Evaluation of the Corrosion Resistance of Different Galvanized Steels Treated in Cerium Salt-Based Solutions

Abstract

Various treatments based on cerium salts have been evaluated in an attempt to develop surface modification treatments for galvanized steels that can replace the presently used processes that are based on hexavalent chromium. The substrates used in this study included pure zinc, electroplated zinc and zinc-nickel as well as hot dipped zinc and zinc-iron. In initial screening tests Zn and hot dipped Zn were immersed in Ce(NO 3 ) 3 or CeCl 3 + H 2 O 2 solutions at different concentrations and treatment times. The films formed in these solutions were sealed in Na 2 SiO 3 . A new conversion coating process for electrogalvanized (EG) steel based on a cerium nitrate solution with several additives such as colloidal silica and silane has been developed that reduces the total treatment time to 70 seconds. The treated EG sample passed the 72 h salt spray test without white rusting. Similar improvements of corrosion resistance were obtained for the other types of galvanized steel. IntroductionRare earth metal salts were introduced as possible candidates for producing chromate-free protective coatings by Hinton et al [1][2]. Mansfeld et al developed the Ce-Mo surface modification process for Al alloys that resulted in excellent pitting resistance in 0.5 N NaCl [3][4][5]. Aramaki reported a process that produced a hydrated Ce 2 O 3 film on pure zinc that had been treated in 1 mM Ce(NO 3 ) 3 [6-9]. The coating layer that consisted mainly of Ce 2 O 3 and contained a small amount of Ce +4 was remarkably protective during exposure to NaCl [6]. Aramaki modified the coating layer with Na 3 PO 4 to produce a self-healing effect that provided good corrosion protection especially in a scratched area [9]. Song and Mansfeld reported that the corrosion resistance and coating structure of cerium-based conversion films depended on the surface chemistry of different types of galvanized steel [10]. Colloidal silica and silane were employed as additives to enhance the corrosion protection of EG provided by a conversion coating that was formed in a molybdate-phosphate solution [11]. In the present study, several cerium salt-based solutions were used to enhance the corrosion resistance of pure Zn, electroplated Zn (EG), electroplated Zn-Ni, hot dipped Zn and hot dipped Zn-Fe. In initial screening tests Zn and hot dipped Zn were immersed in Ce(NO 3 ) 3 or CeCl 3 + H 2 O 2 at different concentrations and treatment times [12]. The films formed in these solutions were sealed in Na 2 SiO 3 . The optimum treatment conditions were determined using a factorial design approach. The effect of the substrate chemistry was studied for electroplated zinc and zinc-nickel as well as hot dipped zinc-iron in a ECS Transactions, 1 (9) 133-152 (2006) 10.1149/1.2215584, copyright The Electrochemical Society 133 ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 134.117.-134.62 Downloaded on 2015-06-13 to IP 134Ce(NO 3 ) 3 solution. Based on the results of these studies two different types o...