Past HF alkylation operation experience and several industry research projects have reported that the carbon steel corrosion rate in HF acid is strongly affected by the residual elements (RE) content (Cu Ni and Cr). Low RE carbon steel has been recommended for new construction of carbon steel equipment and piping in main acid service trace acid service and dilute HF service in API 751.However one aspect that is often overlooked is the effect of the local concentration of HF acid on low RE steel. At low concentrations low RE carbon steel can be subject to preferential corrosion when it is coupled with high RE carbon steel. Normally only the bulk HF concentration in a process stream is available. It is challenging to determine the actual local HF acid concentration in a practical manner. This paper discusses the possible variations of HF acid concentration in typical HF alkylation processing systems and the corresponding effects of RE on HF corrosion. Possible electrochemical mechanisms of RE on HF corrosion are also studied. Several RE related preferential HF corrosion cases reported in the past have also been reevaluated from the HF acid concentration point of view. Evaluation of actual local HF acid concentration is strongly recommended when low RE carbon steel is applied. This can be more critical to the modified HF alkylation process which utilizes modified HF with a lower volatility and can have different HF concentration in the fractionation section.
Carbon steel is the main construction material in HYDROFLUORIC ACID (HF) alkylation units. Carbon Steel has good corrosion resistance to anhydrous HF (AHF) below 160 degrees fahrenheit (71 C). The corrosion resistance is due to the formation of an inorganic iron fluoride scale on the carbon steel surface that protects the steel from futher corrosion. The presence of an adherent and continuous scale is essential in keeping the corrosion rate at a minimum.