Electrochemical machining: The role of steel microstructure in high-rate anodic dissolution

Electrochemical machining (ECM) of metals, particularly steels, plays an important role in many industrial microsystem technologies. This paper presents an overview of investigations into the anodic metal dissolution at high current densities of the technically important carbon steel 100Cr6 in aqueous NaCl- and NaNO₃-electrolytes. The practice of ECM was simulated in flow channel experiments, where high current densities (up to 70 A/cm2) and turbulent electrolyte flow velocities (7 m/s) were applied. Insoluble carbide particles, enriching at the substrate surface, cause an apparent current efficiency > 100% in NaCl and > 67% in NaNO₃ at high current densities. The role of the steel microstructure as controlled by its prior heat treatment is discussed with reference to a qualitative erosion-corrosion model proposed for the anodic dissolution process.     

Fe-30Ni-5NiO alloy as inert anode for low-temperature aluminum electrolysis

Fe-30Ni-5NiO alloy anodes were prepared by a spark plasma sintering process for aluminum electrolysis. NiO nano-particles with the size of ∼20 nm were dispersed in the anodes. The oxidation behaviors of the anodes were investigated at 800°C and 850°C, respectively. The electrolysis corrosion behaviors were tested in a cryolite-alumina electrolyte at a low temperature of 800°C with anodic current densities of ∼0.5 A/cm². The results indicated that the oxidation kinetic of the anodes followed a parabolic law. A continuous Fe₂O₃ film selectively formed on the surface of the anode during the electrolysis process. A semi-continuous Al₂O₃ layer was observed at oxide film/alloy interface, probably caused by an in-situ chemical dissolution process.     

Changes in mass and stress during anodic oxidation and cathodic reduction of the Cu/Cu2O multilayer film

The anodic oxidation and cathodic reduction processes of the Cu/Cu₂O multilayer film and pure Cu film in pH 8.4 borate buffer solution were analyzed by electrochemical quartz crystal microbalance (EQCM) for gravimetry and bending beam method (BBM) for stress measurement. The mass loss of the multilayer film during anodic oxidation at 0.8 V (SHE) in the passive region was less than that of the pure Cu film. The comparison between current transients and mass changes during anodic oxidation has succeeded in separating the anodic current density into two partial current densities of oxide film growth, i_O^2-, and of Cu²⁺ dissolution through the passive film, i_Cu²⁺. As a result, in the case of the pure Cu film, the anodic current density was mainly due to i_Cu²⁺, while in the case of the multilayer film, i_Cu²⁺ was almost equal to i_O^2-. The compressive stress for the multilayer film was generated during anodic oxidation, while the tensile stress for the pure Cu film was generated.The mass loss of the multilayer film during cathodic reduction at a constant current density (i_c = −20 μA cm⁻²) was significantly less than that estimated from coulometry, suggesting that H₂O produced by cathodic reduction remained in the multilayer film. The compressive stress was generated during cathodic reduction of the multilayer film, which was ascribed to H₂O remained in the multilayer film.     

The breakdown of passivity of iron and nickel by fluoride

The breakdown of passivity of iron and nickel by fluoride is investigated in acid solutions. Localized corrosion is observed for iron at pH 5 whereas nickel shows general corrosion. In strong acids only general corrosion for both metals is obtained. The breakdown of passivity of iron in acids occurs in two stages. Fluoride leads first to an increased passive current density (≈ 0.5 mA cm⁻²) where Fe³⁺ ions are formed. After a non-reproducible induction period of minutes to hours a steep increase of the dissolution current density is obtained going along with Fe³⁺ ion production. Apparently, at this stage the passive layer is removed leading to a similar situation as on the surface of pits during localized corrosion caused by other halides.     

The breakdown of passivity of iron and nickel by fluoride

The breakdown of passivity of iron and nickel by fluoride is investigated in acid solutions. Localized corrosion is observed for iron at pH ? 5 whereas nickel shows general corrosion. In strong acids only general corrosion for both metals is obtained. The breakdown of passivity of iron in acids occurs in two stages. Fluoride leads first to an increased passive current density ( 0.5 mA cm^?2) where Fe³⁺ ions are formed. After a non-reproducible induction period of minutes to hours a steep increase of the dissolution current density is obtained going along with Fe³⁺ ion production. Apparently, at this stage the passive layer is removed leading to a similar situation as on the surface of pits during localized corrosion caused by other halides.     

Passivation and local activation of α-Fe + Fe<Subscript>3</Subscript>C-based nanocomposites in neutral media

Regularities of the anodic dissolution and processes of local activation of α-Fe + Fe₃C nanocomposites containing 9 to 92 mol % of cementite are studied. It is shown that the sequence of anodic processes in borate solutions is the same for pH 6.3 and 7.4, namely, the dissolution and passivation of the ferrite component, dissolution and passivation of the cementite component, and anodic oxygen evolution at the nanocomposites’ passive surface. With an increase in the pH the passivation potentials decrease, which agrees with a thermodynamic model of the formation of two-layer passive film Fe₃O₄/γ-Fe₂O₃. It is found that the chloride concentration range, in which the local activation of α-Fe + Fe₃C is possible, narrows with an increase in the cementite content.     

Characterization of corrosion of X70 pipeline steel in thin electrolyte layer under disbonded coating by scanning Kelvin probe

Scanning Kelvin probe technique was used to characterize the electrochemical corrosion behavior of X70 steel in a thin layer of near-neutral pH and high pH solutions, respectively. Results demonstrate that passivity can be developed on steel in the near-neutral pH solution layer as thin as 60 μm, which is attributed to the fact that Fe²⁺ concentration in aqueous phase could reach saturation in the thin solution layer. The solubility of FeCO₃ is reached to drop out of solution as a precipitate. With the increase of solution layer thickness, it becomes more difficult for Fe²⁺ concentration to reach saturation. Consequently, the passivity cannot be maintained, and the steel shows an active dissolution state. Anodic dissolution rate of steel increases with the immersion time. The electrochemical polarization behavior of X70 steel in high pH solution is approximately independent of the solution layer thickness and immersion time. In thin solution layer, diffusion and reduction of oxygen dominate the cathodic process, as demonstrated by the presence of cathodic limiting diffusive current. In the bulk solution, the absence of limiting diffusive current density in cathodic polarization curve indicates that the main cathodic reaction is reduction of H₂CO₃ and , and the formed film is thus mainly FeCO₃.     

High-speed anodic dissolution of heat-resistant chrome-nickel alloys containing tungsten and rhenium: II. Nitrate solutions

Anodic dissolution of two heat-resistant chrome-nickel alloys containing 12% (weight) tungsten and 8% tungsten with 6% rhenium in a 2 M NaNO₃ solution was investigated using the rotating disk electrode method at current densities up to 30 A/cm². It is shown that anodic dissolution of these alloys occurs in the transpassive region of potentials with transition of the components in solution to forms with the highest oxidation level. Various mechanisms of alloy dissolution determining the processing speed (together with electrochemical dissolution) are proposed, including disintegration of the hardening phase, chemical oxidation of low-valence intermediates by solution components, and electrochemical formation of surface oxide layers. The results of change in the chemical composition of surfaces depending on the processing regimes are presented. Some variants of control by regimes of electrochemical dimensional processing (ECDP) of details from these alloys to achieve the optimal parameters of ECDP are proposed.     

Synergistic corrosion inhibition study of Armco iron in sodium chloride by piperidin-1-yl-phosphonic acid-Zn system

Electrochemical techniques, weight loss method and surface analysis were used to study the synergistic inhibition offered by Zn²⁺ and piperidin-1-yl-phosphonic acid (PPA) to the corrosion of Armco iron in 3% chloride solution. It is observed that the combination between PPA and Zn²⁺ shows excellent inhibition efficiency. The potentiodynamic polarization curves reveal that 5 × 10⁻³ mol l⁻¹ of PPA has only 76.7% inhibition efficiency whereas the mixture containing 5 × 10⁻³ mol l⁻¹ PPA -20%Zn²⁺ has 90.2% inhibition efficiency. This suggests that a synergistic effect exists between Zn²⁺ and PPA. The Fourier transform infrared (FTIR) spectrum of the film formed on iron indicates phosphonates zinc salt formation. A suitable mechanism of corrosion inhibition is proposed based on the results obtained. The surface film analysis showed that in the absence of Zn²⁺, the protective film consists of Fe²⁺-PPA complex formed on the anodic sites of the metal surface, whereas in the presence of Zn²⁺, the protective film consists of Fe²⁺-PPA complex and Zn(OH)₂.     

The difference in the types of intermetallic compound formed between the cathode and anode of an Sn–Ag–Cu solder joint under current stressing

An investigation of microstructural evolution with various current densities in a lead-free Cu/SnAgCu/Au/Cu solder system was conducted in this study. Current stressing induced migration of Cu toward the anode and resulted in the formation of Cu₆Sn₅ at the interface. The consumption rates of Cu were calculated to be 2.24 × 10⁻⁷ μm/s and 5.17 × 10⁻⁷ μm/s at 1.0 × 10³ A/cm² and 2.0 × 10³ A/cm², respectively, while the growth rates of Cu₆Sn₅ were 6.33 × 10⁻⁷ μm/s and 7.72 × 10⁻⁷ μm/s. The atomic fluxes of Cu were found to be 2.50 × 10¹² atom/cm² s and 5.88 × 10¹² atom/cm² s at the above-mentioned current densities. The diffusivities of Cu in Cu₆Sn₅ were 2.02 × 10⁻¹¹ cm²/s and 2.38 × 10⁻¹¹ cm²/s under 1.0 × 10³ A/cm² and 2.0 × 10³ A/cm² of current stressing. Current stressing effectively enhances the migration of Cu in Cu₆Sn₅ and results in a 1000-fold increase of magnitude in diffusivity compared to thermal aging. (Cu_1−x,Au_x)₆Sn₅ compound was formed near the anode after a long period of current stressing.     

Comparative analysis of the kinetics of dissolution of oxides Y<Subscript>2</Subscript>O<Subscript>3</Subscript> and Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> in the iron matrix upon mechanical alloying

The kinetics of low-temperature dissolution of oxides Y₂O₃ and Fe₂O₃ in an iron matrix during mechanical alloying has been studied using electron microscopy. It has been shown that the dissolution rate upon deformation of primary coarse oxides Fe₂O₃ in α iron (and, hence, saturation of the α matrix with oxygen) during treatment in a ball mill for up to 10 h is several times higher than the dissolution rate of Y₂O₃ oxides. The high-temperature (1100°C) annealing of a mechanoalloyed mixture of Fe + 1.5% Y + 1.35% Fe₂O₃ leads to the precipitation of 60% (of the total number of particles) secondary oxides 2–5 nm in size and only of 5–7% secondary nanooxides in a mechanoalloyed mixture of Fe + 2% Y₂O₃.     

Protection of passivated iron against corrosion in a 0.1 M NaNO3 solution by coverage with an ultrathin polymer coating of carboxylate SAM

An ultrathin, ordered and two-dimensional polymer coating was prepared on passivated iron by modification of 16-hydroxyhexadecanoate ion HO(CH₂)₁₅CO₂⁻ self-assembled monolayer (SAM) with 1,2-bis(triethoxysilyl)ethane (C₂H₅O)₃Si(CH₂)₂Si(OC₂H₅)₃ and octadecyltriethoxysilane C₁₈H₃₇Si(OC₂H₅)₃. Protection of passivated iron against passive film breakdown and corrosion of iron was examined by monitoring of the open-circuit potential and repeated polarization measurements in an aerated 0.1 M NaNO₃ solution during immersion for many hours. Passive film breakdown on the polymer-coated electrode in the solution was not observed during immersion for 480 h, whereas that of the passivated one occurred at 18.1 h, indicating protection of the passive film from breakdown by coverage with the polymer coating. The protective efficiencies of the passive film covered with the coating were extremely high, around 99.9% in the initial region of the immersion time up to 72 h and more than 98.3% thereafter, indicating prominent cooperative suppression of iron corrosion in 0.1 M NaNO₃ by coverage with the passive film and polymer coating. The polymer-coated surface was characterized by contact angle measurement and electron-probe microanalysis.     

Passivation and local activation of α-Fe + Fe3C-based nanocomposites in neutral media

Regularities of the anodic dissolution and processes of local activation of α-Fe + Fe₃C nanocomposites containing 9 to 92 mol % of cementite are studied. It is shown that the sequence of anodic processes in borate solutions is the same for pH 6.3 and 7.4, namely, the dissolution and passivation of the ferrite component, dissolution and passivation of the cementite component, and anodic oxygen evolution at the nanocomposites’ passive surface. With an increase in the pH the passivation potentials decrease, which agrees with a thermodynamic model of the formation of two-layer passive film Fe₃O₄/γ-Fe₂O₃. It is found that the chloride concentration range, in which the local activation of α-Fe + Fe₃C is possible, narrows with an increase in the cementite content. Original Russian Text ? A.V. Syugaev, S.F. Lomaeva, S.M. Reshetnikov, 2008, published in Zashchita Metallov, 2008, Vol. 44, No. 1, pp. 58–64.     

In‐situ investigation of the surface‐topography during anodic dissolution of copper under near‐ECM conditions

Electrochemical machining is characterized by deliberate high speed anodic dissolution. The distance between anode and cathode are only a few hundred microns. Additionally, the electrolyte flow rate is in the range of approximately meter/second. A special electrochemical cell design was developed to investigate the surface topography during the anodic dissolution simultaneously and in‐situ under near‐ECM conditions. The anode was directly placed under the cathode with a gap of 350 µm. We carried out pulse experiments at 15 A/cm² and 25 A/cm² with an electrolyte flow rate of 4m/s. First‐time it was possible to observe the surface changing of the anode directly during current pulses by using a video camera. Different stages e.g. roughening, film formation or gas evolution could be distinguished.     

X-ray photoelectron spectrum of Fe2+ state in iron oxides

As a standard for identification of iron oxides by X-ray photoelectron spectroscopy, Fe²⁺ spectra are extracted from mixed Fe 2p_3/2 spectra of Fe³⁺, Fe³⁺ and metallic states. The peaks of Fe²⁺ spectra are all located at binding energy of 708·5 eV. The width of Fe²⁺ spectrum seems to be dependent on crystallinity, and is 2·2 eV for a bulk crystalline oxide and 2·9 eV for an amorphous thin film under instrumental condition with FWHM of 1·3 eV for Au 4f_7/2.     

Nature of the Oxide Film Present on Iron after Inhibition in 0·05 M Chromate Solutions

Abstract

Chemical and microprobe analyses have been used to study the composition of the oxide films formed on iron by 0·05 M potassium chromate, pH 4–8.

Chemical analyses have shown that the air-formed oxide film was thinned and evenly reinforced with a normal iron chromium spinel having a composition in the range Fe²⁺ (Fe³⁺_0·5 Cr³⁺_1·5)O₄ — Fe³⁺ Cr³⁺ O₃

Microprobe analyses indicated enrichment of chromium at scratch lines, but the effect was small compared with the overall thickening of the films.     

Anodic dissolution of electrochemical chromium coatings in electrolytes for electrochemical machining: The dissolution rate and surface roughness

By the example of dissolution under conditions of controlled hydrodynamics (a rotating disk electrode), the distinctions of the surface formation after the anodic dissolution of electrolytic chromium films in electrolytes for electrochemical machining (ECM) (in chlorides and nitrates at current densities of 0.1–15 A/cm²) have been shown. The roughness of the produced surface was not considered intrinsic and correlated with the nature of the solution anion. In the chlorides, it was determined by the fracture formation due to the surface weakening and by the pitting formation, while, in nitrates, actually only by pitting formation. The influence of the current density and the surface heating at high current densities, along with the dimensions of the layers remaining after the chromium film dissolution, on the degree of the surface weakening and its roughness has been shown. The expedience of the nitrate solutions use at the high electrolyte flow rates during the electrochemical microprocessing of such surfaces has been demonstrated.     

Corrosion prevention of passivated iron in 0.1 M NaCl by coverage with an ultrathin polymer coating and healing treatment in 0.1 M NaNO3

An ultrathin, ordered and two-dimensional polymer coating was prepared on a passivated iron electrode by modification of 16-hydroxyhexadecanoate ion HO(CH₂)₁₅CO₂⁻ self-assembled monolayer with 1,2-bis(triethoxysilyl)ethane (C₂H₅O)₃Si(CH₂)₂Si(OC₂H₅)₃ and octadecyltriethoxysilane C₁₈H₃₇Si(OC₂H₅)₃. Subsequently, the electrode was healed in 0.1 M NaNO₃. Protection of passivated iron against passive film breakdown and corrosion of iron was examined by monitoring of the open-circuit potential and repeated polarization measurements of the polymer-coated and healed electrode in an aerated 0.1 M NaCl solution during immersion for many hours. Localized corrosion was markedly prevented by coverage with the polymer coating and the healing treatment in 0.1 M NaNO₃. Prominent protection of iron from corrosion in 0.1 M NaCl was observed before the breakdown occurred. The electrode surface covered with the healed passive film and polymer coating was analyzed by contact angle measurement, X-ray photoelectron spectroscopy and electron-probe microanalysis.     

Anodic treatment of strengthening the electrochemical coatings in electrolytes for electrochemical machining: 1. Micromachining of CoW coatings in nitrate and nitrate-alkaline solutions

Polycrystalline CoW coatings (with a 5–6 at % content of W) were found to dissolve in a 2M NaNO₃ solution with 100% current efficiency while reaching the conditions of thermokinetic instability (TKI) upon attaining the anode limiting currents resulting from the salt’s passivation. The anodic micromachining of nanocrystalline coatings (22–25 at % of W) in a nitrate solution also occurs with a 100% current efficiency, though at a very high degree of dissolution instability prior to attaining the TKI. The electrochemical micromachining of nanocrystalline coatings in a nitrate-alkaline solution (2M NaNO₂ + 0.5 M KOH) at low current densities occurs at a current efficiency close to zero, but, in all the cases under the TKI conditions, the current efficiency (upon the coatings treatment with different W contents in different solutions) exceeds the 100% value. A procedure for the removal of the coating material under the TKI conditions is offered that suggests the formation of an oxide-salt film and its periodic destruction due to a thermal explosion. It is shown that the minimal surface roughness is registered after the coating dissolution under the TKI conditions. Results are reported that confirm the possibility of controlling the strengthening (weakening) process of a surface layer after micromachining in different electrolytes using constant and pulse currents.     

Electrochemical study on hot corrosion of Si-modified aluminide coated In-738LC in Na2SO4-20 wt.% NaCl melt at 750 °C

The corrosion performance of the slurry Si-modified aluminide coating on the nickel base superalloy In-738LC exposed to low temperature hot corrosion condition has been investigated in Na₂SO₄-20 wt.% NaCl melt at 750 °C by combined use of the anodic polarization and characterization techniques.The coated specimen showed a passive behavior up to −0.460 V vs. Ag/AgCl (0.1 mol fraction) reference electrode, followed by a rapid increase in anodic current due to localized attack in the higher potential region. In the passive region, the anodic dissolution of constituents of the coating occurred through the passive film, probably SiO₂, at slow rate of 20-30 μA/cm². The passive current for the Si-modified coating was two orders of magnitude smaller than that for bare In-738LC, which is known as Cr₂O₃ former in this melt. This indicates that the SiO₂ film is chemically more stable than Cr₂O₃ film under this condition. However, pitting-like corrosion commenced around −0.460 V and proceeded at the high rate of 100 mA/cm² in the higher potential region than +0.400 V. The corrosion products formed on the coating polarized in different anodic potentials were characterized by SEM, EDS and XRD. It was found from the characterization that oxidation was dominant attack mode and no considerable sulfidation occurred at 750 °C. The SiO₂ oxide was not characterized in the passive region because the thickness of the passive film was extremely thin, but was detected as the primary oxide in the localized corrosion region, where the selective oxidation of Al was observed by further progress of the corrosion attack front into the inner layer of coating.