Which chemical decreases corrosion rates?

• A. Calcium carbonate CaCO3
• B. Sodium chloride NaCl
• C. Magnesium hydroxide Mg(OH)2
• D. Ammonium sulfate NH4SO4

Answer: (A)

Calcium carbonate decreases corrosion rates because it buffers the water and helps form a protective scale on the inside of pipes. When CaCO3 is present, it raises the water’s alkalinity and pH, reducing the aggressive acidic conditions that drive metal dissolution. If the water is near CaCO3 saturation, calcium carbonate can precipitate as a thin, protective film on the pipe surface, limiting direct contact between the metal and the water and thereby slowing corrosion.

Sodium chloride adds chloride ions that can promote corrosion, particularly pitting in metals. Ammonium sulfate and magnesium hydroxide do not reliably provide the same protective carbonate scale under typical distribution-system conditions, so they’re not as effective for consistently reducing corrosion as calcium carbonate.