Carbonic Acid

Definition - What does Carbonic Acid mean?

Carbonic acid is an inorganic compound with the formula H2CO3. It is a very weak acid that forms when water and carbon dioxide combine. This acid can corrode, rust and pit steel but the extent of those effects depends upon the chemical composition of the steel.

In geology, carbonic acid causes limestone to dissolve, producing calcium bicarbonate which leads to many limestone features such as stalactites and stalagmites. Carbonic acid increases water's ability to leach minerals from rock.

It is also a name sometimes given to solutions of carbon dioxide in water, because such solutions contain small amounts of H2CO3.

Corrosionpedia explains Carbonic Acid

Carbonic acid is a diprotic acid from which two series of salts can be formed—namely, hydrogen carbonates, containing HCO3-, and carbonates, containing CO32-. The acid-base behavior of carbonic acid depends on the different rates of some of the reactions involved, as well as their dependence on the pH of the system.

Carbonic acid is used in the making of:

  • Soft drinks
  • Artificially carbonated sparkling wines
  • Other bubbly drinks

In aqueous solutions, a small portion of carbonic acid further dissociates to form H+ and bicarbonate (HCO3) ions, which causes corrosion of carbon steel. Carbonic acid corrosion is the final result of piping being chemically dissolved and then being deposited in steam traps, condensate returns and any other point where condensate can collect.

Carbon steel corrodes 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.

Carbonic acid also triggers pitting, another specialized type of corrosion driven by electrochemical process. When 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.

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