Investigation of Pretreatment Primer Performance by Electrochemical Techniques and Environmental Weathering

Four systems, based on a solventborne polyester/ melamine topcoat applied over one benchmark solventborne primer and three related experimental pretreatment primers, were exposed in natural and accelerated environments. Their performances were characterised by electrochemical impedance spectroscopy (EIS) and focused ion beam/scanning electron microscope (FIB/SEM) analysis to determine the extent and mode of corrosion. The coatings performance were evaluated by measuring the impedance modulus |Z|, phase angles, the degree of blistering and analysis of exposed area. The solventborne primer performed excellently in natural and accelerated environments. However, non-pigmented waterborne pretreatment primer performed poorly in natural environment, but performed similarly to the two other waterborne pretreatment primers in accelerated tests. The two waterborne pretreatment primers containing anticorrosive pigments with and without conversion technology, show some improvement in the corrosion protection of hot dipped galvanised steel. The performance of different coatings tested in electrochemical cells, neutral salt spray, humidity and under outdoor weathering conditions in Bohus Malmön, Sweden (BM) were characterised using EIS. Impedance modulus, |Z|, indicates the overall performance of the system, and phase angles were indicative of the presence of corrosion product. The total impedance was a good indicator in distinguishing the performance of different coatings. The other components in equivalent circuit models were also useful in determining the performance of coatings FIB/SEM analysis developed in this project was found to be a useful technique to study cross-sections of pre-painted metal. Information from secondary and the backscatter electron images can reveal the thickness, the quality of the coating and specific areas of interest such as defects, contamination and corroded areas. EDS measures the distribution and quantitation of elements within coatings can also be studied. FIB/SEM was found to correlate to EIS data and supports the work of other researchers whereby they used total impedance values to indicate the presence of corrosion underneath the coating. Corrosion starts at defects in the zinc layer, not necessarily at the zinc/ primer interface. Corrosion products were found to separate away from the zinc layer and not from the coating as previously understood for blisters and delaminated coatings.