**2.6 Plant extracts**

Plants naturally synthesize chemical compounds in defence against fungi, insects and herbivorous mammals. Some of these compounds or phytochemicals such as alkaloids, terpenoids, flavonoids, polyphenols and glycosides prove beneficial to humans in unique manner for the treatment of several diseases. These compounds are identical in structure and function to conventional drugs. Extracts from parts of plants such as roots, stems, and leaves also contain such extraordinary phytochemicals that are used as pesticides, antimicrobials, drugs and herbal medicines.

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

Plant extracts are excessively used as corrosion inhibitors. An interesting review in this context is compiled by Raja & Sethuraman, 2008. Plant extracts contain a variety of organic compounds such as alkaloids, flavonoids, tannins, cellulose and polycyclic compounds. The compounds with hetero atoms-N, O, S, P coordinate with (corroding) metal atom or ion consequently forming a protective layer on the metal surface, that prevents corrosion. These serve as cheaper, readily available, renewable and environmentally benign alternatives to costly and hazardous corrosion inhibitors (e.g., chromates). Plant extracts serve as anticorrosion agents to various metals such as mild steel, copper, zinc, tin, nickel, aluminium and its alloys. Literature reveals that there are exhaustive numbers of plant extracts that have shown proven anticorrosion activity as corrosion inhibitors. Examples are *Swertia angustifolia, Accacia conicianna, Embilica officianilis, Terminalia chebula, Terminalia belivia, Sapindus trifolianus, Pongamia glabra, Eucalyptus* leaves*, Annona squamosa, Eugenia jambolans, Azadirachta indica, Accacia Arabica, Vernonia amydalina, Carica papaya, Rosmarinus officinalis, Hisbiscus subdariffa, Opuntia extractd, Mentha pulegium, Occium viridis, Datura* metel, *Ricinus communis, Chelidonium majus, Papaia, Poinciana pulcherrima, Cassia occidentalis and Datura stramonium* seeds, *Papaia, Calotropis procera B, Azydracta indica, Justicia gendarussa, Artemisia pallens*, *Auforpio turkiale* sap, Black pepper extract, henna extract and several others (Zucchi & Omar, 1985; Dahmani et al., 2010; Ostovaria et al., 2009; Satapathy et al., 2009).

#### **2.7 Vegetable oils [VO]**

VO are triglycerides of fatty acids (Fig. 7). They find versatile applications as biofuel, lubricants, adhesives, antimicrobial agents, coatings and paints [Mar et al., 2007; Bruning, 1992. The extensive utilization of VO in several diverse fields is manifested in their rich chemistry-a storehouse of functional groups such as esters, carboxyls, hydroxyls, oxirane, double bonds, active methylenes and others. These functional groups on VO backbone may undergo a host of chemical transformations yielding "green" polymer derivatives, e.g, alkyds, epoxies, polyols, polyurethanes, polyesters, polyesteramides, polyetheramides and others, with versatile applications.

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

458 Corrosion Resistance

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

Plants naturally synthesize chemical compounds in defence against fungi, insects and herbivorous mammals. Some of these compounds or phytochemicals such as alkaloids, terpenoids, flavonoids, polyphenols and glycosides prove beneficial to humans in unique manner for the treatment of several diseases. These compounds are identical in structure and function to conventional drugs. Extracts from parts of plants such as roots, stems, and leaves also contain such extraordinary phytochemicals that are used as pesticides,

Plant extracts are excessively used as corrosion inhibitors. An interesting review in this context is compiled by Raja & Sethuraman, 2008. Plant extracts contain a variety of organic compounds such as alkaloids, flavonoids, tannins, cellulose and polycyclic compounds. The compounds with hetero atoms-N, O, S, P coordinate with (corroding) metal atom or ion consequently forming a protective layer on the metal surface, that prevents corrosion. These serve as cheaper, readily available, renewable and environmentally benign alternatives to costly and hazardous corrosion inhibitors (e.g., chromates). Plant extracts serve as anticorrosion agents to various metals such as mild steel, copper, zinc, tin, nickel, aluminium and its alloys. Literature reveals that there are exhaustive numbers of plant extracts that have shown proven anticorrosion activity as corrosion inhibitors. Examples are *Swertia angustifolia, Accacia conicianna, Embilica officianilis, Terminalia chebula, Terminalia belivia, Sapindus trifolianus, Pongamia glabra, Eucalyptus* leaves*, Annona squamosa, Eugenia jambolans, Azadirachta indica, Accacia Arabica, Vernonia amydalina, Carica papaya, Rosmarinus officinalis, Hisbiscus subdariffa, Opuntia extractd, Mentha pulegium, Occium viridis, Datura* metel, *Ricinus communis, Chelidonium majus, Papaia, Poinciana pulcherrima, Cassia occidentalis and Datura stramonium* seeds, *Papaia, Calotropis procera B, Azydracta indica, Justicia gendarussa, Artemisia pallens*, *Auforpio turkiale* sap, Black pepper extract, henna extract and several others (Zucchi & Omar, 1985; Dahmani et al., 2010; Ostovaria et al., 2009;

VO are triglycerides of fatty acids (Fig. 7). They find versatile applications as biofuel, lubricants, adhesives, antimicrobial agents, coatings and paints [Mar et al., 2007; Bruning, 1992. The extensive utilization of VO in several diverse fields is manifested in their rich chemistry-a storehouse of functional groups such as esters, carboxyls, hydroxyls, oxirane, double bonds, active methylenes and others. These functional groups on VO backbone may undergo a host of chemical transformations yielding "green" polymer derivatives, e.g, alkyds, epoxies, polyols, polyurethanes, polyesters, polyesteramides, polyetheramides and

of artificially aged paints (Tangestaniana et al., 2001).

antimicrobials, drugs and herbal medicines.

**Use in corrosion resistance** 

Satapathy et al., 2009).

**2.7 Vegetable oils [VO]** 

others, with versatile applications.

**2.6 Plant extracts** 

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.

Fig. 7. Chemical structure of VO.

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.

Renewable Resources in Corrosion Resistance 461

While the virgin Soy alkyd coating succumbed to corrosion resistance tests in different corrosive media after 2 h, relatively, Soy alkyd/PANI showed higher performance as monitored for a period of 960 h. The corrosion rate decreased with increased concentration of PANI, being minimum for the highest PANI loading in alkyd. Minimum corrosion rate of 35 × 10-2 mpy in 5% HCl, 32 × 10-2 mpy in 5% NaOH and 30 × 10-2 mpy in 3.5% NaCl was observed for 2.5%-Soyalkyd/PANI (Alam, et al., 2009). Metal containing VO coatings have shown antimicrobial behavior due to the presence of metal, either embedded or incorporated into the matrix. Metal/VO corrosion resistant materials interact with the microbes by adhering to their surface, the long hydrophoebic VO chains engulf the microbes

*Mesuea ferrea* L. seed oil polyester/clay silver nanocomposite coatings have shown antimicrobial behavior against *Escherichia coli* and *Psuedomonas aeruginosa* (Konwar, et al., 2010). Zafar et al have reported antibacterial activity of Zn containing Linseed polyesteramide coatings (Zafar, et al., 2007, 2007). Sharmin and co-workers recently investigated the coating properties of copper oxide containing poly (ester urethane) metallohybrids from Linseed oil (Sharmin, et al., 2012). Castor polyurethane organo clay composite coatings prepared by Heidarani et al showed good corrosion resistance properties (Heidariani et al., 2010). At 3wt% loading of clay, good corrosion resistance properties could be achieved as determined by PP and EIS. The composite showed icorr (nA/cm2)=0.139, Rp (MΩcm2) polarization resistance= 3819.41, Eocp (mV/Ag|AgCl) (open circuit potential)= −132 after 30 days immersion of samples in 5wt% NaCl. At higher clay loading (>3wt%), the coating material became viscous and the adhesion of the coatings to the substrate deteriorated. The composites prepared through ultrasonication technique did not show any phase separation contrary to their counterparts prepared by mechanical agitation. Zafar et al have for the first time reported the microwave assisted preparation and characterization of Castor oil based zinc containing metallopolyurethane amide coating material. Metallopolyurethaneamide containing 5% metal showed the best performance. The coatings showed good SH (3.5kg), IRt (150lb/inch), FL (1/8in.) and gloss (tested by standard methods and techniques). The coatings were tested by PD in 3.5% HCl, 3.5% NaOH, and 3.5% NaCl solutions. IE% in 3.5% HCl, 3.5% NaOH, and 3.5% NaCl were found as 96.23, 90.81, and 94.50, respectively [Zafar, et al., 2011]. Ahmad et al recently reported the preparation and corrosion resistance performance of Linseed oil based polyurethanefattyamide/ tetraethoxyorthosilane [TEOS-20, 25, 30 phr] based organicinorganic [PULFAS] prepared at ambient temperature (Ahmad et al., 2012). PD measurements were conducted in HCl (3.5%), NaOH (3.5%), NaCl (5%) and tap water (Clion 63mg/l; conductivity 0.953 mS/A). PULFAS hybrid coatings with 30 phr inorganic content showed the best coating properties, icorr (A/cm2) 2.65x10-8 and IE% 99.77 in 3.5% HCl, icorr (A/cm2) 1.09x 10 -7, IE% 99.34 in 3.5% NaOH. Salt spray test of PULFAS coatings was carried out in 3.5% NaCl solution; while the hybrid coatings could withstand the test for 240h, the coatings of virgin polyurethaneamide showed loss in weight and gloss after this time period. Araujo and co-workers investigated the influence of the type of VO on the barrier properties of alkyd paints pigmented with zinc phosphate. They selected Linseed

completely cutting off their nutrients, making the cell weak and finally dead.

and Soybean oils as modifiers of alkyd paints (Araujo, et al., 2010).

The research work on the use of VO in corrosion resistance is exhaustive. Numerous innovations in the field have occurred in recent years and still more is yet to take place.

Fig. 8. VO derivatives used in corrosion resistance.

Aigbodion et al prepared WB coatings from rubber seed oil (Aigbodion et al, 2000, 2001, 2003, & 2010). WB polyurethanes with dimer fatty acids showed excellent water and hydrolytic resistance (Liu et al, 2011). Commercially procured acrylated soybean oil modified with acrylated sucrose [ACSU] and hyperbranched acrylates [HYAC], was formulated into UV curable coatings, (Chen, et al., 2011). The addition of HYAC and ACSU improved the adhesion and toughness of coatings, respectively. ACSU acted as reactive flexibilizers in coating formulations. ACSU (in an optimum concentration) modified coatings showed good stability in water, after immersion for seven days, except for slight haziness in smaller portion of the films. Soy alkyd/ PANI conducting coatings showed good SH, IRt, FL and conductivity due to good adhesion between PANI and metal substrate.

Fig. 8. VO derivatives used in corrosion resistance.

Aigbodion et al prepared WB coatings from rubber seed oil (Aigbodion et al, 2000, 2001, 2003, & 2010). WB polyurethanes with dimer fatty acids showed excellent water and hydrolytic resistance (Liu et al, 2011). Commercially procured acrylated soybean oil modified with acrylated sucrose [ACSU] and hyperbranched acrylates [HYAC], was formulated into UV curable coatings, (Chen, et al., 2011). The addition of HYAC and ACSU improved the adhesion and toughness of coatings, respectively. ACSU acted as reactive flexibilizers in coating formulations. ACSU (in an optimum concentration) modified coatings showed good stability in water, after immersion for seven days, except for slight haziness in smaller portion of the films. Soy alkyd/ PANI conducting coatings showed good SH, IRt, FL and conductivity due to good adhesion between PANI and metal substrate. While the virgin Soy alkyd coating succumbed to corrosion resistance tests in different corrosive media after 2 h, relatively, Soy alkyd/PANI showed higher performance as monitored for a period of 960 h. The corrosion rate decreased with increased concentration of PANI, being minimum for the highest PANI loading in alkyd. Minimum corrosion rate of 35 × 10-2 mpy in 5% HCl, 32 × 10-2 mpy in 5% NaOH and 30 × 10-2 mpy in 3.5% NaCl was observed for 2.5%-Soyalkyd/PANI (Alam, et al., 2009). Metal containing VO coatings have shown antimicrobial behavior due to the presence of metal, either embedded or incorporated into the matrix. Metal/VO corrosion resistant materials interact with the microbes by adhering to their surface, the long hydrophoebic VO chains engulf the microbes completely cutting off their nutrients, making the cell weak and finally dead.

*Mesuea ferrea* L. seed oil polyester/clay silver nanocomposite coatings have shown antimicrobial behavior against *Escherichia coli* and *Psuedomonas aeruginosa* (Konwar, et al., 2010). Zafar et al have reported antibacterial activity of Zn containing Linseed polyesteramide coatings (Zafar, et al., 2007, 2007). Sharmin and co-workers recently investigated the coating properties of copper oxide containing poly (ester urethane) metallohybrids from Linseed oil (Sharmin, et al., 2012). Castor polyurethane organo clay composite coatings prepared by Heidarani et al showed good corrosion resistance properties (Heidariani et al., 2010). At 3wt% loading of clay, good corrosion resistance properties could be achieved as determined by PP and EIS. The composite showed icorr (nA/cm2)=0.139, Rp (MΩcm2) polarization resistance= 3819.41, Eocp (mV/Ag|AgCl) (open circuit potential)= −132 after 30 days immersion of samples in 5wt% NaCl. At higher clay loading (>3wt%), the coating material became viscous and the adhesion of the coatings to the substrate deteriorated. The composites prepared through ultrasonication technique did not show any phase separation contrary to their counterparts prepared by mechanical agitation. Zafar et al have for the first time reported the microwave assisted preparation and characterization of Castor oil based zinc containing metallopolyurethane amide coating material. Metallopolyurethaneamide containing 5% metal showed the best performance. The coatings showed good SH (3.5kg), IRt (150lb/inch), FL (1/8in.) and gloss (tested by standard methods and techniques). The coatings were tested by PD in 3.5% HCl, 3.5% NaOH, and 3.5% NaCl solutions. IE% in 3.5% HCl, 3.5% NaOH, and 3.5% NaCl were found as 96.23, 90.81, and 94.50, respectively [Zafar, et al., 2011]. Ahmad et al recently reported the preparation and corrosion resistance performance of Linseed oil based polyurethanefattyamide/ tetraethoxyorthosilane [TEOS-20, 25, 30 phr] based organicinorganic [PULFAS] prepared at ambient temperature (Ahmad et al., 2012). PD measurements were conducted in HCl (3.5%), NaOH (3.5%), NaCl (5%) and tap water (Clion 63mg/l; conductivity 0.953 mS/A). PULFAS hybrid coatings with 30 phr inorganic content showed the best coating properties, icorr (A/cm2) 2.65x10-8 and IE% 99.77 in 3.5% HCl, icorr (A/cm2) 1.09x 10 -7, IE% 99.34 in 3.5% NaOH. Salt spray test of PULFAS coatings was carried out in 3.5% NaCl solution; while the hybrid coatings could withstand the test for 240h, the coatings of virgin polyurethaneamide showed loss in weight and gloss after this time period. Araujo and co-workers investigated the influence of the type of VO on the barrier properties of alkyd paints pigmented with zinc phosphate. They selected Linseed and Soybean oils as modifiers of alkyd paints (Araujo, et al., 2010).

The research work on the use of VO in corrosion resistance is exhaustive. Numerous innovations in the field have occurred in recent years and still more is yet to take place.

Renewable Resources in Corrosion Resistance 463

fillers, additives and hardener (aromatic polyamine). The coated panels were subjected to immersion tests in water, 5% NaCl, urea and di-ammonium phosphate for 180 days and humidity cabinet test at 100%RH at 42- 48oC. The coatings showed good SH, adhesion, FL; coatings with micaceous iron oxide showed minimum blistering in immersion and humidity cabinet tests (Aggarwal, et al., 2007). CNSL is also used as a modifier for phenolformaldehyde [PF] resin. CNSL-PF modified natural rubber has shown improved physico-

mechanical performance compared to pure CNSL (Menon, et al., 2002).

Fig. 9. Corrosion resistance by the formation of biofilm.
