INTERFACES OF POLYPHENYLETHERETHERKETONE (PEEK) AND POLYPHENYLENE SULFIDE (PPS) COATED ZINC PHOSPHATED STEELS AFTER HEATING-COOLING CYCLES IN A WET, HARSH ENVIRONMENT

To evaluate the ability of polyphenyletheretherketone (PEEK) and polyp henylene sulfide (PPS) thermoplastic coatings that protect zinc phosph ate (Zn.Ph)-treated steels from corrosion in a wet, harsh environment (1.0 wt% H2SO4, 3.0 wt% NaCl, and 96.0% wt% water at temperatures from 25 degrees to 200 degrees C), we exposed them in autoclave to attempt heating-cooling cyclic fatigue tests (1 cycle - 12 hr at 200 degrees C + 12 hr at 25 degrees C) up to 90 times. Although no changes in appe arance were seen in the PEEK specimens after 60 cycles, extending the cycles to 90 caused the delamination of the coating film from the Zn.P h. The major reason for this delamination was the degradation of the P EEK polymer caused by its hydrothermal-catalyzed esterification. In th e PPS-coating systems, chemical reactions at the interfaces between th e PPS and Zn and Fe in the Zn.Ph layer during cycling led to the forma tion of ZnS and FeS reaction products, which enhanced the Zn.Ph-to-PPS adhesive bond; correspondingly, there were no signs of peeling and se paration of the coating after 90 cycles. In addition, because these in termediate reaction products are insoluble at high pH, they minimized the rate of delamination of the PPS coating caused by the cathodic rea ction, H2O + 1/20(2) + 2e(-)' 2OH(-), at the corrosion side of a defec t in the film. In contrast, PEEK coatings containing non-reactive Zn.P h underwent cathodic delamination, because of the susceptibility of Zn .Ph to alkali dissolution. Thus, we believe that combined deters of PP S and Zn. Ph provide great protection to steels in a harsh environment with temperatures up to 200 degrees C.