Active and Passive Corrosion

Figure 22 Schematic anodic polarization curve indicating active and passive corrosion regions.

A fundamental distinction between corrosion modes is the division into active and passive corrosion. The active mode is characterized by the loss of material to the environment and results in the decrease of size and weight of the specimen. The loss may be in form of gaseous or dissolved species. 

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

Both molybdate and orthophosphate are excellent passivators in the presence of oxygen. Molybdate can be an effective inhibitor, especially when combined with other chemicals. Orthophosphate is not really an oxidizer per se but becomes one ia the presence of oxygen. If iron is put iato a phosphate solution without oxygen present, the corrosion potential remains active and the corrosion rate is not reduced. However, if oxygen is present, the corrosion potential iacreases ia the noble direction and the corrosion rate decreases significantly. 

Spontaneous Passivation The anodic nose of the first curve describes the primary passive potential Epp and critical anodic current density (the transition from active to passive corrosion), if the initial active/passive transition is 10 lA/cm or less, the alloy will spontaneously passivate in the presence of oxygen or any strong oxidizing agent. 

Two basic mechanisms cause biological corrosion. Biologically produced substances may actively or passively cause attack. Each mechanism either accelerates preexisting corrosion or establishes a new form of metal loss. Often the distinction between active and passive attack is vague. 

The active and passive electrochemical processes on which present-day corrosion protection is based were already known in the 19th century, but reliable protection for pipelines only developed at the turn of the 20th century. 

Corrosion protection measures are divided into active and passive processes. Electrochemical corrosion protection plays an active part in the corrosion process by changing the potential. Coatings on the object to be protected keep the aggressive medium at a distance. Both protection measures are theoretically applicable on their own. However, a combination of both is requisite and beneficial for the following reasons ... 

Since the object to be protected represents a cell consisting of active and passive steel, considerable IR errors in the cell current must be expected in measuring the off potential. The considerations in Section 3.3.1 with reference to Eqs. (3-27) and (3-28) are relevant here. Since upon switching off the protection current, 7, the nearby cathodes lead to anodic polarization of a region at risk from corrosion, the cell currents and 7, have opposite signs. It follows from Eqs. (3-27) and (3-28) that the 77 -free potential must be more negative than the off potential. Therefore, there is greater certainty of the potential criterion in Eq. (2-39). 

It is known that the common austenitic stainless steels have sufficient corrosion resistance in sulfuric acid of lower concentrations (<20%) and higher concentrations (>70%) below a critical temperature. If with higher concentrations of sulfuric acid (>90%) a temperature of 70°C is exceeded, depending on their composition, austenitic stainless steels can exhibit more or less pronounced corrosion phenomena in which the steels can fluctuate between the active and passive state [19]. 

Potential-current density (E-i) curves, which have been determined for a number of the austenitic cast irons and also for the nickel-free ferritic irons, indicate that in general the austenitic cast irons show more favourable corrosion characteristics than the ferritic irons in both the active and passive states. 

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. 

Oxide stabilized refers to materials, such as aluminum and the stainless steels, whose corrosion resistance depends on the formation and stability of a very thin surface oxide layer that is inert, easily healed if damaged, and tenacious. When the oxide layer has been disrupted and not healed, the material usually has little corrosion resistance. Both active and passive states sometimes exist adjacent to each other on the surface, resulting in rapid local corrosion. Crevice corrosion in stainless... 

For crevices such as in those in socket welds, the metal in the crevice is likely to be anodic. Crevice corrosion and under-deposit corrosion can be serious problems in oxide-stabilized materials such as aluminum and the stainless steels. Crevices and deposits can also accelerate corrosion in metals (such as carbon steel) that do not exhibit both active and passive states. However, the rate of corrosion is much slower in such materials because they lack the galvanic driving force of the active-passive states characteristic of the oxide-stabilized metals and alloys. The anode areas in crevices and under deposits are typically smaller than the cathode areas. This difference accelerates the corrosion rate. 

Based on the data presented in Fig. 5.42, for each element/electro-lyte listed below, state whether active or passive corrosion occurs and give the corrosion current density, icorr- In each situation, assume the worst-case condition. 

Anodic and cathodic processes may take place preferentially on separate areas of the surface of the reinforcement, leading to a macrocell. This can be established, for instance, between active and passive areas of the reinforcement. Current circulating between the former, which are less noble and thus function as anodes, and the latter, which are more noble and thus function as cathodes, accelerates the corrosion attack on active surfaces while further stabilising the protective state of passive ones. The magnitude of this current, known as the macrocell current, increases as the difference in the free corrosion potential between passive and active rebars increases, and decreases as the dissipation produced by the current itself at the anodic and cathodic sites and within the concrete increases. 

J. A. Gonzalez, S. Feliu Jr., The effect of macrocells between active and passive areas of steel reinforcements, Corrosion Science, 1992, 33,... 

Half-cell potentials cannot be correlated directly with the corrosion rate of the rebars. By excavating suitable inspection windows in the transition areas between very negative (active) and passive regions, the intensity of corrosion attack can be determined, and a correlation between potential and corrosion state can be established. This is valid only for the specific structure investigated. 

The effect of the oxidizer on the corrosion rate is shown in Fig. 4.13. In regions 1 through 3, both active and passive states are present. At the critical current density (the passivation potential), point 4, the corrosion current drops to passivation current. The stability of the system in this region is controlled by the voltage span of the passive region. In the transpassive state, the corrosion current starts to increase at point 7. 

The area ratio between active and passive regions may approach different values depending on the circumstances. Anyway, under these conditions the form of corrosion is not typically uniform, because the nominal state is so close to the passivity region in the Pourbaix diagram. The result is a transition form between uniform and localized corrosion that often occurs under real conditions. 

Under the influence of static mechanical tensile stress, various types of stress corrosion cracking can arise while alternating mechanical stress leads to corrosion fatigue, which occurs in both fully active and passive alloy systems. 

The development of pits starts with a crack or a hole of atomic dimension in the passive film caused, e.g., by tensions or by local chemical dissolution of the fihn. Permanent pitting corrosion can start above a critical potential and a critical concentration of the chloride ions. Above these critical values repassivation is prevented by the adsorption of the aggressive anions in the crack or the hole. The small dimensions of the crack or hole stabilize the large potential drop between active and passive surface. 

But even under steady-state conditions there is a profound influence of physical boundary conditions on corrosion behavior. The most widely known example of this is the boundary between active and passive oxidation of silica-formers. The classic modeling has been done by Wagner [11] for silicon. 

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