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The exposure environment of an engineering material quite often has a large impact on how that material behaves over time. Environments are distinguished by differences in meteorological patterns, geography, salinity, Ultraviolet (UV) radiation, etc1-3. Thus, the degradation of various materials scales proportionately to the characteristics of the exposure site, with more severe sites leading to worse degradation. Developing an understanding of how the local environment impacts the corrosion rates of metals and the deterioration of anti-corrosion coatings is critical for informing asset maintenance schedules and lifetime predictions4.
Naval Research Laboratory (NRL)-Key West is a state-of-the-art testing facility for atmospheric and marine corrosion. The site naturally has elevated Ultraviolet (UV) intensity, similar to operational areas for the Navy. Recently, testing capabilities were expanded to include the ability to rinse samples with clear water (to mimic preventative maintenance washing), to shelter samples within a protective aircraft cover (to mimic preventative maintenance storage), and to spray samples with sea water (to increase the severity of the exposure). It is hypothesized that the Key West site can be tuned so as to mimic the exposure conditions of other sites of relevance to Navy operations.
A 2-Phase, multi-year site severity assessment was conducted to compare sixteen sites to Key West. Two standard Navy coatings (chrome and chrome-replacement) were exposed in addition to steel and silver corrosion coupons. This evaluation will update the relative ranking of these sites in terms of environmental severity index (ESI) and Total Corrosion Risk (TCR). Data presented will include steel mass loss, galvanostatic reduction of silver, and evaluation of the coated samples. In addition, select sites also received environmental dataloggers which will be compared against the exposure samples and nearby publicly available weather data in a follow up study. An understanding of how different local environments impact site-specific corrosion is developed and although many of the 16 test site environments are very different from the ambient conditions in Key West, there is an opportunity to simulate the environments of various bases at this one location. This can be achieved by altering the Key West environment through sea water application and sample sheltering or by adjusting the location and duration of exposure to match the base of interest.
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.