Inspectioneering Journal

99 Diseases of Pressure Equipment: Hydrofluoric Acid (HFA)

By John Reynolds, Principal Consultant at Intertek. This article appears in the September/October 2004 issue of Inspectioneering Journal.

Corrosion in the refining industry from HFA is not as widespread a problem as it is with HCl because it is only associated with HF Alkylation Units, which are usually fairly carefully controlled in order to avoid potential for a toxic HFA cloud after a leak. However, that said, HF Alky Units also can have problems with process control that results in wet acid carryover into carbon steel sections, which were never intended to be exposed to HFA, of the plant that results in corrosion and leaks. Whenever the temperature of the acid phase exceeds 150 F or water content of the acid exceeds 3% (or some combination thereof), there is the likelihood of higher corrosion rates on carbon steel equipment. In areas prone to corrosion on carbon steel, Alloy 400 is widely used with success, if oxygenates are minimized. Alloy 400 is also resistant to hydrogen blistering and cracking associated with HFA processes, but that topic will be covered separately. Carbon steel areas prone to corrosion include flange faces, deadlegs, overhead systems, and some heat exchanger bundles. One of the interesting aspects of HFA corrosion on carbon steel is that corrosion rates can vary considerably depending on the residual elements content of the steel. Some evidence exists that the total % of Cu, Ni, and Cr need to be below 0.20% in order to avoid accelerated corrosion of carbon steel in specific parts of the plant. API RP 751, Safe Operation of HFA Alkylation Units (2) is a good reference on the subject.

For those that operate HFA Alkylation Units, are you closely monitoring the susceptible areas on carbon steel equipment and piping that may be prone to higher than expected corrosion rates if process conditions are not as the designers thought they would be?

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