Industrial & Engineering Chemistry Research, Vol.41, No.21, 5232-5239, 2002
Effects of catalyst and pigment on polyester-melamine in situ phosphatizing coating on a cold-rolled steel system
A pigmentless polyester-melamine coating catalyzed with p-toluenesulfonic acid (p-TSA) shows high impedance (similar to10(10) Omega(.)cm(2)) at low frequency (0.1 Hz). After the addition of pigment and additives, the impedance drops to similar to10(5) Omega(.)cm(2). However, using an in situ phosphatizing reagent (ISPR) for the dual purpose of catalyzing the polyester-melamine cross-linking reaction and phosphatizing the metal surface does not show this detrimental lowering of the impedance at low frequency. Results of electrochemical impedance spectroscopy measurements and the corresponding electrical equivalent circuits show that the in situ phosphatizing coating (ISPC) applied to both bare and pretreated steel panels provides superior corrosion protection. At low frequency, the panels coated with the ISPC show 1000 times the resistance of the bare and pretreated steel panels coated with the p-TSA-catalyzed polyester-melamine paint. The simultaneous chemical process of the ISPR catalyzing desirable cross-linking reactions and forming the substrate-phosphate layer at the substrate-paint interface is the reason for the superior paint performance of ISPCs.