EIS study of organic coating on zinc surface pretreated with environmentally friendly products

Abstract

The life time of many steel structure can be remarkably improved by protecting the steel with zinc layers. However, also the zinc coating can be involved by corrosion phenomena with the consequence that some steel surface is unprotected. The reduction of the corrosion rate of zinc is therefore an important topic. These results can be obtained by introducing zinc alloys with lower corrosion rate (ZnNi, ZnFe, etc.) or by protecting the zinc surface with organic or inorganic layers able to reduce the corrosion rate. In the past a very popular way to reduce the corrosion rate of zinc was the use of chemical conversion layers based on Cr 6+, able to increase the passivation tendency of the zinc (chromating). This procedure is quite effective also for improving the adhesion of organic coatings deposited on the zinc surface, but there is the important problem that the use of chromium salts is now restricted because of environmental protection legislation. It is therefore very important to develop new zinc surface treatments environmentally friendly to improve the corrosion resistance of zinc and the adhesion with the final organic protective layer. In this paper a characterisation of environmentally friendly conversion treatments based on Cr 3+ for zinc surface will be reported in comparison with traditional based Cr 6+ pretreatments on different zinc layers protected by organic coatings. The samples were studied using EIS measurements, and the data analysis was mainly based on the discussion of the mathematical combination (ratio, product, etc.) of different parameters of the equivalent electrical circuit model. This approach was found more useful, in order to compare the performance of different materials, in comparison to the simple discussion of the numerical values of the parameters, being these values generally influenced by random defects present in the samples, affecting the measured impedance. The results showed that the performance (adhesion and corrosion protection) of good formulated Cr 3+ based pretreatments are not far from the results, which it is possible to obtain with industrial Cr 6+ pretreatments and therefore Cr 3+ conversion layers can be considered an interesting alternative to the traditional ones.