Corrosion cathodic depolarization

Oxygen dissolved in aqueous solutions, even in very low concentrations, is a leading cause of corrosion problems (i.e., pitting) in drilling. Its presence also accelerates the corrosion rate of other corrodents such as hydrogen sulfide and carbon dioxide. Oxygen plays a dual role both as a cathodic depolarizer and an anodic polarizer or passivator. Within a certain range of concentration the... 

Chloride is both a cathodic depolarizing agent (depolarizer) and a depassivating agent, and the presence of high chlorides in steam-water circuits significantly increases the risk of stress corrosion cracking of austenitic steels (type 300 stainless). 

Desulfovibrio desulfuricans Anaerobic sulfate reducer that causes corrosion and precipitation of iron by sulfide. Corrosion can be serious, developing deep pits under slime it is especially troublesome in paper mills, steel plants, refineries, and other large cooling systems. Corrosion rates are enhanced by the removal of hydrogen from the cathode by sulfate reducers (cathodic depolarization), as follows ... 

The CEFM is based upon the differential aeration hypothesis (DAH) for localized corrosion that was first enunciated by U. R. Evans at Cambridge University in the 1920s. The DAH postulates that there exists a spatial separation between the local anode, which is located in that region of the system that has the least access to the cathodic depolarizer (e.g., O2), and the local cathode, which forms at the location that has the greatest access to the cathodic depolarizer, with the proviso that both regions must be in electronic and electrolytic communication (Fig. 7). This arrangement results in the flow of positive current in the electrolyte phase... 

Polarization occurs because of ion concentration buildup near the anode and/or cathode. Once the ion concentration reaches saturation, corrosion essentially stops. Polarization can occur when (1) Hydrogen ions concentrate at an active cathode in the absence of a cathodic depolarizer. Dissolved oxygen acts as a cathodic depolarizer. (2) Metal ions saturate the electrolyte around an anode. (Soluble Fe++ may saturate the anode, perhaps as the result of the precipitation of an insoluble iron salt, inhibiting the diffusion of Fe++. For example, insoluble surface compounds such as carbonate scales in a fresh water often occur on carbon steel.)... 

Cathodic depolarization was observed in high SRB activity which did not always result in an accelerated corrosion because of concurrent activation of anodic polarization. 

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... 

Alternative Theories to Cathodic Depolarization Theory Discovering shortcomings of the classical theory helped shift the paradigm of involvement of SRB in corrosion to what we collectively refer to as alternative theories to emphasize that they are essentially different from the classical theory. These theories cover a wide range of research whose main common point is that they try to explain MIC by SRB, althongh not directly involving the bacteria itself. 

Research has shown thaU in the absence of sulfate, SRB can switch to fermentation and ferment a variety of substances and produce hydrogen, carbon dioxide, and acetate. These products can then be used by other bacteria such as methane-producing bacteria (methanogenes), which, by consuming hydrogen, can accelerate corrosion through mechanisms such as cathodic depolarization. Therefore, fermentative capabilities of SRB allow them to use alternative substances other than sulfate. 

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]. 

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. 

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