Stress Corrosion Cracking(SCC)
Definition
A damage mechanism involving the initiation and propagation of cracks in a material under the combined action of tensile stress (applied or residual) and a specific corrosive environment. SCC is insidious because it can occur in materials that are resistant to general corrosion, at stress levels below the yield strength, and with minimal visible surface degradation. Common forms include chloride SCC (austenitic SS), caustic SCC, amine SCC, polythionic acid SCC, and sulfide stress cracking.
Context
SCC is a critical damage mechanism in API 571 that requires specific inspection techniques (surface NDE — MPI, PT; advanced UT — TOFD, PA-UT) because the cracks are tight and may not cause measurable wall thinning. SCC susceptibility depends on the specific combination of material, environment, and stress — changing any one factor can eliminate the mechanism.
Examples
- Chloride SCC in austenitic stainless steel heat exchanger tubes exposed to cooling water with >50 ppm chlorides at temperatures above 60C, requiring replacement with duplex stainless steel or titanium.
- Amine SCC in carbon steel welds in an amine treating unit — detected by wet fluorescent MPI during turnaround, requiring weld grinding, NDE, and post-weld heat treatment to prevent recurrence.
Applicable Standards
- API 571
- NACE SP0472
- NACE MR0103
Related Terms
A specific process or phenomenon that causes degradation of material properties, leading to eventual failure of equipment. Damage mechanisms include corrosion (uniform, localized, galvanic), erosion, fatigue, creep, hydrogen damage, stress corrosion cracking, embrittlement, and other metallurgical or environmental attack modes. API 571 catalogues over 60 damage mechanisms relevant to refining and petrochemical equipment.
API 571Damage Mechanisms Affecting Fixed Equipment in the Refining Industry — a comprehensive catalogue of over 60 damage mechanisms that can affect refinery and petrochemical equipment. Each mechanism is described with its appearance, critical factors, affected materials, prevention methods, and inspection techniques.
Magnetic Particle Inspection(MPI/MT)An NDE method for detecting surface and near-surface discontinuities in ferromagnetic materials. The test surface is magnetized (using a yoke, prods, or coil) and ferromagnetic particles (dry powder or wet suspension) are applied. Particles accumulate at areas of flux leakage caused by discontinuities, forming visible indications. MPI is highly sensitive to surface-breaking cracks, particularly fatigue cracks, stress corrosion cracks, and weld defects.
Non-Destructive Examination(NDE/NDT)A group of analysis techniques used to evaluate the condition of materials, components, and assemblies without causing damage or altering their serviceability. NDE methods detect surface and subsurface flaws, measure wall thickness, characterize material properties, and assess the integrity of welds and joints. Common NDE methods include ultrasonic testing (UT), radiographic testing (RT), magnetic particle inspection (MPI/MT), liquid penetrant testing (PT), visual testing (VT), eddy current testing (ECT), and acoustic emission testing (AE).
SusceptibilityThe degree to which an equipment item is vulnerable to a specific damage mechanism, based on material of construction, process conditions (temperature, pressure, chemistry), design features, and operating history. Susceptibility is typically rated as None, Low, Medium, or High, and drives the selection of damage factors and inspection techniques in RBI assessments.
Where this appears in Reliatic
This concept is used in the Inspections module.