A Proposed Mechanism for Corrosion of C-Steel in Trace HF-acid and the Effect of Residual Elements

摘要

The deleterious effect of the 'residual elements' Cu, Ni and Cr (RE) on the corrosion resistance of C-steel against hydrofluoric acid (HF), though unexplained, has been demonstrated convincingly by many failure cases in the HF-alkylation industry. The effect of RE, however, is not consistent. In some failure cases RE was low or absent, in some non-failures RE was high. In anhydrous hydrofluoric acid or plant acid RE does not appear to have any effect at all. Seamless pipe and tube could suffer severe internal corrosion until paper-thin walls remained, flange faces would suffer corrosion, but plate fabrications hardly or none at all. Other strange characteristics and features of HF-induced corrosion cases remain unexplained: 1. The uncommonly extensive and uniform thinning of corroding pipe. 2. The role of RE in the corrosion mechanism and the reasons for its inconsistency. 3. Why severe corrosion may persists long after HF is removed from the environment. 4. Why scale build-up on corroded areas may be very thick, or entirely absent. In this paper, a hypothetical model for an occluded cell corrosion mechanism is proposed. The model is based on the characteristics and features of a wide range of failure cases and comprehensively explains each of these uncommon features of corrosion in HF-alkylation service.