A model for the release of chromate from organic coatings

Abstract

The kinetics of chromate release from a polyester isocyanate has been studied as a model of the inhibitor transport from organic coatings, which is essential for corrosion protection of bare metals at scribes and cut-edges. The primer was pigmented with strontium chromate and applied on hot-dip galvanized steel. The influence of temperature from 3 to 60 °C, pH and chloride concentration in the leaching solution on the dissolution kinetics and total amount of chromate released during 15 days of exposure was determined. The results were compared with four commercial line-coated primers. The rate of release of chromate from the polyester coating into water solutions showed a logarithmic dependence with time with an apparent break connected to the diffusion of the leaching solution through the coating. Comparatively very high amount of chromate was released during the first minutes of the exposure, being almost immediately available for the corrosion inhibition. The highest amount of chromate released during the first 15 days of exposure was found at 20 °C; both an increase and decrease of temperature lowered the rate of chromate release. The effect of temperature was likely due to a modification of the strontium chromate particle/organic matrix interface in the presence of water in the coating structure. The acidic environment, typical for defects in organic coatings, increased the chromate dissolution in comparison with neutral solutions. Diffusion of water and chromate solution through the investigated polyester primer played an important role in chromate release, especially at the beginning of the process. However, the rate-controlling step of the leaching was very probably related to the chromate release at the pigment particle and coating matrix interface.