Hydrofluoric Acid Corrosion
General Information
Hydrofluoric acid (HF) finds application in diverse industrial processes, serving as a crucial component in the production of fluorinated compounds. Moreover, it plays a key role in the electronics industry, where it is employed for the precision etching of glass and silicon. Additionally, HF is instrumental as a catalyst in specific chemical reactions, particularly in the alkylation process. The following chapter is mostly oriented to refining applications (alkylation), however some information is common for other process industries.
Hydrofluoric acid (HF), belonging to the family of halide acids alongside HCl or HBr, generally exhibits lower aggressiveness towards metallic materials, particularly carbon steel, when in its dry form. In comparison to dry HCl, dry HF is typically less corrosive. Nevertheless, it is crucial to acknowledge that even dry HF can induce corrosion in carbon steel. Therefore, caution should be exercised, and the potential for dry HF corrosion should not be underestimated.
It is also important to highlight that HF environment may also generate potential for cracking, especially on welds between dissimilar metals exposed to HF.1 Atomic hydrogen, generated as a byproduct of the reaction between hydrofluoric acid (HF) and steel, has the potential to diffuse through the crystal matrix of the metal. Following recombination into molecular hydrogen, the resultant hydrogen can induce typical cracking phenomena and associated damages, including stress-oriented hydrogen-induced cracking (SOHIC) and stepwise cracking (SWC).2
Both dry (anhydrous) hydrofluoric acid (HF), with water content typically less than 400 ppm, and aqueous HF, often shipped in concentrations of 49% and 70%, exhibit extremely hazardous properties. These substances have the potential to cause severe burns to skin tissue, eyes, and lungs. Therefore, specialized safety precautions and handling procedures are necessary when dealing with hydrofluoric acid due to its high toxicity and corrosive nature.3 4
Due to its extreme health hazards, corrosion prevention and mitigation in HF alkylation units constitute a critical aspect of the overall process safety approach for these units. Table 1 shows the most common areas in alkylation unit that are prone to HF corrosion.
Table 1 Potential locations for HF corrosion in HF-alkylation units.1 2
| Process Unit | Operation area affected by hydrofluoric acid corrosion |
|---|---|
| Piping and equipment | • High corrosion observed over 65°C (150°F) • Dead legs (PSVs loops, drains etc.) |
| Rerun/Regenerator internals | • The primary factors: temperature and contaminants • O2, oxygenates and sulfur species may promote corrosion |
| Main fractionator/iso-stripper | • Feed line • Top column section above and close to inlet • OVHD condensers and pipelines • Feed/bottoms exchanger tubes |
| Depropanizer | • Feed line • Top column section above and close to inlet • OVHD condensers and pipelines • Feed/bottoms exchanger tubes |
| Acid Relief System (ARS) | • ARS pipelines (in particular: flare lines after scrubber) • Dead legs |
| Acid Regeneration/Rerun System | • Regeneration Tower • OVHD piping • Drain lines |
| C3-C4 Rundown Systems | • Pipelines and condensers (upstream and downstream) of defluorination |
| All areas | • Flange face corrosion (carbon steel) |
Hydrofluoric Acid Corrosion is governed by a combination of several factors like chemical species and concentration, temperature, materials and/or flow regimes.
To find out more information about Hydrofluoric Acid Corrosion and impact of various parameters register for free or buy a subscription.
References
Paid Subscribers can have access to the list of references.