THE EFFECT OF DIELECTRIC COATINGS ON IGSCC IN SENSITIZED TYPE-304 SS IN HIGH-TEMPERATURE DILUTE SODIUM-SULFATE SOLUTION

Zirconium dioxide(ZrO2)coatings were applied to sensitized Type 304SS C(T) crack growth rate specimens and to small coupons for corrosion po tential measurements via electrophoretic deposition. The interfacial r esistance of the coatings was evaluated by electrochemical impedance s pectroscopy (EIS). Electrochemical corrosion potentials (ECPs) of un-c oated and coated samples were measured against an external pressure ba lanced reference electrode (EPBRE) in dilute Na2SO4 (0.0005 m) solutio n at 288 degrees C, and it was found that the ECP of the coated sample s were consistently 200 to 400 mV more negative than that of uncoated samples, ii coated C(T) specimen and an identical, uncoated specimen w ere exposed simultaneously under load in a refreshed autoclave, in ord er to measure the crack growth rates. The results from the long-term e xperiments (400 hours) showed that the surface coatings reduced the IG SCC crack growth rate (CGR) by a factor of more than 10. The reduction in crack growth rate and the negative shift in the ECP were found to scale with interfacial resistance as predicted by the Coupled Environm ent Fracture Model and the Mixed Potential Model, respectively, assumi ng that the exchange current density for oxygen reduction scales inver sely with the interfacial resistance. (C) 1998 Elsevier Science Ltd. A ll rights reserved.