Citric Acid — Frequently Asked Questions

Passivation removes free iron from the surface so the protective chromium oxide layer can re-form uniformly. Citric acid chelates that free iron and lifts it away without attacking the chromium, achieving the same result as nitric acid passivation. It is increasingly preferred because it avoids nitric acid's fume hazard, its aggressive effluent and the nitrate discharge that goes with it.

By sequestering hardness ions. A detergent builder's job is to tie up calcium and magnesium so they cannot consume surfactant by forming insoluble scum. Citric acid and its sodium salt do that through chelation, and being readily biodegradable and non-nutrient they avoid the eutrophication problem that led to phosphate restrictions in cleaning products.

Yes, which is a large part of its appeal. It contains no chloride, so it does not risk the pitting and stress corrosion cracking that hydrochloric acid can initiate in austenitic stainless. That makes it suitable for clean-in-place descaling in dairy, brewing and food plant where chloride-based acids are excluded.

Water of crystallisation. The monohydrate carries one water molecule per acid molecule and is around 91 percent citric acid by weight; the anhydrous form is effectively 100 percent. Anhydrous is preferred where water content matters or where the material is being dry-blended; monohydrate is often cheaper per tonne and is widely used in solution work.