Carbonate stress corrosion cracking is the term applied to surface breaking flaws or cracks that occur adjacent to carbon steel welds in carbonate systems, under the combined action of tensile stress and corrosion.
Carbonate Stress Corrosion Cracking | Materials And Corrosion Control
| Damage Mechanism | Carbonate Stress Corrosion Cracking (SCC) |
| Damage Description | · Carbonate stress corrosion cracking is the term applied to surface breaking flaws or cracks that occur adjacent to carbon steel welds in carbonate systems, under the combined action of tensile stress and corrosion.
· It occurs in sour water piping and equipment in the fractionation and light ends section of Fluid Catalytic Cracking units, as well as gas scrubbing units using hot carbonate solutions to remove CO2. · Cracking susceptibility increases with increasing pH and carbonate (CO3) concentration and can occur under the following conditions: o Non-stress relieved carbon steel, o pH > 9.0 and CO3 > 100ppm, or o 8<pH 9.0 and CO3 > 400ppm. |
| Affected Materials | Carbon steel and low alloy steels. |
| Control Methodology | · Application of a post-fabrication stress-relieving heat treatment of about 1150°F (621°C) is a proven method of preventing carbonate cracking.
· Cracking can be eliminated through the use of effective barrier coatings, solid or clad 300 Series stainless steels, Alloy 400 or other corrosion resistant alloys in lieu of carbon steel. · Water wash non-PWHT’d piping and equipment prior to steam-out or heat treatment. · A metavanadate inhibitor can be used in CO2 removal units in hydrogen reforming units to prevent cracking. Proper dosing and oxidation of the inhibitor must be maintained. |
| Monitoring Techniques | · The cracking can involve sporadic and rapid growth depending on subtle changes in the process, so monitoring is not typically done.
· Although cracks may be seen visually, crack detection is best performed with Wet Fluorescent Magnetic Particle Testing, · Usually there is not much branching so crack depths can be measured with a suitable Ultrasonic Testing technique including external Shear Wave Ultrasonic Testing. · This cracking is not susceptible to extension by grinding. Grinding out the cracks is a viable method of crack depth determination. · Acoustic Emission Testing can be used for monitoring crack growth and locating growing cracks. |
| Inspection Frequency | At every T&I |
| KPIs | · Maintain pH > 9.0 and CO3 > 100 ppm
· Maintain 8<pH 9.0 and CO3 > 400 ppm · Maintain water wash below 50 wppm H2S, and pH below 6.7 to avoid cracking susceptibility in equipment/piping. · In gas scrubbing units, maintain CO2 below 2% and temperature below 200°F (93°C). |
| Reference Resources (Standards/GIs/BPs) | · API RP 571 (DM #21)
· NACE RP0296, Guidelines for Detection, Repair and Mitigation of Cracking of Existing Petroleum Refinery Pressure Vessels in Wet H2S Environments, NACE International, TX, latest edition |
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