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

RISK-BASED INSPECTION METHODOLOGY, PART 2, ANNEX 2.B—DETERMINATION OF CORROSION RATES 2.B-55
2.B.6
2.B.6.1
Hydrofluoric Acid Corrosion
Description of Damage
Concentrated hydrofluoric acid (HF) is used as the acid catalyst in HF alkylation units. The alkylation reaction
chemically combines an alkane (usually isobutane) with an olefin (butylene, propylene, amylene) in the
presence of the acid catalyst. HF presents severe health hazards as both a liquid and vapor. If spilled, HF
may form a dense, low lying, toxic cloud. Extreme caution should be exercised when using HF.
Corrosion of materials in HF primarily depends on the HF-in-water concentration and the temperature. Other
variables, such as velocity, turbulence, aeration, impurities, etc., can strongly influence corrosion. Some
metals will form a protective fluoride film or scale that protects the surface. Loss of this protective film,
especially through high velocity or turbulence, will likely result in greatly accelerated corrosion rates.
Corrosion in 80 % and stronger HF-in-water solutions is equivalent to corrosion in AHF (200 ppm H 2 O).
Below 80 % HF, the acid is considered aqueous, and metal corrosion is highly temperature and velocity
dependent and usually accelerated. The usual HF-in-water concentrations in typical HF alkylation units are
96 % to 99+ %, and the temperatures are generally below 66 °C (150 °F). Under these conditions, carbon
steel is widely used for all equipment except where close tolerances are required for operation (i.e. pumps,
valves, instruments). Where close tolerances are required and at temperatures over 66 °C (150 °F) to
approximately 149 °C (300 °F), Alloy 400 is typically used.
Accelerated corrosion from water dilution of the acid is often encountered in low points (bleeders, line
pockets, etc.) if unit dry out leaves residual free water in these areas.
2.B.6.2 Basic Data
The data listed in Table 2.B.6.1 are required to determine the estimated corrosion rate for HF acid service. If
precise data have not been measured, a knowledgeable process specialist should be consulted.
2.B.6.3 Determination of Corrosion Rate
The steps required to determine the corrosion rate are shown in Figure 2.B.6.1. The corrosion rate may be
determined using the basic data in Table 2.B.6.1 in conjunction with Tables 2.B.6.2 through 2.B.6.3.
It is important to note that the corrosion rate is very high in the initial stages of exposure to HF as the
protective fluoride scale is being established. Once established, the fluoride scale protects the steel resulting
in low corrosion rates unless the scale is disturbed or removed.
Alloy steels have been found to exhibit higher corrosion rates than mild carbon steel in both dilute and
concentrated HF and generally are not specified for this service. Higher alloys are sometimes used in HF
service, and corrosion rates, if unknown, should be obtained from published literature or from the
manufacturer [105] . It is important to consider the galvanic effects of welding carbon steel to Alloy 400 or other
corrosion-resistant alloys. Accelerated and localized attack of the carbon steel may result from galvanic
coupling. Increased rates of corrosion have also been reported in carbon steels that contain high levels of
residual elements, notably Cu, Ni, and Cr [16] .
Corrosion caused by HF results in general thinning except in the event of potential galvanic attack. The
presence of HF may also result in hydrogen stress cracking and blistering. These degradation modes are
considered in Part 2, Section 14.1.