Step wise cracking
STEP WISE CRACKING
The term Step Wise Cracking (SWC) generally refers to fracture propagation mechanisms in carbon steel structures, triggered by hydrogen embrittlement phenomena. This technique received significant industrial attention only after the rupture of a monoethanolamine (MEA) absorption tower at the Lemont, Illinois refinery in 1984. On that occasion, the explosion and subsequent fire caused numerous casualties and severe plant damage.
Potential Issues
According to test results conducted after the 1984 incident, most damage phenomena occur in environments containing wet H₂S. The most well-known of these are:
- Sulfide Stress Cracking (SSC): This refers to the fracture of a metal due to the combined effects of stress and corrosive phenomena occurring in the presence of water and H₂S. SSC results from hydrogen embrittlement, where atomic hydrogen—produced by corrosion processes—diffuses into the metal. This issue is commonly found in high-yield-strength steels or heat-affected zones of welds.
- Hydrogen Blistering: The formation of planar cavities beneath the metal surface (blisters) results from high molecular hydrogen pressures. This phenomenon, typical of low-yield-strength steels, occurs when atomic hydrogen—generated on the surface by corrosion—diffuses into the metal and recombines internally into molecular hydrogen, accumulating at non-metallic inclusions, rolling discontinuities, etc.
- Alkaline Stress Corrosion Cracking (ASCC): A metal fracture caused by the combined effect of stress and corrosive phenomena occurring in alkaline solutions containing H₂S and CO₂.
- Hydrogen Induced Cracking (HIC): This refers to the formation of stepwise internal fractures that connect adjacent hydrogen blisters on different planes. No stress state is required for HIC to occur. Internal fractures tend to link up due to a transgranular plastic deformation mechanism, driven by the high pressure resulting from hydrogen accumulation. HIC is typical in steels with high impurity levels, a high density of planar inclusions, and/or microstructural anomalies caused by impurity or alloying element segregation.
- Stress-Oriented Hydrogen Induced Cracking (SOHIC): SOHIC refers to a sequence of nearly perpendicularly aligned fractures caused by the linking of small HIC fractures under applied or residual stresses. It is commonly observed in heat-affected zones adjacent to welds and is oriented along the metal thickness.
Sicil Service CND’s Commitment
The German company Verkade N.D.T. Service and Sicil Service CND have developed an ultrasonic inspection method capable of performing in-service equipment checks. This method can detect and size cracks and blistering in welds and base metal associated with H₂S attack. Once defects are classified using fracture mechanics calculations, the criticality level is determined, assessing whether the equipment can continue operating or if repairs are necessary.
Using this methodology, Verkade has conducted inspections on approximately 6,000 pressure vessels worldwide and has developed a comprehensive database. Since 1999, its collaboration with Sicil Service CND has enabled fracture prevention inspections in Italy, particularly at petrochemical plants and refineries, earning the trust of major oil companies.
