Concepts of steel protection by coatings with a reduced content of zinc pigments

Abstract

In zinc-rich primers Zn acts in the initial stage as sacrificial anode (cathodic protection). During lifetime Zn is transformed into its corrosion products, which seal the coating's pores (barrier protection). Such coatings are typically very highly pigmented which could cause the deterioration of cohesion and/or adhesion as well as mechanical properties. The formulations of different Zn pigments in epoxy matrices were developed with concentration of zinc particles reduced to about 50 wt% in the dry coatings. As Zn pigments were used: mixtures of zinc pigments with different particle size and particle shape (dust, flakes), zinc dust with organic surface treatment, zinc alloy pigments – ZnMg and ZnSn as well as Zn dust with an addition of graphene or carbon nanotubes.The mechanisms of protection by coatings with a reduced content of zinc pigments were studied in comparison with that of commercial zinc rich primers – epoxy and ethyl silicate – by the following methods: Scanning Kelvin Probe (SKP) detection of artificially inflicted samples after impact of salt spray test (SST), polarization experiments and EOC detections in combination with different corrosive impact, as well as by SST and the exposure to the natural marine environment (sea water splash zone).It was found that cathodic protection is shown by primer pigmented with Zn flakes, with zinc phosphate and barium sulphate addition as well as by primer containing Zn dust and graphene. They reach an open circuit potential (EOC) significantly below −0.86 V. Primers pigmented with ZnSn or Zn dust with an addition of carbon nanotubes (CNT) keep their EOC in the transition phase between −0.7 V and −0.86 V. Other primers with a reduced content of zinc pigments show only barrier protection.Coatings characterized by EOC in the range of cathodic protection or in the transition phase have very good protective properties in the corrosive environment. However, good anticorrosive properties were found also in the case of primers which do not show any cathodic protection, such as the primer pigmented with a mixture of Zn particles with different size and the primer containing Zn dust with chemical surface treatment. This shows that good anticorrosive properties can be obtained even if cathodic protection is not observed.