Cathodic protection minimum current

Iron corrodes in a sea water solution of pH 7. Assuming corrosion proceeds by oxygen depolarization, calculate the minimum current required for complete cathodic protection. Exchange current density, °, for oxygen reduction is... 

Table 10.14 Guidance on minimum design current densities for cathodic protection of...

Once the corrosion (mixed) potential is known, the estimation of the cathodic protection current is relatively simple the cathodic Tafel line is extended until the ordinate reaches the anode equifibrium value. The current corresponding to that ordinate value is the minimum value of the external current that must be suppfied to stop the corrosion process. For processes in which there are multiple species undergoing cathodic or anodic reactions, the resultant cathodic and anodic Tafel curves are calculated by adding the individual polarization curves within the respective potential range. 

If the metal is polarized slightly beyond the open-circuit potential, of the anode, the corrosion rate remains zero. Net current flows from the electrolyte to the metal hence, metal ions cannot enter the solution. Current in excess of the required does no good, however, and may damage amphoteric metals and coatings. In practice, therefore, the impressed current is kept close to the theoretical minimum. Should the applied current fall below that required for complete protection, some degree of protection nevertheless occurs. For example, if the corrosion potential is moved from to a in Fig. 5.15, by appUed current e-b, the corrosion current decreases from Icon to b. As the applied current e-b is increased, the potential a moves to more active values, and the corrosion current b becomes smaller. When a coincides with < )x, the corrosion current becomes zero, and the applied current for complete cathodic protection equals /appi. 

There are several methods that can be used to control corrosion of steel reinforcements in concrete. First, the design of the structure should provide for drainage of salt-containing waters away from the reinforced concrete. Second, concrete of adequate thickness, high quality, and low permeability should be specified to protect the reinforcements from the environment. Third, chloride content of the concrete mix should be kept to a minimum. For further protection, the steel reinforcements can be epoxy-coated. In many parts of North America, steel reinforcements used in bridge decks are now epoxy-coated as a standard construction procedure. Cathodic protection is also being used, both with impressed current anodes and with sacrificial anodes [61]. (See Chapter 13.)... 

Iron corrodes in seawater at a rate of 2.5 gmd. Assuming that all corrosion is by oxygen depolarization, calculate the minimum initial current density (A/m ) necessary for complete cathodic protection. 

The potential necessary to protect buried steel is —0.85 V, however, in the presence of sulfates, reducing bacteria a minimum potential of-0.95 V with respect to copper sulfate electrode would be necessary. Approximately 15-100 mA/ft current is needed for protection of bare steel in sluggish water. In rapidly moving water, 1-10 mA/ft for bare steel in a soil would be necessary. Current requirements in various environments can be foxmd abundantly in the literature as well as cathodic protection specifications. For submarine pipeline, a current density of 5 mA/ft is required. 

The dc power supplies used in anodic protection systems have similar design and requirements as the rectifiers for cathodic protection, with one exception. Because of the nature of the active-passive behavior of the vessel, the currents required to maintain the potential of the vessel wall in the passive range can become very small. Some designs of dc power supplies must be specially modified to reduce the minimum amount of current put out of the power supply ... 

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