Which chemical decreases corrosion rates?

• A. Calcium carbonate CaCO3
• B. Sodium bicarbonate NaHCO3
• C. Sodium chloride NaCl
• D. Magnesium sulfate MgSO4

Answer: (A)

Corrosion rates in water systems are slowed when a protective mineral scale forms on metal surfaces. Calcium carbonate tends to precipitate from water that is rich in calcium and carbonate, creating a thin, adherent film that acts as a barrier between the metal and the corrosive components in the water. This film also helps stabilize pH, which further reduces the electrochemical driving forces of corrosion. Because of this tendency to form protective scale, calcium carbonate is the best choice for decreasing corrosion rates.

Sodium chloride supplies chloride ions that readily attack protective films and promote corrosion, especially pitting and general surface attack. Sodium bicarbonate acts mainly as a buffering agent, helping to maintain a stable pH, but it does not reliably form a protective, adherent scale in the same way calcium carbonate does. Magnesium sulfate is a sulfate salt and does not generally provide the same protective scaling effect, so it does not reduce corrosion rates in the same manner.