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Typical service lifetimes for protective coating systems range from 10-50 years, depending on how extreme the service environment is, as well as on all the details of substrate preparation, coating composition, and coating application.4 While the primary consideration for specifiers of protective coating systems is the service life related to corrosion protection, there is often also a requirement of durability of decorative properties, e.g. color and gloss. This is true not only for monumental steel structures, but also for instance for industrial and offshore structures where “safety colors” are used. This segment therefore has many similarities to the “architectural coatings” segment (coatings for monumental buildings), where the substrates may be different, but where a multi-layer system approach is still used, and owners expect the durability of both the protective and decorative functions.
This paper describes recent work to develop 2-component aqueous topcoat formulations, based on PVDF-acrylic hybrid dispersions crosslinked with polyisocyanates, which meet the requirements of the new SSPC Paint 471 standard for highly weatherable fluoropolymer topcoats, and when combined with conventional primer and midcoat technology, also meet the requirements of IEEE(1) standards C57.12.282 and C57.12.293 for pad-mounted transformer enclosures. Enhanced weatherability is primarily conferred by having relatively high levels of fluoropolymer in the coating binder. Chemical resistance is obtained by a combination of high fluoropolymer level plus crosslink density. Barrier properties contributing to corrosion resistance depend on both these compositional factors, as well as on other formulation details.
The alloys used as clad material for this study are members of the so-called “C-family”. It consists of Ni-Cr-Mo alloys, which are known for combining the corrosion resistance of Ni-Cr alloys in oxidizing media with corrosion resistance of Ni-Mo alloys in reducing media. As a result, these materials have proven to be extremely durable in a wide range of highly aggressive media. The development of these materials started in the 1930s with Alloy C. This alloy showed remarkable corrosion resistance in a wide spread of media, low sensitivity for pitting or crevice corrosion and virtual immunity to chloride induced stress corrosion cracking.
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Corrosion of metallic structures is a ubiquitous problem in industries such as power generation, oil and gas, pulp and paper, metals processing etc. which also results in significant financial losses. According to the National Association of Corrosion Engineers (NACE) International report, the global cost of corrosion was ~ 2.5 trillion USD in 2013 - close to 3.4 percent GDP of the entire world. The use of corrosion inhibitors is one of the most effective and economical ways to mitigate corrosion of metal and alloy components. Corrosion inhibitors are substances that are added in small quantities in corrosive media to protect metal and alloy components from corrosion.
Steel rebars in concrete structures are usually protected from corrosion by a thin layer of passive film, which is formed due to the high alkalinity of concrete pore solution.1-2 However, this protective passive film could be damaged by penetration of chloride into concrete structures in marine environments or exposure to the use of de-icing salt for the removal of snow and ice in winter times.3 Penetration of chloride would impair the passive film locally and initiate pitting corrosion.