I disagree with the answer below. Citric acid chelates iron and this increases its corrosion potential far beyond the simple pH issue of the answer. It can also corrode some stainless steels, notably ordinary stainless steel kitchen sinks. "Citric Acid corrosion of mild steel piping" Question: "I am a PSU student interning at an aluminum rolling plant. In our water based mill we use citric acid to lower the pH of the water, allowing it to seperate more easily from the mineral oil for filtration. The pH of the water is maintained between 4-5. The problem is the mild steel piping carrying the water is being corroded at a rediculous rate. I was curious if another acid may be used that would prove less corrosive. Some of the other plants within our corporation use Sulfuric acid, is the best choice. Would sulfuric acid corrode at a slower rate ?" Answer: Which acid, citric or sulfuric acid, would be more corrosive to carbon steel depends on concentration, temperature and contaminants. In general, I would not expect citric acid to be more corrosive than dilute sulfuric acid.