API RP 581 - 3rd Ed.2016 - Add.2-2020 - Risk-Based Inspection Methodology

2.B-4 API RECOMMENDED PRACTICE 581
2.B.2
Hydrochloric Acid Corrosion
2.B.2.1 Description of Damage
HCl corrosion is a concern in some of the most common refining process units. HCl is aggressive to many
common materials of construction across a wide range of concentrations and is often localized in nature,
particularly when it is associated with localized or shock condensation or the deposition of chloride
containing ammonia or amine salts. Austenitic stainless steels will often suffer pitting attack and may
experience crevice corrosion and/or ClSCC. Some of the nickel-based alloys may experience accelerated
corrosion if oxidizing agents are present or if the alloys are not in the solution annealed heat treatment
condition.
The primary refining units where HCl corrosion is a concern are crude distillation, hydrotreating, and catalytic
reforming. HCl forms in crude units by the hydrolysis of magnesium and calcium chloride salts and results in
dilute HCl in the overhead system. In hydrotreating units, HCl may form by hydrogenation of organic
chlorides in the feed or can enter the unit with hydrocarbon feed or hydrogen and condense with water in the
effluent train. In catalytic reforming units, chlorides may be stripped off of the catalyst and hydrogenate
resulting in HCl corrosion in the effluent train or regeneration systems.
2.B.2.2 Basic Data
The data listed in Table 2.B.2.1 are required to estimate the rate of corrosion in dilute HCl. More
concentrated acid is outside the scope of this section. If precise data have not been measured, a
knowledgeable process specialist should be consulted.
2.B.2.3 Determination of Corrosion Rate
The steps required to determine the corrosion rate are shown in Figure 2.B.2.1. The HCl corrosion rate may
be determined using the basic data in Table 2.B.2.1 in conjunction with Tables 2.B.2.3 through 2.B.2.6.
2.B.2.4 References
See References [46], [78], [79], [80], [92], and [93] in Section 2.2.