Abstract
The material of low density with naturally-formed protective oxide film and good mechanical properties is widely used as an engineering material, namely Aluminum and its alloys. The passive layer, however, is not stable in strong acidic or alkaline medium, leading to increased corrosion rates and reduced durability of the material. The conventional corrosion inhibitors have raised the environmental issues and led to much research on plant extracts, natural compounds, amino acids, biopolymers, agricultural wastes, and others as environmentally friendly alternatives. This review critically compares the green corrosion inhibitor (GCI) systems for the protection of aluminum and its alloys in acidic and alkaline media, focusing on the type of inhibitor, the composition of the substrate, corrosive environment, GCI efficiency, adsorption behavior of the inhibitor, electrochemical response of the alloy, surface protection, thermodynamic interpretation, and computational analysis. The literature reviewed indicated that inhibition efficiencies of more than 90% have been demonstrated in both acidic and alkaline conditions for optimal laboratory conditions. Despite this, it is not possible to make direct comparisons of inhibitors based on the maximum inhibition efficiency alone, as the performance of an inhibitor is strongly dependent on the concentration of the inhibitor, the composition of the alloy, the electrolyte, its concentration, the temperature, the duration of exposure to the electrolyte, and the method of measurement. Both environments have adsorption-mediated protection, but the mechanism of protection is quite different because of the different interfacial chemistry of acid-assisted oxide dissolution and hydroxide-assisted aluminate formation. The main research gaps are the lack of standardized comparisons between acidic and alkaline, lack of reproducibility for the crude plant extracts, lack of identification of active phytochemicals, lack of consistent use of adsorption-thermodynamic treatment, lack of long-term and industrial validation, lack of thorough environmental assessment and lack of simplified computational representation of realistic Al surfaces. Thus, future studies need to shift from empirical extract screening to chemically characterized, mechanism validated, and environmentally verified and standardized inhibitor development strategies. This method is crucial for the development of successful and sustainable corrosion control technologies from promising laboratory-proven green inhibitors.