**11.1 Definition of inhibitor**

An inhibitor is a substance which when added to an environment in small concentration minimizes the loss of metal, reduces the extent of hydrogen embrittlement, protects the metal against pitting, reduces over pickling and acid fumes resulting from excessive reaction between the acid and basic metals and reduces acid consumption. They reduce corrosion by either acting as a barrier by forming an adsorbed layer or retarding the cathodic, the anodic or both processes [10]. A schematic representation of inhibitor process is shown in **Figure 12**.

Any corrosion retardation process or the reduction in the oxidation rate of the metal by addition of a chemical compound to the system is caused by corrosion inhibitors. Inhibitors are often easy to apply and offer the advantage of in-situ application without causing any significant disruption to the process. The use of corrosion inhibitors is one of the best methods of combating corrosion [11].

In order that they can be used effectively, three factors must be considered, namely:

• Identification of the corrosion problems.

Generally, the components of a corrosion cell (anode, cathode, electrolyte and electronic conductor) may be affected by corrosion inhibitors in order to reduce corrosion. The inhibitor may cause:


However there are several factors to be considered when choosing an inhibitor.


In order to avoid or reduce the corrosion of metallic materials, inhibitors used in cooling system must satisfy the following criteria

