Active-passive behavior

Certain metal-electrolyte combinations exhibit active-passive behavior. Carbon steel in concentrated sulfuric acid is a classic example. The surface condition of a metal that has been forced inactive is termed passive. 

FIG. 25-2 (a) Active-passive behavior, (b) Application of anodic protection. 

Chemical passivity corresponds to the state where the metal surface is stable or substantially unchanged in a solution with which it has a thermodynamic tendency to react. The surface of a metal or alloy in aqueous or organic solvent is protected from corrosion by a thin film (1—4 nm), compact, and adherent oxide or oxyhydroxide. The metallic surface is characterized by a low corrosion rate and a more noble potential. Aluminum, magnesium, chromium and stainless steels passivate on exposure to natural or certain corrosive media and are used because of their active-passive behavior. Stainless steels are excellent examples and are widely used because of their stable passive films in numerous natural and industrial media.6... 

Figure 6.3 Schematic of anodic polarization curve of iron,10 showing active-passive behavior of iron in sodium borate-boric acid buffer solution at pH 8.4...

Electrochemical testing and determination of polarization characteristics of every component are recommended. If one of the metals has active-passive behavior, the state of the contact material should be considered for the expected active and passive states. Both Pourbaix pH diagrams and the potential of the passive metal or alloy can be helpful for this purpose. Bacterial corrosion in case of intended media and conditions should be investigated. 

For materials that exhibit classical active-passive behavior, passivation is more conducive under static rather than dynamic conditions. For the latter, the frequency of cyclic loading is often one of the critical factors that influences CF in corrosive environments. Cathodic protection generally mitigates CF and SCC, but increases the probability of HEC of susceptible materials. 

The Figure is a schematic polarization curve for a metal exhibiting typical thin-film active-passive behavior (e.g., Ni or Cr in sulfuric acid). Note that this diagram is for a single redox system, namely M/M+ (i.e.,... 

Most often, it is the anodic polarization behavior that is useful in understanding alloy systems in various environments. Anodic polarization tests can be conducted with relatively simple equipment and the scans themselves can be done in a short period of time. They are extremely useful in studying the active-passive behavior that many materials exhibit. As the name suggests, these materials can exhibit both a highly corrosion-resistant behavior or that of a material that corrodes actively, while in the same corrodent. Metals that commonly exhibit this type of behavior include iron, titanium, aluminum, chromium, and nickel. Alloys of these materials are also subject to this type of behavior. 

Active-passive behavior is dependent on the material-corrodent combination and is a function of the anodic or cathodic polarization effects, which occur in that specific combination. In most situations where active-passive behavior occurs, there is a thin layer at the metal surface that is more resistant to the environment than the underlying metal. In stainless steels, this layer is composed of various chromium and/or nickel oxides, which exhibit substantially different electrochemical characteristics than the underlying alloy. If this resistant, or passive, layer is damaged while in an aggressive environment, active corrosion of the freshly exposed surface will occur. The damage to... 

Even with an established anodic polarization behavior, the performance of a material can vary greatly with relatively minor changes in the corrodent. This is also illustrated in Fig. 3. Frame 1 illustrates the case where the anodic and cathodic polarization curves intersect much as in materials with no active-passive behavior. The anode is actively corroding at a high, but predictable, rate. 

Frame 2 represents the condition often found perplexing when using materials that exhibit active-passive behavior. With relatively minor changes within... 

Anodic protection finds its basis in the understanding of active-passive behavior. By increasing the potential of the component to be protected, it moves from an actively corroding situation to one where passivity can be induced. Such techniques can be quite cost-effective, but must be applied under well-controlled operating conditions because slight overprotection or... 

Passivation can be induced and observed in electrochemical experiments. Figure 26.34 shows a typical E versus log / plot for a metal that exhibits active-passive behavior [157]. The metal... 

Fig. 6.3 Schematic experimental polarization curves (solid curves) assuming active-passive behavior for the individual metal-oxidation curve and Tafel behavior for the individual cathodic-reactant reduction curve (dashed curves)...

Anodic protection is effective only for metal/environment combinations in which passivity is achievable and maintainable. If, for any reason, the passive film is damaged and breaks down, the application of anodic protection can result in greater damage than would be observed with no protection at all. This situation is shown schematically in Fig. 2 for a metal exhibiting active-passive behavior. The application of anodic protection is good if the passive film is developed, and a low current is achieved. If a sustained breakdown of the passive film occurs, however, then no decrease in the current is observed, and the corrosion current follows the dashed path indicated on the diagram. In this latter case, the increase of potential for oxidizing values win accelerate the corrosion as indicated by the X marked bad . 

Cyclic potentiodynamic polarization used in determining pitting potential consists of scanning the potential to more anodic and protection potentials during the forward and return scans and compare the behavior at different potentials under identical conditions. The polarization curve of an alloy (with or without coating showing active-passive behavior may be obtained in a chosen medium as a function of chloride concentration). E, or Ep represent pitting potential or breakdown potential,... 

Localized Biological Corrosion of Stainless Steels There are three sets of conditions under which localized biological corrosion of austenitic stainless steel occurs. These conditions should be examined for metals that show active-passive behavior. Microbiological corrosion in austenitic steel weldments has been studied and documented (4,48). 

Active-Passive Behavior and Susceptible Zone of Potentials... 

FIGURE 15.12 Schematic Evans diagram illustrating the influence of the rate of the reduction reaction (dotted lines) on active-passive behavior of a metal (solid line). ,ed> reversible potential for the reduction reaction oi, 02, 03, increasing exchange current densities for the reduction reaction (m/m+)> reversible potential for the M/M couple corr(i) and corr(2) are stable corrosion potentials. Concentration polarization is assumed to be absent. 

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