Polarization behavior, corrosion rate

A. A. Aksut, W. J. Lorenz, and F. Mansfeld, "The Determination of Corrosion Rates by Electrochemical DC and AC Methods II. Systems with Discontinuous Steady State Polarization Behavior," Corrosion Science, 22 (1982) 611-619. 

Environmental tests have been combined with conventional electrochemical measurements by Smallen et al. [131] and by Novotny and Staud [132], The first electrochemical tests on CoCr thin-film alloys were published by Wang et al. [133]. Kobayashi et al. [134] reported electrochemical data coupled with surface analysis of anodically oxidized amorphous CoX alloys, with X = Ta, Nb, Ti or Zr. Brusic et al. [125] presented potentiodynamic polarization curves obtained on electroless CoP and sputtered Co, CoNi, CoTi, and CoCr in distilled water. The results indicate that the thin-film alloys behave similarly to the bulk materials [133], The protective film is less than 5 nm thick [127] and rich in a passivating metal oxide, such as chromium oxide [133, 134], Such an oxide forms preferentially if the Cr content in the alloy is, depending on the author, above 10% [130], 14% [131], 16% [127], or 17% [133], It is thought to stabilize the non-passivating cobalt oxides [123], Once covered by stable oxide, the alloy surface shows much higher corrosion potential and lower corrosion rate than Co, i.e. it shows more noble behavior [125]. 

There are several factors that can lead to non-Tafel behavior. Diffusion limitations on a reaction have already been introduced and can be seen in the cathodic portion of Fig. 27. Ohmic losses in solution can lead to a curvature of the Tafel region, leading to erroneously high estimations of corrosion rate if not compensated for properly. The effects of the presence of a buffer in solution can also lead to odd-looking polarization behavior that does not lend itself to direct Tafel extrapolation. 

Polarization of the galvanic cell. The different phenomena of polarization of the anodic and cathodic reactions (activation, diffusion, convection, etc.), should be well known as a function of the evolution and change of the properties of the interface as a function of time. The polarization behavior of the cathodic and anodic reactions on the two electrodes should be examined (see Figure 6.5). In natural atmospheres, the cathodic reaction controls frequently the attack rate. The diffusion of oxygen is an important parameter to avoid control and polarization of the corrosion by the rate of the cathodic reaction (Figure 6.12).7... 

The electrochemical behavior of copper and tantalum in different solutions was investigated using DC polarization experiments. From the polarization data, the corrosion current density and hence the corrosion rate of copper were calculated using the Stem-Geary Equation ... 

The earlier sections of this chapter discuss the mixed electrode as the interaction of anodic and cathodic reactions at respective anodic and cathodic sites on a metal surface. The mixed electrode is described in terms of the effects of the sizes and distributions of the anodic and cathodic sites on the potential measured as a function of the position of a reference electrode in the adjacent electrolyte and on the distribution of corrosion rates over the surface. For a metal with fine dispersions of anodic and cathodic reactions occurring under Tafel polarization behavior, it is shown (Fig. 4.8) that a single mixed electrode potential, Ecorr, would be measured by a reference electrode at any position in the electrolyte. The counterpart of this mixed electrode potential is the equilibrium potential, E M (or E x), associated with a single half-cell reaction such as Cu in contact with Cu2+ ions under deaerated conditions. The forms of the anodic and cathodic branches of the experimental polarization curves for a single half-cell reaction under charge-transfer control are shown in Fig. 3.11. It is emphasized that the observed experimental curves are curved near i0 and become asymptotic to E M at very low values of the external current. In this section, the experimental polarization of mixed electrodes is interpreted in terms of the polarization parameters of the individual anodic and cathodic reactions establishing the mixed electrode. The interpretation then leads to determination of the corrosion potential, Ecorr, and to determination of the corrosion current density, icorr, from which the corrosion rate can be calculated. 

Molybdenum exhibits unusual polarization behavior. The initial portion of the curve, shown dashed in Fig. 5.20, is very difficult to determine experimentally because it occurs at very low current densities indicating that the passive state is very rapidly established by traces of dissolved oxygen or by very low concentrations of other cathodic reactants. In fact, many of the published curves show only the transpassive range over which the current density rapidly increases. The implication is that as long as the potential is below 200 mV (SHE), the corrosion rate of molybdenum would be very low and this is observed. 

Kaesche and Hackerman (13) have investigated the inhibition of several aliphatic and aromatic amines on pure iron corroding in IN hydrochloric acid. These authors observed in thirteen out of fourteen cases that the inhibition was both anodic and cathodic, albeit predominantly anodic. The exception was methylamine which acted only cathodically. In the case of the corrosion inhibition on pure iron by B-naphthoquinoline in sodium sulfate/sulfuric acid solution (13). one observes a simple parallel shift of the anodic and cathodic Tafel lines towards smaller values of current density. Here the effect is almost symetrical, indicating that this inhibitor acts to the same extent upon anodic and cathodic reaction rates. Therefore, the effect of B-naphthoquinoline can be explained on the basis that its adsorption blocks a fraction 0 of the metal surface for all electrode reactions. If equation 9 describes the external polarization behavior in terms of a function of the partial current potential relationship for the anodic and cathodic reactions in the usual terms ... 

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