surfaces are typically not possible in conventional DC ECM. The PRC waveforms control the ECM process and consequently ECM with PRC is adaptable to a variety of electrolytes. To develop a (R)ECM process, we select an electrolyte whereby the machined material is soluble and avoids sludge formation by precipitation. The ECM unit operation is coupled to an EW unit operation where the machined material is collected as metal. The ECM and EW unit operations are adjusted to keep the soluble metal concentration in a range where the ECM process is not adversely effected and the EW operation is efficient and economical. Consequently, metals are recovered, waste is avoided and water usage is minimized. In the presentation, we will summarize the initial electrolyte selection studies for C18000, SAE4150, IN718 and Ta10W materials. In additionally, we will demonstrate integrated operation of the (R)ECM process where the soluble metal concentration is maintained in a pre-determined range by 1) simultaneous (R)ECM operation by adjusting the ECM unit operation in the presence of constant EW unit operation, 2) simultaneous (R)ECM operation by adjusting the EW unit operation in the presence of constant ECM unit operation, and 3) sequential operation of the ECM and EW unit operations. Finally, the status (R)ECM system design and installation at Benet Laboratories will be reviewed.

Precipitation and surface adsorption of metal complexes during electropolishing. Theory and characterization with X-ray nanotomography and surface tension isotherms.

Electropolishing of metals often leads to supersaturation conditions resulting in precipitation of complex compounds. The solubility diagrams and Gibbs adsorption isotherms of the electropolishing products are thus very important to understand the thermodynamic mechanism of precipitation of reaction products. Electropolishing of tungsten wires in aqueous solutions of potassium hydroxide is used as an example illustrating the different thermodynamic scenarios of electropolishing. Electropolishing products are able to form highly viscous films immiscible with the surrounding electrolyte or porous shells adhered to the wire surface. Using X-ray nanotomography, we discovered a gel-like phase formed at the tungsten surface during electropolishing. The results of these studies suggest that the electropolishing products can form a rich library of compounds. The surface tension of the electrolyte depends on the metal oxide ions and alkali-metal complexes.

Magnetic Fields for Electropolishing Improvement: Materials and Systems

The paper aims to present the main objectives for using magnetic fields to improve process of electropolishing (EP), firstly by focusing on materials and electrochemical systems. The general introduction has been concerned on the sample surface treated under pseudopassivity conditions, in the process generally known as magnetoelectropolishing (MEP). Long-term up-to-date experiments have shown profound changes observed on metals and alloys. The advantageous effects gained by applying MEP to metals and alloys cover: improvement of corrosion resistance, bio- and haemocompatibility, roughness by modification of their surfaces. The improvements are also referred to the mechanical properties of metals and alloys treated by MEP namely: removal of hydrogen, fatigue resistance enhancement, etc. Further developments and the effects of magnetic fields on electropolishing of metals and alloys are to be presented in the next publications.

Magnetic Fields for Electropolishing Improvement: Materials and Systems

The paper aims to present the main objectives for using magnetic fields to improve process of electropolishing (EP), firstly by focusing on materials and electrochemical systems. The general introduction has been concerned on the sample surface treated under pseudopassivity conditions, in the process generally known as magnetoelectropolishing (MEP). Long-term up-to-date experiments have shown profound changes observed on metals and alloys. The advantageous effects gained by applying MEP to metals and alloys cover: improvement of corrosion resistance, bio- and haemocompatibility, roughness by modification of their surfaces. The improvements are also referred to the mechanical properties of metals and alloys treated by MEP namely: removal of hydrogen, fatigue resistance enhancement, etc. Further developments and the effects of magnetic fields on electropolishing of metals and alloys are to be presented in the next publications.

Concept of microsmoothing in the electropolishing process

Electropolishing of metals and numerous alloys can provide smooth, bright and reflective surfaces that exhibit superior corrosion resistance compared with untreated materials. These features are achieved mainly after electropolishing pure metals and single-phase alloys. Earlier researchers have indicated that, in comparison with other treatments (mechanical polishing, chemical etching), electropolishing produces a much thinner surface layer with changed and specific properties. This implies a difference in the solid surface chemistry which is valuable from the viewpoint of the technical usefulness of such surfaces. Experimental investigations carried out on various metals and alloys have proved that both electrochemical and hydrodynamic conditions are responsible for the positive results obtained in the process. The aim of the present work is to provide a new concept of electropolishing covering transport phenomena such as diffusion and adsorption as well as the hydrodynamic conditions.

On the Solution of Equations of Diffusion Presented in Their Paper “Diffusion and Solid‐Film Formation during Electropolishing of Metals”

not Available.

Diffusion and migration in the electropolishing and anodic dissolution of metals

The solution of the Nernst-Planck equation for the electropolishing of metals is presented. The migration and diffusion equations are solved for the cases where the metal ion is transported independently of the anion of the electrolyte and when ion pairing occurs. It is concluded that under polishing conditions, the surface concentration of acid is decreased by the salt solubility. It is shown that this has important practical consequences for defining the conditions required for electropolishing and in the selection of the acid used.

Mass transfer effects in the electropolishing of metals

SummaryThe mechanism of electropolishing of iron and cobalt from methanolic solutions of H2SO4 and 10M o-phosphoric acid is discussed. It is found that it is not necessary to have a duplex film for...

Diffusion and Solid‐Film Formation during Electropolishing of Metals

Various metals and electrolytes were used for potentiostatic electropolishing. When measuring the current‐time behavior over several orders of magnitude and for the different temperatures, three stages were observed: (i) a short stage I , during which the current density was approximately constant; (ii) during the following stage II saturation of the dissolved metal ions (reaction products) was obtained in the vicinity of the anode surface, and the current density decreased with the reciprocal square root of time; and (iii) after sufficiently long times, during stage III a steady‐state current density was approached by an exponential time law. The known theoretical models, based on the work of Elmore, are able to explain the relations between current density, time, and temperature for stage I and II . For stage III a new model is presented which takes convection within the electrolyte into account. The possible formation of a solid film at the transition from stage I to stage II is thought to be responsible for characteristic deviations from the expected current‐time behavior. A mathematical treatment of this formation yields film thicknesses of 3–30 nm.

Electropolishing of metals in alcoholic solution of sulphuric acid

The potentiodynamic polarization curves of iron, nickel, cobalt, molybdenum, copper and iron-nickel and cobalt-nickel alloys were measured in the polishing solution of 1M H 2SO 4 in CH 3OH. The above mentioned metals and alloys may be divided into three groups: metals and alloys with high dissolution rate in the active state in which the polishing follows immediately behind the active dissolution region (Fe, Co and Ni-Fe and NiCo alloys with a low nickel content), metals and alloys with low passivating current density that become passive at first and the polishing takes place behind the region of localized corrosion (nickel and NiFe and NiCo alloys with a high nickel content) are metals that are polished in the transpassive region (molybdenum).

Role of semiconducting films in electropolishing of metals in sulphamic-acid-formamide solutions

In the anodic electropolishing of Cu, Pb, Sn, W, Mo, Cd, Ag, Ti, U, In and Zn in anhydrous sulphamic-acid-formamide solutions, the polishing proceeds via formation of semiconducting anodic films. It is shown that the solid state properties (eg band gaps, lattice energies) of these films may be correlated with electropolishing rates of these metals obtained from the potentiostatic polarization curves of Menzies, Marshall and Griffin. The theoretical significance of these correlations is discussed. The analysis presented is similar in approach to our previous interpretation of anodic dissolution of 23 materials, mostly metals, in anhydrous hydrogen fluoride. The considerations developed here further support the previously presented viewpoint, namely, that in electroinorganic reactions involving solid reaction films, the solid state properties of these films influence profoundly the electrochemical behaviour of film-covered electrodes.

A method for rapid electropolishing of metals for optical metallography

A simple laboratory device for the rapid preparation of high quality polished surfaces from relatively rough metal specimens has been described. Even though, compared with other methods, high current density is used deep polishing takes place without pitting.