Failure Mechanisms and Inspection Strategies for Carbon Steel and Stainless Steel Pressure Vessels under High Temperature, Pressure, and Insulation

Failure Mechanisms in Pressure Vessels

Pressure vessel inspection is a critical part of mechanical integrity programs across refining, petrochemical, and power industries. Vessels operating at high temperature and pressure while insulated face some of the most aggressive degradation mechanisms encountered in service. Carbon steel and stainless steel—the two most common materials of construction—each exhibit characteristic damage modes under these conditions. This article reviews the dominant failure mechanisms affecting both materials, with focus on corrosion under insulation (CUI), high-temperature hydrogen attack (HTHA), sulfidation, chloride stress corrosion cracking (Cl-SCC), sensitization, creep, and thermal fatigue. It further discusses best-practice inspection and risk-based approaches per API 510 and API RP 571, and explains how acoustic emission (AE) testing and fitness-for-service evaluations per API 579 can be applied to detect, monitor, and mitigate these damage mechanisms.

1. Introduction

Effective pressure vessel inspection ensures safe, reliable, and compliant operation of critical process equipment. The API 510 Pressure Vessel Inspection Code provides the industry standard framework for in-service inspection, repair, alteration, and rerating of pressure vessels, while API Recommended Practice 571 defines common damage mechanisms and the associated inspection methods.

When pressure vessels are insulated to maintain process efficiency or protect personnel, the insulation system often traps moisture and contaminants. Combined with elevated temperature and pressure, these conditions accelerate corrosion and cracking, particularly when insulation integrity deteriorates. Understanding how different materials—particularly carbon steel and stainless steel—degrade under these service conditions is essential for implementing effective inspection, risk assessment, and maintenance programs.

Figure 1: Acoustic emission test setup for carbon and stainless steel pressure vessels.

2. Failure Mechanisms in Carbon Steel

2.1 Corrosion Under Insulation (CUI)

CUI is one of the most pervasive failure mechanisms in carbon steel pressure vessels. Water ingress beneath insulation introduces oxygen and salts that drive corrosion at temperatures between 25–120 °C, with attack possible up to 175 °C. Damage is typically concentrated at supports, nozzles, and low points where water accumulates.

2.2 High-Temperature Sulfidation and Oxidation

In refinery and hydrocarbon service, sulfur species and oxygen at temperatures above 260 °C react with steel to form iron sulfides and oxides, causing rapid wall thinning. Alloy upgrades or protective coatings are recommended mitigation measures.

2.3 High-Temperature Hydrogen Attack (HTHA)

HTHA occurs when hydrogen at elevated pressure and temperature (> 205 °C) diffuses into the steel, reacting with carbides to form methane. The resulting micro-fissures and decarburization cause severe embrittlement. Assessment and control should follow API 941 – Steels for Hydrogen Service.

2.4 Creep and Stress Rupture

Long-term exposure above 400 °C leads to time-dependent deformation (creep). Creep cracking is often found near welds and stress concentrators. Creep life evaluation forms a key part of API 579-1/ASME FFS-1 fitness-for-service assessments.

2.5 Thermal Fatigue

Cyclic thermal stresses from startup and shutdown produce fatigue cracking, typically at weld toes and nozzle intersections.

Figure 2: Stress corrosion cracking in stainless steel pressure vessel.

3. Failure Mechanisms in Stainless Steel

3.1 Chloride Stress Corrosion Cracking (Cl-SCC)

Cl-SCC is the most critical damage mechanism for insulated stainless steel pressure vessels. Chlorides from insulation materials or the environment, combined with tensile stress and temperatures between 60–200 °C, promote cracking that often goes undetected until leakage occurs.

3.2 Localized Corrosion Under Insulation

Even in the absence of cracking, stainless steels may experience deep pitting or crevice corrosion under chloride-laden insulation. These localized attacks can rapidly compromise wall integrity.

3.3 Sensitization and Intergranular Corrosion

Exposure to 500–800 °C can cause chromium carbide precipitation at grain boundaries, leading to sensitization and intergranular attack. Low-carbon or stabilized grades (L, Ti, Nb) mitigate this risk.

3.4 Sigma-Phase Embrittlement

Long-term exposure between 565–980 °C promotes sigma-phase formation, reducing toughness and ductility, especially in duplex grades.

3.5 Creep and Thermal Fatigue

At sustained service above 540 °C, stainless steels can experience creep, while cyclic operations continue to promote fatigue cracking at welds and geometric transitions.

4. Shared Degradation Challenges

Both carbon and stainless steels are susceptible to thermal fatigue and corrosion under insulation. Because insulation conceals external surfaces, degradation may remain hidden until significant wall loss occurs. This highlights the importance of on-stream inspection (OSI) and risk-based inspection (RBI) planning in accordance with API 510.

5. Inspection and Mitigation Strategies

Carbon Steel:

• Perform ultrasonic thickness mapping and corrosion rate trending.

• Remove insulation selectively for visual external inspection.

• Use guided-wave ultrasonics (GWUT) or profile radiography for inaccessible areas.

• Conduct HTHA assessments per API 941 and evaluate life reduction via API 579 Level 2 FFS.

  
  

Stainless Steel:

• Apply penetrant testing (PT) and radiography (RT) for Cl-SCC detection.

• Use eddy current or phased-array ultrasonic testing (PAUT) to identify pitting and wall loss.

• Inspect insulation supports and terminations where water ingress and chloride buildup are common.

  
  

General Recommendations:

• Implement risk-based inspection (RBI) to determine intervals based on damage likelihood and consequence.

• Perform fitness-for-service (FFS) evaluations per API 579-1/ASME FFS-1 when wall loss or cracking exceeds code limits.

• Maintain insulation systems to prevent moisture ingress, and avoid chloride-bearing insulation on stainless equipment.

6. Acoustic Emission Testing in Pressure Vessel Inspection

Acoustic emission (AE) testing is an advanced, on-stream inspection method used to detect active damage in operating pressure vessels. AE is a passive nondestructive testing (NDT) technique that listens for high-frequency stress waves (20 kHz–1 MHz) generated by the material during crack propagation, hydrogen blistering, corrosion pitting, or leak development. Piezoelectric sensors mounted on the vessel surface detect these signals, which are then analyzed to locate and evaluate active damage zones.

AE testing is particularly valuable for API 510 inspection programs because it can be performed while the vessel is in service or during controlled pressurization, eliminating the need for internal access or insulation removal. It is most effective for detecting HTHA, stress corrosion cracking, and incipient leaks. AE provides real-time, whole-vessel coverage and early detection of flaws at initiation, long before they become critical.

However, AE testing does not measure wall thickness or flaw dimensions, and background process noise must be filtered carefully. It is therefore used primarily as a screening tool to identify suspect areas for follow-up examination using ultrasonic, radiographic, or phased-array methods. When integrated with API 579 fitness-for-service evaluations, AE testing significantly enhances a facility’s capability to monitor vessel integrity under live operating conditions.

For high-risk equipment in hydrogen, high-pressure, or cyclic service, AE serves as a predictive maintenance tool within risk-based inspection programs, providing early warnings of active degradation and supporting proactive repair planning.

7. Conclusion

Pressure vessels operating at high temperature and pressure under insulation are subject to multiple interacting damage mechanisms. For carbon steel, the major threats are CUI, sulfidation, HTHA, creep, and thermal fatigue; for stainless steel, Cl-SCC, pitting, sensitization, sigma-phase embrittlement, and creep dominate. Both materials are prone to thermal fatigue and hidden corrosion.

By integrating acoustic emission testing, risk-based inspection (RBI), and fitness-for-service (API 579) evaluations into the API 510 inspection program, operators can detect early signs of degradation, extend service life, and reduce unplanned shutdowns. A holistic approach that combines material knowledge, inspection technology, and data-driven risk assessment represents the most effective strategy for ensuring long-term reliability of pressure vessels.

References

1. American Petroleum Institute (API). API 510: Pressure Vessel Inspection Code – In-Service Inspection, Rating, Repair, and Alteration. 11th Edition, 2022.

2. American Petroleum Institute (API). API RP 571: Damage Mechanisms Affecting Fixed Equipment in the Refining Industry. 3rd Edition, 2020.

3. American Petroleum Institute (API). API 579-1 / ASME FFS-1: Fitness-for-Service. 2021.

4. American Petroleum Institute (API). API 941: Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants. 8th Edition, 2016.

5. NACE International. Corrosion Under Insulation and Fireproofing (CUI), SP0198-2017.

6. T. T. Allen, “Application of Acoustic Emission Testing in Pressure Vessel Integrity Assessment,” Journal of Pressure Vessel Technology, ASME, 2021.

7. TÜV SÜD, “On-Stream Inspection and Acoustic Emission Testing of Pressure Equipment,” Technical White Paper, 2020.

8. Inspectioneering Journal. “Corrosion Under Insulation and Risk-Based Inspection Planning.” Vol. 27, No. 4, 2022.