Current Efficiency and Metal Removal Rate
Depending on the metal-electrolyte combination and operating conditions, different anodic reactions take place at high current densities. Rates of these reactions are dependent to a great extent on the ability of the system to remove the reaction products as soon as they are formed and supply fresh electrolyte to the anode surface. All of these factors influence the machining performance namely: dissolution rate, shape control, and surface finish of the workpiece. An understanding of the kinetics and stoichiometry of anodic reactions and their dependence on mass transport conditions is, therefore, essential to optimize the operating parameters.
Knowledge of anodic reactions that take place at high potentials is mostly derived from weight loss measurements and by applying Faraday’s law [6-9]. The current efficiency for metal dissolution, 0 is related to weight loss, AW, by
where / is the applied current, t is the time, F is the Faraday constant, and n is the valence of metal dissolution (the number of electrons removed from dissolving metal atoms by anodic oxidation and M is the atomic weight). For alloys, the atomic weight is calculated by using МЛ]„У = , with Xj being the mole fraction of the
component j and Mj its atomic weight.
The use of weight loss as a measure of dissolution valence is strictly applicable to anodic reactions involving metal dissolution only. In the presence of other reactions simultaneously occurring at the anode, such as oxygen evolution, it is essential to get a complete analysis of the reaction products including collecting the gas to determine the contribution of each reaction to the current efficiency. Accordingly, the weight loss measurements have been frequently used to determine an “apparent dissolution valence” [6-9].
The value of n provides information on the overall reaction stoichiometry and is useful for the calculation of the metal removal rate. Material removal rate, MRR, is related to the dissolution valence, n, according to Equation 8.8:
where the material removal rate, MRR, is in cm/s, A is the surface area in cm2, and p is density in g/cm3.
Determination of current efficiency for metal dissolution in nonpassivating electrolytes, such as chloride solutions, is somewhat easier since there is no simultaneous anodic oxygen evolution occurring in these systems and the value of в can be directly obtained from weight loss measurements. Figure 8.4 shows metal dissolution efficiency as a function of the current density for a few selected materials in nonpassivating and passivating electrolytes. For nickel dissolution in chloride solutions (Figure 8.4a), the current efficiency based on divalent nickel formation is 100% independent of current density and electrolyte flow rate . Cr dissolution in chloride solution yields a value of n = 6 independent of current density . For iron dissolution in chloride solutions, the current efficiency based on divalent iron (Fe2+) is 100% at low current densities, but at current densities higher than the limiting current, the value of which depends on the electrolyte flow rate, the current efficiency based on Fe2+ decreases. Complete analysis of reaction products showed no evidence of anodic oxygen evolution . The decrease in current efficiency, therefore, is attributed to the simultaneous production of Fe2+ and Fe3+, with an average value of 2.5 . This behavior is shown by curve b in Figure 8.4. The dissolution behavior of Fel3Cr alloys is similar to that of pure Fe with the measured values of n being 2.1 at low current densities and 2.7 at high current densities above the limiting current. The measured values for Fel3Cr are explained by postulating formation of Fe2+, Cr2+, and Cr3+ below' the limiting current and formation of Fe2+, Fe'+, and Cr6+ (chromate) above the limiting current. Anodic dissolution of Fe24Cr alloy yields « = 3.6 independent of current density , which corresponds to the formation of Fe3+ and Cr6+. For 304 stainless steel, the experimental data yields n = 3.5 independent of current density. This corresponds to the formation of Fe3+, Cr6+, and Ni2+ . In summary, anodic dissolution of nickel, Cr, Fe24Cr, and 304 SS in NaCl (nonpassivating) electrolyte correspond to the curve (a) in Figure 8.4a where current efficiency is showm to be independent of current density. On the other hand, for anodic dissolution of Fe and Fel3Cr in NaCl, the current density-dependent dissolution stoichiometry leads to a decreased current efficiency at high currents (Figure 8.4b).
Anodic reaction stoichiometry during the high-rate dissolution of iron and nickel in passivating ECM solutions (sodium nitrate and sodium chlorate) has been extensively studied [7,10,12,13]. In these studies, oxygen evolution has been found to be the predominant anodic reaction at low' current densities in the transpassive potential region. At higher current densities, the relative rate of metal dissolution increases
FIGURE 8.4 Current efficiency for metal dissolution as a function of current density in passivating and nonpassivating electrolytes: (a) nickel in 5 M NaCl, (b) iron in 5 M NaCl, and (c) nickel and iron in NaNO, and NaCIO, electrolytes [6,7].
with increasing current density (Figure 8.4c). The transition from predominantly oxygen evolution to high-rate transpassive dissolution is governed by the specific metal-electrolyte interactions which influence passivation and depassivation processes. Different results have indicated that even in passivating systems, the metal dissolution current efficiency reaches close to 100% at high current densities under ECM conditions [7,10,12,13]. The most important aspect of passivating electrolytes is that the dissolution efficiency is an increasing function of current density which plays a significant role in achieving machining accuracy.
Mass Transport and Surface Finish
The technical feasibility of an electrochemical metal shaping process is determined by the metal removal rate, shape profile, and surface finish. These criteria are dependent on the ability of the system to provide desired mass transport, current distribution, and surface film properties at the dissolving workpiece surface. In most applications, mass transport and current distribution are intimately related. Mass transport conditions at the dissolving anode not only affect the current distribution on a macroscopic and a microscopic scale, but they are also crucial for the surface texture resulting from dissolution.
The rate of dissolution at any given point on the electrode surface is proportional to the local current density. The shape evolution of an anodically dissolving metal, therefore, depends on the current distribution. In ECM, the macroscopic current distribution on the workpiece is most important because it determines the final shape resulting from anodic dissolution. Because of the high current densities used, current distribution problems can be well described by the primary current distribution approximation. However, local conductivity changes resulting from the presence of gas bubbles in the interelectrode gap or from Joule heating create complications that need to be considered. Furthermore, in many systems, the current efficiency varies with the current density. For example, high-rate transpassive dissolution of nickel or iron in passivating (sodium nitrate and sodium chlorate) electrolytes, current efficiency increases with an increase in current density as discussed above . In these systems, closer tolerances have been achieved in practice due to this effect. For surface finishing operations, both the macroscopic and the microscopic current distribution are of importance. While macroscopic current distribution depends on the potential distribution and on the hydrodynamic conditions, the current distribution on a microscopic scale is mostly governed by mass transport. From a mathematical point of view, the current distribution on a micro profile for a purely diffusion-controlled process is the same as the primary current distribution and, therefore, Laplace’s equation can be used to simulate the rate of anodic leveling. However, although anodic leveling under primary current distribution conditions and under mass transport-limited conditions are mathematically the same, the resulting surface finish differs.
Metal dissolution at high current densities in both passivating and nonpassivating systems leads to an increase in the metal ion production at the anode. When the metal ion concentration at the surface exceeds the saturation limit, precipitation of a thin salt film occurs. The polarization curve under these conditions exhibits a limiting current plateau. The limiting current density has been found to increase with increasing electrolyte flow in a channel cell or with increasing rotation speed in an RDE system. The limiting current density is, therefore, controlled by convective mass transport. The formation of salt films on the anode influences the surface morphology of a dissolved workpiece [6,7,9,13]. Different studies have conclusively demonstrated that two distinctly different surface morphologies result from dissolution. At current densities lower than the limiting current density, extremely rough surfaces are obtained which, depending on the metal-electrolyte combination, reveals crystallographic steps and etch pits, preferred grain boundary attack or finely dispersed microstructure. On the other hand, dissolution at or higher than the limiting current leads to electropolished surfaces.
Surface finishing is a combination of leveling and brightening effects. Anodic dissolution results in surface smoothing because protrusions of a rough surface are more easily accessible than recesses. This is known as the leveling effect. At the mass transport-controlled limiting current, the influence of charge-transfer kinetics is negligible and differences due to grain orientation, grain boundaries, dislocations, or small inclusions, therefore, do not play a significant role. This leads to a brightening effect caused by the random removal of atoms from the metal surface. In ECM electrolytes, the surface smoothing is identified to be the dissolution product limited transport known as the salt-film mechanism in which the rate of transport of dissolving metal ions from the anode surface into the bulk is rate-limiting. At the limiting current, a thin salt film is present on the surface and the concentration of the dissolving metal ions at the surface corresponds to the saturation concentration of the salt formed with the electrolyte anions. The plateau current, due to the resistive salt film, may extend over several volts, depending on the salt-film properties. Surface brightening of nickel, Fe, and stainless steel under ECM conditions in NaCl, NaC10„ and NaNO, electrolytes has been shown to occur when the limiting current leading to salt-film formation is reached or exceeded [6,7]. Similar behavior has been observed for copper , titanium , and stainless steel . The solubility of the dissolving metal ions in ECM electrolytes is generally high and, in addition, high flow rates and/ or pulsating current lead to a small diffusion layer thickness at the anode. As a consequence, the limiting current densities needed to achieve surface brightening under ECM conditions are usually high, typically on the order of several A/cm2.
Bright and polished surfaces often exhibit pits and pits with tail in the flow direction. These phenomena have been observed with many metals and alloys, and it has been shown that these tails may give rise to so-called flow streaks mentioned in the ECM literature . It has been established that pit tailing and flow streak formation are observed only during transpassive dissolution with the direct current when anodic dissolution is entirely, or partially mass transport controlled . The formation of pit tails as an initial step to flow streak formation is therefore due to a local disturbance of hydrodynamic conditions introduced by the presence of a pit. This is possible if flow separation  caused by the presence of pits creates small vortices and eddies that increase the local mass transport rate at the downstream end. However, flow streaks formation is restricted to direct current ECM under brightening conditions where the anode surface is covered with a salt film that leads to surface brightening. On the other hand, surfaces dissolved in the active mode or during transpassive dissolution with pulsating current do not show pit tails. The fact that under pulsating current no tails are observed even under transpassive dissolution conditions is also consistent with the fact that the effective diffusion layer thickness during dissolution with the pulsating current is much smaller than the steady- state diffusion layer thickness and is essentially determined by the pulse parameters. Therefore, local variations of hydrodynamic conditions have little influence on the dissolution rate. This aspect is one of the important virtues of using pulsating current in metal shaping and finishing operations.