Study guide
8.3 Amine, Caustic & Chloride Stress Corrosion Cracking
API 510 Pressure Vessel Inspector Exam
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1 Chapter 1: Introduction, Scope & Certification Requirements
1.1 Exam Structure, Administration & Study Strategy 1.2 API 510 Code Scope, Exclusions & Jurisdictional Boundaries 1.3 Roles, Responsibilities & Inspector Qualification
2 Chapter 2: Inspection Planning, Data Evaluation & Remaining Life
2.1 Corrosion Rates, Remaining Life & Corrosion Allowance Calculations 2.2 Inspection Intervals, On-Stream Inspection & Interval Deferrals 2.3 Condition Monitoring Locations (CMLs) & Data Analysis
3 Chapter 3: Vessel Evaluation, Corrosion Averaging & Fitness-for-Service
3.1 Corrosion Averaging & Minimum Thickness Evaluation 3.2 Pitting Evaluation & Corroded Area Limits 3.3 Fitness-for-Service (API 579-1) & Risk-Based Inspection (API 580)
4 Chapter 4: ASME Section VIII Div 1 Design & Thickness Calculations
4.1 Internal Pressure Design & Cylindrical Shell Calculations 4.2 Ellipsoidal & Hemispherical Head Calculations 4.3 Spherical Shells, Static Head & Vessel MAWP 4.4 Openings, Nozzle Reinforcement & External Pressure Basics
5 Chapter 5: ASME Section VIII Fabrication, Welds & Joint Efficiency
5.1 Weld Joint Categories (UW-3) & Joint Efficiencies (UW-12) 5.2 Radiographic Examination (UW-11, UW-51, UW-52) 5.3 Post-Weld Heat Treatment (PWHT) & Impact Testing (UCS-56, UCS-66)
6 Chapter 6: Welding Inspection & Metallurgy (API 577 & ASME Sec IX)
6.1 Welding Procedures (WPS/PQR) & Essential Variables 6.2 Welder Performance Qualification (WPQ) & Testing 6.3 Welding Processes, Metallurgy & Weld Discontinuities (API 577)
7 Chapter 7: Nondestructive Examination (ASME Section V)
7.1 Radiographic Examination Techniques & Image Quality Indicators (Article 2) 7.2 Ultrasonic Thickness Measurement (Article 23, SE-797) 7.3 ASME Section V Articles 1, 6 & 7: General Requirements, PT & MT
8 Chapter 8: Damage Mechanisms (API RP 571)
8.1 Atmospheric Corrosion & Corrosion Under Insulation 8.2 HTHA, High-Temperature H2/H2S Corrosion & Sulfidation 8.3 Amine, Caustic & Chloride Stress Corrosion Cracking 8.4 Wet H2S Damage, Brittle Fracture & Erosion-Corrosion 8.5 Boiler-Water, Steam-Condensate & Hydrochloric Acid Corrosion 8.6 Caustic Corrosion & Naphthenic Acid Corrosion
9 Chapter 9: Pressure Vessel Inspection Practices (API RP 572 & API RP 578)
9.1 Internal & External Vessel Inspection Practices (API 572) 9.2 Structural Components, Internal Trays, Linings & Skirts 9.3 Material Verification Programs & Positive Material Identification (API 578)
10 Chapter 10: Pressure-Relieving Devices (API RP 576)
10.1 PRD Types, Construction & Operating Principles 10.2 PRD Causes of Failure, Shop Testing & Overhaul 10.3 PRD Inspection Intervals, Records & Rupture Disks
11 Chapter 11: Repairs, Alterations, Rerating & Pressure Testing (API 510 & PCC-2)
11.1 Repair vs. Alteration Authorization, Execution & Methods 11.2 PCC-2 Core Welded Repairs: Articles 101, 201, 202, 211 & 212 11.3 PCC-2 Special Welding: Articles 209, 210, 215 & 216 11.4 PCC-2 Excavation, Flange, Exchanger & Test Articles 304, 305, 312, 501 & 502 11.5 Vessel Rerating Requirements & Engineering Calculations 11.6 Pressure Testing: Hydrostatic, Pneumatic & NDE in Lieu of Testing
8.3 Amine, Caustic & Chloride Stress Corrosion Cracking
Key Takeaways Explain This Topic AI
All three mechanisms require a susceptible material, a specific environment, and sustained tensile stress; removing one leg of that combination prevents SCC.
Amine SCC primarily affects carbon steel welds and heat-affected zones in amine service and is strongly associated with residual stress in non-stress-relieved fabrication.
Caustic SCC produces surface-connected cracking in susceptible carbon steel, low-alloy steel, and some stainless services where caustic concentrates.
Chloride SCC commonly produces branched transgranular cracks in 300-series austenitic stainless steels exposed to aqueous chlorides and tensile stress.
PT or MT can find surface-breaking cracks on suitable materials, but angle-beam UT or other qualified volumetric methods are needed to characterize crack depth; selection depends on material and access.
Last updated: August 2026
Amine, Caustic & Chloride Stress Corrosion Cracking
The current API 510 blueprint selects three stress-corrosion mechanisms from API RP 571: amine stress corrosion cracking , caustic stress corrosion cracking , and chloride stress corrosion cracking . Each requires the same three-part framework:
- a susceptible material or microstructure;
- a specific chemical environment; and
sustained tensile stress, either applied or residual.
These are cracking mechanisms, not ordinary uniform corrosion. A component can retain nearly full general wall thickness while a tight crack threatens integrity. Inspection plans must therefore target crack location and orientation, not only average thickness.
1. Amine Stress Corrosion Cracking
Amine SCC is most often associated with carbon steel equipment and piping in aqueous alkanolamine systems used to remove acid gases. Cracking commonly develops at or near welds, attachments, heat-affected zones, and highly stressed cold-worked regions. Residual welding stress is a major susceptibility factor, so cracking is frequently associated with weldments that were not effectively stress relieved.
The environment and operating history matter. Concentration, acid-gas loading, contaminants, temperature, local wetting, and solution chemistry can change susceptibility. Do not convert a risk-based mitigation recommendation into a universal rule that every amine-service weld has exactly the same mandatory heat treatment. The examination task is to recognize the mechanism and apply the effective edition of RP 571 and the applicable construction, repair, and engineering requirements to the actual service.
Typical morphology is surface-connected, oxide-filled cracking near welds and HAZs. Cracks may run parallel to a weld and can be difficult to see after ordinary surface cleaning.
Inspection: Wet fluorescent magnetic particle testing is effective for surface-breaking cracks in ferromagnetic carbon steel when surface condition permits. Angle-beam ultrasonic techniques can help size or detect cracks below the surface. Visual examination and thickness readings alone are not sufficient to rule out SCC.
Mitigation: Effective stress-relief heat treatment, control of solution chemistry and contaminants, appropriate material selection, and reducing stress concentrators can reduce susceptibility. Repairs must address both the crack and its cause; simply grinding or welding over a crack without changing the environment/stress combination invites recurrence.
2. Caustic Stress Corrosion Cracking
Caustic SCC occurs when concentrated sodium or potassium hydroxide contacts a susceptible material under tensile stress. Carbon steel and low-alloy steel are common concerns; some stainless steels and nickel alloys have service-dependent susceptibility ranges. Caustic can concentrate at heat-transfer surfaces, beneath deposits, at leaks, in crevices, or where evaporation occurs, so the local chemistry may be much more severe than a bulk sample suggests.
Cracking in carbon steel is commonly surface connected and may be predominantly intergranular. High-residual-stress locations—weld toes, HAZs, attachments, bends, and cold-worked details—deserve priority. A network of fine cracks can be hidden by deposits or corrosion scale.
Inspection: After suitable cleaning, WFMT is often selected for carbon steel. PT may be used on nonmagnetic alloys with an appropriate surface. Angle-beam UT can characterize deeper cracking when the procedure is qualified for the geometry and expected orientation.
Mitigation: Control caustic concentration and temperature, prevent local concentration beneath deposits or at heat-transfer hot spots, reduce residual stresses where engineering rules call for it, and select a more resistant material when service severity warrants. Because susceptibility depends on material, concentration, temperature, and stress, avoid memorizing one universal temperature boundary.
Caustic corrosion and caustic SCC are separate RP 571 mechanisms. Caustic corrosion is metal loss; caustic SCC is cracking under tensile stress. They can occur in related environments, but their damage morphology and inspection response differ.
3. Chloride Stress Corrosion Cracking
Chloride SCC primarily affects 300-series austenitic stainless steels exposed to aqueous chlorides while under tensile stress. Oxygen or another oxidizing influence, increasing temperature, chloride concentration, evaporation, and wet-dry cycling can increase likelihood. It can occur at temperatures below commonly quoted screening values when conditions are severe, so a single temperature should not be treated as an absolute threshold.
Cracks are classically highly branched and predominantly transgranular . They often initiate at pits, crevices, deposits, insulation breaches, or other locations where chlorides concentrate. Weld residual stress can make HAZs and attachment details vulnerable even without high applied stress.
Inspection: PT can reveal surface-breaking cracks on clean stainless steel. Eddy current may be useful for thin sections or tubing. Angle-beam UT can assist with crack detection and sizing, although complex branching and geometry demand a suitable technique and qualified examiner. Ordinary UT thickness readings do not establish the absence of chloride SCC.
Mitigation: Exclude chlorides and moisture where practical, prevent concentration under deposits or insulation, use compatible insulation and coatings, control wash-water quality, reduce tensile stress, or upgrade to a more resistant alloy based on engineering evaluation.
4. Comparison and Inspection Logic
Mechanism Typical susceptible material Key environment Common morphology High-priority locations
Amine SCC Carbon steel Aqueous amine service Surface-connected cracking near stressed weld regions Welds, HAZs, attachments, cold work
Caustic SCC Carbon/low-alloy steel; service-dependent alloys Concentrated NaOH or KOH Often intergranular in carbon steel Hot spots, deposits, crevices, welds
Chloride SCC 300-series austenitic stainless steel Aqueous chlorides plus tensile stress Branched, usually transgranular Pits, deposits, insulation breaches, HAZs
When an exam scenario presents cracking with little general wall loss, identify the material first, then the environment, crack morphology, and stress source. Select an examination method capable of finding cracks in that material. Finally, choose mitigation that removes or controls at least one of the three necessary factors. This structured approach is more reliable than relying on a single remembered temperature or concentration.