CATHODIC DELAMINATIONS OF POLY(PHENYL ETHER ETHER KETONE) (PEEK) COATINGS OVERLAID ON ZINC PHOSPHATE-DEPOSITED STEELS

The melt-crystallized poly(phenyl ether ether ketone) (PEEK) polymer w as overlaid on crystalline zinc phosphate (Zn . Ph) conversion coating -deposited and nondeposited cold-rolled steels at 400-degrees-C in air or in N2 environments. The ability of these coating systems to protec t the steel against corrosion was evaluated from the rate of cathodic delamination of the coating layer from the steel. Because the cathodic reaction, H2O + 1/2O2 + 2e- = 2OH-, at the corrosion side of a defect in the coating layer creates a high pH environment at the interfacial boundary between the coating and steel, the magnitude of susceptibili ty of the interfacial intermediate layers to the alkali-induced degrad ation played a key role in suppressing cathodic failure. The lowest le vel was observed in the N2-induced PEEK/Zn . Ph/steel joint systems, t hereby decreasing the rate of delamination. By contrast, oxidized PEEK induced by air in same joint systems was susceptible to the alkali-ca talyzed hydrolysis. This phenomenon significantly promoted the alkali dissolution of Zn . Ph crystals, reflecting a high rate of delaminatio n. Nevertheless, the introduction of Zn . Ph as interfacial tailoring material into the intermediate layers markedly acted in reducing the c athodic delamination of PEEK/steel joint systems. (C) 1993 John Wiley & Sons, Inc.