What Does Internal Corrosion Mean?
Internal corrosion refers to corrosion occurring on the inside of a pipeline.
This type of corrosion often results from the presence of molecules such as carbon dioxide (CO2), hydrogen sulfide (H2S), water, organic acids and other molecules. Typically, these molecules react with the internal pipe surface through anodic and cathodic reactions. The product of these reactions may deposit within the pipe, creating a protective layer that inhibits further corrosion. In other cases, the products do not precipitate and facilitate high rates of corrosion.
Internal corrosion can also be caused by microorganisms.
The rate of internal corrosion depends on the concentration of these corrosive molecules, the temperature, the flow velocity and the surface material.
Corrosionpedia Explains Internal Corrosion
It is important for pipeline installations to provide monitoring systems to detect internal corrosion in pipes. Early detection of corrosion can prevent costly repairs and improve control measures.
An illustrative example of internal corrosion is the oxidation of iron by aqueous CO2 to produce iron carbonate (FeCO3). Although the mechanism of this reaction varies upon conditions such as pH, temperature and pressure, we can understand the key electrochemical half-reactions. The anodic reaction is the oxidation of metallic iron by two electrons to produce Fe2+:
Fe = Fe2+ + 2e-
The formation of Fe2+ results in the deterioration of the pipe's metal surface because this process removes an iron atom away from the bulk metal. In compliment, we have the cathodic reaction:
2H2CO3 + 2e- = H2 + 2HCO3-
Combining the two redox half-reactions and simplifying with aqueous CO2 hydration and acid-base equations produces the following overall equation:
Fe + CO2 + H2O = FeCO3 + H2
The byproduct, FeCO3, is insoluble and may precipitate on a steel surface in conditions of high pH or temperature, blocking CO2 from reacting with the iron underneath.
Scientists study corrosion reactions like the example above to create models for the rate of corrosion. By understanding the relevant factors involved, methods can be developed to slow down the rate of corrosion and prevent serious damage from occurring. For example, using a high pH in the water phase of gas condensate pipelines can reduce corrosion rates by CO2 to below 0.1 mm/year. From the discussion above, we know that the rate reduction was achieved by facilitating the formation of protective FeCO3 layers at high pH. Another prominent rate reduction method is the addition of chemical compounds inside the pipes. These chemicals inhibit corrosion by suppressing chemical corrosion mechanisms.