The Effect of Particle Size on the Oxidation Resistance of a Nanoceria-Coated 304 Stainless Steel
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THE effect of cerium and other ‘‘reactive elements’’ (RE) on the oxidation behavior of high temperature alloys has been the subject of numerous studies.[1–4] Over 6 decades ago, it was found that additions of cerium to iron-chromium alloys improved the adhesion of the chromia scale while reducing its growth rate.[5] Also, it was found that a minimum cerium content in the alloy of 0.3 at. pct was needed in order to observe the REE.[6] Among the findings on the role of Ce additions are (a) a change in the scale growth mechanism from outward cation diffusion to inward oxygen anion diffusion, (b) interface segregation, and (c) improved scale plasticity. In addition, it has been found that when nanoceria particles are used as coatings for high temperature protection, the oxidation rates can be reduced by 1 to 2 orders of magnitude.[7–10] Apparently, most of the effects associated with cerium additions in high temperature alloys are also active when nanoceria coatings are implemented for oxidation protection. From the published literature, it is generally agreed that nanoceria coatings[11–14] provide appreciable oxidation protection HUGO F. LOPEZ, Professor, is with the Department of Materials Science and Engineering, CEAS, University of Wisconsin-Milwaukee, 3200 N. Cramer Street, Milwaukee WI 53211. Contact e-mail: [email protected] HAIYING ZHANG, Research Professor, is with Civil and Materials Engineering Department, University of Illinois at Chicago, Chicago, IL. Manuscript submitted April 7, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
to high temperature alloys including stainless steels through the reactive element effect (REE). Nevertheless, there are distinctive differences as the Ce4+ ion source will require active dissolution of the nanoceria particles. According to the phase diagram,[15] there is no measurable dissolution of cerium ions in chromia scales. Nevertheless, ceria ions have been found to be present at the gbs of growing scales in Ni and Cr alloy substrates.[1,2] Apparently, dissolution of nanoceria particles plays a key role in promoting preferential segregation of Ce4+ ions at the grain boundaries (gbs) of the newly formed scale.[7–10] These ions are thought to hinder outward gb diffusion of metal ions while favoring gb oxygen ion inward diffusion. The dissolution of nanoceria is thermodynamically feasible when the particles reach sizes below 10 nm, particularly 5 nm as reported in the literature.[7–10] Nevertheless, no explanation has been given on any possible dissolution mechanisms. In addition, nanoceria particles provide numerous heterogeneous nucleation sites for the development of the chromia scale. In turn, total coverage by a finegrained scale is found to take place during the early stages of oxidation.[1] In general, the ceria nanoparticles are assumed to act as ‘‘inert markers’’ during scale growth,[1,2] suggesting that they are highly stable at the oxidation temperatures. Nevertheless, it has been found that nanoceria coatings are effective only when the average nanoceria particles are b
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