Electrochemical properties of organic-inorganic hybrid (OIH) coatings obtained by sol-gel process on galvanized steel

Abstract

Among the possible anti-corrosion preventive measures, galvanized steel reinforcement is usually employed to extend the service life of infrastructures in aggressive environments. Chromate conversion treatments have been employed as pre-treatments to reduce corrosion activity during the initial stages of contact of the reinforcing steel with the concrete alkaline environment, hindering the hydrogen evolution process during the formation of the protective layers on the surface of the galvanized steel. New environmentally friendly pre-treatments are however necessary since Cr(VI) is a carcinogen and its use is strongly restricted within the European Union [1]. The properties of organic-inorganic hybrid materials attracted significant attention over the past decades as a new class of materials through the novel properties that can arise from the combination of organic polymer and inorganic material and the production of hybrid protective thin coatings to apply on different metal substrates to prevent corrosion has been widely studied [2-4]. This class of materials could be easily obtained by sol-gel method at mild synthesis conditions and the large variety of available chemical precursors allows producing a diversity of coating materials with tuned mechanical and thermal properties. In this work a functionalized metal alkoxide (3-isocyanatopropyltriethoxysilane) and five oligopolymers (Jeffamine 130, 300, 600, 900 and 2000), identified as having potential to be used as an alternative pre-treatment to prevent the corrosion of galvanized steel in concrete, are tested as source of sol-gel precursors. Five hybrid matrix films were produced, doped with sodium dichromate acting as inhibitor. The coatings, doped and undoped with inhibitor, were applied using a single and a triple dip step. Dip-coating was done using a draw speed of 10 mm/min in both cases. Coatings resistance, with a minimum thickness, low permeability and high stability were evaluated by electrochemical studies, namely EIS (electrochemical impedance spectroscopy) and corrosion potential monitoring. The studies were carried out in Portland cement pastes and mortars. SEM/EDS analyses of the coatings were performed after 7, 14 and 28 days of exposure to the cement based materials