Well-Controlled Organic Monolayers as Model Inhibitors for Dealloying Corrosion of Noble Metal Alloys

Abstract

Materials degradation due to corrosion is one of the most critical problems on our way to a more sustainable society because of the extensive use of alloys in sensors, electronic appliances, fuel storages and supplies, chips and micro - nano devices. Understanding the fundamental microscopic processes in corrosion initiation and development at the atomic level is essential to improve corrosion protection and resistance by inhibitors. Despite the fact that a substantial number of reports on corrosion testing are available, only a few of them aspired to understand the basic processes driving materials degradation on an atomic or molecular level. Many researches have been devoted to find possible organic inhibitors to protect metallic alloys. An interesting type of alloy corrosion, localized dealloying, takes place in alloy surfaces protected by self-assembled monolayers (SAMs) of organic inhibitors such as alkanethiols and selenols but, the mechanism behind the localized dealloying initiation still remains unexplored. Therefore, dealloying initiation at the nanoscale can be addressed by controlling the spatial distribution and molecular organization of SAMs at nano and micro length scales. The aim of this PhD thesis is to gain insights into localized dealloying initiation of noble metal alloys, mainly Cu3Au, as a model system by using well-controlled alkanethiol SAMs. Similarly, the nanoscale inhibitor stability and corrosion efficiency in pure Cu and Cu-based alloys was also investigated. Initially, different chain-length alkanethiols were tested on a reference surface, ultraflat gold (UFG), by using a multistep approach of microcontact printing (µCP) combined with subsequent backfilling. The molecular organization of single and double printed (2-µCP) followed by further backfilling to produce complex alkanethiol SAMs was characterized by atomic force microscope (AFM) at the nanoscale. By using double printing, both alkanethiol forms coexist as compact SAMs with well-defined thickness. Microcontact printing and further backfilling with the same molecule resulted in well-delimited nanometer-thick domain boundaries of micrometer-range lateral dimension in between patterned and backfilled areas. This finding provides an opportunity to explore localized dealloying at the nanoscale level in the presence of well-defined complex alkanethiol inhibitor layers. Microcontact printing and subsequent backfilling of different chain-length alkanethiol creates artificial heterogeneous molecular interfaces in well-controlled way which trigger localized dealloying corrosion at the artificially created defects such as the patch backfilled boundaries. Such heterogeneous inhibitor assemblies strongly depend on nanoscale geometrical and chemical features of the metal-film interface and are in turn ultimately important for surface processes such as corrosion and corrosion inhibition. Electrochemical dealloying in the presence of such complex alkanethiol inhibitor revealed that localized dealloying initiation starts at the boundary region between two differently approached molecules, as well as from the junction between two print patches. Besides this, the Cu dissolution also initiates from the SAM defect areas on regions with a high density of atomic terraces, developing either large circular or more delimited star-shaped dealloying pits. Thus, localized dealloying initiation is strongly dependent on the alloy surface morphology but also the stability of the applied inhibitor molecule. This work uses model systems to reach better insights at the atomic or molecular level of localized corrosion initiation process. In addition, SAMs of mercapto-based imidazole derivatives were used as model corrosion inhibitors for Cu and Cu-Zn alloys. Several selected imidazole derivatives were chosen to gain understanding of the monolayer stability at the molecular and nanoscale by in-situ force spectroscopy AFM (FS-AFM) and its influence on macroscale corrosion inhibition. In this work, two imidazole-based inhibitors were studied, commonly being used for Cu surface protection. The molecular stability and intermolecular forces of SAMs consisting of imidazole derivatives, namely, 5-methoxy-2-mercapto-benzimidazole (SH-BimH-5OMe) and 5-amino-2-mercapto-benzimidazole (SH-BimH-5NH2) on Cu surfaces were characterized experimentally by AFM, and theoretically by density functional theory (DFT) in collaboration. The inhibitor-covered Cu surfaces were characterized by AFM imaging and by tip-sample force measurements using Quantitative Imaging (QI) mode AFM. For a SH-BiMH-5NH2 molecular layers, molecular fishing by the AFM tip frequently occurred with about 20% of total events, displaying a ‘fishing’ force magnitude representative of intermolecular attractions rather than a surface bond strength. For SH-BimH-5OMe molecules, no molecular fishing events were observed, indicating more stable layers in line with its better Cu corrosion efficiency. Both, computational modeling and electrochemical corrosion measurements also proved that the SH-BiMH-5OMe-covered Cu surface is more protective. This study helped to gain knowledge on noble metal alloy corrosion, which contributes towards an understanding of more complex and more reactive alloys mostly found in real-world applications.