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Phosphoric acid
Phosphoric acid is less corrosive than other mineral acids. Many materials demonstrate useful resistance in phosphoric acid at least at low temperatures. Corrosion rates often increase with temperature and impurities in the acid.
Uses
Phosphoric acid is a component of fertilizers (80% of total use), detergents, and many household cleaning products. Dilute solutions have a pleasing acid taste; thus, it's also used as a food additive, lending acidic properties to soft drinks and other prepared foods, and in water treatment products. It is also used in rust proofing, engraving, and metal coating and is an intermediate or reagent in many manufacturing processes. Phosphoric acid also occurs naturally in many fruits and their juices. Apart from use of phosphoric acid itself, the greatest consumption of phosphoric acid is in the manufacture of phosphate salts. Taking advantage of its ability to lower blood pH, phosphoric acid has been used therapeutically to treat lead poisoning.
Many types of phosphoric acids are used in industrial applications. The common classification of these is wet-process phosphoric acids (WPA) and usually means that the acid contains impurities in combination with phosphoric acid. Some triggers to increased corrosiveness in technical phosphoric acids are fluorides, chlorides, and bromides. Care has to be taken while using phosphoric acids containing chlorides and at increased temperatures. A temperature change of 50–100 °C would cause the corrosion rate to increase more than 10-fold on a typical stainless steel grade, even where only low amounts of chlorides are present in the acid. If the chloride content were to increase to values exceeding 500 mg l−1 (ppm), even the S31600 grade would be unsuitable due to the high corrosion rate at this temperature range.