Influence of Surface Preparation on the Durability of Repair Coatings for Prepainted Galvanised Steel

Abstract

The performance of repair coating solutions at damaged areas on prepainted galvanised steel was assessed in a cyclic corrosion test and after outdoor exposures in marine atmosphere. Different surface preparations at corroded defects before the application of the repair coatings were tested, for example, water-jetting, the use of commercial cleaning solutions either acidic or alkaline, a chrome-free conversion coating and the application of Zn-Al primer coating. A poorer performance of the repair coatings was observed on damaged and corroded systems compared to damaged-only panels. This was connected to the presence of remaining chlorides and corrosion products at scribes on corroded panels before repair coating application, which indicated the need for careful surface preparation to minimise the concentration of soluble salts. This was achieved by water-jetting and the use of alkaline or acidic cleaning which provided satisfying corrosion performance of the repair coatings particularly with a two-layer system as expected from the thicker layer of the reparation and presence of a primer. The weakest protection was observed when the one-layer repair coating was applied on a chemically treated surface, which may be related to the weakness of the coating.