Application of Mössbauer spectroscopy to the study of tannins inhibition of iron and steel corrosion

Abstract

The inhibitory effect of tannins was investigated using, among others, potentiodynamic polarizations and Mössbauer spectroscopy. These techniques confirmed that the nature, pH and concentration of tannic solution are of upmost importance in the inhibitory properties of the solutions. It is observed that at low tannin concentration or pH, both, hydrolizable and condensed tannins, effectively inhibit iron corrosion, due to the redox properties of tannins. At pH ≈ 0, Mössbauer spectra of the frozen aqueous solutions of iron(III) with the tannin solutions showed that iron is in the form of a monomeric species [Fe(H 2 O) 6 ] 3+ , without coordination with the functional hydroxyl groups of the tannins. The suspended material consisted of amorphous ferric oxide and oxyhydroxides, though with quebracho tannin partly resulted in complex formation and in an iron (II) species from a redox process. Other tannins, such as chestnut hydrolysable tannins, do not complex iron at this low pH. Tannins react at high concentrations or pH (3 and 5) to form insoluble blue-black amorphous complexes of mono-and bis-type tannate complexes, with a relative amount of the bis-ferric tannate generally increasing with pH. Some Fe 2+ in the form of hydrated polymeric ferrous tannate could be obtained. At pH 7, a partially hydrolyzed ferric tannate complex was also formed. The latter two phases do not provide corrosion protection. Tannin solutions at natural pH react with electrodeposited iron films (approx. 6 μm) to obtain products consisting only on the catecholate mono-complex of ferric tannate. Some aspects of the mechanism of tannins protection against corrosion are discussed.

Synthesis and characterization of ferric tannate as a novel porous adsorptive-catalyst for nitrogen removal from wastewater

“…With strong reducibility, the phenolic hydroxyl groups in tannic acid can reduce Fe 3+ to Fe 2+ . [35][36][37] In the case of the 1 : 10 ratio, the excess tannic acid can produce a dissolved ferrous tannate, with an absorption peak appearing at 302 nm. 38 In the cases of the 1 : 30 and 1 : 40 ratios, the ferric ion was in excess, which could lead to the formation of Fe(OH) 3 .…”

Electrocatalytic Nanostructured Ferric Tannates: Characterization and Application of a Polyphenol Nanosensor

“…According to the literature, it has been shown that tannins are able to react with iron to form insoluble amorphous compounds of mono‐ and bis‐type tannate complexes, for example, bis‐ferric tannate. According to the Mössbauer and XRPD results, Fe 3+ ions can be considered the most important precursors for the formation of ferric tannate complexes on the surface of SAMNs.…”

Natural Ingredients in Functional Coatings—Recent Advances and Future Challenges

“…For wooden cultural heritage objects, the appropriate materials for coating layers are natural products, such as proteins, lipids, polysaccharides, or terpenoids, having a waterproofing role and, occasionally, as ingredients of binding media [169], while for iron artefacts, adding natural compounds (tannins) into commercial resins can inhibit their corrosion [170]. For conservation studies, natural polymer coatings are preferred instead of resins, for indoor objects, due to their water-solubility, thus avoiding the use of harmful solvents, necessary for the application and removal of commonly used commercial protective coatings.…”