Mass transport control, anodic dissolution

The fit to the Koutecky-Levich equation, Fig. 9, demonstrates that the anodic dissolution of Cu occurs under mass-transport control, and extrapolation of these fits to co 1/2 = 0 yields kinetically controlled currents, 7k, free from transport effects and appropriately used in Tafel plots. 

Under mass transport control, an anodic layer (salt film or ion-concentrated layer) forms on the anode surface. The electrochemical dissolution process is then controlled by the diffusion and migration of mass transport limiting species in the anodic layers. In the case of Cu in concentrated phosphoric acid solution, it is said to be acceptor (water molecules) diffusion controlled. Since any arbitrary surface profile can be obtained by superimposing a series of sine waves, it is reasonable to simplify the anode surface profile to a single sine wave of wavelength a and amplitude b. Figure 10.8 shows a sinusoidal anode surface profile with anodic layer in the case of Cu in concentrated phosphoric acid solution that is, the mass transport limiting species is acceptor. Cu and acceptor concentration profiles are also illustrated in Fig. 10.8. The surface profile can be mathematically expressed as... 

Ironically, once a crevice has initiated, the flow of solution across the fully exposed surface generally acts to increase the propagation rate. This effect results from the effect of increased flow on cathodic reactions on the fully exposed surface that are mass transport controlled, such as oxygen reduction. As the cathodic reaction rate increases, the polarization of the internal, crevice anode increases as well, leading to increased dissolution rates. This effect is mitigated to the extent that the... 

The oxidation of trivalent chromium oxide to hexavalent oxide is also observed in passive films on stainless steel. An interesting application of this effect is electropolishing. The object made of stainless steel to be polished is connected as the anode in an electrochemical cell that contains a suitable electrolyte, normally a mixture of concentrated sulfuric and phosphoric acids. Transpassive dissolution then leads to electropolishing, provided the dissolution reaction is under mass transport control. Protrusions on the surface are favored by mass transport and therefore they dissolve more rapidly, leading to leveling of the surface [34]. 

Just as in the limiting case of ohmic control, the depth of the corrosion pit increases with the square root of the time. However, the proportionality constant here differs, it includes the saturation concentration rather than the potential and the diffusion coefficient rather than the electrolyte conductivity. Figure 7.64 shows experimental results for potentiostatic anodic dissolution of nickel in chloride solution. The geometry of the electrochemical cell corresponds to the one-dimensional pit model represented in Figure 7.62. The results show that, after a certain time, the current density decreases according to the square root of the polarization time, independent of potential. Mass-transport controlled growth of corrosion pits is favored by a highly electrolyte conductivity and weakly soluble dissolution products. 

The effects of pulsed waveforms are extremely complex and poorly understood, but the following effects are generally accepted. During the off period of a pulse, no net electron transfer can take place and the cathode surface is refreshed with metal cations as a result of convective diffusion. During the on period, the surface metal ion concentration will initially approach the bulk solution value but will decay with time, i.e. the technique involves non-steady state diffusion. A limiting case is a surface metal ion concentration of zero, i.e. complete mass transport control. The reverse (anodic) current may lead to selective dissolution of high points on the deposit due to their enhanced current density, producing a more compact or smooth surface. 

Anodic Dissolution Under Mass Transport Control... 

P. Glartsdorfif and 1. Prigogine, Structures, stabilite etfluctuations, Masson, Paris, 1971. O. E. Baida, O. R. Mattos, and B. Tribollet, Anodic dissolution of iron in acidic sulfate tmder mass transport control, J. Electrochem. Soc. 759 446 (1992). 

Anodic Dissolution under Mass Transport Control.188... 

Under diffusion-controlled dissolution conditions (in the anodic direction) the crystal orientation has no influence on the reaction rate as only the mass transport conditions in the solution detennine the process. In other words, the material is removed unifonnly and electropolishing of the surface takes place. 

In acidic solutions, the corrosion rate is relatively high. Studies on cadmium monocrystals and polycrystals in acidic chloride solutions revealed anodic dissolution independent of the crystallographic orientation the dissolution rate was controlled by the mass transport of CdCl" ions [331]. The inhibitive influence of adsorbed organic substances, for example, alcohols [332], phenotiazine [333], and some polymers (e.g. poly (vinyl alcohol), poly(acrylic acid), sodium polyacrylate. 

Etching kinetics of semiconductors may be diffusion-controlled in two ways (24). In the first case, the reduction reaction is diffusion limited and controls the etching kinetics. Under these conditions a well defined crystallographic facet is obtained. This behavior is observed at low pH. In the second case of high pH, the rate of anodic dissolution of the semiconductor wafer depends on the mass transport of OH ions to the electrode. Electroless etching based on this limitation shows rounded profiles typical of diffusion-controlled dissolution. 

---