Cooling- and boiler-water corrosion inhibitors u2014 sodium molybdate, nitrite, zinc, and the yellow-metal azoles benzotriazole, tolyltriazole, and mercaptobenzothiazole u2014 for multi-metal water systems.
Corrosion inhibitors for water treatment are chemicals dosed into cooling,nboiler, and closed-loop water systems to slow the electrochemical attack on steel, copper,nand other metals in contact with the water. They work in three ways: anodicnpassivators such as molybdate and nitrite, a cathodic film-former such as zinc, andnadsorption films from the azoles u2014 benzotriazole, tolyltriazole, and mercaptobenzothiazole u2014nthat protect copper and brass. A multi-metal system usually needs a blend, because no singleninhibitor protects every metal in the loop.
nnThe first choice is usually nitrite versus molybdate. Sodium nitrite is inexpensive andneffective on steel, but it oxidizes and is consumed by nitrifying bacteria, so it needs anbiocide and frequent makeup; reserve it for closed loops where water loss is low. Sodiumnmolybdate passivates at a low dose and suits open recirculating systems, often blended with anphosphonate and zinc. The trade-off is cost and discharge: molybdenum draws effluent scrutinynin some jurisdictions and is the more expensive inhibitor.
nnBenzotriazole (BTA), tolyltriazole (TTA), and mercaptobenzothiazole (MBT) are yellow-metalninhibitors. They form a thin protective film on copper, brass, and bronze and do little fornbare steel, so pair them with an anodic or cathodic inhibitor in a multi-metal loop. Where annoxidizing biocide such as chlorine is held in the system, specify TTA: it tolerates a halogennresidual better than BTA, which a free-chlorine residual can degrade.
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