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51315-5560-Occluded Cell Corrosion Mechanism for C-steel in HF-acid Environment

Picture for Occluded Cell Corrosion Mechanism for C-steel in HF-acid Environment
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Product Number: 51315-5560-SG
ISBN: 5560 2015 CP
Author: Henk Helle
Publication Date: 2015
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Although the deleterious effect of the residual elements Cu Ni and Cr (RE) in C-steel on its corrosion resistance against HF-acid has clearly been established in numerous failure cases in HF-alkylation installations and in laboratory testing the responsible mechanism for this effect was undisclosed. Moreover other factors had to be active as well. Evidently RE could not be the only cause of failures or even a predictor for failure. In some failure cases RE was low or absent in some non-failures RE was high. In plant acid RE did 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.At the at the UOP Worldwide HF Alkylation symposium in June 2014 the hypothetical model for an occluded cell corrosion mechanism was presented by the author. The proposed mechanism is driven by water formed from certain oxide inclusions in steel underneath a fluoride film which is iron-deficient and semi-conductive for Fe2+ cations. The model is based on both theoretical considerations and the features and characteristics of a wide range of failure cases. The hypothesis comprehensively explains a number of peculiar features of corrosion in HF-alkylation service:

  • The uncommonly extensive and uniform thinning of affected pipe sections in trace acid or intermittent acid environment
  • The mechanism that explains the role of RE in corrosion and its inconsistency
  • The mechanism that explains the effect of elevated temperatures on corrosion
  • The cause for extensive scale build-up or the absence of scale build-up on corroded areas.

To attendees of the symposium have been asked to provide metallographic support for the conjecture by revisiting their old HF-alkylation failure cases and check the presence or absence of oxides. I am confident that corroborating or refuting evidence will materialize.

Although the deleterious effect of the residual elements Cu Ni and Cr (RE) in C-steel on its corrosion resistance against HF-acid has clearly been established in numerous failure cases in HF-alkylation installations and in laboratory testing the responsible mechanism for this effect was undisclosed. Moreover other factors had to be active as well. Evidently RE could not be the only cause of failures or even a predictor for failure. In some failure cases RE was low or absent in some non-failures RE was high. In plant acid RE did 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.At the at the UOP Worldwide HF Alkylation symposium in June 2014 the hypothetical model for an occluded cell corrosion mechanism was presented by the author. The proposed mechanism is driven by water formed from certain oxide inclusions in steel underneath a fluoride film which is iron-deficient and semi-conductive for Fe2+ cations. The model is based on both theoretical considerations and the features and characteristics of a wide range of failure cases. The hypothesis comprehensively explains a number of peculiar features of corrosion in HF-alkylation service:

  • The uncommonly extensive and uniform thinning of affected pipe sections in trace acid or intermittent acid environment
  • The mechanism that explains the role of RE in corrosion and its inconsistency
  • The mechanism that explains the effect of elevated temperatures on corrosion
  • The cause for extensive scale build-up or the absence of scale build-up on corroded areas.

To attendees of the symposium have been asked to provide metallographic support for the conjecture by revisiting their old HF-alkylation failure cases and check the presence or absence of oxides. I am confident that corroborating or refuting evidence will materialize.

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