Inspectioneering Journal

99 Diseases of Pressure Equipment: Hydrochloric Acid

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

Corrosion from HCl is a significant problem in many refining and chemical process units, and often the materials solution to HCl corrosion is rather expensive, since the lower cost, more available alloys are usually not resistant to most concentrations of HCl. Typically process unit construction materials are chosen to resist HCl when the designers know where HCl will be present in the process, and at what temperature and concentration. The trouble comes when low pH HCl fluids show up where they were not expected or process changes occur causing unexpected HCl corrosion.

In the refining industry, the most common reason for HCl corrosion is dew point formation in the overhead (O/H) of distillation towers. Most crudes contain inorganic salts that give rise to dew point HCl 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. Because of the mechanics/ chemistry of dew point corrosion, the point where the dew point first occurs in the O/H steams is normally the most corrosive and lowest pH. If that point keeps shifting upstream or downstream (because of process variations) from where it was expected by the plant designers, then unexpected leaks can occur.

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 of it. 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 recycle hydrogen streams. If chloride beds/ treaters are not present or not maintained, then excessive chlorides can break through and cause corrosion wherever the dew point is reached downstream.

Recently the refining industry has experienced a number of cases of severe corrosion from organic chloride contamination of process crudes. These organic chlorides are not effectively removed during normal desalting and often migrate downstream to naphtha hydrotreaters (NHT), where they hydrolyze into low pH streams that can cause corrosion rates into the 3- 4 digit range, and leaks in equipment within a very short period of time. Unfortunately these organic chlorides are not easily detected, and are not revealed in typical crude assays. But there are effective means to test for them and to monitor wash water pH in NHT streams.

Prevention of HCl failures involves knowledge of where the low pH solutions are likely to exist and selection of a number of potential mitigation strategies. These strategies include, choosing the right alloys to resist HCl 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.

HCl 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.

Do you know where your HCl dew points, mix points and other HCl containing steams may cause accelerated corrosion in your process steams and do you have the right mitigation and monitoring strategies in place to prevent leaks form HCl?

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