The materials most vulnerable to naphthenic acid corrosion are carbon steel and the iron-chrome (5 to 12 %Cr) alloys commonly used in corrosive refining services. 12Cr may experience corrosion rates greater thanthat of carbon steel. Type 304 stainless steel offers some resistance to naphthenic acid corrosion at loweracid levels, but normally the molybdenum containing austenitic stainless steels (Type 316 or Type 317 SS)are required for resistance to greater acid concentrations. It has been found that a minimum Mo content of2.5 % is required in Type 316 SS to provide the best resistance to naphthenic acids.