A project was carried out to study concurrent naphthenic acid and sulfidation corrosion of mild steel (CS-A106) and 5Cr steel. Three different systems were used: standard stirred autoclaves, a Flow Through Mini-Autoclave (FTMA), and a High Velocity Rig (HVR). Standard stirred autoclave tests provided basic information under all conditions. FTMA tests avoided accumulation of H₂S and degradation of naphthenic acid by a continuous flow of fresh test media. HVR tests focused on the effects of shear stress by a using rotating cylinder. The effects of test duration, temperature, total acid number and sulfur content were determined.  Some tests in real crude fractions were also conducted for comparing with the model oil tests. The ultimate objective of this project is to develop a practical model for predicting concurrent naphthenic acid and sulfidation corrosion in crude oil refining environment.