Chrome free pigments for corrosion protection in coil coated galvanised steels

Abstract

A range of bentonite clays such as Wyoming (W), hybond (H), cat litter (CL) and versions exchanged with Ce³⁺, Y³⁺, La³⁺ and Ca²⁺ cations used as the corrosion inhibitors in the 5 μm polyester primer layer applied to 0·7 mm gauge hot dip zinc (0·15%Al) galvanised steel materials, over coated with a 20 μm architectural polyester coating. Similar primer systems have been prepared containing strontium chromate (Cr), Shieldex (Shd) and titanium dioxide (TiO₂) for the purposes of comparison. The scanning vibrating electrode technique (SVET) was used to probe the corrosion kinetics at sample cut edges immersed in 5%NaCl on a limited number of the coatings based on the Wyoming bentonite. The SVET determined zinc losses for the pigments tested over 24 h were as follows: Shd (920 μg), Cr (740 μg), W–Y (655 μg), W–Ce (560 μg), W (550 μg) and W–Ca (496 μg). The same samples together with all of the other pigmented coatings were also subjected to salt spray testing. The average maximum cut edge corrosion delamination distances were: Cr (14 mm), Shd (20 mm), TiO₂ (100 mm), W (13 mm), H (17 mm), CL (14 mm), W–Ca (10 mm), H–Ca (15 mm) W–Ce (9 mm), H–Ce (10 mm), W–Y (13 mm) and H–Y (15 mm). The performance trends over 1000 h of salt spray testing were in broad agreement with the 24 h SVET data for those samples tested in both ways. The ion exchanged and naturally occurring bentonites appear to be promising corrosion inhibitors for coated galvanised steel. In order to maximise performance, a fully formulated system has been developed on a 4·5Al–95·5Zn galvanised substrate which has a PVC plastisol top coat. A bentonite matrix with higher ion exchange capacity (>1 mequiv g⁻¹) and low silica content has been developed (IM–X, where X is Ce³⁺, Ca²⁺ or Zn²⁺). In this system delamination distances have been reduced to extremely low levels of IM–Ce 0·3 mm, IM–Ca 0·45 mm and IM-Zn 0·37 mm.