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-71
2.B.9.5 Figures
Start
• Maximum metal
temperature
• Material
Determine oxidation
rate using Table
2.B.9.1, 2.B.9.2 and
2.B.9.3.
Estimated
corrosion rate
Figure 2.B.9.1—High Temperature Oxidation—Determination of Corrosion Rate
2.B.10 Acid Sour Water Corrosion
2.B.10.1
2.B.10.1.1
Description of Damage
Overview
Acid sour water is defined as water containing H 2 S and with pH below neutral pH (7). The primary refining
units or gas fractionator plants where acid sour water corrosion is a concern are sour distillation overhead
systems in gas fractionation plants. Most other refining distillation units contain chlorides or ammonia and are
covered in Sections 2.B.2 and 2.B.7, respectively.
Corrosion damage from acid sour water is typically general thinning. Above pH of about 4.5, a protective thin
iron sulfide layer limits the corrosion rate. In some instances at pH above 4.5, a thicker porous sulfide film
layer can form. This can promote pitting under sulfide deposits yet generally this does not affect the general
corrosion rate. Sour water condensates in equilibrium with a vapor containing H 2 S at a partial pressure
below 1 atm will usually have a pH between 4.5 and 7. At H 2 S partial pressures greater than 1 atm, a pH <
4.5 can be achieved and are outside the scope of this paragraph.
2.B.10.1.2
Scope and Limitations
The scope of this paragraph is to give guidance in determining a conservatively estimated corrosion rate
driven by acid sour water, under the following conditions and assumptions.
a) Primary corrosive is H 2 S. Assuming no significant amounts of ammonia, carbon dioxide, chlorides, or
cyanides are present.
b) The corrosion mechanism is general corrosion, with some occurrence of pitting in the presence of
oxygen.
c) This paragraph contains guidance for estimating corrosion rates of carbon steel. Copper alloys and
nickel alloys (not containing chromium) can be used and are generally not susceptible to acid sour water
corrosion in the refinery applications where acid sour water is found. Stainless steel can be used where
the temperature is low enough that ClSCC is not likely. Where the material of construction is other than
carbon steel, it is assumed that the selected material is suitable for the service and that the corrosion
rates will not exceed 0.05 mm/y (2 mpy).
d) Cracking is not considered. Wet H 2 S cracking mechanisms are addressed elsewhere in this document.