It is true that stainless steel is highly resistant to many types of corrosion, even at elevated temperatures. However, there are several effects that can be manifested in stainless steel at elevated temperatures, even those easily accessible in a home oven. Depending upon the grade of the steel, exposure of temps as low as 350 C (662 F) can lead to significant reduction in corrosion resistance. Furthermore, processes such as embrittlement and intergranular corrosion will occur in temperatures as low as 425 C (797 F), which can result in a significant reduction of corrosion resistance. It is worth noting that heating stainless steel to these temps in an environment characterized by moisture or volatile materials will pronounce the effects. Exposure to higher temps can bring on more problems yet, again, depending upon the grade of stainless steel that is used.
My guess is that stainless steel flower stakes are likely to be made of fairly low grade material, as they would not be expected to be exposed to harsh conditions. Add to the oven environment any residual fertilizer salts and the like, not to mention moisture being baked out of clay pots, and it is not surprising that corrosion occurs.
Jeff Stuart
-- Jeffrey A. Stuart Research Assistant Professor W. M. Keck Center for Molecular Electronics Department of Chemistry Syracuse University Syracuse, NY 13244-4100
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