Hydrofluoric (HF) Acid Corrosion

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Hydroflouric (HF) Acid Corrosion is an acidic environment damage mechanism associated almost exclusively with HF Alkylation Units. Although these units are usually carefully controlled, unexpected problems with process control can lead to wet acid carryover into carbon steel sections, which were never intended to be exposed to HF acid, resulting in corrosion and leaks. Carbon steel areas prone to HF acid corrosion include flange faces, dead legs, overhead systems, and some heat exchanger bundles.

Corrosion and fouling in HF Alkylation Units tends to be closely linked to feed quality and operating conditions. Prior to entering the Reaction Section, the olefin and isobutane feed should be treated to remove water, sulfur and other contaminants.

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 has been very successful in minimizing and preventing corrosion in situations where oxygenates are minimized.

While HF acid corrosion isn’t widespread, there is no reason to get complacent. It is important for operators to continue to closely monitor the susceptible areas on carbon steel equipment and piping.

API RP 751, Safe Operation of Hydrofluoric Acid Alkylation Units, is a recommended practice developed and published by the American Petroleum Institute (API) that provides guidance to refinery operators on the safe operation of HF alkylation units, including recommendations (“should”) and certain requirements (“shall”) on hazards management, operating procedures, material inspection and maintenance, and corrosion & materials considerations specific to HF alkylation units. 

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Articles about Hydrofluoric (HF) Acid Corrosion

A question was posed to me regarding guidelines for routine external inspection of spheres, including procedural approaches along with any nondestructive examination (NDE). Here are two approaches to inspection of spheres, and a mixture of the two,...

Authors: Greg Alvarado
September/October 2004 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...

Authors: John Reynolds
July/August 2004 Inspectioneering Journal

The following is the final part of a 2-part series outlining the relationship between key operating parameters and corrosion that has been used to develop a set of guidelines to define an operating envelope.

May/June 2004 Inspectioneering Journal

Corrosion and fouling in HF Alkylation Units are closely linked to feed quality and operating conditions. This article outlines the relationship between key operating parameters and corrosion that has been used to develop a set of guidelines to...

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