SMART FILMS FOR CORROSION PROTECTION

Most biological membranes respond to environmental stimuli with the purpose to satisfy a particular requirement of the organism. Therefore, the concept of “intelligence” is often attributed to biological membranes due to their ability to respond, e.g. to a change in pH, variation in salt concentration, and physical or chemical damage, among others. The so-called smart materials are inspired in the natural intelligence of living systems.

For corrosion-protection of metals,. Aluminum alloys of the 2000 and 7000 series are widely used in structural applications, particularly in the aircraft industry due to their remarkable weight / mechanical resistance ratio and to the well defined physical properties of these materials. The presence of alloying elements and the thermal treatment that these alloys are subjected to, make them highly susceptible to corrosion, that is, they experience damage in moist environments of varied composition [Hatch 1984, Chen 1996]. Although the traditional method of aluminum protection has been anodizing, the methodology is restricted to the local and global behavior of the alloying elements constituents from the 2000 and 7000 series during the electrolytic process [Páez 1996, Habazaki 1996 a, Páez 2000*, Zhou 2000, Habazaki 1996 b]. Such behavior gives rise to fragile anodic films associated with pore branching during film growth. Indeed, these alloys are protected by the multilayer paint system [Hatch 1984], where the first and second layers are chromate-based. Since the process is currently under scrutiny, an alternative methodology to protect alloy materials is of an utmost importance. Also worth mentioning are the costs associated to waste disposal of current metal coating processes which present a constant issue for the industry.

Replacement of chromate-based coatings is a noteworthy challenge, since these coatings have the ability of self-healing in case of chemical or mechanical damage of the film. Furthermore, chromate based coatings inhibit bacterial proliferation, which has been found to influence corrosion processes (Microbiological Influenced Corrosion, MIC). For the aforementioned reasons, an alternative type of coating, which would effectively replace those chromate based, should be ideally compact, with self-healing abilities and resistant to biocorrosion. Recently, the development of a new generation of self-repairing coatings and bulk materials [Andreeva 2008*] has driven investigations on future high-tech functional surfaces for corrosion protection of aluminum alloys. These smart coatings have both passive mechanical characteristics originated from matrix material (mainly polymeric matrixes of organic-inorganic sol-gel type) and an active response sensitive to changes in the local environment or to the integrity of the passive matrix. The progress in this area of research is in resonance with the advances that nanotechnology has generated in the development of such coatings. Indeed, nowadays coatings can be generated with specific properties in relation to their particular application. The main concept is to modify polymer hybrid coatings through nanoparticle incorporation, which requires the investigation of synthesis combination and modification that will ultimately enable the generation of a film with high-quality mechanical properties. Another important concept is the nanoparticle functionalization to be incorporated in the coating. In this case, both the type of interaction between the nanoparticle and the functionalizing species, and between the modified nanoparticle and the polymer matrix, are determinant for the properties conferred to the coating [Feng 2007].

In the present proposal, the different functionalities and specificities that can be accomplished with the incorporation of nanoparticles to films shall be applied to develop smart coatings for the protection of aluminum alloys. To simulate the bactericidal property of chromate species, the present project, based on preliminary investigations of our laboratory, proposes the study of the incorporation of copper and silver nanoparticles into inorganic-organic hybrid sol-gel type polymers. The metallic nanoparticles will be directly or, through prior encapsulation with silica, incorporated into the polymeric matrix. Recent advances in different types of nanomaterials [Gu 2003; Gong, 2007*] have shown that silver nanoparticles have antibacterial, antiviral and antimycotic properties [Gong, 2007*; Rai 2009]. Even though silver nanoparticles have been incorporated in coatings designed for medical equipment, clothing and textile fabrics [Gong, 2007*; Rai,2009], there is no information of their incorporation in anti-corrosion hybrid coatings of the sol-gel type. On the other hand, for introducing self-healing properties to the coating, incorporation of inorganic nanoparticles doped with corrosion inhibitors into the polymeric matrix will be also considered. Incorporation of both types of nanoparticles (functionalized inorganic and silver) in hybrid polymeric films are expected to reproduce the performance of chromate base coatings as starting point.

In addition to the properties already mentioned, several aspects such as extensibility, adherence, and coating tension are also addressed with the purpose of ensuring the operative life of the coated material. The combination between specific and mechanical properties, which are required for a great deal of material applications, constitute a permanent challenge. In consequence, the necessity to develop new effective and environmentally friendly coatings has significantly increased. The mechanical resistance will be used as a parameter to study the coatings for their potential application in material protection. Coating adherence, thickness, fracture resistance, hardness, residual tension and ageing are key features that critically determine the final efficiency of these films. Regardless of the significance of evaluating these mechanical behaviors, the existing methods for their assessment are limited by resolution or they require film destruction. This collaboration project aims to connect investigators that use non-destructive methods to obtain mechanical characterization of thin films with investigators who are developing new types of coatings