Cooling- and boiler-water corrosion inhibitors — sodium molybdate, nitrite, zinc, and the yellow-metal azoles benzotriazole, tolyltriazole, and mercaptobenzothiazole — for multi-metal water systems.
Corrosion inhibitors for water treatment are chemicals dosed into cooling, boiler, and closed-loop water systems to slow the electrochemical attack on steel, copper, and other metals in contact with the water. They work in three ways: anodic passivators such as molybdate and nitrite, a cathodic film-former such as zinc, and adsorption films from the azoles — benzotriazole, tolyltriazole, and mercaptobenzothiazole — that protect copper and brass. A multi-metal system usually needs a blend, because no single inhibitor protects every metal in the loop.
The first choice is usually nitrite versus molybdate. Sodium nitrite is inexpensive and effective on steel, but it oxidizes and is consumed by nitrifying bacteria, so it needs a biocide and frequent makeup; reserve it for closed loops where water loss is low. Sodium molybdate passivates at a low dose and suits open recirculating systems, often blended with a phosphonate and zinc. The trade-off is cost and discharge: molybdenum draws effluent scrutiny in some jurisdictions and is the more expensive inhibitor.
Benzotriazole (BTA), tolyltriazole (TTA), and mercaptobenzothiazole (MBT) are yellow-metal inhibitors. They form a thin protective film on copper, brass, and bronze and do little for bare steel, so pair them with an anodic or cathodic inhibitor in a multi-metal loop. Where an oxidizing biocide such as chlorine is held in the system, specify TTA: it tolerates a halogen residual better than BTA, which a free-chlorine residual can degrade.
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