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. 

The accountables Passive behavior Rational Most likely short... 

Passivating behavior of self-assembled octadecanethiol on pc-Au electrodes has been tested by CV using Ru(NH3)6Cl3, K4Fe(CN)6, and benzoquinone [106] as the redox probes. Inhibiting properties toward these reactants were found to be different. 

The protons of the hydroxy groups were deuterated by dissolving BP(OH)2 in cyclohexane and shaking the solution with deuterated water for several hours. After precipitation pump-probe measurements of BP(OD)2 in cyclohexane were recorded and are compared to BP(OH)2 in Fig. 4. Both samples were excited at 350 nm and probed at 505 nm. The delay of the emission rise of about 50 fs is equal in both cases and the coherent excitation of the vibrations is identical with respect to frequencies, phases and amplitudes. The ESIPT dynamics is obviously not altered by the deuteration and the mass of the proton has no influence on the transfer speed. This excludes that tunneling of the proton determines the speed of the transfer and the measurements provide the first proof for the passive behavior of the proton in the ESIPT. 

The critical heat release rate following the Frank-Kamenetskii theory (see Section 13.4), which describes the passive behavior of the reactor without fluid circulation, when heat transfer occurs by thermal conduction only. The critical heat release rate is the highest power that does not lead to a thermal explosion and varies with the inverse of the squared radius ... 

In the last chapter, Strehblow provides a review of experimental methodology and theoretical concepts of passivation and passivity of metals. The topics of emphasis include growth and composition of passive layers, their structure and electronic properties, and their breakdown. Current accomplishments are discussed in detail for a selected number of key metal and alloy systems. Summarized in some detail are the most important analytical methods for elucidation of chemical composition, electronic properties and structure of passive layers. It is shown for many systems that the application of multiple combinations of electrochemical and spectroscopic methods provide many insights and confidence in the interpretation of the passive behavior of metals. 

It is important to note that passive behavior of the metal also depends upon the electrolytic environment. As an example mild steel is passivated in pure nitric acid, but not in dilute aqueous nitric acid. The current density required to passivate pass can be high and the current density to maintain the film r fiim, may be small. 

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.,... 

Deactivation, or "passivation , behavior has been found to be a feature of the oxygen evolution process on these electrodes [284], While apparently not related to oxide dissolution, the loss of activity for oxygen evolution is evidently related to the formation of a hydrated Co-rich oxide multilayer film over a significant portion of the LaosBojCoOg electrode surface [284]. Lowering of the electrode potential restored the initial activity for oxygen evolution. Further study of this deactivation process in warranted since similar deactivation processes may occur at other oxide catalysts. 

Ee° - Bmp at the mixed potential as a passivity factor. The greater die factor the higher degree of passivity. In this way he has classified the metals in a given solution according to their tendency to show passive behavior. 

UNSTABLE PASSIVE BEHAVIOR = CASE II ACTIVE BEHAVIOR = CASE I... 

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... 

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