Oxide Formed in High Temperature Corrosion By Model Naphthenic Acids

Naphthenic acid (NAP) refers to the indigenous carboxylic acid in the crude oil. High-temperature NAP corrosion of steel is one of major concerns for chemical/corrosion engineers in refineries. Structures of indigenous NAP in crude oil are very complex, which hinders the accurate evaluation of their corrosive properties. In this study, several model NAP, including 4-phenylbutyric acid, 1-naphthoic acid, 4-cyclohexyl pentanoic acid, and palmitic acid, and dodecyl sulfide were used to prepare solutions simulating the content of NAP and sulfur compounds in real crude oil. Corrosion of these solutions was investigated under two conditions - a stagnant autoclave and a flow-through system. The corrosion product layer on the steel surface was analyzed by microscopic techniques including Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). It was found that an oxide layer on the steel surface was formed in the corrosion by the solution containing only model NAP in the stagnant autoclave. This was quite different from the traditional understanding the NAP corrosion did not leave any corrosion product on steel surface.

No oxide layer was found after the corrosion by the model NAP only solution in the flow-through system. However, an oxygen-containing layer adjacent to the steel surface was found after the corrosion by the solution containing both model NAP and dodecyl sulfide. This layer was found to be protective against NAP corrosion. It was proposed that the oxide was formed due to thermal decomposition of iron naphthenate, the product of NAP corrosion.