Use of the scribe and spotface techniques to assess the painted galvanised steel underfilm corrosion

摘要

The increasing interest of the steel industry on materials with higher corrosion resistance has led to a continuous development of the zinc-coated steels as well as the corrosion process evaluation techniques. One major goal of the new analysis methods is to reduce the assessing time. A common technique consists in scribing the painted galvanised steel surface in order to expose the steel substrate to the corrosion environment. In this work, an alternate analysis technique for assessing the underfilm corrosion propagation is used with the major benefit of reducing the time of analysis, without altering the basic corrosion mechanisms. In this method, industrially-painted galvanised steel sheets have the painting and coating removed from a 13-mm diameter spot in order to increase the cathodic/anodic area ratio. This increase leads to a lower sacrificial layer protection power thus reducing significantly the total time of the corrosion test as compared to the time consumed during the "scribe" method test. In addition, a new method is proposed for measuring the corrosion propagation which consists in measuring the corroded area under the organic painting thus determining the average corrosion propagation under the organic coating.