Metallic corrosion polarization curves

Corrosion protection of metals can take many fonns, one of which is passivation. As mentioned above, passivation is the fonnation of a thin protective film (most commonly oxide or hydrated oxide) on a metallic surface. Certain metals that are prone to passivation will fonn a thin oxide film that displaces the electrode potential of the metal by +0.5-2.0 V. The film severely hinders the difflision rate of metal ions from the electrode to tire solid-gas or solid-liquid interface, thus providing corrosion resistance. This decreased corrosion rate is best illustrated by anodic polarization curves, which are constructed by measuring the net current from an electrode into solution (the corrosion current) under an applied voltage. For passivable metals, the current will increase steadily with increasing voltage in the so-called active region until the passivating film fonns, at which point the current will rapidly decrease. This behaviour is characteristic of metals that are susceptible to passivation. 

Electrochemical methods of protection rest on different precepts (1) electroplating of the corroding metal with a thin protective layer of a more corrosion-resistant metal, (2) electrochemical oxidation of the surface or application of other types of surface layer, (3) control of polarization characteristics of the corroding metal (the position and shape of its polarization curves), and (4) control of potential of the corroding metal. 

Electrochemical corrosion processes also include a number of processes in organic chemistry, involving the reduction of various compounds by metals or metal amalgams. A typical example is the electrochemical carbonization of fluoropolymers mentioned on p. 316. These processes, that are often described as purely chemical reductions, can be explained relatively easily on the basis of diagrams of the anodic and cathodic polarization curves of the type shown in Fig. 5.54. 

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]. 

The polarization curve (polarization current i, versus polarization potential E) of a corroding metallic electrode can be measured by polarizing the electrode in the anodic and cathodic directions. In the range of electrode potential a short distance away from the corrosion potential, the polarization curve follows the Tafel relation as shown in Fig. 11-6. Here, the polarization current, ip, in the anodic direction equals the dissolution current of the metal i and the polarization current, ip, in the cathodic direction equals the reduction current of the oxidant i. In the range of potential near the corrosion potential, however, the polarization current, ip, is the difference between the anodic dissolution current of the metal... 

Fig. 11-8. Polarization curves for a corroding metallic electrode of which corrosion rate is controlled by diffusion of oxidants in aqueous solution solid curve = observable polarization curve.

As described in Sec. 11.3, the spontaneous corrosion potential of a corroding metal is represented by the intersection of the anodic polarization curve of metal dissolution with the cathodic polarization curve of oxidant reduction (Figs. 11—5 and 11-6). Then, whether a metal electrode is in the active or in the passive state is determined by the intersection of the anodic and cathodic polarization curves. 

Fig. 11-14. (a) Corrosion rate of metallic iron in nitric acid solution as a function of concentration of nitric add and (b) schematic polarization curves for mixed electrode reaction of a corroding iron in nitric add W p, = iron corrosion rate CHNO3 = concentration of nitric add t" (t ) = current of anodic iron dissolution (cathodic nitric add reduction) dashed curve 1= cathodic current of reduction of nitric add in dilute solution dashed ciuve 2 s cathodic current of reduction of nitric add in concentrated solution. [From Tomashov, 1966 for (a).]... 

Fig. 11-16. Corrosion rate of metallic nickel in sulfate solutions (0.5 M NsjSO ) as a function of pH at 25 C inserted sub-figures are polarization curves of nickel electrodes in acidic solution and in basic solution. [From CScamoto-Sato, 1959.]...

Consider first the polarization curve (i.e., Tafel plot) for the anodic halfreaction occurring in corrosion of stainless steels (Fig. 16.8). The diagram for the active region is much the same as has been seen for other anodes (Figs. 15.4 to 15.7). As Eh is increased to a certain specific value, however, a sudden and dramatic drop in the anodic current density i occurs, corresponding to formation of an oxide film. At higher Eh, i remains constant at a very low level (the horizontal scale in Fig. 16.8 is logarithmic), and the metal has become passive, that is, effectively immune from corrosion. 

Fig. 2 distinguishes the domains of immunity, corrosion and passivity. At low pH corrosion is postulated due to an increased solubility of Cu oxides, whereas at high pH protective oxides should form due to their insolubility. These predictions are confirmed by the electrochemical investigations. The potentials of oxide formation as taken from potentiodynamic polarization curves [10] fit well to the predictions of the thermodynamic data if one takes the average value of the corresponding anodic and cathodic peaks, which show a certain hysteresis or irreversibility due to kinetic effects. There are also other metals that obey the predictions of potential-pH diagrams like e.g. Ag, Al, Zn. 

An additional interpretation issue involves the presence of oxidation reactions that are not metal dissolution. Figure 28 shows polarization curves generated for platinum and iron in an alkaline sulfide solution (21). The platinum data show the electrochemistry of the solution species sulfide is oxidized above -0.8 V(SCE). Sulfide is also oxidized on the iron surface, its oxidation dominating the anodic current density on iron above a potential of approximately -0.7 V(SCE). Without the data from the platinum polarization scan, the increase in current on the iron could be mistakenly interpreted as increased iron dissolution. The more complex the solution in which the corrosion occurs, the more likely that it contains one or more electroactive species. Polarization scans on platinum can be invaluable in this regard. 

One example of the application of polarization curves in a predictive manner involves their use in galvanic corrosion. Galvanic corrosion occurs when two dissimilar metals are in electrical and ionic contact as is schematically shown in Fig. 29. Galvanic corrosion is used to advantage in sacrificial anodes of zinc in seawater and magnesium in home water heaters. It slows corrosion of millions of tons of structural materials. The darker side of galvanic corrosion is that it also causes major failures by the accelerated dissolution of materials that are accidentally linked electrically to more noble materials. 

The open circuit Ecm values for each metal are the entries in the traditional galvanic series. Kinetic information is also available via analysis of the polarization curves. The 4ouPie can be used to calculate the increased corrosion rate of Metal 2. Because of the coupling to Metal 1, the dissoultion rate has increased from 4cathodic kinetics on Metal 1 must now be satisfied. In addition to determining the increase in the corrosion... 

In the presence of oxidizing species (such as dissolved oxygen), some metals and alloys spontaneously passivate and thus exhibit no active region in the polarization curve, as shown in Fig. 6. The oxidizer adds an additional cathodic reaction to the Evans diagram and causes the intersection of the total anodic and total cathodic lines to occur in the passive region (i.e., Ecmi is above Ew). The polarization curve shows none of the characteristics of an active-passive transition. The open circuit dissolution rate under these conditions is the passive current density, which is often on the order of 0.1 j.A/cm2 or less. The increased costs involved in using CRAs can be justified by their low dissolution rate under such oxidizing conditions. A comparison of dissolution rates for a material with the same anodic Tafel slope, E0, and i0 demonstrates a reduction in corrosion rate... 

Armed with an understanding of the underlying physical processes, the electrochemical phenomenology of localized corrosion can be better understood. Figure 23 shows three schematic polarization curves for a metal in an environment in which it spontaneously passivates and (1) can be anodized, (2) transpassively dissolves at higher potentials, and (3) pits upon further anodic polarization. We have discussed cases 1 and 2 in the section on passivity. For case 3, the region of passivity extends from to a potential labeled EM at which point the current increases dramatically at higher potentials. 

In this experiment, polarization curves for carbon steel and copper in 3.5 wt% NaCl will be determined. From these data, the corrosion rates will be estimated for the individual metals freely corroding in solution and for the metals electrically coupled in solution as would be the case for an immersed, riveted connection, for example. 

Corrosion — Corrosion current density — Figure. Polarization curves of a metal/metal ion electrode and the H2/H+ electrode including the anodic and cathodic partial current curves, the Nernst equilibrium electrode potentials E(Me/Mez+) and (H2/H+), their exchange current densities / o,M> o,redox and related overpotentials Me) and 77(H), the rest potential r, the polarization n and the corrosion current density ic at open circuit conditions (E = Er) [i]... 

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