Stress Corrosion (Environmental) Cracking

Stress corrosion cracking can be a very serious form of deterioration that can result in the catastrophic failure of a pressure vessel. It is highly dependent on the vessel material, as well as on process environment and operating conditions. Proper materials selection and fabrication procedures effectively prevent stress corrosion cracking, but the possibility of its occurrence in severe process environments should not be ignored.

The major types of stress corrosion cracking are:
1. H2S stress cracking of carbon steel and low-alloy Cr – Mo steels.
2. Chloride stress corrosion cracking of austenitic stainless steels.
3. Ammonia stress corrosion cracking of carbon steels.
4. Caustic embrittlement of carbon steel.

An in-service inspection program should include nondestructive examination procedures that can detect stress corrosion cracks, regardless of the preventive measures taken. Stress corrosion cracking can be caused by process environments that do not normally contain, but are occasionally contaminated by, the chemical compounds that cause the phenomenon.

Stress corrosion cracks most often originate at the surface exposed to the process environment. Welds and weld heat affected zones can be especially susceptible. However, some types of stress corrosion cracks caused by process environments containing H2S can originate below the surface.

Visual examination (VT) is not reliable for detecting stress corrosion cracks in a pressure vessel, because the cracks tend to be very fine. However, magnetic particle examination (MT) or dye-penetrant examination (PT) are usually reliable. Frequently, MT or PT examinations will reveal a great number of very closely spaced cracks. Many of these cracks can be superficial and not have a significant effect on the integrity and reliability of the vessel unless they grow to a larger size. Ultrasonic examination (UT) using shear wave procedures can be used to determine the depth of the cracks to evaluate if they jeopardize the vessel. However, the typical fine, closely spaced, and branched morphology of stress corrosion cracks makes them very difficult to reliably detect and accurately size by UT examination. Specialized UT techniques are usually required to obtain satisfactory results. UT must be used to detect any stress corrosion cracks that originate below the surface, and it may be used to detect those that originate at a surface that is not accessible for MT or PT examination.

Categories: In-Service Inspection | Leave a comment