Corrosion reactions, cathodic depolarization

Use cathodic inhibitors to combat cathodic depolarization reactions, minimizing galvanic corrosion. 

Ferrous ions from the anodic reaction Fe Fe + 2e react with from the cathodic depolarization reaction and with OH from the water dissociation reaction and form ferrous sulphide, FeS, and hydroxide, Fe(OH)2. FeS can play an important role. Where the sulphide forms a continuous film on the surface it acts as protection and as an effective site for the cathodic reaction. If the film is injured or there is a lack of continuity in the film for other reasons, local galvanic corrosion will occur. Experiments and experience indicate that also the anodic reaction (Fe —> Fe +2e ) is depolarized as a result of the SRB environment. This is of interest in connection... 

In nitric acid, the cathodic depolarizer (passivator) is nitrous acid, HNO2. This must form first in sufficient quantity by an initial rapid reaction of iron with HNO3. As nitrous acid accumulates, anodic current densities increase, eventually reaching 4riacai- Passivity is then achieved, and the corrosion rate falls to the comparatively low value of about 2gmd [14]. 

Equation (29.10) is the Stern-Geary equation. Should the cathodic reaction be controlled by concentration polarization, as occurs in corrosion reactions controlled by oxygen depolarization, the corrosion current equals the limiting diffusion current (Fig. 29.2). This situation is equivalent to a large or infinite value of Pc in (29.10). Under these conditions, (29.10) becomes... 

Due to its simplicity, open circuit corrosion potential measurements (see Chapter 20 of this manual) have been used in MIC studies for many years. Corrosion potential measurements as a function of time have been used to obtain information on MIC of steel, aluminum alloys, stainless steels, and other passive alloys. By itself, the corrosion potential of plain carbon and low alloy steels indicates very little because these steels can corrode at a wide range of potentials. Rapid changes in the corrosion potential, however, can be used to indicate cathodic depolarization, or an enhancement of the anodic reaction, or to the formation of a semi-protective film. 

In connection with the above statements a next step has been made in the investigations, namely studying the influence of the Ce and Ce + ions as components of the corrosion medium (O.IN H2SO4) on the anodic behavior of stainless steel. These investigations were provoked by the observed occurrence of cathodic depolarization reaction of Ce + (CeCb) reduction, as a result of which the surface concentration of cerium is decreasing and theoretically it should approach zero value (Stoyanova et al., 2010). For this purpose an inverse experiment was carried out at different concentrations of Ce + ions in the corrosion medium we monitored the changes in the stationary corrosion potential of the thermally treated steel by the chronopotentiometric method. The aim of this experiment was to prove the occurrence of a reversible reaction of reduction of Ce + Ce +- e <-> Ce , (instead of the reaction of hydrogen depolarization), which in its turn creates also the option to form a film (chemically insoluble) of cerium hydroxides/oxides on the active sections of the steel surface. 

During oxidation of reduced sulfur compounds, more corrosive sulfides are produced under anoxic conditions, causing cathodic reactions. The corrosion rate increases as the reduced and oxidized FeS concentrations increase (Lee et al. 1993a, b). Cathodic depolarization processes also can yield free O2 which reacts with polarized hydrogen on metal surfaces. 

Figure 82. Depiction of the Differential Aeration Hypothesis (DAH) for localized corrosion showing the coupling currents (electronic through the metal, ionic through the solution) from the corrosion cavity to the external surface, where the two currents annihilate via a charge transfer reaction involving a cathodic depolarizer (e.g., reduction of oxygen). Reprinted from Corrosion Science, 32 (1991) 51, Copyright (1991), with permission from NACE International...

At high temperatures the reverse reaction occurs, and heated surfaces become coated with CaCOj. A protective scale is also produced when CafHCOjlj becomes alkaline in the region of a cathodic area. The scale thus deposited inhibits corrosion by reducing the cathodic area, restricting diffusion of cathodic depolarizers, and increasing ohmic resistance. 

It is therefore now widely accepted that it is rather unlikely that the cathodic reaction is affected directly by the enzymatic consumption of hydrogen as described in the "cathodic depolarization theory." This theory is therefore reported here more for historical interest than as a potential mechanism for pitting corrosion of iron in presence of SRB. 

The corrosion rate is controlled mainly hy cathodic reaction rates. Cathodic Reactions 5.2 and 5.3 are usually much slower than anodic Reaction 5.1. The slower reaction controls the corrosion rate. If water pH is depressed. Reaction 5.3 is favored, speeding attack. If oxygen concentration is high. Reaction 5.2 is aided, also increasing wastage hy a process called depolarization. Depolarization is simply hydrogen-ion removal from solution near the cathode. 

---