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Azoles

 
  1. Azoles are a class of organic compounds used as corrosion inhibitors, especially for yellow metals like copper and brass.
  2. The most common ones:
    1. Benzotriazole (BZT)
    2. Tolyltriazole (TTA)
    3. Mercaptobenzothiazole (MBT) (used less often now due to environmental concerns)
  1. Why Use Azoles in Closed-Loop Systems?
    1. Closed loops often have a mix of metals: carbon steel, copper, brass, sometimes aluminum.
    2. While nitrite, molybdate, or phosphate may protect steel, azoles are the go-to for protecting copper alloys.
  1. Mode of action
    1. Formation of a thin protective film on the metal surface
    2. Preventing galvanic corrosion between dissimilar metals
  2. They’re often used in chilled water and hot water loops, especially where copper/brass heat exchangers or components are present.
  3. Typical Dosage of Azoles:
Azole Type Target Concentration (mg/L or ppm )
Benzotriazole (BZT) 5–10 ppm (as active ingredient)
Tolyltriazole (TTA) 3–10 ppm (as active)
 
  1. For initial passivation or new system start-up, higher doses (up to 25–50 ppm as active) may be used temporarily, then dialed back.