Carbonate Stress Corrosion Cracking (ACSCC) is a common problem in the oil and gas production, petroleum refining, and petrochemical/chemical processing industries. ACSCC is a form of alkaline stress corrosion cracking that often occurs more aggressively at higher pH and high concentrations of carbonate solutions. It’s very tricky to deal with since only very low levels of residual stress are required for carbonate cracking to occur.
ACSCC appears frequently in some refinery cat crackers, especially in fractionator and gas processing overheads. Likewise, some gas scrubbing units are also susceptible to the mechanism.
One of the more unusual characteristics of ACSCC compared to other damage mechanisms is that it often extends out several inches away from the welds and is not just a phenomena in heat affected zones. In fact, carbonate cracking usually consists of multiple, tight, fine, parallel cracks and is often described as looking like a spider web. For this reason, it is often difficult to detect without careful surface preparation and wet fluorescent magnetic particle testing methods. ACSCC can also be tested for via other methods such as shear wave ultrasonic testing, acoustic emission testing, and radiographic testing, although these tend to be less effective than WFMPT. Visual examinations can be used as well, but these tend to be ineffective except in more advanced cases.
When done properly, post weld heat treatment is an effective way to prevent ACSCC. In situations where post weld heat treatments are ineffective, barrier coatings are also an adequate method of prevention.
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