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Abstract
Giving a “new life” to wastes should be the golden rule for all production processes in the forthcoming future, aiming at making them more sustainable and environmentally friendly. In the corrosion science field, the ambitious circular economy paradigm has recently led to the employment of extracts from plants (and, in less extent, from agri-food wastes) as green inhibitors against corrosion of metals. However, in spite of the number of scientific papers published in the field, a deep revision of the scientific approach is needed both in the execution of experiments and in the critical analysis of the results. Starting from some discrepancies in published data, the corrosion inhibition effect induced by a well-characterized methanolic extract from wastes of fermented Punica granatum and by its main component (ellagic acid, EA) was validated. The corrosion behaviour of Armco® pure iron in the presence of small concentrations of ellagic acid and extract (containing ca. 10 µM and 100 µM EA) was studied by combining results from mass loss tests, at free corrosion potential, and from polarization tests, by linearly sweeping the potential applied to the metal substrate. Experiments were carried out both in acidic medium (typical for a general corrosion phenomenon) and in near-neutral chloride bearing solution (characteristic of a localized pitting corrosion phenomenon). Neat conflicts with already published data have been identified first in the solubility of the inhibitor and then in the inhibition efficiency (around 40% in a 0.05 M HCl). The very limited solubility in aqueous environment was identified as the main drawback, hindering any possible exploitation of ellagic acid and pomegranate extract as promising green corrosion inhibitors. Results point to the necessity to establish clear and rigorous good laboratory practices to follow while reporting results on such complex matrices like vegetable extracts.
Highlights
The biggest challenge of the present and near future is the preservation of our planet
This general trend has been experimentally confirmed for ellagic acid: the equilibrium solubility of the species increases by two orders of magnitude by passing from pH 5.5 to 7.5 [13]
PEG300 works itself as an effective corrosion inhibitor of iron (IE ca. 80%), in good agreement with theChoice behaviour evidenced by glycols ofInhibition different molecular weights towards mild of a Co-Solvent forpolyethylene
Summary
The biggest challenge of the present and near future is the preservation of our planet. The related green chemistry principles [1], represent a valuable route to reduce wastes, to favor a cleaver reuse of by-products and to decrease both pollution and dependence on primary resources. The label “green”, or sometimes “eco-friendly”, is commonly added to the noun inhibitor if the molecule, in addition to meeting the previous definition of inhibitor, has a biocompatibility in nature [5]. Part of this class of inhibitors are complex matrices like plant extracts due to their biological origin and, by extension, their single components like flavonoids, alkaloids, polyphenols and glycosides
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