Chapter5 - Classification of corrosion inhibitors

Abstract

Corrosion inhibitors are chemical compounds that decrease the corrosion rate at low concentration. Numerous classifications have been proposed to classify corrosion inhibitors depending upon their properties. One of the most common and broad classifications of corrosion inhibitors is based on their inorganic or organic nature. Inorganic corrosion inhibitors may be anodic- or cathodic-types depending upon their major influence on either type of polarization reactions. Inorganic compounds that passivate metal surfaces, i.e., form protective oxide film in the absence of oxygen are called oxidizing anions. Some of the inorganic corrosion inhibitors essentially require the presence of oxygen for the passive film. These anions are known as nonoxidizing anions. Cathodic inorganic inhibitors can be classified as cathodic poisons that decrease the rate of cathodic reaction, cathodic precipitates that precipitate at cathodic site, and avoid the diffusion of corrosion species and scavengers that react with the corrosive species such as oxygen and decrease the rate of cathodic reaction. Various classifications have been proposed for organic corrosion inhibitors. Broadly, organic corrosion inhibitors can be divided as aliphatic or aromatic types. Obviously, aliphatic corrosion inhibitors contain aliphatic hydrophobic carbon chain(s) along with at least one polar functional group. Aromatic corrosion inhibitors can be further divided as hetero- or homo-aromatic corrosion inhibitors. More so, organic compounds can also be classified as natural or synthetic types depending upon their mode of origin. Based on their mechanism of corrosion mitigation, organic corrosion inhibitors can be classified as anodic-, cathodic-, or mixed-type. The present chapter describes the various methods of classification of inorganic and organic corrosion inhibitors.