Timescale Correlation Between Marine Atmospheric Exposure and Accelerated Corrosion Testing

Evaluation of metal-based structures has long relied on atmospheric exposure sites to determine corrosion resistance in marine environments.  This approach is an invaluable method to understand the life of a metal material.  The primary disadvantage to this approach is the long timescale required for evaluation, often lasting 3 to 5 years.  A faster method that successfully correlates to atmospheric exposure results is needed to expedite material evaluation.  Traditional accelerated corrosion testing relies on mimicking the exposure conditions, often incorporating salt spray and UV, and exposing the metal to continuous or cyclic conditions of the corrosive environment.  Their success in correlation to atmospheric exposure is often a concern in terms of exposure conditions and when determining the timescale to which the accelerated tests can be related.  Accelerated laboratory testing, which often focuses on the electrochemical reactions that occur during corrosion conditions, has yet to be universally accepted as a useful tool in predicting the life of a metal despite its ability to rapidly induce corrosion.  Although visual and mass loss methods of evaluating corrosion are the standard and their use is imperative, a method that correlates timescales from atmospheric exposure to accelerated testing,based on the chemical changes that occur during corrosion, would be very valuable.  This work makes use of RAMAN and electrochemical techniques to interpret the chemical changes occurring on steel and aluminum substrates during atmospheric and accelerated corrosion conditions.  The results of correlating these four marine-based corrosion conditions, atmospheric exposure at the Kennedy Space Center beach site, alternating seawater submersion, salt fog via ASTM B117, and continuous and alternating immersion in typical electrochemical laboratory conditions, will be presented.