1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) is another phosphonic acid-containing compound that is used as a water treatment agent. It is an effective chelating agent and sequestrant, and is widely used to prevent scale deposition and other mineral deposits on metal surfaces in cooling water systems, boiler water, and oil field operations. HEDP is also used as a detergent builder, a corrosion inhibitor in metalworking fluids, and an ingredient in personal care products. It is highly soluble in water and has excellent thermal stability, similar to ATMP. HEDP and ATMP have similar properties and are often used in combination to provide synergistic benefits.