ABSTRACT
Laboratory assessment of the performance of zinc-rich paint (ZRP) coatings on naval steel in seawater has been carried out using electrochemical impedance spectroscopy (EIS) combined with cathodic protection potential measurements. Different formulations of painted naval steel samples were investigated during exposure times up to 50 days. ZRP coating formulations included those with the zinc pigment content covering a wide w/w-concentration range as well as those with different organic/inorganic binders based on chlorinated-rubber, epoxy-polyamide, ethyl-silicate and alkyd-resins. Experimental impedance diagrams have been quantitatively explained in terms of a transfer function analysis using identification procedures and non-linear fit routines. Information concerning the influence of both binder type and zinc pigment concentration on the corrosion protection behaviour of these ZRP coatings was obtained from the exposure time dependence of the system parameters, which were derived from the fitting procedure.
Characteristics and properties of the different naval steel/ZRP coating/seawater systems have been determined according to the application of an impedance transfer function model which describes the reactions taking place at the metal/film and film/solution interfaces as well as the diffusion processes through the active ZRP film. The kinetic parameters correlate well with gradual system deterioration and reflect the way in which the galvanic effect and barrier-action properties of the ZRP employed as primers progressively diminished. The whole set of results can be interpreted in terms of corrosion processes going on extensively through pores and cracks of these coatings.
