CORROSION INHIBITORS

Understanding how various corrosion inhibitors work is directly related to understanding the corrosion process. In the previous section we discussed in detail the corrosion process and the various forms in which it can appear. However, we can recap this discussion by reiterating that for corrosion to take place we need: an anode, a cathode, a circuit through the metal for electrons to flow, and our "wire" to complete the circuit, the cooling water. I

f any of these items are missing, corrosion will not take place. Once we understand these basics of the corrosion process, it becomes obvious that the formation of a chemical inhibition film at either the anode, cathode, or both of these sights will ultimately minimize the corrosion process. Therefore, corrosion inhibitors are classified as anodic, cathodic, or both, depending upon the corrosion reaction each controls.

Corrosion inhibition usually results from one or more of three general mechanisms. In the first, the inhibitor molecule is adsorbed on the metal surface by the process of chemisorption, forming a thin protective film either by itself or in conjunction with metallic Ions. Some inhibitors, however, merely cause a metal to form its own protective film of metal oxides, thereby increasing its resistance; this constitutes the second mechanism. In the third, the inhibitor reacts with a potentially corrosive substance in the water. 

Choice of the proper inhibitor is determined by the cooling system design parameters and water composition. The type of metals in the system, stress conditions, cleanliness and designed water velocity all affect inhibitor selection. In addition, other factors to be considered include treatment levels required, pH, dissolved oxygen content and, salt and suspended matter composition.

Corrosion inhibition usually results from one or more of three general mechanisms. In the first, the inhibitor molecule is adsorbed on the metal surface by the process of chemisorption, forming a thin protective film either by itself or in conjunction with metallic Ions. Some inhibitors, however, merely cause a metal to form its own protective film of metal oxides, thereby increasing its resistance; this constitutes the second mechanism. In the third, the inhibitor reacts with a potentially corrosive substance in the water.