My mother always thought that I was a genius. She decided that I was a genius based on one incident that happened when I was six years old.
My mother called me into the bathroom. “Look at the sink.” The sink was discolored by brown, rusty stains from the old pipes in our ancient apartment house.
“Mom, I think my sister did that.”
“My son! No one is blaming you for the stains. Stop blaming your sister. What I want is for you to get the stains off.”
So I went into the kitchen, got a bottle of Coca-Cola, poured it over the stains, and the sink was clean. From this single incident, my mother decided I was a genius.
What was it about the Coca-Cola that removed the iron deposits from our sink? It was carbonic acid (H2CO3). (Coca-Cola contains lots of phosphoric and citric acid too.)
Carbonic acid is formed when CO2 dissolves under pressure in water. The resulting acid has a 5 to 6 pH, even at relatively high acidic concentrations. The acid readily dissolves iron to form water-soluble iron carbonate, Fe(HCO3).
This is a problem in process plant steam heaters. There are always some residual carbonates in boiler feed water. When the water is turned into steam, some of these carbonates decompose into CO2.
Thus, all steam is contaminated with CO2. The CO2 being far more volatile than water gets trapped and accumulates in the high points of steam heaters. With time, the CO2 condenses in the water to form carbonic acid. This causes corrosion and tube leaks. To avoid CO2 accumulation, the exchanger high points can be vented.
Taken (with minor changes) from: A Working Guide to Process Equipment, Norman and Elizabeth Lieberman, McGraw-Hill, 2008
