Sour Water Corrosion
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. Sour waters containing significant amounts of ammonia, chlorides or cyanides may significantly affect pH but are outside the scope of this section.
Primarily affects carbon steel. Stainless steels, copper alloys and nickel base alloys are usually resistant.
At a given pressure, the H2S concentration in the sour water decreases as temperature increases. 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. Above a pH of about 4.5, a protective, thin iron sulfide layer limits the corrosion rate. 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.
Critical factors include H2S content, pH, temperature, velocity and oxygen concentration. The H2S concentration in the sour water is dependent on the H2S partial pressure in the gas phase as well as temperature and pH. 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. The presence of air or oxidants may increase the corrosion and usually produces pitting or underdeposit attacks.
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.
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. 300 Series SS are susceptible to pitting attack and may experience crevice corrosion and/or chloride stress corrosion cracking.
UT, RT, Corrosion Probes/Coupons – Evidence of locally thinned areas can be found using scanning ultrasonic thickness methods or profile radiography. For carbon steel, damage is usually in the form general thinning but may be highly localized to specific areas of high velocity or turbulence, typically where a water phase is condensing. Process and corrosion monitoring are important aspects of a well-developed program to minimize the effects of acidic sour water corrosion. The water draws of overhead accumulators should be monitored periodically to measure pH. Properly placed corrosion probes and corrosion coupons provide additional information on the rate and extent of potential damage.
300 Series SS can be used at temperatures below about 140°F (60°C) where Chloride Stress Corrosion Cracking (CSCC) is not likely. 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.
Other factors to consider in these environments include wet H2S damage and carbonate SCC.