**2.8 Biofilms**

A biofilm consists of a highly organized bacterial community with cells entrapped in an extracellular polymer matrix. Bacteria in biofilms show higher resistance to antibiotics, increased production of exopolysaccharide, morphological changes in cells, different responses to environmental stimuli, and distinct gene expression profile (Zuo, 2007; O'Toole et al., 2000; Videla & Characklis, 1992) (Fig. 9). Biofilm formation on metal surfaces may enhance or hamper corrosion process. The bacterial colonies on metal substrates form anodic (area below thicker colonies, due to more respiration activity and lower oxygen concentration) and cathodic (areas below thinner colonies due to less respiration activity and higher oxygen concentration) areas, resulting in the corrosion of metal surface. The biofilm matrix itself, contrarily, forms a transport barrier, impeding the penetration of corrosive agents (such as oxygen, chloride, and others), decreasing their contact with the metal surface, thus reducing corrosion. Often, the corrosion products themselves form a passive layer that may impede corrosion. The overall process (corrosion or anti-corrosion) depends upon the type of metal and activity of microbes. Some bacteria may become protective or corrosive, depending upon the pH of the medium (Zuo, 2007; O'Toole et al., 2000; Videla & Characklis, 1992; Videla & Herrera, 2005; Lopes et al.; 2006). The mechanism involves the removal of corrodents such as oxygen by aerobic respiration of biofilms, elimination of corrosion causing bacteria by biofilms generated antimicrobials, biofilm secreted corrosion inhibitors form passive layer decreasing contact of metal and corrodents. Such corrosion inhibiting microbes include *Pseudomonas cichorii*, *Bacillus mycoides, Bacillus licheniformis* and several others. The use of biofilms as anti-corrosion agents requires extensive research to be focussed mainly on interactions between bacteria within the microbial community and interactions between certain bacteria and metal. This requires the collaboration of microbiologists and corrosion chemists for further fruitful results in the field.

## **2.9 Cashew nut shell liquid (CNSL)**

CNSL is obtained as a by-product of the cashew nut industry, mainly containing anacardic acid 80.9%, cardol 10-15%, cardanol, and 2-methyl cardol (Fig. 10). CNSL occurs as a brown viscous fluid in the shell of cashewnut, a plantation product obtained from the cashew tree, *Anacardium oxidentale* (Bhunia, et al., 2000). CNSL is used in the manufacture of industrially important materials such as cement, primers, specialty coatings, paints, varnishes, adhesives, foundry core oils, automotive brake lining industry, laminating and rubber compounding resins, epoxy resins, and in the manufacture of anionic and non-ionic surface active agents. CNSL modified phenolic resins are suitable for many applications and perform improved corrosion and insulation resistance.

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

CNSL has excellent combination of functional groups viz., hydroxyls, double bonds, long aliphatic chain, aromatic ring. It can impart good adhesion to coating material due to its structural attributes. Aggarwal et al prepared epoxy-cardanol resin based paints from epichlorohydrin, bisphenol-A and cardanol (Aggarwal, et al., 2007), in presence of Zn powder, Zn phosphate, micaceous iron oxide and synthetic iron oxide as pigments, some

A biofilm consists of a highly organized bacterial community with cells entrapped in an extracellular polymer matrix. Bacteria in biofilms show higher resistance to antibiotics, increased production of exopolysaccharide, morphological changes in cells, different responses to environmental stimuli, and distinct gene expression profile (Zuo, 2007; O'Toole et al., 2000; Videla & Characklis, 1992) (Fig. 9). Biofilm formation on metal surfaces may enhance or hamper corrosion process. The bacterial colonies on metal substrates form anodic (area below thicker colonies, due to more respiration activity and lower oxygen concentration) and cathodic (areas below thinner colonies due to less respiration activity and higher oxygen concentration) areas, resulting in the corrosion of metal surface. The biofilm matrix itself, contrarily, forms a transport barrier, impeding the penetration of corrosive agents (such as oxygen, chloride, and others), decreasing their contact with the metal surface, thus reducing corrosion. Often, the corrosion products themselves form a passive layer that may impede corrosion. The overall process (corrosion or anti-corrosion) depends upon the type of metal and activity of microbes. Some bacteria may become protective or corrosive, depending upon the pH of the medium (Zuo, 2007; O'Toole et al., 2000; Videla & Characklis, 1992; Videla & Herrera, 2005; Lopes et al.; 2006). The mechanism involves the removal of corrodents such as oxygen by aerobic respiration of biofilms, elimination of corrosion causing bacteria by biofilms generated antimicrobials, biofilm secreted corrosion inhibitors form passive layer decreasing contact of metal and corrodents. Such corrosion inhibiting microbes include *Pseudomonas cichorii*, *Bacillus mycoides, Bacillus licheniformis* and several others. The use of biofilms as anti-corrosion agents requires extensive research to be focussed mainly on interactions between bacteria within the microbial community and interactions between certain bacteria and metal. This requires the collaboration of microbiologists and corrosion chemists for further fruitful results in the

CNSL is obtained as a by-product of the cashew nut industry, mainly containing anacardic acid 80.9%, cardol 10-15%, cardanol, and 2-methyl cardol (Fig. 10). CNSL occurs as a brown viscous fluid in the shell of cashewnut, a plantation product obtained from the cashew tree, *Anacardium oxidentale* (Bhunia, et al., 2000). CNSL is used in the manufacture of industrially important materials such as cement, primers, specialty coatings, paints, varnishes, adhesives, foundry core oils, automotive brake lining industry, laminating and rubber compounding resins, epoxy resins, and in the manufacture of anionic and non-ionic surface active agents. CNSL modified phenolic resins are suitable for many applications and

CNSL has excellent combination of functional groups viz., hydroxyls, double bonds, long aliphatic chain, aromatic ring. It can impart good adhesion to coating material due to its structural attributes. Aggarwal et al prepared epoxy-cardanol resin based paints from epichlorohydrin, bisphenol-A and cardanol (Aggarwal, et al., 2007), in presence of Zn powder, Zn phosphate, micaceous iron oxide and synthetic iron oxide as pigments, some

**2.8 Biofilms** 

field.

**2.9 Cashew nut shell liquid (CNSL)** 

**Use in corrosion resistance** 

perform improved corrosion and insulation resistance.

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

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

Renewable Resources in Corrosion Resistance 465

In an excellent review by Shchukin and Mçhwald(Shchukin & Mçhwald, 2007), they have discussed about the nanoreservoirs containing active materials (corrosion inhibitors) for self-repairing coatings and surfaces. Such an approach can be employed on renewable resources in corrosion. In another review by Nimbalkar and Athawale, they have elaborated the use of VO in WB coatings (Athawale & Nimbalkar, 2011). In another excellent report, use of plant extracts as natural corrosion inhibitors has been briefly described (Raja & Sethuraman, 2008). The target of corrosion engineers and chemists, beyond the boundaries, is to achieve and come to a cost effective, environment friendly, user-friendly and long term solution to corrosion-the metallic cancer. For the present, persistent ongoing research efforts in the direction have shown proven results. The substitution of renewable resources based binders and corrosion inhibitors to conventionally used chemicals will pave way for a fruitful utilisation of our naturally available bioresources. With innovative technologies in hand, green chemistry, nanotechnology and green anticorrosion methods and materials as our tools, we can be fully equipped to combat corrosion and related problems, in near future.

Renewable resource based derivatives are cost-effective, abundantly available, biodegradable, environmentally benign alternatives for corrosion resistant coatings, paints and inhibitors. With advancements in knowledge and updated instruments and techniques available, further research in the field may be focussed on the enhanced use of the lesser and highly explored biomaterials for the development of anticorrosion agents in hand with "green" coating technology, for high performance high solids, hyperbranched, waterborne, hybrid and composite coatings that may compete with their petro-based counterparts, both in the terms of cost and performance, in near future. Though we have come a long way,

much remains to be done on our palette; we still have a long way to go and explore.

Dr.Eram Sharmin (Pool Officer) and Dr Fahmina Zafar (Pool Officer) acknowledge CSIR, New Delhi, India for Senior Research Associateships against grant nos. 13(8464-A)/2011- POOL and 13(8385-A)/2010-POOL, respectively. They are also thankful to the Head, Department of Chemistry, Jamia Millia Islamia (A Central University), for providing

Abdallah, M. (2004). Antibacterial Drugs as Corrosion Inhibitors for Corrosion of

Aggarwal, L.K.; Thapliyal, P.C. & Karade, S.R. (2007). Anticorrosive properties of the epoxy-

Ahmad, S.; Zafar, F.; Sharmin, E.; Garg, N. & Kashif, M. (2012). Synthesis and

Aluminium in Hydrochloric Solution. *Corrosion Science,* Vol. 46, No. 8, (August

cardanol resin based paints. *Progress in Organic Coatings*, Vol. 59, No. 1, (April

Characterization of Corrosion Protective Polyurethanefattyamide/ Silica Hybrid Coating Material. *Progress in Organic Coatings*, Vol. 73, No. 1, (January 2012),

**3. Conclusion**

**4. Acknowledgements** 

support to carry out the work.

2004), pp. 1981-1996, ISSN 0010-938X

2007), pp. 76-80, ISSN 03009440

pp.112– 117,ISSN 03009440

**5. References** 

Fig. 10. Chemical structure of the constituents of CNSL, (a) anacardic acid, (b) cardanol, (c) cardol and (d), 2-methyl cardol.
