The often neglected closed loops are becoming increasingly critical to the performance of environmental control systems in commercial operations; due to this, increased awareness of the need to properly maintain them is becoming more common. Closed system inhibitors
have ranged from a variety of heavy metal chemistries based on oxidizing approaches to other treatment programs which rely primarily on film formation for corrosion inhibition. The role of other inhibitor components, including pH control and yellow metal inhibitors are discussed. This paper reviews these different technologies and highlights their
advantages and disadvantages. Keywords: azole, closed system, glycol, molybdate, nitrite, phosphate, phosphonate.