Corrosion inhibitors is “a substance which retards corrosion when added to an environment in small concentrations” (by NACE)
The corrosion inhibitors function by:
1) Adsorption to form a thin film on the surface of metal
2) Introducing an corrosion reaction barrier by corrosion products
3) Changing local corrosivity by removal, neutralising or inactivating an aggressive corrodant
There are various way to categorise corrosion inhibitors, the most common way is by their function as described above:
1) adsorptive corrosion inhibitor
2) film forming inhibitors
They may also be categorised with their impact on corrosion electrochemical reaction mechanisms:
1) anodic corrosion inhibitors
2) cathodic corrosion inhibitors
3) mixed reaction corrosion inhibitors
Neutralising corrosion inhibitors are good example of changing local corrosive environment: lime, soda ash, caustic are the common chemicals for the neutralisation. In refining environment, the hydrolysis of chlorides to hydrochloric acid at elevated process temperature is the major cause for metal loss. Addition of alkali to suppress the hydrolysis is the major mitigation, However, the most common associated side effect is scaling. The scaling issue is aggravated by using high pH boiler blowdown or high pH effluents from sour water strippers as feed. The other side effect is foaming with the high alkalinity and presence of surfactant. This is typically serious in refinery desalting unit. Ammonia is the most common neutralising corrosion inhibitor used for overhead stream. The side effects of using ammonia are mainly from its corrosvity to copper alloy and forming of ammonium chloride (NH4Cl) fouling deposit. The first drawback is usually managed by tightly controlling of fluid pH, while the later is mitigated by intermittent or continuous washdown.