Electrochemical quartz crystal microbalance in modern corrosion research Study of the pretreatment of galvanized steel

Abstract

The electrochemical quartz crystal microbalance (EQCM) is applied in order to study the technical pretreatment processes of galvanized steel such as alkaline cleaning, chromating and phosphating in modern trication solutions. The mass change associated with these reactions is monitored insitu under conditions which are very similar to those of a coil-coating line. It is shown that cleaning of the zinc surface in an alkaline cleaning solution is a two-step process: removal of the zinc oxide layer by a chemical dissolution followed by electrochemical dissolution of zinc, the rate of which is controlled by the diffusion of zincate ions into the electrolyte. During phosphating of the sample, three subsequent steps are observed: zinc dissolution during the first seconds followed by precipitation of a phosphate layer and then cementation of Ni, which leads again to increasing dissolution of the base material.