Inhibition of Filiform Corrosion on Organic-Coated Magnesium-Containing Galvanized Steel By Smart-Release Ion Exchange Pigments

Abstract

Zinc-(1-2 w%) Magnesium-(1-2 w%) Aluminium coated galvanized steel presents a superior corrosion resistance over conventional galvanized steel as a result of the Al and Mg additions to the bulk composition of the coating. It has been previously demonstrated that cathodic delamination does not occur on organically-coated ZMA galvanized steel due to the unfavourable potential gradient formed between the intact interface and the defect. Contrary, the potential at the intact interface is more negative than the potential at the defect thus triggering anodic undermining in the form of filiform corrosion. Smart-release exchange pigments have been proven to be a promising approach towards the suppression of undermining corrosion. In the current research, the ability of hydrotalcite (HT) based pigments on the inhibition of induced filiform corrosion on organic coated ZMA galvanized steel has been investigated. This paper describes a systematic kinetic study on the inhibitory effect of anion exchange HT-based pigments on filiform corroding hot dipped Zn-(1-2 w%)Mg-(1-2 w%)Al galvanized steel. The samples were alkaline degreased in 1M KOH and 1M NaOH at 70oC and 3 min immersion time and were coated with a model organic coating of 15.5% PVB containing the exchange pigments via bar casting. Filiform corrosion was then initiated via scribing the samples and introducing a corrosive electrolyte in the defect. The kinetics of filiform corrosion were followed by optical recording over a period of six weeks. It is positively demonstrated that the addition of smart-release pigments in the organic coating greatly suppresses the rate of filiform corrosion. The inhibitory effect of the pigments has been further investigated by systematically varying the volume fraction of the pigment on the organic coating presenting the reduction of filiform corrosion rate with respect to the concentration of the pigment in the organic coating. The effect of anion exchange HT-based pigments on filiform kinetics is analysed and described in terms of an inhibition mechanism.