Evaluation of corrosion protection properties of additives for waterborne epoxy coatings on steel

The development of environment compatible additives for corrosion inhibition in waterborne coatings requires test methods which yield significant results on a short time scale. The present study aims at the evaluation of the effect of corrosion inhibiting model additives on the performance of a waterborne epoxy coating using electrochemical and non-electrochemical methods which measure different properties. Electrochemical impedance spectroscopy (EIS), linear sweep voltammetry, mechanical pull-off tests and scanning acoustic microscopy (SAM) in combination with image analysis are used. Two kinds of corrosion inhibiting additives are employed: an organic inhibitor based on a carboxylic acid neutralized by a polysiloxane base, and ZPA, an inorganic pigment with inhibiting properties. The results obtained show that corrosion inhibiting additives drastically modify the adhesion, water uptake, blistering behavior and substrate protection of waterbome epoxy coatings. Both additives improved the dry adhesion and reduced blistering under cathodic polarization conditions. The experimental approach described in this paper should be useful for additive development and for coating formulation because it yields a more complete picture than can be obtained by single methods of how a given additive affects the coating performance in a corrosive environment. (C) 2002 Elsevier Science B.V. All rights reserved.