Thin Glass Coatings for the Corrosion Protection of Metals

Abstract

This dissertation presents the research work aimed at developing functional submicrometer thick SiOx barrier coatings for the corrosion protection of stainless steel substrates in chloride containing media, which may enable the use of stainless steels as plate material for marine heat exchangers, and thus lower the component cost with respect to incumbent materials such as titanium alloys. The technology is of particular interest for the application on heat exchanger plates and components, since the thin coating films are expected to serve as efficient ionic barrier coatings, which prevent issues with localized corrosion and do not impact the heat transfer or the component performance. The herein presented approach focuses primarily on the formation of SiOx-like thin films from Hydrogen Silsesquioxane (HSQ) –based “spin-on-glass” (SOG) precursor. The technology is well known for the deposition of dielectric films in microelectronic applications and has been recently introduced as industrial surface finish for molding tools due to its ability to form stable surface films with excellent levelling. Within conventional SOG processing, a liquid precursor is deposited on the substrate and subsequently cured to form a continuous polymeric surface film. It is the aim of this work to transfer the existing technology to stainless steel substrates and establish an understanding of the effect of the curing conditions, the performance and failure mechanisms of SOG coatings on stainless steel substrates in corrosion sensitive applications. Since the deposition of SiOx thin films is a well-established technology, the SOG technology was directly benchmarked to PVD-based SiO2 coatings. The coating adhesion was assessed by cross cut testing and increasing load scratch testing and the efficiency of the sub-micrometer thick coatings was assessed by potentiodynamic anodic polarization measurements, EIS and neutral salt spray testing. Further, localized coating failures were investigated by the SVET and spot testing and the coating microstructure was investigated by (FIB-) SEM and a variety of analytical TEM methods. The coating chemistry was studied by FT-IR and XPS and the coating properties were characterized by water contact angle measurements, nanoindentation and AFM. Overall, the results indicated that SOG deposition may yield well adherent coatings with excellent coverage and substrate levelling. The process yielded highly resistive coatings; however, all coatings allowed penetration of electrolyte to the substrate and no ideal barrier behavior was observed. Further, the results stress the particular importance of the interaction between the coating and the substrate for the coating performance: Oxidative curing led to pronounced interface oxidation, and thus to de-alloying of the substrate surface. As a consequence, the pitting resistance of the coated systems was found decreased or void formation between the coating and the substrate was observed. While interface oxidation can be suppressed by curing in non-oxidizing atmosphere, void formation due to coating delamination may induce critical coating defects, leading to the stabilization of growing pits. In consequence, it was shown that use of a bright surface finish minimizes the risk of coating failure. Moreover, it was shown that the degree of coating polymerization is crucial for the chemical stability of the coatings and that coating imperfections lead to significant coating dissolution in near-neutral aqueous media.