Effect of Environment on the Oxide Layer Formation in AFA vs CFA Materials

Reaction vessels used in petrochemical processes undergo high temperatures and are exposed to atmospheres of hydrocarbons, air, steam, and miscellaneous reaction byproducts. The traditional materials used are Chromia-Forming Alloys(CFA). These alloys form a layer of chromium oxide, which protects the metal from sustaining further damage from hot corrosion effects and carbon deposition. Of the cost of these petrochemical processes, those relating to maintenance have the highest impact in the economy of the process. Therefore, maintenance cost is the performance metric used to evaluate and compare materials. Consequently, temperatures and gasses contacting the vessel are constraints to this performance metric because of the formation and longevity of the oxide layer; it affects reaction efficiency and costs of maintenance. Studies have shown that Alumina-Forming Alloys (AFA) seem to perform better than the traditional CFAs in many petrochemical processes. These AFA steels form an aluminum oxide layer that is more stable chemically and electrically insulating, resulting in better protection against corrosive factors. The oxidation of CFAs and AFAs is compared. The aim of these experiments was to evaluate the effects of oxidation using two environments, 100% steam and 100% air, to determine whether the environment has a significant impact on the formation of the corresponding layer. Oxidation experiments were done at 900°C and 10 h of exposure. The materials were analyzed using SEM and EDS to characterize the oxide layer formation. The specific change in mass was recorded and the general linear model was used to evaluate if the material and/or environment had a significant impact on the change of mass. The overall oxide formation of AFA alloys relative to traditional CFA alloys is described. Additionally, it was determined that the environment had a significant effect on the change in mass and the developed oxide scale varies with it