Technological Innovations in Coatings forCombating Corrosion
Mr. Shrikant M. Naik
Chief Manager - Technology
Asian Paints Limited
Dr. Suman Mitra
Lead Researcher
Asian Paints Limited

Corrosion causes enormous industrial losses with a depletion of our natural resources. Organic coatings are one of the most widely used methods for corrosion protection of metallic substrates. Over the last few decades various types of technologies have been explored for corrosion protection with the aim of improving the service life and exterior durability of the system. Some of the disruptive approaches to achieve durable mitigation of corrosion are immensely attractive both for technological and commercial reasons. This paper highlights few of the recent advancements in coating technologies for corrosion control.

Corrosion is a natural process which leads to energy and material loss. Globally, the annual cost related to corrosion and corrosion mitigation has been estimated to constitute 3-4 per cent of the gross domestic product (GDP)1. As per one conservative estimate, India with a GDP of around USD 2 trillion loses as much as USD 100 billion (more than 6 lakh crore) every year on account of corrosion. In addition to the economic costs corrosion can lead to structural failures that have catastrophic consequences for humans and the surrounding environment e.g. bridge collapse, leakages in gas pipelines etc.2, 3

Coatings for corrosion inhibition purposes are usually applied as functional barriers in various environments eg, immersion in water, buried in soils, exposed in industrial areas where it has to encounter ultraviolet radiation, hot corrosive liquids, and air pollution. Over the years various chemistries have been used for protection of steel in adverse climatic conditions. 2k Epoxy and 2k Polyurethane are the front runner here and are used extensively across the segment. Apart from this Inorganic & Organic Zinc rich primer find major use in protection of steel.

In this article we will be touching upon two of the upcoming technologies in corrosion protection viz. "Smart coatings" and "Nano materials based coatings"

Smart Coatings:

In the recent years there has been major development in the area of smart coatings category. Depending on the formulation, smart coatings are special films that exhibit auto-responsive characteristics when in contact with an aggressive environment. Smart coatings respond to the aggressive changes caused as a result of a change in pH, temperature, pressure, surface tension, ionic strength, electrical or magnetic fields, acoustics, light, mechanical forces including abrasions etc. resulting in certain photochemical, acid-base, complexation, bond formation/ breakage, electrochemical reactions etc.4. Few examples of smart coatings are self- cleaning coatings and super hydrophobic systems, self-healing coatings, anti-fogging coatings, corrosion sensing coatings, intumescent fire-retardant coatings, radio frequency identification coatings, etc.

The corrosion sensing coatings are mostly pH sensitive coatings required to expose oxygen reduction following the oxidative corrosion reaction. The onset of corrosion on metals and alloys coated with such coating experience an increased pH at the site. These coatings contain some color changing dyes and/or compounds within the film matrix that fluoresce or change color due to oxidation at high pH values or complex formation with metal cations upon possible mechanical damage. Mostly transparent matrices are used so the color change or fluorescence is visible upon reaction with the corroding species. The color dyes contain anticorrosive species and additives within microcapsules. They are also pigments formulated without observable color changes to release anticorrosive species upon damage or sense the presence of corroding species. This area has encouraged innovations in the formulation of coatings and paints from polymeric materials. Compounds such as Schiff bases, hydroxyquinolines, fluorescein, phenolphthalein, oxines, bromothymol blue, 7-amino-4-methylcoumarin, 7-diethylamino-4-methylcoumarin etc. have been consolidated with the primer coatings and inhibiting additives for this purpose.

Most fluorescent systems are classified as active and the color changing types are said to be passive. This corrosion sensing indicators are better placed in the primer layer next to the substrate and covered with preferably a transparent top coat as shown in Figure 1.











Figure 1: Illustration showing active corrosion sensing and inhibition mechanism. (Adapted from Ref. 4, Progress in Organic Coatings 111 (2017) 294–314)

Self-healing is another class of coatings in the smart category. Self-healing properties are required for the total or partial repair of coated areas damaged by ageing or unexpected aggressive events. Concerning self-healing coatings for corrosion protection, two main strategies have been pursued: (i) mending of defects formed in the polymeric coating matrix via addition of polymerisable agents and (ii) inhibition of corroding areas due to the presence of corrosion inhibitors 4, 5, 6. By definition, these are coatings that ensure that the polymer matrix is constructively repaired after damage occurs to maintain its mechanical properties and deter the onset of corrosion. The functional nature of the self-healing coating depends on its chemical composition and structure and is modeled after the biological wound healing process. One of the most applicable areas of structural self-healing is that of corrosion protection, where active agents therein are inhibitors with reactive groups such as free radicals, aromatics, −OH, −Si-O, −C=C−, −COOH, −NH2, −SH, −S−S−, −C=O, etc. either in micro or nano-forms.

Self-cleaning anti-corrosive is another class of coating which is inspired by nature, wherein it clean its surface through either hydrophobic or hydrophilic action of water. Their smart nature allows response to external factors such as electric field, temperature, light, etc. The geometric structure and chemical composition of the solid coating surface influence wettability and water contact angle. Also, both surface roughness and surface energy have a great role to play. The hydrophobic properties exhibited by hydrophobic coatings is that of water droplets rolling off the cured surface along with any surface contaminant present leaving the surface dry and clean. Hydrophilic coatings clean via photocatalysis process and its wettability is high. These classes of coatings requires polymers with excellent film forming properties alongside flexibility and toughness etc. such that a small quantity is sufficient to provide coatings with improved mechanical properties such as durability and optical transparency. TiO2 is an established major component of hydrophilic coatings due to its favorable physical and chemical properties. The photocatalytic and hydrophilic property of TiO2 in a self-cleaning coat has been reported to be responsible for the contact angle change between a water droplet and dirt on the substrate.

Nano Material based Coatings

Nanotechnology is the science to describe effects which arise from the quantum structure of nano-sized particles. Nanoparticles are generally considered atoms or molecules with at least one dimension of less than 100 nm. The interesting properties of nanoparticles are due to the high surface area to volume ratio. The extremely small sizes of nanoparticles over high surface to volume ratio provide the desired functionality when present in a very small concentration, compared to the bulk counterpart. It has well been proven that materials with high surface areas have enhanced physical, chemical, mechanical, optical or magnetic properties. There are several examples of addition of nanoparticles such as nano-ZnO, nanoalumina, nano silica etc. which have helped in enhancing properties such as corrosion resistance, mechanical properties and UV blocking effect7.

Anti-corrosive coatings should possess sufficient mechanical strength as they are often exposed to abrasive and erosive application environments. In auto industry, corrosion takes place at the location of scratches on the auto paint. The use of nanoparticles in coatings formulations can significantly improve scratch resistance. Nanoparticle coating composites prepared using nano grades of aluminum oxide in a solvent borne transparent melaminepolyol coating have shown significant improvement in scratch-resistance properties8. A comparative study of scratch-resistance properties of alumina nanoparticles against silica particles at equivalent loading levels has shown that the nano-alumina particles provide much better scratch-resistance protection for the UV-curable coating compared to silica particles at equivalent particle loadings. The combination of alumina nanoparticles and polysiloxane-based additives gives drastic improvement in scratch resistance. Another example is cerium oxide nanoparticles. It has been shown in the literature that surface mechanical properties (hardness and scratch resistance) improve by addition of these nanoparticles.

In the past decade, Graphene, a novel two-dimensional material with single layer having a thickness of around 0.335 nm and a diameter ranging from several microns to several hundred microns, has received worldwide attention due to its extraordinary properties arisen from its unique structure. Some of the important properties are excellent physical-mechanical properties, large specific surface areas, super hydrophobic property and good compatibility with polymer matrix9, 10. Graphene-based materials, which include graphene, graphene oxide, reduced graphene oxide, and graphene- embedded polymers, have demonstrated potential for applications in anti-corrosive coatings. Graphene-based paints can be used for conductive ink, antistatic, electromagnetic-interference shielding, and gas barrier applications. Graphene is highly inert, and so can also act as a corrosion barrier against water and oxygen diffusion. Besides the impermeable nature, the high conductivity of graphene also contributes in the corrosion protection. The higher conductivity provides an alternative path for electrons to travel, other than metal substrate, hence retarding the overall corrosion process. Again the high surface area of graphene also contributes in corrosion protection as it provides the tortuous path for the water and hence reduces the water permeation. Graphene coatings could be ideal corrosion- inhibiting coatings in applications where a thin coating is favorable, such as microelectronic components (eg, interconnects, aircraft components and implantable devices). Graphene is found to be effective towards the microbial induced corrosion also. Microbial corrosion is lesser understood form of corrosion where micro-organism affects the metallic surface and the damaged is unnoticeable until the major loss occurs.

Summary

Corrosion protection will always remain a challenge in highly aggressive environment. The recent development in the areas of smart coatings and the use of nano materials for specific properties enhancement in coatings have shown promising results. While some of the nano materials are now being made available for commercial use this new class of coatings will play a significant role in corrosion protection in the coming years.

References:

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2. Fragata, F, Salai, RP, Amorin, C, Almeida, E, "Compatibility and Incompatibility in Anticorrosive Painting—The Particular Case of Maintenance Painting." Prog. Org. Coat., 56 257 (2006).

3. P. A. Sørensen, S. Kiil, K. Dam- Johansen, C. E. Weinell, "Anticorrosive coatings: a review", J. Coat. Technol. Res., 6 (2) 135–176, 2009.

4. Sarah B. Ulaeto, Ramya Rajan, Jerin K. Pancrecious, T.P.D. Rajan, B.C. Pai, "Developments in smart anticorrosive coatings with multifunctional characteristics" Progress in Organic Coatings 111 (2017) 294–314.

5. R.P. Wool, Self-healing materials: a review, Soft Matter 4 (3) (2008) 400–418.

6. J. Yang, M. Huang, One-part selfhealing anticorrosive coatings: design strategy and examples, in: A. Tiwari, J. Rawlins, L.H. Hihara (Eds.), Intelligent Coatings for Corrosion Control, Elsevier Inc., Oxford UK, 2015, pp. 491–535.

7. V.S. Saji, J. Thomas, Nanomaterials for corrosion control, Curr. Sci. 92 (1) (2007) 51–55.

8. Deepak Shanbhag and Prashant Dhamdhere, "Recent Developments to Improve Scratch and Mar

9. Resistance in Automotive Coatings", PCI Magazine, June 2012.

10. R.K. Singh Raman and A Tiwari, "Graphene: The Thinnest Known Coating for Corrosion

11. Protection", JOM: the journal of the Minerals, Metals & Materials Society · April 2014

12. [10] K. S. Aneja and A.S. Khanna, "Graphene based anti-corrosive coatings for Cr (vi) replacement, Nanoscale, 7 (2015), 17879-17888.