Strateško opredjeljenje Europske unije u pogledu postizanja cilja klimatske neutralnosti do 2050. godine zahtijeva preobrazbu svih postojećih gospodarskih sektora. Putanja napretka u tom smislu zahtijeva i revolucionarne inicijative koje će rezultirati novim tehnološkim saznanjima i proizvodima. Unaprijeđena, ekonomski učinkovita i okolišno prihvatljiva tehnološka rješenja predstavljat će temelj tranzicije prema cjelokupnom održivom gospodarstvu, a posebice sektora energetike, prometa i industrije. U sklopu ovog doktorskog rada razvijeni su samoobnovljivi premazi za zaštitu konstrukcijskih materijala od korozije, čija bi potencijalna komercijalizacija u konačnici mogla pridonijeti promjeni dugoročnih trendova u energetskom i neenergetskom sektoru. Razvijeni premazi sadrže čahure kontrolirane propustljivosti, koje su modificirane tako da omogućuju aktivni odziv u slučaju pojave korozijskih reakcija.
Eksperimentalni rad počiva na nizu ulaznih varijabli koje podrazumijevaju sintezu i primjenu čahura različitih dimenzija, kapaciteta, koncentracija i svojstava, primjenu različitih inhibitora korozije te izloženost različitim agresivnim okruženjima. Provedeni su procesi sinteze poliurea čahura i mezoporoznih čestica silicijevog dioksida te proces učahurivanja inhibitora korozije cink oleata, 8-hidroksikinolina i 1-hidroksibenzotriazola. Disperzija čahura provedena je u matrici vodorazrjedivog epoksidnog premaza. Učinkovitost zaštite razvijenih premaza ispitana je na podlozi niskougljičnog čelika. Provedena je opsežna karakterizacija, pri čemu su određeni kemijski sastavi, parametri dimenzijske distribucije, stabilnosti disperzija i količina učahurenih inhibitora. Osim toga, određena su fizikalna svojstva premaza te je ustanovljena aktivna i pasivna učinkovitost u zadanim korozivnim uvjetima. Korištene su analitičke metode pretražne elektronske mikroskopije i energijski disperzivne rendgenske spektroskopije, dinamičkog i elektroforetskog raspršenja svjetlosti, termogravimetrijske analize, infracrvene spektroskopije Fourierovom transformacijom, ultraljubičaste vidljive spektroskopije, adsorpcije dušika, elektrokemijske impedancijske spektroskopije te metode ubrzanih korozijskih ispitivanja u slanoj i vlažnoj komori, uranjanja u NaCl otopinu i određivanja prionjivosti i degradacije premaza. Definirani su i modeli kinetike oslobađanja inhibitora iz čahura i prioritetni faktori učinkovitosti. Djelotvornost razvijenih samoobnovljivih sustava je potvrđena, a ujedno je i ustanovljena nedvosmislena dominantnost u odnosu na referentna trenutno dostupna rješenja.
When looking at the development and policy processes currently taking place in the energy and non-energy sector, one can unequivocally conclude that we are in a phase of historical transformation towards a climate-neutral economy. Achieving energy and climate goals requires the necessary technological improvements in the form of commercialization of existing technologies, as well as in the form of the development of new innovative solutions. Technology innovation should be based on the principles of reducing lifetime costs, reducing harmful environmental effects and improving functional characteristics.
Protective coatings are one of the indispensable foundations of the general economy. Extremely high costs of protection of finished structures are one of the limiting factors in the further development and application of renewable energy sources and the economy in general based on environmentally friendly solutions. Current protection systems require rapid and well-designed changes, aimed at a cleaner and more sustainable future. Achieving a technological breakthrough in terms of protective coatings will directly affect the cost, safety and social aspects, as well as climate change, which is considered one of the greatest threats to modern humanity. The attractiveness of the potential economic and environmental benefits arising from the idea of self-healing coatings is almost irresistible to the academic and industrial community. This fact has resulted in a world race in which one of the goals is the development of self-sustaining, responsive, feedback protective systems with wide application area.
Although global scientific research activities related to such "new generation" systems have intensified (as evidenced by a significant number of scientific publications over the past few years), a number of additional insights are needed to enable potential wider commercialization. This work represents a significant step forward in terms of knowledge about technological processes and the application of the final products in different environmental conditions. The conducted scientific research within this dissertation resulted in new and valuable findings in the field of autonomous active self-healing coatings for corrosion protection. Experimental activities for the first time compare two different technological processes related to the synthesis of the containers. Likewise, the properties of the developed composite coatings were correlated for the first time with respect to application in different corrosive environments. Models of active response in terms of kinetics of release of corrosion inhibitors from containers have been defined. With regard to a number of input variables, priority factors that affect the overall protection efficiency of developed composite systems have been identified.
The main goal of the research activities was primarily related to the development of an effective self-healing coating for corrosion protection. The developed protection system is based on a matrix of environmentally acceptable water-based coating in which the containers with corrosion inhibitor are dispersed. In the case of the coating rupture due to exposure to the aggressive environment, the autonomous response of the containers (in the form of the release of corrosion inhibitors) and active reaction occurs with the aim of increasing corrosion resistance. Further research objectives relate to the evaluation of the effectiveness of developed corrosion protection systems due to changes in key variables such as type, dimensions and concentrations of containers, types and concentrations of encapsulated inhibitors and the diversity of environmental exposure conditions.
As part of the doctoral dissertation, two types of containers (inorganic and polymer shell containers) were successfully synthesized. The dimensions of the containers range in the micro and nano range and contain various corrosion inhibitors. Successful synthesis and morphological features have been confirmed and defined by several scientifically experimental techniques. The containers were dispersed within the matrix of the water-based coating in various concentrations, and the composite systems thus obtained were applied to a low-carbon steel substrate. A comprehensive evaluation of the resistance of developed coatings in different corrosive environments was performed.
Experimental activities in the first step involve the synthesis of the polyurea containers loaded with 8-hydroxyquinoline and zinc oleate inhibitors, and mesoporous silica particles containing 1-hydroxybenzotriazole and 8-hydroxyquinoline inhibitors. Extensive characterization of the synthesized capsules or particles was performed, whereby the morphological properties, chemical compositions, dimensional distributions and stability of the container / particle dispersions and the quantity of encapsulated inhibitors were determined. The synthesized capsules / particles containing inhibitors were dispersed in a matrix of water-based epoxy coating in various concentrations, and thus were applied to the substrate. The physical properties of dry film thickness and adhesion of the coating were determined. The samples were exposed to different corrosive environments, and the application of electrochemical test methods detected an active response in terms of release and action of inhibitors from the containers / particles. The resistance of composite coatings to corrosion was defined.
Using various analytical methods, comprehensive information on capsules / particles, composite coatings and their protection effectiveness in aggressive corrosive conditions was
determined. The structure of key information was obtained on the basis of experimental methods like scanning electron microscopy (SEM, FE-SEM), energy dispersive X-ray spectroscopy (EDS), dynamic and electrophoretic light scattering (DLS, ELS), dynamic thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FT-IR), ultraviolet visible spectroscopy (UV-VIS), nitrogen adsorption, salt spray and humidity chamber testing, exposure to 3.5% NaCl solution, electrochemical impedance spectroscopy (EIS) and coating adhesion / degradation tests. Interpretation of information was performed using scientific research methods of description, compilation, analysis and synthesis, and in addition, comparative and inductive methods were used.
Predefined goals in terms of original scientific contribution have been successfully achieved. The research resulted in new findings in the context of the development and application of composite protection systems based on autonomous containers. Two different technological processes of capsule synthesis were demonstrated, which enable active response in an aggressive corrosive environment. By incorporating the containers into the water-based coating matrix, with the application of specific variables, new composite self-healing corrosion protection systems have been developed. The systems differ from each other according to the type and dimension of the containers, their quantity share in the coating and according to the type and concentration of encapsulated inhibitors.
The corrosion resistance of the coating in terms of barrier effect, active inhibitor effect and matrix integrity was determined. The knowledge on effectiveness of the used corrosion inhibitors has been deepened. Priority factors that affect the effectiveness of complete corrosion protection by applying developed coatings have been defined. A clear distinction was made between the morphological characteristics of the containers synthesized by different technological processes. Successful incorporation of corrosion inhibitors into containers has been confirmed, and the volume capacity of specific containers has been determined. Significant scientific contribution has been made in terms of the impact of individual variables, which affect the developed protective systems, in various corrosive environments. The research included a detailed analysis of the impact of the incorporation of the containers on the adhesion and degradation of the coating. The compatibility of the containers / particles with the polymer coating matrix was determined and guidelines were given for the further development of such systems with respect to the achievement of optimal corrosion protection.
Considering the possibility of application in various spheres such as energy, biotechnology or medicine, responsive active protection with coatings has a very significant scientific and technological potential. Despite the fact of currently fast-growing research activities, the area of self-healing corrosion protection coatings is still insufficiently explored. The conducted experimental research primarily defines the methodological approach to evaluating the effectiveness of developed protective systems based on autonomous containers. The defined parameters of analytical methods for the characterization of containers and the determination of the resistance of composite systems are applicable for further similar research. The dissertation provides a systematic and detailed insight into the technological process of preparation of self-healing composite coatings, methods for evaluating their durability in various corrosive environments and the key factors influencing the effectiveness of developed protection systems. The results of the research are useful primarily to the scientific research community and specifically to industrial manufacturers engaged in the protection of metal structures from corrosion using coatings. By respecting the concluding guidelines, it is possible to generate a more significant step towards a market-competitive product, with more dominant positive characteristics compared to the existing solutions. The cost-effectiveness of developed systems is one of the key factors that opens significant potential for positioning new products on the market. For example, corrosion protection of wind farms requires a cost-intensive maintenance regime. The application of autonomous response protection would greatly affect the predefined inspection periods, and consequently increase the internal rate of return of such projects. Such fact, as technological parameter input in long-term planning in the energy system, significantly influences the choice of scenarios for achieving the reduction of harmful emissions and achieving climate neutrality for the energy sector. Given that this is also a more environmentally friendly solution, the target group of the presented results are also the creators of environmental and climate policy.
Sensitivity analysis was performed by varying the key parameters affecting the autonomous response system. The influence of the variability of individual parameters has been clearly confirmed, with indicative trends that occur, for example, due to a decrease or increase in the concentration of capsules / particles in the coatings. A more detailed analysis of the sensitivity to changes in the results, which occurs due to, for example, a decrease or increase in the range of used concentrations of containers or corrosion inhibitors, can be performed based on the default settings within the conducted research.
The dissertation is divided into seven basic chapters. Introductory considerations, together with hypotheses and objectives, achieved scientific contributions and target groups are described in the first chapter. The second chapter describes the basic mechanisms of corrosion and the possibility of protecting the substrate by applying organic water-based coatings. The theory of the action of corrosion inhibitors is explained with emphasis on the specific formulations used in the experimental research.
The complexity of self-healing coatings with an overview of possible structures is described in chapter three. Special emphasis is placed on an approach based on autonomous containers / particles with an active response in the form of release and action of corrosion inhibitors.
Previous scientific achievements in the field of active self-healing coatings are described in chapter four. Coatings are classified according to the type of stimulus that elicits an active response. The types of possible containers, technologies of their preparation and results of their application are presented. In addition, key considerations and basic determinants related to the justification of conducting experimental work are given.
Chapter five presents the methodological concept of the experimental research. The materials used, settings of technological processes, analytical and measurement methods and expected research results are described. Chapter six refers to a detailed analysis of the obtained results with the corresponding discussion, while in chapter seven there are concluding considerations with guidelines for conducting further research activities.