Modes and mechanisms for the degradation of fusion-bonded epoxy-coated steel in a marine concrete environment

Abstract

Fusion-bonded epoxy is used extensively to protect steel reinforcing bars from corrosion. This coating has proven to be a cost-effective material for increasing the service life of reinforced concrete structures. However, field observations have reported premature corrosion of fusion-bonded epoxy-coated reinforcing bars used in marine concrete environments, leading to severe cracking and spalling of the reinforced concretes. This study was initiated to provide a better understanding of the modes and mechanisms of the corrosion of fusion-bonded epoxy-coated steel exposed in a marine concrete environment. Grit-blasted steel panels, were coated with two commercial fusion-bonded powder epoxy coatings at two thicknesses. Half of the coated panels were scribed, the other half were non-scribed. The panels were immersed in a saturated calcium hydroxide aqueous solution containing 0.6 mole/liter sodium chloride maintained at either 35°C or 50°C. Degradation was characterized and followed by infrared thermography, wet adhesion, and microscopic and analytical chemical techniques. Unscribed panels exhibited only water-induced adhesion loss, most of which was recovered after drying. However, in addition to the water-induced adhesion loss, scribed panels exhibited two modes of failure: anodic blistering near the scribe mark and cathodic delamination around the anodic blisters. Anodic blistering was attributed to localized crevice corrosion under the coating followed by blistering via an osmotic pressure mechanism. Cathodic delamination was probably induced by the alkaline cathodic reaction products, and water-induced adhesion loss was due to the presence of a water layer at the coating/steel interface.