Estudio de la resistencia a la corrosión de nuevos recubrimientos con el empleo de líquidos iónicos

Resumo

Los recubrimientos protectores se emplean ampliamente para el control de la corrosión proporcionando una protección de larga duración bajo un amplio rango de condiciones corrosivas. Es válido destacar, que a pesar de existir una amplia diversidad de recubrimientos anticorrosivos, el método más utilizado para la protección contra la corrosión es la aplicación de pinturas. La necesidad de la protección del medio ambiente y de la salud humana condujo a la completa prohibición de muchas de las formulaciones tradicionales de pinturas, debido a que incluían productos tóxicos y/o carcinogénicos en su composición. Por lo que durante los años 1990 ocurrió un cambio radical en las tecnologías de pinturas que motivó la reformulación y aparición de las pinturas ecológicas, dentro de ellas, las que poseen un bajo contenido de compuestos orgánicos volátiles (COV). Los sistemas de pinturas anticorrosivos del tipo epoxi/poliuretano han sido ampliamente usados en la protección de estructuras metálicas de acero expuestas a diferentes ambientes. Sin embargo, el uso de isocianatos en el proceso de curado y el alto contenido de compuestos orgánicos volátiles (COV) hacen necesaria la búsqueda de nuevas pinturas libres de isocianato por su reconocida toxicidad. Los recubrimientos híbridos orgánico-inorgánico tales como los epoxi-siloxano, constituyen un avance importante en el campo de los recubrimientos anticorrosivos de pintura. Estas nuevas pinturas híbridas libres de isocianato presentan un bajo nivel de COV, buena estabilidad al calor y a los rayos UV, etc. Además, el empleo de las nuevas resinas inorgánicas de polisiloxano mejora el comportamiento anticorrosivo de las resinas orgánicas tradicionales. No obstante, la bibliografía consultada contiene muy poca información acerca de la resistencia anticorrosiva de este tipo de recubrimientos, así como del proceso de entrada de agua. Es bien conocido que todas las técnicas electroquímicas tradicionales empleadas para el estudio del mecanismo de entrada de agua en una película de pintura, dependen principalmente del uso de electrolitos acuosos. Estos electrolitos presentan el inconveniente de que pueden penetrar en la matriz polimérica de las pinturas afectando a las mediciones. Los líquidos iónicos se establecen como una posible alternativa para mejorar el estudio de este proceso, ya que se caracterizan por ser muy buenos conductores iónicos, tener un amplio rango de potencial electroquímico, etc, pero sobre todo, porque no penetran directamente en la pintura, como sí ocurre en presencia de los medios electrolíticos tradicionales. En la bibliografía consultada se han encontrado pocos trabajos donde se estudie el proceso de entrada de agua en la pintura empleando los líquidos iónicos. Por lo antes expuesto, es objetivo de la presente investigación estudiar el comportamiento anticorrosivo de recubrimientos híbridos orgánico-inorgánicos, así como el mecanismo de entrada de agua con el empleo de las técnicas electroquímicas tradicionales y la introducción de los líquidos iónicos y compararlo con el de las pinturas anticorrosivas tradicionales.Os recubrimentos protectores empréganse amplamente para o control da corrosión proporcionando unha protección de longa duración baixo un amplo rango de condicións corrosivas. É válido destacar, que a pesar de existir unha ampla diversidade de recubrimentos anticorrosivos, o método máis utilizado para a protección contra a corrosión é a aplicación de pinturas. A necesidade da protección do medio e da saúde humana conduciu á completa prohibición de moitas das formulacións tradicionais de pinturas, debido a que incluían produtos tóxicos e/ou carcinoxénicos na súa composición. Polo que durante os anos 1990 aconteceu un cambio radical nas tecnoloxías de pinturas que motivou a reformulación e aparición das pinturas ecolóxicas, dentro delas, as que posúen un baixo contido de compostos orgánicos volátiles (COV). Os sistemas de pinturas anticorrosivos do tipo epoxi/poliuretano foron amplamente usados na protección de estruturas metálicas de aceiro expostas a diferentes ambientes. Non obstante, o uso de isocianatos no proceso de curado e o alto contido de compostos orgánicos volátiles (COV) fan necesaria a busca de novas pinturas libres de isocianato pola súa recoñecida toxicidade. Os recubrimentos híbridos orgánico-inorgánico tales como os epoxi-siloxano, constitúen un avance importante no campo dos recubrimentos anticorrosivos de pintura. Estas novas pinturas híbridas libres de isocianato presentan un baixo nivel de COV, boa estabilidade á calor e aos raios UV, etc. Ademais, o emprego das novas resinas inorgánicas de polisiloxano mellora o comportamento anticorrosivo das resinas orgánicas tradicionais. Non obstante, a bibliografía consultada contén moi pouca información acerca da resistencia anticorrosiva deste tipo de recubrimentos, así como do proceso de entrada da auga. É ben coñecido que todas as técnicas electroquímicas tradicionais empregadas para o estudo do mecanismo de entrada da auga nunha película de pintura, dependen principalmente do uso de electrólitos acuosos. Estes electrólitos presentan o inconveniente de que poden penetrar na matriz polimérica das pinturas afectando ás medicións. Os líquidos iónicos establécense como unha posible alternativa para mellorar o estudo deste proceso, xa que se caracterizan por ser moi bos condutores iónicos, ter un amplo rango de potencial electroquímico, etc, pero sobre todo, porque non penetran directamente na pintura, como si acontece en presenza dos medios electrolíticos tradicionais. Na bibliografía consultada encontráronse poucos traballos onde se estude o proceso de entrada da auga na pintura empregando os líquidos iónicos. Polo antes exposto, é obxectivo da presente investigación estudar o comportamento anticorrosivo de recubrimentos híbridos orgánico-inorgánicos, así como o mecanismo de entrada da auga co emprego das técnicas electroquímicas tradicionais e a introdución dos líquidos iónicos e comparalo co das pinturas anticorrosivas tradicionais.Atmospheric corrosion is a subject of a great scientific interest for many researchers. This concern is even greater in coastal areas under the influence of marine aerosol, where the major corrosion losses takes place. Several authors have demonstrated that chloride ions, coming from the salt spray of the sea, play a major role in the corrosion of steel, as well as the sulphate ions (from the same source). In addition, SO2, mainly associated with industrial environments, substantially increase the corrosion rate. A previous study has noted that chloride and sulfate ions maintain the (Cl ? / SO2­4 = 7 w/w ratio in sea water and in the marine aerosol. Hence, the transformation of SO2 into SO42­ ions lead to relationship values lower than 7, thus suggesting the presence of anthropogenic SO42­ ions, in addition to the SO42­ ions coming from the marine source. Also, the poor mathematical correlations obtained between the corrosion rate and the chloride ions and sulphur compounds (from different sources) suggest the existence of some limitations in the determination of the ions concentration in the atmosphere with marine influence. Furthermore, the search for reliable methods for the determination of these ions is crucial for developing accurate investigations in corrosion. Among the normalized methods to measure the deposition rate of chloride ions and sulphur compounds, the wet candle (WC) and the alkaline sulphation plates (ASP), respectively, are the most commonly used. The Cuban standard contemplates also the use of the dry plate (DP) method to determine chloride ions. Bearing in mind the abovementioned, it is intended to demonstrate the importance of determining the chloride and sulphate ions in a same collection device for corrosion researches. The experiments were carried out for three years in outdoor conditions, by using three different collection methods (already mentioned). At the same time, the determination of the corrosive aggressiveness is required for further corrosion studies. In environments of high, very high and extreme corrosive aggressiveness the search of efficient protection methods is an issue in the limelight. Organic coatings are known as the most widespread method for preventing atmospheric corrosion of a metallic substrate, where the surface preparation before painting is crucial to accomplish a correct performance and the expected durability of the paint system. Among the existing method, abrasive blasting is the most employed probably due to its effectiveness, simplicity and durability. However, environmental, health and economic reasons have hindered its extensive application in some installations such as hotels. Accordingly, the search of competitive surface treatments motivates the scientific community to investigate new alternatives such as phosphating. Furthermore, it is aimed to evaluate the effectiveness of different surface treatments, mainly focused in phosphate conversion layers, applied to three coating systems before painting. This will allow selecting the most appropriate combination (pretreatment/paint system) to be applied at real scale both in samples and civil structures located in the open air, under an aggressive marine atmosphere for two years. Among the paint systems studied, epoxy/polyurethane ones have been widely employed for corrosion protection of steel structures in aggressive environments. However, more recently, hybrid organic-inorganic coatings such as epoxy-siloxane have outperformed the former eliminating the isocyanate and reducing considerably the content of volatile organic compounds (VOCs). These new coatings offer undoubted benefits since they sinergically combine the advantages of silicone and epoxy resins which result in a polymer with unique properties. According to the consulted bibliography, there is a lack of information about the study of the anticorrosive protection of this coatings with or without the combination of other protective layers, by electrochemical impedance spectroscopy (EIS). It is well known that this technique offers valuable information of the electrochemical mechanism . Hence, it is pretended to assess the protective properties of an epoxy-siloxane hybrid coating (as a single layer and also forming part of coating systems as topcoat), where an epoxy zinc rich paint (ZRP) and an epoxy polymer pigmented with micaceous iron oxide (MIO) and zinc phosphate (ZP) will be the primers. Additionally, and for comparative purposes, a traditional polyurethane topcoat with the same primers already mentioned and an epoxy/polyurethane water-based paint system will be tested by EIS, salt spray test, physical tests and surface characterization techniques (SEM, EDX and XRM) will support the anticorrosive performance study. Some common failures (pinholes, voids, mechanical scratches, edge effect) in coatings lead to their deterioration owing to the anticorrosive design problems, surface preparation and/or fails in the paint system. The delamination is another typical defect in coatings, which is critical in the presence of the failures previously mentioned and it is also increased due to cathodic polarization, promoting cathodic disbondment. This is very common in metallic structures embedded in electrolytes which are protected against corrosion by a combination of a paint system and cathodic protection. Several authors have converged upon the idea that the driving force for the disbonding is the cathodic potential, which activates the cathodic reaction in the defects. One of the alternatives for disbonding mitigation have been pigments inclusion in coating formulation. In addition, the study of these phenomena in multilayer coating systems has been less addressed and most of the studies have been dedicated to the influence of cathodic polarization and not to natural disbondment. Therefore, the role played by zinc pigments in the resistance of epoxy zinc rich paint (ZRP)/epoxy/polyurethane coating system against cathodic and natural disbondment will be assessed. EIS measurements, physical-mechanical tests (adhesion, radial cut), together with surface characterization methods (optical microscopy, SEM and EDX) will support the investigation. Some coatings outperform paints motivated to their low cost, flexibility, impact-resistance and low permeability";" which support specific applications in combination with paint systems. In this work, new coatings of Polymer Modified Asphalts (PMAs) will be developed and applied on steel samples and in bodyworks. Accelerated tests, adherence and impact, as well as the behaviour of the macroscopic failures for two years-exposure, under a very high aggressive marine zone, is pretended to carry out. The use of recycled rubber as a raw material in the PMA formulation contribute to waste valorisation which is a topic in the limelight. Water constitutes an appropriate medium for species diffusion, which leads to coating delamination, the establishment of pathways for entering aggressive media and other effects. Among the experimental methods available for evaluating water absorption in coatings, EIS is the most common technique. However, this method require the use of water as a conductive medium, thus affecting the protective properties of the polymer due to water uptake. This experimental handicap is overcome with the employment of ionic liquids (ILs), salts that are liquids at temperatures under 100ºC and have good conductivity (over 0.01 s cm-1), promoting their application in electrochemistry as non-aqueous medium. According to the consulted bibliography, there is a lack of knowledge in this topic, which encourage the study of water absorption-desorption process in an epoxy¿siloxane coating, as well as in a ZRP by using, for the first time, an imidazolium-based IL. 1-ethyl-3-methyl imidazolium ethylsulfate was selected due to this family is one of the most important ILs already produced at an industrial scale. Besides, it can be easily synthesized, at a reasonable cost and it shows high chemical and thermal stabilities. It is intended to monitor, both impedance and capacitance of coatings during inward/outward water diffusion by EIS measurements. On the other hand, the beneficial influence of the IL for improving the cathodic protection of ZRP will be evaluated by EIS. Additionally, surface characterization techniques (SEM, EDX, FTIR) will prove the presence or not of the selected IL.