INFLUENCE OF GALLIC ACID ON THE PHASE-TRANSFORMATION IN IRON-OXIDE LAYERS BELOW ORGANIC COATINGS STUDIED WITH MOSSBAUER-SPECTROSCOPY

The effect of gallic acid on the corrosion reactions underneath paint films on steel during immersion experiments in the presence and absenc e of oxygen is investigated using Moessbauer spectroscopy. The mass ev olution as a function of immersion time is measured for different soli d phases such as iron, lepidocrocite, magnetite and ferrous hydroxide. Results show that in the presence of oxygen, gallic acid enhances the initial dissolution of iron due to complexation with ferrous ion. The ferrous complex is subsequently oxidized into an insoluble ferric com plex leading to a reduction in the anodic reaction rare. No magnetite is formed under these conditions. in the absence of oxygen, magnetite is formed by reduction of lepidocrocite on precorroded steel coated wi th films not containing gallic acid. When gallic acid is present in th e paint film the reductive transformation of lepidocrocite to magnetit e is inhibited. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.