99 Diseases of Pressure Equipment: Carbonate Cracking

Carbonate cracking (CC) of carbon steel has seen an increase recently in frequency and severity in some refinery cat crackers, especially in fractionator and gas processing overheads. Some gas scrubbing units are also susceptible. CC is a form of alkaline stress corrosion cracking that often occurs more aggressively at higher pH and higher concentrations of carbonate solutions. One of the more unusual characteristics of carbonate cracking is that it often extends out some distance from the welds (inches away) and is not just a HAZ phenomena. The appearance can look like a spider web maze of tight, fine cracks, making CC difficult to detect without careful surface preparation and wet fluorescent magnetic particle examination. UT shear-wave has been effective in finding piping cracks from CC onstream. Effective PWHT is usually a good way to prevent CC, but if it isn't done right with sufficient soak time and higher temperatures than normal, then cracking may reappear in replacement equipment. It appears that it takes only very low levels of residual stress to initiate carbonate cracking. If PWHT is not effective, then we can alloy up or apply effective barrier coatings to protect the steel (that is until the coating breaks down).

Do you have equipment that is susceptible to carbonate cracking and are you looking for it during turnarounds with the right techniques and meticulous surface preparation?