**5. General extraction methods and challenges faced for sustainability**

The most crucial step is the field of green inhibitors is the extraction of active substances. In general, extraction is a separation process, in which the active ingredients are isolated from the plant. Proper extraction processes are needed to extract the required active ingredients from the plants. The suitability of the extraction method depends on the polar or non-polar nature of the target compound, the size of the sample particles, and the presence of interfering materials. In addition, the selection of the solvent for extraction has to be rigorous and guarded as it should be based on the type of plant, part to be extracted, the availability of solvent, and the nature of the bioactive compounds [7]. Generally, extraction of polar compounds involves polar solvents, while nonpolar solvents are applied in extraction of nonpolar compounds. Several commonly used techniques [7, 91] can be applied to separate and extract the required extract from the plants among them:


*Controlling Corrosion Using Non-Toxic Corrosion Inhibitors DOI: http://dx.doi.org/10.5772/intechopen.109816*

Each extraction method has its own characteristics. The type of extraction process can greatly affect the final natural products obtained. It should be chosen with caution according to the objective of the study. It has an effect on purity, price and yield and depends on the compound of interest and the required degree of purity. However, the choice of extraction and purification methods is another important fact. Some process is tedious, cumbersome, energy-consuming, time-consuming, and expensive. For example, plant extracts comprise only tiny portion of active constituents, therefore, a large amount of plants is mandatory to achieve satisfactory inhibition ability which results in a high cost. High temperature could lead to the deterioration of the sensitive active constituents and thus reduce the relative inhibition efficiency. In addition, the extraction process is too complex to be appropriate for large-scale applications in industries. Extraction requires relatively the use of high-level organic solvents that emit greenhouse gases that threatened humans, agriculture, and microorganisms. Moreover, excessive use of solvents leads to enormous waste of by-products. Unlike these dangerous techniques, further research is needed to introduce efficient and environmentally friendly processes such as "green processing," "green solvents," and "green products."
