About this subject please note the following:
“5.1.1.10 Sour Water Corrosion (Acidic)
5.1.1.10.1 Description of Damage.
a) Corrosion of steel due to acidic sour water containing H2S at a pH between 4.5 and 7.0. Carbon dioxide (CO2) may also be present.
b) Sour waters containing significant amounts of ammonia, chlorides or cyanides may significantly affect pH but are outside the scope of this section.
5.1.1.10.2 Affected Materials
a) Primarily affects carbon steel.
b) Stainless steels, copper alloys and nickel base alloys are usually resistant.
5.1.1.10.3 Critical Factors
a) H2S content, pH, temperature, velocity and oxygen concentration are all critical factors.
b) The H2S concentration in the sour water is dependent on the H2S partial pressure in the gas phase as well as temperature and pH.
c) At a given pressure, the H2S concentration in the sour water decreases as temperature increases.
d) Increasing concentrations of H2S tend to decrease solution pH down to about 4.5. Streams with a Ph below 4.5 indicate the presence of a strong acid which would be the main corrosion concern (see 5.1.1).
e) Above a pH of about 4.5, a protective, thin iron sulfide layer limits the corrosion rate.
f) In some instances at a pH above 4.5, a thicker, porous sulfide film layer can form. This can promote pitting under sulfide deposits. Typically, this does not affect the general corrosion rate.
g) Other contaminants have a significant affect on water pH. For example, HCl and CO2 lower pH (more acidic). Ammonia significantly increases pH and is more often associated with alkaline sour water where the main concern is ammonia bisulfide corrosion (see 5.1.1.2).
h) The presence of air or oxidants may increase the corrosion and usually produces pitting or underdeposit attacks.
5.1.1.10.4 Affected Units or Equipment
Acid sour water corrosion is a concern in overhead systems of FCC and coker gas fractionation plants with high H2S levels and low NH3 levels.
5.1.1.10.5 Appearance or Morphology of Damage
a) Corrosion damage from acidic sour water is typically general thinning. However, localized corrosion or localized underdeposit attack can occur, especially if oxygen is present. Corrosion in CO2 containing environments may be accompanied by carbonate stress corrosion cracking (see 5.1.2.5).
b) 300 Series SS are susceptible to pitting attack and may experience crevice corrosion and/or chloride stress corrosion cracking (see 4.5.1).
5.1.1.10.6 Prevention / Mitigation
a) 300 Series SS can be used at temperatures below about 140oF (60oC) where Chloride Stress Corrosion Cracking (CSCC) is not likely.
b) Copper alloys and nickel alloys are generally not susceptible to acid sour water corrosion. However, copper alloys are vulnerable to corrosion in environments with ammonia”
Reference
Damage Mechanisms Affecting
Fixed Equipment in the
Refining Industry
RECOMMENDED PRACTICE 571
FIRST EDITION, DECEMBER 2003
