Comparison of Atmospheric Chloride Deposition Measurement Techniques in a Multitude of Microclimates

Atmospheric corrosion rates can be influenced greatly by the deposition of airborne chlorides.  Therefore, the ability to measure atmospheric chloride deposition rates as an environmental parameter can be very useful in assessing the corrosivity of the environment for purposes such as designing for corrosion control, etc.  Outdoor exposure and subsequent coulometric reduction of silver coupons is a technique currently being used to obtain atmospheric chloride deposition rates (ASTM B825-02); however, when these measurements are compared to results obtained using the wet chloride candle method (ASTM G140), discrepancies are often observed.  In order to gain an understanding of the differences between chloride-deposition measurements using coulometric reduction and chloride-candle techniques, silver coupons and dry chloride candles were deployed side-by-side on portable exposure racks for a period of one year in a variety of environments.  Dry-type chloride candles were used since wet candles could not be maintained for the one-year exposure.  Experimental results showed that wet and dry chloride candles recorded similar chloride levels during monthly exposures.  Thirty four sets of silver coupons and dry candles were exposed on the island of Oahu, Hawai‘i, and three sets were also deployed at Picatinny Arsenal and Sea Isle City in New Jersey, and Cape Canaveral, Florida.  After the exposure, the silver coupons were analyzed using x-ray diffraction (XRD) and coulometric reduction was performed using a modified version of ASTM B825-02.  Silver coupons at selected sites were also analyzed using scanning electron microscope (SEM) and energy dispersive x-ray analysis (EDXA) techniques.  Ion chromatography was used to identify any aerosols in addition to chlorides deposited onto the candles.