**2.4 Chitosan [CHTO]**

454 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.

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 (Lundvall et al., 2007).

Fig. 5. Structure of chitin and chitosan.

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

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

Renewable Resources in Corrosion Resistance 457

the reactions of their hydroxyl groups with functional groups of the synthetic polymers, such as carboxylic acids, anhydrides, epoxies, urethanes, oxazolines, and others. Another alternative method is via free-radical ring-opening polymerization occurring between their glucose rings and vinyl monomers. Sugama et al carried out the preparation of polyorganosiloxane grafted starch coatings for the protection of aluminium from corrosion (Sugama & DuVall, 1996). The protocol involved the modification of potato starch [PS] with N-[3-(triethoxysilyl)propyl]-4,5,-dihydroimidazole [TSPI]. The constant threat with the use of PS was active bacterial and fungal growth, which caused diminution of its corrosion resistance behavior. TSPI protects the bacterial and fungal growth on PS solution; this was analysed by SEM technique (Sugama & DuVall, 1996). The grafting of organosiloxane occurred by the opening of glycosidic rings. The coating properties were investigated by EIS and salt spray test. PS/TSPI 85/15 and 90/10 ratio-derived coatings displayed good protection of Al against corrosion (salt spray test-288 hours, impedance >105 Ω cm2). In another report, Sugama attempted to investigate the effect of cerium (IV) ammonium nitrate

Bello et al. used modified cassava starch as corrosion inhibitor of carbon steel in an alkaline 200mgL−1 NaCl solution (chemical composition of tap water) in contact with air at 25oC. One was cassava starch modified through gelatinization and activation [GAS] and carboxymethylated starch [CMS] with different degrees of substitution [DS]. These were characterized by NMR spectroscopy; estimation of DS was also performed, which was about 0.13±0.03 (CMS 0.13) and 0.24±0.04 (CMS 0.24). Electrostatic potential [V(r)] mapping of the repetitive unit of GAS and CMS was based on the model proposed by Politzer and Sjoberg (Bello et al., 2010). Corrosion studies were performed by EIS coupled with a rotating disk electrode with a fixed rotation speed of 1000 rpm. The polarization resistance values followed the order CMS 0.13 <CMS 0.24 < GAS. The studies confirmed that starch acts as corrosion inhibitor of carbon steel; the extent of protection against corrosion depended on the amount and type of active groups present [carboxylate (–COO−) and alkoxy (–CO−) groups for CMS, and alkoxy (–CO−) groups for GAS] and also on DS (Bello et al., 2010).

Rosliza and Nik studied the corrosion resistance conferred by tapioca starch [TS] to AA6061 alloy in seawater. The weight loss of AA6061 alloy specimens in seawater diminished with increasing TS concentration as a result of corrosion deposits. PD results revealed that as the concentration of TS increased, corrosion potential [Ecorr] values shift to more positive value, corrosion current density (icorr) reduced remarkably, the numerical values of both anodic and cathodic Tafel slopes decreased, polarization resistance [Rp ] value of AA6061 alloy increased (higher the Rp value, lower the corrosion rate), double layer capacitance value [Cdl] decreased, indicating that anodic and cathodic processes are suppressed by TS, that acts as corrosion inhibitor, preferentially reacting with Al3+ to form a precipitate of salt or complex on the surface of the aluminum substrate (Rosliza & Nik, 2010). Inhibition efficiency [IE(%)] values obtained from all the measurements viz. gravimetric, PD, linear polarization resistance [LPR] and EIS were in close agreement with each other. IE (%) of TS increased with the corrosion inhibitor concentrations ranging from 200 to 1000 ppm. The protection conferred by TS is attributed to the adsorption on AA6061 alloy surface through all the functional groups present in starch (linear amylose constituted by glucose monomer units joined to one another head to tail forming alpha-1, 4 linkage, and highly branched

modified PS as primer coatings for aluminium substrates (Sugama, 1997).

a remedial approach to employ CHTO as an environmentally green water-based coating system for aluminum (Al) substrates, Sugama et al modified CHTO with polyacid electrolyte, poly(itaconic acid) [PI], containing two negatively charged carboxylic acid groups, with CHTO: PI ratio of 100:0, 90:10, 80:20, 70:30, 50:50, 30:70, and 0:100, by weight, applied on 6061-T6 aluminum (Al) sheet by a simple dip-withdrawing method. –COOH and –NH2 groups of PI and CHTO, respectively, formed (hydrophoebic) secondary amide linkages, which lead to the grafting of PI on CHTO backbone, and at higher temperature crosslinking occurred. Increased "grafts" and "crosslinks" formed coatings that were less susceptible to moisture and prevented the penetration of corrosive electrolyte species, providing good corrosion protection to the substrate. CHTO:PI ratio 80:20 was found to be an ideal composition for efficient corrosion protection (Sugama & Cook, 2000). Sugama et al also modified CHTO with corn-starch derived dextrin and applied on Al-6063. CHTO:dextrin ratio 70/30 provided low moisture resistance and could withstand salt spray test upto 720 h (Sugama & Milian-Jimenez, 1999). CHTO shows high hydrophilicity and poor adhesive strength with Al 2024 T3 alloy. CHTO was modified with epoxy functional silanes [2-(3,4 epoxycyclohexyl)-ethyltrimethoxysilane and (3-Glycidoxypropyl)-trimethoxysilane] as coupling agents and vanadates as corrosion inhibitor (Kumar & Buchheit, 2006). The derivatives of CHTO such as acetylthiourea CHTO, carboxymethyl CHTO are used as efficient corrosion inhibitors as assessed by PD, EIS, SEM, weight loss measurements, conductometric titrations and other studies (Fekry & Mohamed, 2010; Cheng et al., 2007). Hydroxyapatite-CHTO composite coatings on AZ31 Mg alloy by aerosol deposition produce well adherent, corrosion resistant biocompatible coatings (Hahn et al., 2011)

#### **2.5 Starch**

As a carbohydrate consisting of a number of glucose units joined together by glycosidic bonds, starch is a low cost, renewable and biodegradable natural polymer. It consists of two types of molecules, amylose (linear) and amylopectin (branched) (Fig. 6). It is the energy store of plants (Sugama & DuVall, 1996). Commercial refined starches are cornstarch, tapioca, wheat and potato starch. Industrial applications include pharmaceutical, papermaking, textile, and in food preparation.

Fig. 6. Structure of starch.

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

Starch is used as a natural corrosion inhibitor. A few reports are available in literature where starch is used to protect metal against corrosion (Sugama & DuVall, 1996). When used at low pH, starch shows low water solubility and poor stability. Thus, for improved performance, certain physical and chemical modifications become necessary. These involve

a remedial approach to employ CHTO as an environmentally green water-based coating system for aluminum (Al) substrates, Sugama et al modified CHTO with polyacid electrolyte, poly(itaconic acid) [PI], containing two negatively charged carboxylic acid groups, with CHTO: PI ratio of 100:0, 90:10, 80:20, 70:30, 50:50, 30:70, and 0:100, by weight, applied on 6061-T6 aluminum (Al) sheet by a simple dip-withdrawing method. –COOH and –NH2 groups of PI and CHTO, respectively, formed (hydrophoebic) secondary amide linkages, which lead to the grafting of PI on CHTO backbone, and at higher temperature crosslinking occurred. Increased "grafts" and "crosslinks" formed coatings that were less susceptible to moisture and prevented the penetration of corrosive electrolyte species, providing good corrosion protection to the substrate. CHTO:PI ratio 80:20 was found to be an ideal composition for efficient corrosion protection (Sugama & Cook, 2000). Sugama et al also modified CHTO with corn-starch derived dextrin and applied on Al-6063. CHTO:dextrin ratio 70/30 provided low moisture resistance and could withstand salt spray test upto 720 h (Sugama & Milian-Jimenez, 1999). CHTO shows high hydrophilicity and poor adhesive strength with Al 2024 T3 alloy. CHTO was modified with epoxy functional silanes [2-(3,4 epoxycyclohexyl)-ethyltrimethoxysilane and (3-Glycidoxypropyl)-trimethoxysilane] as coupling agents and vanadates as corrosion inhibitor (Kumar & Buchheit, 2006). The derivatives of CHTO such as acetylthiourea CHTO, carboxymethyl CHTO are used as efficient corrosion inhibitors as assessed by PD, EIS, SEM, weight loss measurements, conductometric titrations and other studies (Fekry & Mohamed, 2010; Cheng et al., 2007). Hydroxyapatite-CHTO composite coatings on AZ31 Mg alloy by aerosol deposition produce well adherent,

As a carbohydrate consisting of a number of glucose units joined together by glycosidic bonds, starch is a low cost, renewable and biodegradable natural polymer. It consists of two types of molecules, amylose (linear) and amylopectin (branched) (Fig. 6). It is the energy store of plants (Sugama & DuVall, 1996). Commercial refined starches are cornstarch, tapioca, wheat and potato starch. Industrial applications include pharmaceutical,

Starch is used as a natural corrosion inhibitor. A few reports are available in literature where starch is used to protect metal against corrosion (Sugama & DuVall, 1996). When used at low pH, starch shows low water solubility and poor stability. Thus, for improved performance, certain physical and chemical modifications become necessary. These involve

corrosion resistant biocompatible coatings (Hahn et al., 2011)

papermaking, textile, and in food preparation.

Fig. 6. Structure of starch.

**Use in corrosion resistance** 

**2.5 Starch** 

the reactions of their hydroxyl groups with functional groups of the synthetic polymers, such as carboxylic acids, anhydrides, epoxies, urethanes, oxazolines, and others. Another alternative method is via free-radical ring-opening polymerization occurring between their glucose rings and vinyl monomers. Sugama et al carried out the preparation of polyorganosiloxane grafted starch coatings for the protection of aluminium from corrosion (Sugama & DuVall, 1996). The protocol involved the modification of potato starch [PS] with N-[3-(triethoxysilyl)propyl]-4,5,-dihydroimidazole [TSPI]. The constant threat with the use of PS was active bacterial and fungal growth, which caused diminution of its corrosion resistance behavior. TSPI protects the bacterial and fungal growth on PS solution; this was analysed by SEM technique (Sugama & DuVall, 1996). The grafting of organosiloxane occurred by the opening of glycosidic rings. The coating properties were investigated by EIS and salt spray test. PS/TSPI 85/15 and 90/10 ratio-derived coatings displayed good protection of Al against corrosion (salt spray test-288 hours, impedance >105 Ω cm2). In another report, Sugama attempted to investigate the effect of cerium (IV) ammonium nitrate modified PS as primer coatings for aluminium substrates (Sugama, 1997).

Bello et al. used modified cassava starch as corrosion inhibitor of carbon steel in an alkaline 200mgL−1 NaCl solution (chemical composition of tap water) in contact with air at 25oC. One was cassava starch modified through gelatinization and activation [GAS] and carboxymethylated starch [CMS] with different degrees of substitution [DS]. These were characterized by NMR spectroscopy; estimation of DS was also performed, which was about 0.13±0.03 (CMS 0.13) and 0.24±0.04 (CMS 0.24). Electrostatic potential [V(r)] mapping of the repetitive unit of GAS and CMS was based on the model proposed by Politzer and Sjoberg (Bello et al., 2010). Corrosion studies were performed by EIS coupled with a rotating disk electrode with a fixed rotation speed of 1000 rpm. The polarization resistance values followed the order CMS 0.13 <CMS 0.24 < GAS. The studies confirmed that starch acts as corrosion inhibitor of carbon steel; the extent of protection against corrosion depended on the amount and type of active groups present [carboxylate (–COO−) and alkoxy (–CO−) groups for CMS, and alkoxy (–CO−) groups for GAS] and also on DS (Bello et al., 2010).

Rosliza and Nik studied the corrosion resistance conferred by tapioca starch [TS] to AA6061 alloy in seawater. The weight loss of AA6061 alloy specimens in seawater diminished with increasing TS concentration as a result of corrosion deposits. PD results revealed that as the concentration of TS increased, corrosion potential [Ecorr] values shift to more positive value, corrosion current density (icorr) reduced remarkably, the numerical values of both anodic and cathodic Tafel slopes decreased, polarization resistance [Rp ] value of AA6061 alloy increased (higher the Rp value, lower the corrosion rate), double layer capacitance value [Cdl] decreased, indicating that anodic and cathodic processes are suppressed by TS, that acts as corrosion inhibitor, preferentially reacting with Al3+ to form a precipitate of salt or complex on the surface of the aluminum substrate (Rosliza & Nik, 2010). Inhibition efficiency [IE(%)] values obtained from all the measurements viz. gravimetric, PD, linear polarization resistance [LPR] and EIS were in close agreement with each other. IE (%) of TS increased with the corrosion inhibitor concentrations ranging from 200 to 1000 ppm. The protection conferred by TS is attributed to the adsorption on AA6061 alloy surface through all the functional groups present in starch (linear amylose constituted by glucose monomer units joined to one another head to tail forming alpha-1, 4 linkage, and highly branched

Renewable Resources in Corrosion Resistance 459

VO is the single, largest, well-established, non-polluting, non-toxic, biodegradable family used in coatings and paints, since primeval times particularly in corrosion resistance. Depending on their Iodine value [IV], VO are classified as non-drying, semi-drying and drying, as indicated by their drying index [DI] (DI=linoleic%+(2linolenic%); "drying" VO : IV>130 and DI> 70); "semi-drying" VO: 115<IV<130 and DI 65-75; "non-drying" VO : IV<115; DI< 65). Usually, drying VO are used in coatings and paints. Drying VO are film formers, ie., they have the tendency to form films over the substrate on drying by themselves, without the use of any drier. In drying VO, drying occurs as a natural phenomenon through auto-oxidation initiating from the active methylene groups on VO backbone. However, since these films are not tough enough to meet the desirable performance characteristics, VO are chemically transformed into several derivatives as polyesters, alkyds, polyesteramides, polyetheramides, polyurethanes (Fig. 8) and others, to meet the stringent environmental conditions. These have been further modified through chemical pathways including acrylation, vinylation, metallation, and others, for improvement in their drying, gloss, scratch hardness [SH], impact resistance [IRt] , flexibility [FL], and corrosion resistance of coatings produced therefrom. The presence of hydroxyls, esters, oxiranes, amides, carbonyls, metals, acrylics, carboxyls, urethanes, imparts good adhesion to the substrate due to good electrostatic interactions with the metal substrate.

Today, the advancements in knowledge, rise of several innovative technologies, human awareness and concerns related to energy consumption and environmental contamination have brought about manifold changes in the world of VO based coatings and paints. They include VO based low/no solvent coatings, high solids coatings, hyperbranched coatings,

WB coatings, UV curable, organic-inorganic hybrids and nanocomposite coatings.

**Use in corrosion resistance**

Fig. 7. Chemical structure of VO.

amylopectin with an alpha-1, 6 linkage every 24–30 glucose monomer units). Other uses of starch include their potential application in edible coatings (Vásconeza et al., 2009; Pagella et al., 2002), coatings for colon-specific drug delivery (Freirea et al., 2009), and in blast cleaning of artificially aged paints (Tangestaniana et al., 2001).
