Ferric-tannates

Tannins are polyphenols that occur only in vascular plants such as leaves, needles, barks heartwood, seeds and flowers. Tannins exist primarily in condensed and hydrolysable forms. Natural tannin extracts have been employed since the turn of the last century in leather industry. Recent development of their industrial uses as adhesives, flocculants, depressants, viscosity modifier agents and more recently as corrosion inhibitors reflects their importance as industrial raw materials. Electrochemical studies have shown that tannins extracted from the barks of mangrove trees are excellent corrosion inhibitors of steel at very low pH. The mechanism of inhibition at this pH was due to the chemisorption of tannin molecules while at higher pH, inhibition was achieved via formation of ferric-tannates [6]. 

From the X-ray diffraction, XRD pattern in Fig. 19.2, the pre-rasted sample was found to consist of mainly lepidocrocite and magnetite and traces of geothite. The XRD pattern indicated the reduction of several lepidocrocite peaks in favom of ferric-tannate formation after the addition of mangrove tannins. 

Fig. 19.3 FTIR spectram of (a) bare rust and (b) converted rust samples FT-ferric-tannate, G-geothite, L-lepidocrocite, M-magnetite...

Fig. 19.5 The most stable ferric-tannate eomplex with B,Bj eonformation...

Partial transformation of rust into ferric-tannates was found to occur with the addition of mangrove tannins leading to a low inhibitive behaviour. The mechanism of adsorption of catechin onto the iron surface has been established via theoretical... 

Ferric Tannate, Ferric gallotannate. Variable composition. Contains 8-10% Fe. 70-80% tannin. 

Gum arable is also at least partially effective as a treatment for ferric casse in red wines. It does not prevent the appearance of a dark, blnish color, due to the formation of colloidal ferric tannate, but it does stop the colloid flocculating. It acts differently from citric acid, which prevents color from changing, as it produces a soluble complex with iron that is no longer capable of reacting with tannins (Section 4.6.2). These two treatments are often complementary (Section 4.6.3). 

Cold stabilization is also partially effective in preventing other types of colloidal precipitation. It helps to prevent ferric casse by insolubilizing ferric phosphate in white wines and ferric tannate in reds. However, even after aeration to promote the formation of the Fe + ions involved in these mechanisms, only small quantities of iron are eliminated. Fining at the same time as cold stabilization improves treatment effectiveness but is never sufficient to prevent ferric casse completely. 

Contradictory opinions have been referred to in the literature particularly on the nature of the iron-tannate and its interaction with the rusted steel due to the diversity of the material used in different studies. Studies have included the use of tannic acid [7-10], gallic acid [11], oak tannin [12, 13], pine tannin [14] and mimosa tannin [15]. In order to establish the correlation between the ferric-tannate formation and the low inhibition efficiency observed at high pH from the electrochemical studies, phase transformations of pre-rusted steels in the presence of tannins were evaluated. In this work the quantum chemical calculations are conducted to analyse the relationship between the molecular structure and properties of ferric-tannate complex and its inhibitory mechanism. 

Ferric tannate is the major ingredient of black ink, and acid destroys it. That s how old-fashioned ink eradicators worked — you sprinkled a little acid on your error and hoped it would destroy the ink before it destroyed the paper. Careful blotting was clearly necessary. 

Metal instability, described as casse, is relatively rare today. When encountered, the metals involved are generally copper and iron. The latter may be present as either ferric phosphate ( white casse) or ferric tannate ( blue casse). Even though ferric phosphate casse is described as white casse, it may assume various shades of blue even in white wines (Toland, 1996 personal communication). Copper casse is present as an initially white and later reddish-brown precipitate in bottled or other wines stored... 

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