Water ionizes into hydronium (H3O+) cations and hydroxide (OH−) anions. The concentration of ionized hydrogen (as protonated water) in a cooling water system is expressed as pH. Low pH values increase rate of corrosion while high pH values encourage scale formation. Amphoterism is uncommon among metals used in water cooling systems, but aluminum corrosion rates increase with pH values above 9. Galvanic corrosion may be severe in water systems with copper and aluminum components. Acid may be added to cooling water systems to prevent scale formation if the pH decrease will offset increased salinity and dissolved solids.