Abstract

Wet hydrogen chloride (HCl) environments present one of the most aggressive corrosion environments in the mineral processing industry, particularly when combined with elevated temperatures, organic contaminants, and thermal cycling. The degradation mechanisms in wet HCl governed by its strong reducing nature and high permeability through most liner materials significantly restrict the range of viable materials of construction.

This case study examines the premature failure of a carbon steel column lined with PFA (perfluoroalkoxy) lining operating at 125 °C in a wet HCl vapor stream. The failure analysis demonstrates that although PFA is chemically resistant to HCl, the thickness of powder spray-applied lining is insufficient, which accelerated HCl permeation leading to under-liner corrosion, blistering, and catastrophic coating failure. These manufacturing-driven vulnerabilities underscore the need to evaluate not only the material but also the method by which it is applied

This study evaluates several alternative systems, including glass lining, PTFE (Polytetrafluoroethylene)/PFA loose linings, rotationally molded PFA, and FRP‑based composite construction. Each option demonstrates that long-term performance in wet HCl is governed as much by manufacturing method as by inherent material properties.

Glass lining offers very low permeation and excellent chemical resistance but requires extensive vessel redesign to eliminate stress concentrators, accommodate swaged nozzles, and manage the thermal expansion mismatch between glass and steel. Inadequate design modification or improper firing can result in cracking, delamination, and costly repairs. Fluoropolymer linings, particularly loose PFA or PTFE systems, provide strong chemical resistance but are highly sensitive to manufacturing quality. Welded sheet linings introduce stress risers and potential leak paths, particularly at nozzles; paste-extruded PTFE offers low permeability but is limited to specific diameters; and rotationally molded PFA provides seamless construction but cannot reliably coat complex internals such as packing supports. Fibre reinforced polymer (FRP) construction with an internal PFA or PTFE liner emerges as a promising alternative, offering structural robustness, reduced risk of catastrophic failure, and improved manufacturability. However, this approach requires careful attention to venting, thermal cycling, and bonding limitations, particularly under vacuum conditions.

Permeation resistance is strongly influenced by liner type, thickness, crystallinity, void content, and processing conditions, all of which vary significantly between manufacturers and fabrication methods. Overall, reliable operation in wet HCl environments requires a holistic approach that integrates materials science with detailed manufacturing engineering. Material selection must be paired with rigorous evaluation of fabrication processes, quality control practices, and design improvements. Failure to address these manufacturing-driven factors can negate the inherent corrosion resistance of even the most chemically robust materials.