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Overview of Hydrochloric (HCl) Acid Corrosion

Hydrochloric (HCl) Acid Corrosion is an acidic environment damage mechanism that can be a significant problem for operators of refining and chemical process units. Typically, process unit construction materials are chosen to resist hydrochloric acid corrosion when the designers know where and how much hydrochloric acid will be present in the process. However, problems arise when low pH hydrochloric acid fluids show up where they were not expected or process changes occur causing unexpected hydrochloric acid corrosion.

In the refining industry, the most common reason for hydrochloric acid corrosion is dew point formation in the overhead (O/H) of distillation towers. Most crudes contain inorganic salts that give rise to dew point corrosion, and while effective desalting removes the vast majority of these salts (thus reducing O/H corrosion), the chlorides from those salts that are not removed often end up in the O/H piping, exchangers and accumulator vessels as low pH corrosive water solutions. Hydrochloric acid corrosion can be both general and highly localized depending upon where a dew point may be forming, where low pH solutions collect, and the presence of hydrodynamic effects.

A similar phenomena can occur at various mix points in process streams where chloride contaminated streams are mixed with lower temperature, wet process streams causing aggressive corrosion with hydrodynamic effects at the mix point and just downstream. Another fairly common source of corrosion from chlorides occurs in cat reforming units where chlorides are stripped from the catalyst and then migrate downstream in recycled hydrogen streams.

Prevention of hydrochloric acid corrosion failures involves knowledge of where the low pH solutions are likely to exist and selection of a number of potential mitigation strategies, including: choosing the right alloys to resist corrosion, proper neutralization and/or inhibitor injection, avoidance of oxygenates that can cause corrosion of otherwise resistant high alloy materials, installation and maintenance of wash water systems, and monitoring pH of water steams.

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