Važnost primjene suvremenih znanstvenih spoznaja o materijalima i njihovoj tehnologiji zaštite u tehnici nezaobilazan je skup metoda i postupaka potrebnih za postizanje sofisticiranog načina projektiranja, eksploatacije, održavanja te odlaganja, pri čemu njihovo istodobno generiranje, razumijevanje i strukturiranje usmjerava njihovu proizvodnju prema ekološki prihvatljivijim proizvodnim procesima i sustavima zaštite materijala, a eksploatiranom materijalu osigurava ponovnu primjenu nakon postupka recikliranja. U tom kontekstu, zbog iznimne važnosti i posebne primjene aluminijske legure 2024-T3, čiji je dosadašnji sustav površinske zaštite uspješno ostvaren sinergijskim djelovanjem organskih i konverzijskih prevlaka sa štetnim sadržajem heksavalentnog kroma, treba provesti niz unaprjeđenja kako bi se izbjegla izrazita opasnost za ljudsko zdravlje i okoliš te postigla potpuna optimizacija i modernizacija cjelokupnog sustava. Cerij (III) klorid heptahidrat (CeCl3 × 7H2O) inovativan je "zeleni'' inhibitor koji ima izvrsna zaštitna svojstva kad je u pitanju korozija aluminijske legure 2024-T3 u kloridnom mediju, a njegov je razvoj posebno intenziviran zbog zakonskih odredaba koje sve više ograničavaju primjenu kroma. Ipak, šira je primjena još uvijek izostala, ponajprije zbog nedovoljno poznatih zaštitnih karakteristika. U cilju proširenja dosadašnjih saznanja u doktorskom je radu ispitan utjecaj koncentracije CeCl3 × 7H2O, temperature i vremenskog perioda uranjanja na korozijski potencijal, brzinu prodiranja korozije i piting potencijal predmetne aluminijske legure te su posebno istraženi zaštitna djelotvornost, stupanj inhibicije i prekrivenost površine kao i mehanizam te vrsta adsorpcije. Za istraživanje su upotrijebljene elektrokemijske mjerne tehnike AC i DC, a svojstva su metalne površine i njezina mikroanaliza ispitani metodom pretražne elektronske mikroskopije (FE-SEM) i energijski disperzivne rendgenske spektroskopije (EDS) te mikroskopije atomskih sila (AFM), a kao krajnji rezultat rekonstruiran je trodimenzionalni prikaz skeniranog ispitnog uzorka. Za definirane koncentracije, vremenske periode uranjanjanja i temperature, nakon provedenih svih predviđenih ispitivanja i analize rezultata, potvrđena je višestruka dobrobit primjene cerijevih iona u budućim suvremenim i ekološki prihvatljivim sustavima zaštite od korozije.
|Abstract (english)|| |
Observing the complex trends of modern times, it can be unequivocally concluded that we are in the most complex period of human development, in which the imperative of sustainable economic development, together with social progress and environmental protection, imposes a number of changes and challenges in all aspects of human life and action, which would ultimately through the necessary scientific and technological improvements result in a climate neutral and circular economy.
Since nowadays corrosion destruction of construction materials is one of the key factors in economic, safety and environmental issues, which has significantly increased interest in this topic, modern solutions of corrosion reduction should be based on the application of highly effective and environmentally friendly solutions, such as green corrosion inhibitors. Their extreme effectiveness and multifunctionality, in addition to individual application, is a fundamental determinant of other surface protection systems, such as conventional and self-healing coatings, so it is not surprising that the great interest of academic and industrial communities encourages a great amount of scientific research in order to find an inhibitor with a wide application area, which at the same time affects the self-sustainability of protective surface systems. Since such sustainable innovations can generally be broadly ranked from incremental to transformative, with each category having its own typical improvement potential, timescale, technological and operational challenges, global research activities are largely focused on the intensive improvement of such modern systems. This work represents a direct contribution to the development and potential commercial application of "Rare-Earth" elements as a promising and environmentally friendly representatives of next-generation corrosion inhibitors. The conducted scientific research within this dissertation has resulted in a new and valuable step forward in terms of sustainable development of modern inhibitors. The importance of applying a wide range of experimental activities confirmed the significant contribution in the context of determination and evaluation of crucial factors influencing the effectiveness and efficiency of corrosion protection and the adsorption mechanism of cerium compounds, which is a significant success in understanding lanthanide elements as environmentally friendly corrosion inhibitors. With regard to a number of input variables, the adsorption mechanism was identified as well as the priority factors influencing the overall protection efficiency of cerium ions, which opens a new chapter in further scientific research
in which many synergistic solutions can be enthusiastically expected to completely eliminate harmful and carcinogenic chromates from existing surface protection systems.
The main goal of the research activities carried out in this paper was primarily to examine and evaluate the impact of crucial variables relevant to potential industrial implementation and application, i.e. the wider commercialization of cerium salts as "green" inhibitors for 2024-T3 aluminum alloy. In this regard, the influence of CeCl3 × 7H2O concentration, temperature and immersion time period on the degree of inhibition and corrosion retardation factor, corrosion potential, corrosion rate and pitting potential of aluminum alloy 2024-T3 was examined. Subsequent analysis and synthesis of the obtained data revealed interactions between cerium (III) chloride heptahydrate inhibitors and the surface of the subject alloy in chloride medium. Further research goals relate to determination of the influence of test variables on thermodynamic parameters, roughness and morphology, as well as on the size, depth and distribution of pit phenomena on the surface of the subject alloy.
The efficiency of conducting experimental activities, and thus the accuracy and value of the obtained results, in addition to the precision of conducting analytical methods, depends primarily on a series of preparatory actions directly related to the establishment of certain surface properties of test samples. In this regard, the test specimens were machine-cut into cylindrical geometric bodies with a diameter of 16 mm. After cutting, they were mechanically treated with sandpaper of different granulation (600 - 2000), using an automatic grinding and polishing device, and subsequently polished with diamond paste in combination with an alcohol-based lubricant. After polishing, the test specimens were washed with deionized water at room temperature for a period of 1 minute, and were ultrasonically degreased in a 96% ethanol bath for ten minutes using a degreasing device.
The implementation of the experimental part of the paper as a crucial segment of scientific research activities is the result of a systematic theoretical analysis of the knowledge gained so far, generated as a result of research by eminent scientific figures on the international scene. In order to determine the type of corrosion process as well as the influence of the inhomogeneity of material surface on the adsorption of cerium ions and compounds, a number of experimental methods like light microscopy - metallography, scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS), atomic force microscopy (AFM), Tafel extrapolation method, cyclic polarization method, electrochemical impedance spectroscopy (EIS) were performed. Considering the totality of methodical procedures, based on rational and
empirical scientific-research methods of phenomena and processes, in order to achieve objective and systematic scientific knowledge, a complete processing and detailed presentation of the obtained experimental data was carried out.
Predefined goals in terms of original scientific contribution have been successfully achieved. The conducted scientific research in the domain of this doctoral thesis has resulted in a number of useful findings in the field of inhibition of corrosion processes by alternative and environmentally friendly inhibitors based on cerium ions (cerium (III) chloride heptahydrate). Since all levels of testing are aimed at confirming the cerium inhibitor as an appropriate replacement for existing aluminum alloy surface protection systems, it is extremely important that the effective inhibitory action of the tested inhibitor represents a promising improvement of the protective properties. Based on the conducted tests, and in order to confirm the research hypothesis, it was proved that cerium ions as a cathodic inhibitor have excellent inhibitory efficacy, which despite a large number of test variables, for certain test parameters, achieves a theoretical maximum.
By analyzing the results obtained in this dissertation, it was proved that the insulating (passive) properties of the inhibited film are more stable, and the degree of inhibition and protective ability is better if the temperature is lower and the concentration and time period of exposure to the inhibitor is higher. It was also found that the mentioned variation of test variables on the metal surface causes a significant reduction in both corrosion rate and frequency, propagation depth of the total and maximum pit area, which results in significant improvements in the topographic and morphological characteristics of the protective film. Such findings suggest that further research is needed to optimize the test variables, which would ultimately for specific test or potentially exploitation conditions, allow to achieve the best possible properties of the barrier layer. Furthermore, by implementing Langmuir's mathematical model as a starting point for a systematic way of calculating and modeling adsorption processes, a quantitative assessment and interpretation of a wide range of experimental data was achieved, and as a final result, a physical and mathematical explanation of the cerium adsorption mechanism on the aluminium surface was presented. Based on the numerical and graphical analysis conducted as part of the dissertation, it is concluded that the forces between the adsorbent and the metal surface have a lower intensity and that there is no electron transfer or exchange, which is a characteristic of physical adsorption. Also, by applying appropriate mathematical relations, a number of thermodynamic parameters were defined, as well as the
adsorption capacity values of the adsorbent and the adsorbate mass as a function of the test variables. In this regard, it was found that the increase in temperature has an inversely proportional effect on the mass of the precipitated adsorbate on the metal surface, which implies that the temperature increase supports the desorption of adsorbents, while the increase in concentration, in contrast to the temperature load, proportionally affects the increase in mass, i.e. the amount of sorption of inhibitors at a certain time.
The results of the current research are useful primarily to the scientific community, and especially to industrial manufacturers of corrosion inhibitors and coatings, whose application occupies a dominant place when it comes to protective metal construction against corrosion. At the same time, emphasizing the ecological and health aspect implies future human health safety and reduction of harmful impact on the environment, while the economic criterion implies finding an appropriate cost-effective substance that would replace their role. In that context, regulations and guidelines implementing the restriction and prohibition of hazardous substances have significantly contributed to the restriction of the use of chromate, which at the same time has stimulated a whole range of research that should have much better characteristics than existing surface protection solutions. Since the conducted experimental research primarily defines a methodological approach to evaluating the effectiveness of cerium ions as an alternative inhibitor of carcinogenic chromium, the defined parameters of analytical methods for determining the resistance of new generation inhibitors are applicable for further similar research.
The dissertation is divided into nine basic chapters. Introductory considerations, together with the motivation for conducting research, hypotheses and objectives and achieved scientific contributions are described in the first chapter. The second chapter describes the classification, mechanisms and key parameters of corrosion processes, with special emphasis to the economic consequences of corrosion that are manifested through direct and indirect losses which significantly hamper the progress of the community and raising living standards.
The third chapter describes in detail the properties of aluminum and aluminum alloys, as well as the effects of important alloying elements and impurities on their characteristics. Also, the division of the subject alloys according to technological processing, possible heat treatment and chemical composition are presented, as well as their application in the areas where they are most represented.
In the fourth chapter, special emphasis is placed on the review of the properties of aluminum alloy 2024-T3 as well as on a detailed description of the intermetallic phases formation, i.e. particles of the second phase whose formation is shown in the equilibrium state diagram of the aluminium alloy. Various types of possible intermetallic particles, whose presence is the result of the precipitation hardening mechanism realized through the process of soluble annealing and age hardening, which from the corrosion point of view represent the starting points of corrosion processes, are also presented and described.
Corrosion resistance and a detailed description of the local corrosion phenomena (pits) formation on the surface of aluminum alloy 2024-T3, immersed in chloride medium, are presented in chapter 5, while chapter six describes the current scientific achievements in the field of inhibition of corrosion processes by the action of cerium ions as a substitute for harmful chromates. Special emphasis is placed on the thermodynamic aspects of the adsorption process as well as on the influence of various factors on its efficiency.
Chapter seven 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 eight refers to a detailed analysis of the obtained results with the corresponding discussion, while chapter nine presents concluding considerations along with guidelines for conducting further research activities.