Naphthenic acid corrosion of steel is a major challenge in oil refineries. Iron sulfide scales, formed by corrosion due to sulfur compounds found in the crude oil, have shown unpredictable behavior when it comes to their protectiveness. Recent results show that simultaneous formation of an iron oxide scale formed by corrosion due to naphthenic acids may explain part of the variability and contribute to corrosion resistance of the scale formed on 5Cr steel. In depth analysis identified magnetite in the sections of the scale just adjacent to the metal surface. Currently reported research focuses on investigating conditions that lead to the formation of the iron oxide layer on carbon steel. A comparison of different oxide layers formed on carbon steel from a pure model acid (palmitic acid) and a commercial naphthenic acid mixture derived from petroleum shows different behavior when it comes to corrosion protection. The two acids also show different behavior in the presence of sulfur containing compounds (a model compound and native sulfur compounds found in a heavy lube basestock). These results suggest that the molecular structure of the acid is an important factor in the formation and the protectiveness of an iron oxide scale.