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

2.B-58 API RECOMMENDED PRACTICE 581
2.B.6.6 Figures
Start
Is the material
of construction Carbon
Steel?
No
Is the material
of construction Alloy
400?
No
Determine corrosion
rate from published
literature.
Yes
Yes
Estimated
Corrosion Rate
• Temperature
• Velocity
• HF concentration
Determine corrosion
rate from Table
2.B.6.3.
Determine corrosion
rate from Table
2.B.6.2.
• Temperature
• Aerated?
• HF concentration
Estimated
Corrosion Rate
Estimated
Corrosion Rate
Figure 2.B.6.1—HF Corrosion—Determination of Corrosion Rate
2.B.7
Alkaline Sour Water Corrosion
2.B.7.1 Description of Damage
Alkaline sour water corrosion is broadly defined as corrosion by water containing H 2 S and NH 3 , and it is
typically a concern for carbon steel above neutral pH. This corrosion is caused by aqueous ammonium
bisulfide (NH 4 HS). The primary variables that influence alkaline sour water corrosion rates are the NH 4 HS
concentration of the water velocity (wall shear stress) and H 2 S partial pressure. Secondary variables include
temperature and hydrocarbon to water ratios. Alkaline sour water corrosion is of concern across a broad
range of the most common refining process units, notably hydroprocessing treating, catalytic cracking, amine
treating, coking, and light ends recovery. H 2 S is typically formed by thermal breakdown or catalytic
conversion of sulfur compounds. NH 3 is similarly formed from nitrogen compounds. NH 4 HS is formed as a
result of the reaction between these two gases and precipitates out of the gas phase in the reactor effluent
stream as temperatures are reduced below about 150 °F (66 °C).
2.B.7.2 Basic Data
The data listed in Table 2.B.7.1 are required to determine the estimated corrosion rate for NH 4 HS service. If
precise data have not been measured, a knowledgeable process specialist should be consulted.
2.B.7.3 Determination of Corrosion Rate
The steps required to determine the corrosion rate are shown in Figure 2.B.7.1. The corrosion rate may be
determined using the basic data in Table 2.B.7.1 in conjunction with the baseline corrosion rates and
equations in Table 2.B.7.2 to correct for H 2 S partial pressure.