Physico-chemical characterisation of mangrove tannins as corrosion inhibitors
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Date
2005
Authors
Abdul Rahim, Afidah
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Abstract
Tannins from mangrove barks have been successfully extracted using 70 % aqueous acetone and purification by LH-20 column chromatography afforded condensed tannins of high purity. The reversed-phase HPLC analysis of condensed tannins following depolymerisation in phloroglucinol and acidic ethanol revealed mainly four terminal units namely catechin, epicatechin, epigallocatechin and epicatechin gallate. Substantial anti-oxidant capabilities have been shown by mangrove tannins which were comparable to the synthetic standards used.
The electrochemical experiments have shown that the inhibitive efficiency of mangrove tannins on steel which was largely contributed by the condensed tannins, increased with increasing concentrations at very low pH. The inhibitive performance of mangrove tannins was comparable with that of commercial mimosa, quebracho and chestnut tannins at pH 0, 0.5 and 2.0. All tannins that were investigated are cathodic inhibitors in acidic media and the inhibition efficiency of all tannins was found to decrease with increasing pH. At pH 0 and 0.5 the adsorption mechanism was attributed to chemisorption of tannin molecules onto the iron surface while at higher pH, the formation of ferric-tannates resulted in a low protective effect. The electrochemical studies have shown that the flavanoid monomers that constitute mangrove tannins are potential inhibitors for steel in acidic medium. All monomers act mainly as cathodic inhibitors of the steel corrosion and its inhibitive performance dependent on concentration in acidic conditions. To explain the adsorptive behaviour of the molecules on the steel surface, a semi-empirical approach involving quantum chemical calculations using HyperChem 6.0 was undertaken. The correlation between the electronic density of the molecule and the inhibiting properties was established. The most probable adsorption centers were found in the vicinity of the phenolic groups.
Inhibition efficiency of pre-rusted steel in 3.5 % NaCl solution containing
3.0 g L-1 tannins was found to be dependent on the concentration of phosphoric acid added and pH of solution. The correlation between ferric-tannate formation and the low inhibition efficiency observed at high pH from the electrochemical studies was established from phase transformation studies, evaluated via FTIR, XRD, SEM, EDS, humidity chamber and salt spray tests. All methods of evaluation inferred a temporary and low corrosion protection by mangrove tannins.
Description
PhD
Keywords
Chemical science , Mangrove tannins , Corrosion inhibitors