Abstract
Corrosion rates for selected carbon steel piping circuits in a petroleum refinery HF (hydrofluoric) Alkylation unit were examined. The HF acid process environment was characterized as being one of three types: liquid HF acid (including saturated and condensing HF vapor services), superheated HF acid vapor, or HF acid dissolved in a hydrocarbon phase. Corrosion data, both from refinery inspection data and from available literature 1-3, was plotted using an Arrhenius type plot in which the logarithm of the corrosion rate is plotted versus the reciprocal of the absolute temperature. Temperature functions for the corrosion rate were developed for the liquid HF acid service and superheated HF acid vapor. The corrosion rate of superheated acid is estimated to be about 20% of the rate of liquid acid at the same temperature. The observed corrosion rate for acid dissolved in hydrocarbon was intermediate between the rates for liquid acid and superheated acid vapor. Using this approach, it was possible to predict the observed circuit average corrosion rate. For the circuits examined, each had a significant range of corrosion rates, typically about 300%, that is, the largest observed individual rate was about 3 times or more that of the smallest observed rate. This range in observed corrosion rates is believed to result from the effect of steel composition, as reported by others4. The characterization of the process environment into either superheated HF vapor or liquid phase also allows for a quantitative evaluation of the effect of process conditions on the resulting corrosion rates.
