The Effects of Carbonic Acid on Steel

Carbonic acid (H2CO3) is a common inorganic compound formed when carbon dioxide (CO2) dissolves in water (H2O). In aqueous solution, a small portion of carbonic acid will further dissociate to form H+ and bicarbonate (HCO3) ions. The resultant weak acid can corrode, rust or pit steel but the extent of those effects depends upon the chemical composition of the steel.

General Corrosion

The most common effect of carbonic acid on steel is general corrosion, the full or partial breakdown of the steel into its constituent chemical components. Carbon steel will corrode very quickly when it comes into contact with carbonic acid. Corroded carbon steel can weaken, bend or break, posing a significant problem in pipes and valves. Stainless steel, in contrast, resists general corrosion caused by carbonic acid.

Mesa Corrosion

At high temperatures, the dissociation of carbonic acid into bicarbonate and H+ ions does not cause general corrosion in steel. Rather, the carbonic acid may interact with the steel, resulting in the creation of iron carbonate (FeCO3). This process is known as mesa corrosion due to the appearance of ridges and valleys upon the steel surface that resemble the mesas caused by erosion.

Rust

Rust is a common chemical byproduct of general or mesa corrosion. Rust is produced by an additional, oxidizing reaction that accompanies the corrosion reaction and produces iron (ferrous) ions. Rust will transform the appearance of the affected steel, producing a red or orange appearance. Rust may develop in any type of steel containing iron, including carbon steel or alloy (non-carbon) steels such as stainless, weathering or tool steel.

Pitting

Carbonic acid may trigger pitting, another specialized type of corrosion driven by electrochemical process. When the carbonic acid comes into contact with a small location on the steel, the acid dissolves the steel into free ions, causing that location to become positively charged. Negatively charged ions are attracted to that area and their migration creates small, distinctive pits within the steel. Alloy steels, such as stainless or nickel-based maraging steels, are more vulnerable to pitting than carbon steel.