By definition, a corrosion inhibitor is a chemical substance that, when added in small concentration to an environment, effectively decreases the corrosion rate. The efficiency of that inhibitor is thus expressed by a measure of this improvement
CRuninhibited = corrosion rate of the uninhibited system
CRinhibited = corrosion rate of the inhibited system
In general, the efficiency of an inhibitor increases with an increase in inhibitor concentration, e.g. a typically good inhibitor would give 95% inhibition at a concentration of 0.008% and 90% at a concentration of 0.004%.
Through the years, sophisticated corrosion inhibitor test methods, typically designed to reproduce the most extreme conditions in a system, have been employed to improve inhibitor capabilities. New and better corrosion inhibitors have been developed as a result of their performance in elaborate laboratory apparatus, yet many have not achieved comparable performance in the field. The inability to transfer inhibitor performance from the laboratory to the field remains a challenge today. However, correlation of laboratory and field performance may be possible once key factors involved in inhibitor chemistry and corrosion theory are considered.