Active-passive corrosion behavior

Localized biological corrosion of stainless steels. There are three general sets of conditions under which localized biological corrosion of austenitic stainless steel occurs (Figure 6.29). These conditions should be examined for metals that show active-passive corrosion behavior. Microbiological corrosion in austenitic steel weldments has been documented. (Wahid)61, (Krysiak)14... 

The second part of the book consists of two chapters namely the forms of corrosion and practical solutions. The chapter, Forms of Corrosion consists of a discussion of corrosion reactions, corrosion media, active and active-passive corrosion behavior, the forms of corrosion, namely, general corrosion, localized corrosion, metallurgically influenced corrosion, microbiologically influenced corrosion, mechanically assisted corrosion and environmentally induced cracking, the types and modes of corrosion, the morphology of corroded materials along with some published literature on corrosion. 

The potential corresponding to the maximum active current density (critical anodic current density) of an electrode that exhibits active-passive corrosion behavior. 

Stainless steels offer useful resistance because they tend to exhibit passive corrosion behavior as a result of the formation of protective oxide films on the exposed surfaces. Under normal circumstances, stainless steels will readily form this protective layer immediately on exposure to oxygen. When this protective film is violated or fails to form, active corrosion can occur. Some fabrication processes can impede the reformation of this passive layer, and to insure that it is formed, stainless steels are subjected to passivation treatments. 

Figore 17.13 Demonstration of how an active-passive metal can exhibit both active and passive corrosion behaviors. 

FIG. 28-10 Six possible types of behavior for an active/passive alloy in a corrosive environment. 

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... 

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. 

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... 

It will be shown later that the values of icrit, Epp, and ip, which are the important parameters defining the shape of the active-passive type of polarization curve, are important in understanding the corrosion behavior of the alloy. In particular, low values of icrit enhance the ability to place the alloy in the passive state in many environments. For this reason, the maximum that occurs in the curve at B (Fig. 5.4) is frequently referred to as the active peak current density or, in general discussion, as the active peak. It is the limit of the active dissolution current density occurring along the A region of the polarization curve. 

Prediction of Corrosion Behavior of Active-Passive Type Metals and Alloys in Specific Environments... 

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