Corrosion under insulation (CUI) happens due to moisture egress and thermal cycling. Chemical inhibitors can be a relatively low-cost method to mitigate CUI. Inhibitors for CUI that are stable at temperatures below 175°C exist. However, many pipes have thermal cycling at temperatures higher than 175°C. When such high-temperature-operating pipes undergo idling and then experience high temperature operation again, the CUI would be more pronounced because higher temperatures entail higher corrosion rates. It would require inhibitors that are both stable at high temperatures and possess vapor phase inhibition. Presented here is a novel vapor phase corrosion inhibitor that is stable up to 662°F (350°C). The thermal stability, diffusion, and the protection of carbon steel (C1018) against chloride-containing liquid by its vapor deposit will be discussed. This inhibitor is nonflammable. It is USDA-certified biobased, and biodegradable.