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Evaluation of Surface Preparation Methods to Remediate Chlorides

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By Bobby Meade, Greenman-Pedersen, Inc.

Presented at SSPC 2017; Session: Surface Preparation: The Foundation of Every Coating Project; Session chair: Kevin Irving

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The Kentucky Transportation Center (KTC) conducted a study for the Kentucky Transportation Cabinet to assess the effectiveness of standard surface preparation methods for bridge painting. The purpose was to identify the best method to reduce or eliminate chloride contamination prior to repainting. Previous work has shown that painting over chloride contamination, even at levels less than 10 g/cm2, can cause premature coating failure. Chlorides on a corroded steel surface tend to collect in high concentration in pits (hot spots) which may be 0.00079cm to 0.00118cm in diameter that are very difficult to clean. Current field test methods will dissolve the chlorides but averages them over a much larger area, ~10 cm2 and does not measure chloride concentrations in pits. Steel panels were preconditioned in 2,000 hours of ASTM B117 Salt Spray (Fog) Testing to produce a contaminated, pitted surface prior to surface preparation. Thirty-two surface preparation methods were evaluated. Those included dry abrasive blasting with variations of media type/size and wet methods with variations in water volume, pressure, and temperature. The wet methods also incorporated the use of abrasives and commercially available chloride stabilizing products. Panels were analyzed for pre-cleaned condition and the condition after surface preparation. Boiling extraction (SSPC GUIDE 15) and a scanning electron microscope (SEM) were used to analyze the panels. Boiling extraction was used to assess the total chlorides before and after surface preparation and the SEM was used to analyze chlorides distribution after surface preparation.


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