What Does Cathodic Inhibitor Mean?
Cathodic inhibitors slow the reaction at the cathode or precipitate cathodic areas in order to increase the impedance on the surface, thus limiting diffusion of reducible species. The nature depends on the material to be protected or agents to be neutralized.
This can also be described as a chemical compound that decreases the rate of corrosion of a given metal or an alloy when added to a liquid or a gas. The effectiveness of these inhibitors depends on the flow regime, water quantity and the fluid composition. The process involves the formation of a passivation layer to prevent the access of the corrosive substance to the metal.
Corrosionpedia Explains Cathodic Inhibitor
Inhibition can be achieved through different mechanisms, namely:
- Cathodic poisons: These are used by stifling the processes of cathodic reduction to balance the reaction at the anode. The susceptibility of the metal to hydrogen-induced cracking can be prone to cathodic inhibition because the metal can absorb hydrogen during cathodic charging or aqueous corrosion. In low-pH solutions, some reduced hydrogen diffuses as atomic hydrogen into the metal instead of forming the gas. This happens during electroplating or pickling of the metal. Cathodic poisons include substances like antimony, arsenic, sulfur, tellurium, selenium and cyanide ions, which hinder the hydrogen atoms from forming hydrogen gas. Environments that contain hydrogen sulfide are dangerous for metals and alloys.
- Oxygen scavengers: These are chemicals that react with the dissolved oxygen for corrosion reduction. Sulfite and bi-sulfite ions are the best examples that form sulphates when reacting with oxygen. The redox reaction is catalyzed by either cobalt or nickel. Before any lowering of oxygen dissolved in mud is done by a scavenger, air is removed from the mud through mechanical foaming and degassing.
- Cathodic precipitates: These include zinc, calcium and magnesium. They are precipitated on the surface of the metal to form a protective layer. Since the work of an inhibitor is to decrease the anodic process rate, the corrosion potential change after an inhibitor has been added indicates a retardation in the process. Positive displacement of the corrosion potential indicates a retardation of the anodic process. The negative displacement of the potential indicates retardation of the cathodic process. If there is only a small change, it means that both the cathodic and the anodic processes have been retarded.