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Degradation of Powder Epoxy Coated Panels Immersed in a Saturated Calcium Hydroxide Solution Containing Sodium Chloride. Final Report. August 1992-July 1994.


pdf icon Degradation of Powder Epoxy Coated Panels Immersed in a Saturated Calcium Hydroxide Solution Containing Sodium Chloride. Final Report. August 1992-July 1994. (1683 K)
Martin, J. W.; Nguyen, T.; Alsheh, D.; Lechner, J. A.; Embree, E. J.; Byrd, W. E.; Seiler, J. F., Jr.

FHWA-RD-94-174; 44 p. October 1995.

Sponsor:

Federal Highway Administration, McLean, VA

Available from:

National Technical Information Service

Keywords:

panels; degradation; anodic blisters; cathodic disbondment; crevice corrosion; epoxy-coated rebars; infrared thermography; peel; wet adhesion; exposure; films

Abstract:

Blasted-steel panels were coated with two commercial powder epoxy coatings. Approximately half (80) of the coated panels were scribed; while the other half remained defect-free. All of the panels were immersed in a saturated calcium hydroxide solution containing 3.5 percent sodium chloride maintained at either 35 or 50 DGC. None of the unscribed panels degraded after 3074 h or immersion at 35 DGC; whereas, all of the scribed panels degraded within 25 h after immersion, regardless of the immersion solution temperature. Scribed panels degraded in three ways: (1) anodic corrosion, (2) cathodic disbondmant, and (3) wet-adhesion loss. Anodic corrosion was attributed to localized crevice corrosion. The rate of anodic growth depended on the immersion solution temperature, but it did not depend on the type of coating or coating thickness. Liquid-filled blisters formed above the anodic sites after approximately 1000 h of immersion at 35 DGC. The chloride concentration of the blister fluid was four times greater than that of the bulk solution and its pH was around 5. The rate of cathodic disbondment was not affected by the type of coating or coating thickness, but it was greatly affected by an increase in the temperature of the immersion solution. Wet-adhesion loss was not affected by coating thickness, but it does depend on the type of coating and immersion temperature. Also, even though the wet-adhesion strength of the two coatings differed by a factor of five, the rate of corrosion for the two coatings was not significantly different.