Corrosion prevention with deoxygenators

Corrosion Inhibition by Oxygen Scavengers

A scavenger reacts with dissolved oxygen in a system, preventing the reduction of oxygen to hydroxide ions. Although the interactions between the metal, the scavenger, the pH of the environment, and other dissolved components are complex, oxygen scavengers can be considered corrosion inhibitors, most of whose inhibition involves chemical interactions between the metal and the inhibitor at the metal-water interface.

A scavenger removes dissolved oxygen in a closed system with a neutral or alkaline ph. As a result, corrosion is stopped because oxygen is not available for a cathodic reaction. Sulfite is a common, inexpensive scavenger and is widely used in water injection for oil recovery.

Scavengers are commonly used in systems involving carbon steels, cast iron, copper, and their alloys, and in industrial plants for the production of oil and steam. Oxygen can either cause or prevent corrosion, depending on the conditions. This behavior has been observed in boilers and has been attributed to the presence or absence of aggressive ions such as chloride. Oxygen-enhanced corrosion is a corrosion process that is significantly reduced by the removal of oxygen from the system. Oxygen corrosion can be seen as general or uniform corrosion, pitting, crevice corrosion, water pipe corrosion, and subsoil corrosion.

Inhibition by oxygen scavengers can be cathodic inhibition, in which the chemical removal of oxygen overcomes the hydroxyl ions, and anodic inhibition occurs through fouling. The rate of oxygen consumption by the oxygen scavenger is important in cathodic inhibition. This is a mode of inhibition in the absence of fouling. In the case of oil-borne water in oil fields and boiler feed water that is not sufficiently deionized, fouling does not occur. In these cases, an oxygen scavenger along with pH adjustment is sufficient to inhibit corrosion.

Examples of oxygen scavengers include sulfites, bisulfites, and hydrazine. Sulfites are converted to sulfates by absorbing oxygen, which cause problems in terms of reducing fluid velocity and heat transfer by depositing on pipe walls and metal surfaces. Hydrazine is often used in steam boilers, especially for high-pressure and high-temperature systems. Hydrazine acts slowly at low temperatures. One of the advantages of using this oxygen scavenger is that it does not leave solid residue. Hydrazine has a high vapor pressure. On the other hand, hydrazine can convert to NH3 and cause corrosion in copper parts. Note: Oxygen scavengers are not usually used with deoxidizers because one tries to prevent oxidation and the other tries to oxidize the metal.