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 potential measurements via electrophoretic deposition. The interfacial resistance of the coatings was evaluated by electrochemical impedance spectroscopy (EIS). Electrochemical corrosion potentials (ECPs) of un-coated and coated samples were measured against an external pressure balanced reference electrode (EPBRE) in dilute Na2SO4 (0.0005 m) solution at 288 degrees C, and it was found that the ECP of the coated samples were consistently 200 to 400 mV more negative than that of uncoated samples, ii coated C(T) specimen and an identical, uncoated specimen were exposed simultaneously under load in a refreshed autoclave, in order to measure the crack growth rates. The results from the long-term experiments (400 hours) showed that the surface coatings reduced the IGSCC 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 Environment Fracture Model and the Mixed Potential Model, respectively, assuming that the exchange current density for oxygen reduction scales inversely with the interfacial resistance. (C) 1998 Elsevier Science Ltd. All rights reserved.