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Cathodic inhibitors either slow the cathodic reaction itself or selectively precipitate on cathodic areas to increase the surface impedance and limit the diffusion of reducible species to these areas. Cathodic inhibitors can provide inhibition by three different mechanisms as:
Cathodic precipitates
Some cathodic inhibitors, such as compounds of arsenic and antimony, work by making the recombination and discharge of hydrogen more difficult. Other cathodic inhibitors, ions such as calcium, zinc or magnesium, may be precipitated as oxides to form a protective layer on the metal. Oxygen scavengers help to inhibit corrosion by preventing the cathodic depolarization caused by oxygen. The most commonly used oxygen scavenger at ambient temperature is probably sodium sulfite (Na2SO3). (reference)
Cathodic poisons are used advantageously as corrosion inhibitors by stifling the cathodic reduction processes that must balance the anodic corrosion reaction. However cathodic poisons can also increase the susceptibility of a metal to hydrogen induced cracking since hydrogen can also be absorbed by metal during aqueous corrosion or cathodic charging.
When corrosion occurs in a low-pH solution, some of the reduced hydrogen does not form gaseous hydrogen, but instead, diffuses into the metal as atomic hydrogen. This can happen during pickling and electroplating of metal. Substances such as arsenic, antimony, sulfur, selenium, tellurium, and cyanide ions prevent the hydrogen atoms from forming hydrogen gas, and are called cathodic poisons. Cathodic poisons facilitate contamination by keeping hydrogen in atomic form, in which hydrogen more readily diffuses into the metal. Environments containing hydrogen sulfide, which contains both hydrogen and a cathodic poison (i.e. sulfur), are especially dangerous for alloys and metals. Hydrogen sulfide is often encountered in the petroleum industry�during the drilling and completion of oil and gas wells, and during the storage and piping of petroleum products containing hydrogen sulfide.
A chemical that reacts with dissolved oxygen to reduce corrosion, such as sulfiteand bisulfite ions that combine with oxygen to form sulfate. This is a redox reaction and requires a nickel or cobalt catalyst. Removal of air from a mud by defoaming and mechanical degassing is an essential first step before a scavenger can lower the dissolved oxygen content.
