**Use in corrosion resistance**

Lignin contains hydroxyl, carboxyl, benzyl alcohol, methoxyl, aldehydic and phenolic functional groups. It adsorbs on the metal surface and is capable of forming a barrier between the metal and corrodents (Altwaiq et al., 2011). Extracted alkali lignin as investigated by Altwaiq et al has shown corrosion inhibition behavior in the corrosion of different alloys immersed in HCl solutions. This was investigated by weight loss analysis, surface analysis on the corroded metals by scanning electron microscope (SEM), and microbeam X-ray fluorescence (μ-XRF), inductively coupled plasma−optical emission spectroscope (ICPOES) and others (Altwaiq et al., 2011). Lignin doped conductive polymers [polyaniline-PANI] are used in corrosion protection. Sulphonated kraft lignin conductive polymers are more dispersible in water and other solvents. Electrochemical analysis revealed that Ligno-PANI is an efficient corrosion inhibitor. A very low loading (1-2%) of the inhibitor brings much (10-20 fold) reduction in corrosion, presumably by the formation of a passive oxide layer (Xu, 2002). Corrosion behavior of Ligno-sulphonate doped PANI coatings on mild steel in neutral saline conditions (salt spray/immersion) was investigated by Sakhri and coworkers by EIS, potentiodynamic measurements [PD] and visual observations. The coatings with highest PANI performed well both in the salt spray and immersion tests (Sakhri et al., 2011).

#### **2.3 Tannic acid [TA]**

TA is commercial form of Tannin. It is a polymer of gallic acid molecules and glucose. The pure form of TA is a light yellowish and amorphous powder. It is contained in roots, husks, galls and leaves of plants. It is also found in bark of trees (oak, walnut, pine, mahagony), in tea, nettle, wood, berries and horse chestnuts. TA has astringent, antibacterial, antiviral and antienzymatic properties. TA is used in tanning of leather, staining wood, a mordant for cellulose fibres, dyeing cloth, disinfectant cleansers, pharmaceutical industry, food additives, metal corrosion resistance as rust convertor, slime treatment of petroleum drilling, paper, ink production and oil industry. The structure of TA is shown in Fig. 4.

Lignin is the second most common organic polymer. About 50 million tons of lignin is produced worldwide annually as residue in paper production processes. It consists of methoxylated phenyl propane structures. The biosynthesis of complex structure of lignin is thought to involve the polymerization of three primary monomers, monolignols: p-coumaryl, coniferyl, and sinapyl alcohols (Figure 3), which are linked together by different ether and carbon-carbon bonds forming a three-dimensional network. The monolignols are present in the form of p-hydroxylphenol, guaiacyl and syringyl residues in lignin structure. Lignin is non-toxic, inexpensive and abundantly available (Sena-Martins et al.; 2008). It is hydrophoebic, smaller in size and forms stable mixtures (Park et al.; 2008). It is used in dye dispersants, dispersants for crop protection products, to produce low molecular weight chemicals like dimethyl sulphoxide. It is also used as filler in inks, varnishes and paints (Belgacem et al., 2003) and as a dispersing agent in concrete, as binders for wood composites, chelating agents, for treating porous materials, in coatings and

Lignin contains hydroxyl, carboxyl, benzyl alcohol, methoxyl, aldehydic and phenolic functional groups. It adsorbs on the metal surface and is capable of forming a barrier between the metal and corrodents (Altwaiq et al., 2011). Extracted alkali lignin as investigated by Altwaiq et al has shown corrosion inhibition behavior in the corrosion of different alloys immersed in HCl solutions. This was investigated by weight loss analysis, surface analysis on the corroded metals by scanning electron microscope (SEM), and microbeam X-ray fluorescence (μ-XRF), inductively coupled plasma−optical emission spectroscope (ICPOES) and others (Altwaiq et al., 2011). Lignin doped conductive polymers [polyaniline-PANI] are used in corrosion protection. Sulphonated kraft lignin conductive polymers are more dispersible in water and other solvents. Electrochemical analysis revealed that Ligno-PANI is an efficient corrosion inhibitor. A very low loading (1-2%) of the inhibitor brings much (10-20 fold) reduction in corrosion, presumably by the formation of a passive oxide layer (Xu, 2002). Corrosion behavior of Ligno-sulphonate doped PANI coatings on mild steel in neutral saline conditions (salt spray/immersion) was investigated by Sakhri and coworkers by EIS, potentiodynamic measurements [PD] and visual observations. The coatings with highest PANI performed well both in the salt spray and

TA is commercial form of Tannin. It is a polymer of gallic acid molecules and glucose. The pure form of TA is a light yellowish and amorphous powder. It is contained in roots, husks, galls and leaves of plants. It is also found in bark of trees (oak, walnut, pine, mahagony), in tea, nettle, wood, berries and horse chestnuts. TA has astringent, antibacterial, antiviral and antienzymatic properties. TA is used in tanning of leather, staining wood, a mordant for cellulose fibres, dyeing cloth, disinfectant cleansers, pharmaceutical industry, food additives, metal corrosion resistance as rust convertor, slime treatment of petroleum drilling, paper, ink production and oil industry. The structure of TA is shown in Fig. 4.

paintings (Stewart , 2008; Park et al., 2008; Mulder et al., 2011).

**2.2 Lignin** 

**Use in corrosion resistance** 

immersion tests (Sakhri et al., 2011).

**2.3 Tannic acid [TA]** 

Fig. 3. Structure of lignin.

#### **Use in corrosion resistance**

TA has been extensively utilized in anticorrosion methods as investigated by infrared, Mössbauer, Raman spectroscopies, EIS, PD and others (Morcillo et al., 1992; Nasrazadani , 1997; Jaén et al., 2003, 2011; Al-Mayouf, 1999; Ocampo et al., 2004; Galván Jr et al., 1992; Chen et al., 2009). TA is used as conversion coating to prevent corrosion of iron, zinc, copper and their alloys. The (ortho) hydroxyls react with metals forming metal-tannic acid complexes, which protect metal from rusting (Chen et al., 2008). TA based conversion coating can be formed on AZ91D magnesium alloy (Sudagar et al., 2011). Chen et al proposed the formation of organic chromium-free conversion coating on AZ91D

Renewable Resources in Corrosion Resistance 455

Chitin and CHTO are polysaccharides. They are chemically similar to cellulose, differing only by the presence or absence of nitrogen. CHTO is deacetylated chitin (degree of deacetylation of chitin ~50%), obtained from the outer shell of crustaceans (crabs, lobsters, krills and shrimps). CHTO primarily consists of β linked 2-amino-2-deoxy-β-Dglucopyranose units. CHTO shows biocompatibility, low toxicity, biodegradability, osteoconductivity and antimicrobial properties (Fig. 5). CHTO is a cationic polyelectrolyte. CHTO forms complexes with metal ions and can gel with polyanions. It contains reactive hydroxyl and amine groups that undergo chemical transformations producing chemical derivatives with plethora of applications. It is used in cosmetics, as preservative, antioxidant, antimicrobial agent and coatings in food, fabrics, drugs, artificial organs and fungicides (Rinaudo, 2006; Bautista-Baños et al., 2006), as metal adsorbants for the removal of metals (mercury, copper, chromium, silver, iron, cadmium) from ground and waste water

CHTO dissolved aqueous solution forms tough and flexible films. CHTO is utilized as anticorrosion material, however, it absorbs moisture from atmosphere, which penetrates the film easily and deteriorates its performance (Lundvall et al., 2007; Sugama & Cook, 2000). As

**2.4 Chitosan [CHTO]** 

(Lundvall et al., 2007).

Fig. 5. Structure of chitin and chitosan.

**Use in corrosion resistance** 

magnesium alloy obtained from solution containing TA and ammonium metavanadate. The corrosion resistance performance of these chromate free coatings was compared with the traditional chromate conversion coating. PD revealed that the said coating showed more positive potential and obvious lower corrosion current density relative to traditional chromate conversion coating; salt spray tests also showed the improved anticorrosive behavior of the former (Chen et al., 2008). In another report, mildly rusted steel surface were pretreated with TA based rust converters followed by the application of a Zn rich coating. The rust converters react with iron and rust to form a sparingly soluble iron tannate film on metal surface, which renders low pH adjacent to corroding interface by the diffusion of the unreacted acidic constituents of the rust converter in alkaline concrete solution. The low pH facilitates the formation of passive hydrozincite layer within 50h of exposure to chloride contaminated concrete pore solution relative to 150h for normal zinc coating without rust converter. The mechanism of film formation was investigated by EIS, Potential-time studies, Raman Spectroscopy, SEM, energy dispersive X-ray analysis [EDXA] and X-ray diffraction studies [XRD] (Singh &Yadav, 2008). Methacrylic derivatives of TA [m-digallic acid], toluylene 2,4-diisocyanate [TDI] and 2-hydroxyethyl methacrylate [HEMA] formed UV curable urethane coatings (in molar ratio 1:3:3). The formation occurred by the coupling reaction between TA and TDI followed by HEMA addition (Grassino et al., 1999).

Fig. 4. Structure of tannic acid.
