Chrome free pigments for corrosion protection in coil coated galvanised steels

• Published in: Corrosion engineering, science, and technology Vol. 41; no. 3; pp. 240 - 248
• Main Authors: Loveridge, M. J., McMurray, H. N., Worsley, D. A.
• Format: Journal Article
• Language: English
• Published: London, England Taylor & Francis 01.09.2006; SAGE Publications; Maney; Taylor & Francis Ltd
• Subjects: Applied sciences; BENTONITE; CHROMIUM FREE PIGMENTS; COIL COATING; Corrosion; Corrosion environments; CORROSION INHIBITION; Corrosion mechanisms; Corrosion prevention; CUT EDGE CORROSION; Exact sciences and technology; GALVANISED STEEL; Metals. Metallurgy; Nonmetallic coatings; Production techniques; SALT SPRAY; Surface treatment; SVET; BENTONITE; SVET; SALT SPRAY; COIL COATING; CORROSION INHIBITION; GALVANISED STEEL; CHROMIUM FREE PIGMENTS; CUT EDGE CORROSION; Galvanized steel; Corrosion inhibition; Corrosion protection; Non metal coating; Surface treatment; Corrosion inhibitor; Galvanised steel; Coil; Salt spray; Cut edge corrosion; Corrosion; Chromium free pigments; Sheet metal; Chromium; Bentonite; Coil coating
• ISSN: 1478-422X; 1743-2782
• DOI: 10.1179/174327806X111243

A range of bentonite clays such as Wyoming (W), hybond (H), cat litter (CL) and versions exchanged with Ce 3+ , Y 3+ , La 3+ and Ca 2+ 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 (TiO 2 ) 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), TiO 2 (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 Ce 3+ , Ca 2+ or Zn 2+ ). 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.