The Study Of Smart Nano-composite Multilayer Coatings For Corrosion Protection Of Stainless Steel

Stainless steel (SS) is widely employed as an anti-corrosive material and considered to be the backbone of industrial and engineering structures because of its high mechanical strength and corrosion resistance. However, SS is thermodynamically unstable and prone to corrosion when exposed to chloride containing environment. The corrosion reaction occurs on the SS surface when SS comes in contact with the chloride environment, which would provoke destruction or deterioration of its structural integrity. This metallic corrosion would result in significant economic losses leading to the expenditure of billions to trillions of dollars per year worldwide. Thus, the development of new, smart and environmentally friendly anticorrosion coatings is an important approach to extend the lifetime of domestic SS used as engineering structures in international market.To minimize the economic losses, the concept of corrosion protection by the use of organic inhibitors, smart and multifunctional coatings was investigated quite intensively in the recent years. To protect SS in different aggressive environments, a number of strategies have been employed. The first strategy is the use of organic hetero atoms as inhibitors, which are considered to be an important protecting material against deterioration due to corrosion, especially in acidic media. Their adsorption on the metal surface as a protective film would result in a physical barrier that hinders the physical contact of the aggressive species with the metal surface in the acidic medium. The second important strategy is the use of organic coatings that would provide long-lasting corrosion protection. Among organic coatings, conductive polymers (CPs) based "smart" composite coatings, which can prevent metallic corrosion even in defect areas where the bare metal surface is exposed to the corrosive environment. Polyaniline (PANI) as a conductive polymer is believed to be the best anti-corrosion material, because it exists in different states (oxidation/reduction state) and can easily exchange between these states under appropriate conditions. PANI acts as an advanced corrosion protection coating material that undergoes redox processes and thereby induces a passive oxide layer on metal surfaces. However, the extremely poor solubility of PANI in aqueous solution limited its application as a coating material and corrosion inhibitor in aggressive medium for SS. However, the process-ability of PANI could be achieved by polymerizing it in an insulating matrix to obtain water soluble composites.The third strategy is the use of functional organic coatings for corrosion protection of the SS in a concentrated salt solution. Recently, the concept of a "smart" coating has been applied to functional coatings, e.g. the coating’s self-healing ability as a response to certain stimuli generated by intrinsic or extrinsic events for corrosion protection. The self-healing property of the anti-corrosion coatings can be achieved by releasing the healing agents extrinsically encapsulated in nano-containers. To avoid the incorporation of healing agents and difficulties involved in their controlled release, intrinsic self-healing materials are the replacement. However, the use of polyelectrolytes self-assembled multilayers (SAMu) capable of intrinsically healing severe damage is still a challenge and the self-healing mechanism is not clear.The fabrication of smart and functional organic coatings with a multilayered structure can be achieved by some common techniques, i.e. spray coating, dip coating and spin coating. Among these methods, dip and spin coatings provide a finer control over thickness and composition. Self-assembled nano-network formed by polyelectrolytes using layer-by-layer (LBL) technique can be achieved by dip coating. Spin coating is also a facile and robust method, but the coating morphology is significantly influenced by solvent type, solution concentration and humidity, thus affects the corrosion resistance.Based on the above discussion, it concludes that the corrosion protection of the SS in aggressive environments by process-able PANI composites based inhibitor and its composite coating is desired. In addition, the processing parameters like; solution concentration (c) and disk rotating speed (co), the electrolyte diffusion behavior within the multilayer structure, pH of the dipping solutions, could affect the thickness and corrosion resistant properties of the coating during the multilayer fabrication. Therefore, to develop a smart anti-corrosion coating, having self-healing and redox catalytic properties, by tuning the processing parameters during fabrication is a key research topic in the field of anti-corrosion application for SS. The aim of the present work is to employ the advanced three strategies (as discussed above) in order to achieve smart anti-corrosion properties to protect SS in different environments.In this work, we have prepared water-soluble polyaniline-polyacrylic acid (PANI-PAA) composites with an excellent process-ability and electro-activity by a one-step in-situ polymerization method. A series of these electroactive composites were studied as an efficient corrosion inhibitor for 316SS in the strong acidic medium. We have also prepared PANI-PAA/PEI multilayer composite coatings by a spin assembly and optimize their corrosion protection performance for 316SS. The corrosion performance of the PANI-PAA/PEI coatings with different n was evaluated. The factors like c and co were optimized during spin assembly, a multilayer structure with controllable thickness and their effects on anti-corrosion properties of the PANI-PAA/PEI coating was investigated. The mechanism involved in the enhanced corrosion resistance of the coating was proposed on the basis of the employed multilayer structure that lengthens the diffusion pathway of electrolyte and its diffusion behavior within the multilayer structure was investigated. In addition, we have also explored the stimuli responsive self-healing behavior of the PDDA/PAA multilayer coatings. Our research work was summarized as follows:(1) Water-soluble PANI-PAA composites were prepared. PAA as a matrix not only improves the solubility of PANI in water, but also prevents the formation of macroscopic PANI clusters. The adsorption of the inhibitor on 316SS surface was fitted for Langmuir adsorption isotherm and the PANI-PAA composite with an optimized concentration of 200 ppm with marked increase in inhibition efficiency was achieved. The enhanced inhibition efficiency is attributed to an insulating interfacial layer formed by the adsorption of PANI-PAA, which obstructs the corrosion reaction at the interface.(2) Polyaniline-polyacrylic acid/polyethyleneimine (PANI-PAA/PEI) composite coatings with a multilayer structure for corrosion protection of 316SS were prepared by an alternate deposition. Spin coating combined with heating assists a removal of residual water that results in a linear increase in thickness with layer number (n). The combination of PANI-PAA composite with PEI and their multilayer structure provides a synergistic enhancement of corrosion resistance. Importantly, the improved corrosion protection properties of the PANI-PAA/PEI coating were optimized at layer number of n= 20. The superior performance was attributed to the formation of an interfacial oxide layer as well as the multilayer structure that extends the diffusion pathway of corrosive ions.(3) The PANI-PAA)/PEI coatings with tunable thickness in a wide range from 0.47 to 2.94 μm were prepared. The PANI-PAA/PEI coatings are electroactive with the redox catalytic ability leading to highly improved corrosion protection efficiency. The coating is still stable after 120 h in 3.5% NaCl solution and electrolyte diffusion behavior within the coating is found to be a function of time. The mechanism based on the diffusion behavior of electrolyte within the multilayer structure was proposed.(4) At last, smart polydiallyldimethylammonium chloride (PDDA)/polyacrylic acid (PAA) multilayer coatings with enhanced stimuli responsive self-healing and anti-corrosion ability were prepared for 430SS. The multilayer coatings were prepared by a layer-by-layer (LBL) technique, which allows a fine control over layer numbers (n) and coating thickness. The dependence of anti-corrosion and self-healing properties on layer number was investigated. The involved self-healing is attributed to the stimuli responsive swelling and electrostatic repairing of the polyelectrolyte multilayers (PEMu) in the vicinity of the scratches. The enhanced corrosion resistance ability is mainly due to the multilayer structure and the self-healing behavior that lengthens and hinders the diffusion pathway of corrosive species.