Performance of Conductive Pre-Primers Applied on Galvanized Steel Sheets for Automotive Bodies

The main driving forces concerning the use of pre-primed automotive sheet are corrosion protection increase and cost reduction during vehicle manufacturing. In this work, the behaviour of two conductive primers, codified by 1 and 2, was studied and compared with that of a conventional electrophoretic primer. Primer 1 is an organic zinc rich silicate with a low thickness (near 4 µm) and primer 2 is a very thin (near 2 µm) water based primer filled with graphite. These primers were applied on electrogalvanized, hot dip galvanized and galvannealed steel sheets. Formability capabilities of pre-primed galvanized steel sheets were evaluated, through cupping and stone chipping tests, according to EN ISO 1520 and ASTM D3170, respectively. To evaluate corrosion protection, the samples were submitted to a corrosion cyclic test according to VDA procedure 621-415 contained in the SEP 1160. Corrosion mechanisms were studied by scanning electron microscopy (SEM) with X-ray microanalysis by dispersive energy spectrometry (EDS) associated. Primers 1 and 2 offer cathodic protection when exposed, however by different manners, and allow previous welding works. Primer 1 acts as a zinc rich primer and primer 2 acts as a sealer of metallic zinc coatings, not allowing zinc depletion and delaying the appearance of iron corrosion from the steel substrate. After some exposure time it was possible to see on the surface of the primer 2, zinc corrosion products arising from the pores of the paint coating. Primers 1 and 2 applied with a low thickness, seem to be good alternatives to the conventional electrophoretic primers.