ELSEVIER Thin Solid Films 286 (1996) 127-135 Dry and wet corrosion behaviour of AISI 304 stainless steel coated by sol-gel ZrO2-CeO2 films R. Di Maggio *, L. Fedrizzi, S. Rossi, P. Scardi Dipartimento di lngegneria dei Materiali, Universit~ di Trento, Trento, Italy Received 29 August 1995; accepted 7 December 1995 Abstract ZrO2---CeO 2 coatings were deposited on 304 stainless steel substrates by dip coating in alkoxide solution. The coatings were transparent and continuous at thicknesses below about 700 nm. The films were used as barriers against oxidation for 304 stainless steel at high temperature. The best performance (no oxidation up to 16 h at 750 *C) was obtained with a coating approximately 1 l~m thick. The corrosion resistance of the coated samples in an aggressive electrolyte ( i 5 wt.% HCI solution) was studied by electrochemical impedance spectroscopy, a technique frequently used to test the protective properties of organic and metallic coatings on metals. The protective action of the deposited coatings was discussed as a function of the film thickness and quality. Keywords: Ceramics; Coatings;Corrosion;Depositionprocess 1. Introduction To improve the resistance against high-temperature oxi- dation and electrochemical corrosion of metals, the surface at which the corrosive agents act may be modified using a ceramic film, Of several deposition techniques, one of the most interesting is sol-gel dip coating [ 1]. The deposition of ceramic coatings on metals by sol-gel processing has been reported by several workers [2-8], and many kinds of sol- gel coating on mild steel and stainless steel have been tested successfully in aggressive environments and at high temper- ature [9-14]. In this paper, we report the results of high-temperature oxidation and corrosion testing of AISI 304 stainless stee,! substrates dip coated with films of CeO2-stabilized ZrO2. The properties of 18 tool.% CeO2-stabilized ZrO2 match the main requirements for a corrosion-resistant coating, e.g. chemical durability and good thermal shock characteristics. Moreover, this oxide has a high mechanical toughness and a thermal expansion coefficient comparable with that of AISI 304 stainless steel [ 15]. The feasibility of the ZrOz--CeO2 coatings depends on the starting materials, since their chemical properties determine the stability of the solutions in air. Moreover, to obtain good coatings with reproducible thickness, the kinetics of the * Corresponding author. 0040-6090/96/$15.00 © 1996ElsevierScienceS.A. All rights reserved SSD10040-6090 (95) 08515-7 hydrolysis and condensation processes of the chosen organometallic compounds must be strictly controlled. In a previous investigation [4], a procedure for the pro- duction of ZrO2--CeO2 thin layers on iron or AISI 304 stain- less steel by dip coating was developed. In particular, a thermal treatment in vacuum for coating densification was demonstrated to reduce metal oxidation. In the present work, coatings with different thicknesses were prepared using a similar procedure. Their protective effect against dry and wet corrosion is presented. The effect of Censification treatment in vacuum or air was also tested. The resistance against high- temperature oxidation was evaluated using the weight gain per unit area recorded during isothermal treatment of the specimen. The improvement of the corrosion resistance of the coated substrates was studied by electrochemical impe- dance spectroscopy, a technique frequently used to test the protective properties of organic and metallic coatings on metals [ 16-18]. 2. Experlmentai details 2.1. Preparation and characterization of solutions Three solutions were prepared with concentrations of 0.3, 0.15 and 0.03 M of 18moi.%CeO2-ZrO2 by adding zirconium tetrabutoxide to cerium 2,4-pentanedionate dissolved in