Corrosion depolarizer

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. 

When water pH is between about 4 and 10 near room temperature, iron corrosion rates are nearly constant (Fig. 5.5). Below a pH of 4, protective corrosion products are dissolved. A bare iron surface contacts water, and acid can react directly with steel. Hydrogen evolution (Reaction 5.3) becomes pronounced below a pH of 4. In conjunction with oxygen depolarization, the corrosion rate increases sharply (Fig. 5.5). 

In the above mechanism, both hydrogen ion and molecule are utilized by SRB to convert SO to H,S. The consumption of hydrogen depolarizes the cathode and leads to an increased rate of corrosion. 

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... 

Generally, the corrosivity of water containing dissolved salts increases at low-salt concentrations, until some maximum is reached, and beyond this maximum the corrosion rate decreases. Corrosion rate throughout the salt concentration range is under the influence of oxygen s ability to depolarize. It is believed that... 

Cl- and S042+ leakage (DI). Anodic depolarization, leading to depassivation and corrosion... 

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). 

The minimum rate of boiler steel corrosion (i.e., the maximum development of dense, adherent, and protective magnetite) occurs at pH of 11 to 12. However, at significantly lower pH values (acid conditions) hydrogen ions are discharged (depolarization), the magnetite layer becomes porous, and corrosion rates increase. 

Other forms of concentration-cell corrosion include Caustic gouging Saline corrosion Combination of free caustic and concentrating effect causes severe metal wastage. High chlorides and sulfates, result in corrosion from depolarization and depassivation effects... 

The effects of impurities are less important for oxygen-depolarized than for hydrogen-depolarized corrosion, since the values of polarization for oxygen reduction found at different metafs differ fess strongfy than those for hydrogen evofution. 

Under most usual oxygenated and alkaline-operating conditions for cooling water, the oxygen cathode provides the rate-determining step, and depolarization by O2 fthe removal of hydrogen ion) will increase the rate of corrosion and hence metal wastage, as below ... 

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 ... 

Corrosion often begins at a location where the metal is stressed in some way or isolated from oxygen, such as between joints or under a paint film. The metal ions dissolve in the moisture film and the electrons migrate to another location where they are taken up by a depolarizer . Oxygen is the most common depolarizer the resulting hydroxide ions react with the Fe2+ to form the mixture of hydrous iron oxides known as rust . 

Corrosion continues as a "depolarizer removes electrons from metal ... 

An -> ideal nonpolarizable electrode is one whose potential does not change as current flows in the cell. Much more useful in electrochemistry are the electrodes that change their potential in a wide potential window (in the absence of a - depolarizer) without the passage of significant current. They are called -> ideally polarized electrodes. Current-potential curves, particularly those obtained under steady-state conditions (see -> Tafel plot) are often called polarization curves. In the -> corrosion measurements the ratio of AE/AI in the polarization curve is called the polarization resistance. If during the -> electrode processes the overpotential is related to the -> diffusional transport of the depolarizer we talk about the concentration polarization. If the electrode process requires an -> activation energy, the appropriate overpotential and activation polarization appear. 

Although corrosion is favored by a large difference of potential between the anodic and cathodic portions of a system, even the smallest of such differences is sufficient to stimulate corrosion in the presence of a depolarizer. In an apparently uniform piece of metal, any portion which has been subjected to strain is less noble than an unstrained portion and small crystals are less noble than large ones further, minute inclusions of noble material are often found in relatively pure metals. These differences permit local voltaic cells to be set up and, in the presence of a depolarizer, corrosion of the baser (anodic) regions will occur. 

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... 

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