Mitigation of Mild Steel Corrosion in Acid by Green Inhibitors: Yeast, Pepper, Garlic, and Coffee

Synthesized organic chemicals, used as inhibitors in mitigating the corrosion of huge quantities of steel articles, pose a major threat to the global environmental problems and health hazards. Naturally occurring products which had been used for natural medication purposes, since the human civilization, are found to inhibit corrosion of steel. Electrochemical studies of the effects of black pepper, garlic, yeast, and coffee on acid corrosion of steel have shown that the corrosion current decreases by manyfold with increase in concentration of the inhibitors. These green inhibitors have been found to get adsorbed maximum up to 70–90%. The polarizing effect is more on cathodic reactions than on anodic reactions, acting as cathodic inhibitor, while a few behaves as anodic to mixed inhibitor. Mechanisms of adsorption are investigated by Frumkin, Temkin, and Langmuir isotherms. The free energy of adsorption is found to be between ？15 and ？40？kJ/m for most inhibitors, indicating the phenomena of physical adsorption. 1. Introduction Mild steel articles are prone to severe degradation in HCl. As a corrosion prevention and protection method, application of inhibitor is very popular. A number of heterocyclic compounds with N, S, and O as hetero atoms are proved to be effective corrosion inhibitors [1, 2], and the screening of synthetic heterocyclic compounds is still being continued. Though many synthetic compounds showed good anticorrosive activity, most of them are highly toxic to both human beings and environment. The safety and environmental issues of corrosion inhibitors arisen in industries have always been a global concern. These inhibitors may cause the reversible (temporary) or irreversible (permanent) damage to organ system, namely, kidneys or liver, or to disturb a biochemical process or to disturb an enzyme system at some site in the body. The toxicity may manifest either during the synthesis of the compound or during its applications. These toxic effects have led to the use of natural products as anticorrosion agents which are ecofriendly and harmless [3]. In recent days many alternative ecofriendly corrosion inhibitors have been developed, they range from rare earth elements [4] to organic compounds [5–8]. A few natural products such as plant extracts and animal proteins were reported [9] to have been used in pickling acid bath. But detailed studies of corrosion rate determination and adsorption of natural products as green inhibitors are very limited [10, 11]. In the present investigation, mitigation of corrosion of low carbon steel has been