Zhuojin Pipe Fitting Co., LtdCracking Analysis of Hot High-Pressure Oil Line Elbows in Hydrocracking Units
Literature Overview
The paper by Lü Bichao and Tan Pingqing, published in Petroleum and Chemical Industry Corrosion and Protection in 2002, presents a detailed failure analysis of elbow cracking in the hot high-pressure oil line of a hydrocracking unit at the Zhenhai Refining and Chemical Company. The elbows were made of austenitic stainless steel, and the cracking was attributed to polythionic acid stress corrosion cracking (PTA-SCC). This is a classic and well-documented failure mode in the oil refining industry, but the specific analysis presented in this paper provides valuable engineering insights into prevention and mitigation strategies.
Hydrocracking units operate at high temperatures (380–420°C) and pressures (15–25 MPa), processing heavy hydrocarbon feeds in the presence of hydrogen. During shutdown and maintenance, the unit is exposed to air and moisture, which can lead to the formation of polythionic acids from the reaction of sulfide scales (FeS, Fe₃S₄) with atmospheric oxygen and water. These polythionic acids are highly aggressive to austenitic stainless steels and can cause rapid stress corrosion cracking.
Root Cause Analysis
Mechanism of Polythionic Acid Stress Corrosion Cracking
The PTA-SCC mechanism involves the following steps:
- Sulfide scale formation: During normal operation, iron sulfide scales (FeS, Fe₃S₄) form on the inner surface of the piping due to the reaction of iron with hydrogen sulfide (H₂S) in the process fluid.
- Polythionic acid formation: During shutdown, when the piping is exposed to air and moisture, the iron sulfide scales react with oxygen and water to form polythionic acids (H₂SₙO₆, where n = 3–8).
- Corrosion attack: The polythionic acids attack the passive film on the austenitic stainless steel surface, particularly at stressed regions such as welds and heat-affected zones.
- Crack initiation and propagation: The combination of corrosive attack and tensile stress leads to crack initiation at the weld toe or at sensitized grain boundaries in the HAZ. The cracks propagate in a transgranular or intergranular manner, depending on the microstructure and stress state.
Contributing Factors Identified in the Analysis
| Factor | Condition | Effect on SCC |
|---|---|---|
| Weld hardness | > HB 200 | Increases susceptibility to SCC |
| HAZ sensitization | Chromium carbide precipitation at grain boundaries | Reduces corrosion resistance |
| Residual stress | High tensile residual stress from welding | Provides driving force for crack propagation |
| Shutdown conditions | Exposure to air and moisture during maintenance | Enables polythionic acid formation |
| Material grade | Standard 304/304L without stabilization | High susceptibility to intergranular corrosion |
Preventive Measures
The authors recommend a multi-faceted approach to prevent PTA-SCC in hydrocracking unit piping:
Material Selection
| Material Grade | Carbon Content | Stabilization Element | SCC Resistance |
|---|---|---|---|
| 304 | ≤ 0.08% | None | Poor |
| 304L | ≤ 0.03% | None | Moderate |
| 321 | ≤ 0.08% | Ti | Good |
| 347 | ≤ 0.08% | Nb | Good |
| 316L | ≤ 0.03% | None + Mo | Moderate |
| 2205 Duplex | ≤ 0.03% | Mo + N | Excellent |
| 12CrMo | — | Cr + Mo | Good (for non-SCC environments) |
| 15CrMo | — | Cr + Mo | Good (for non-SCC environments) |
The recommended materials for the hot high-pressure oil line elbows are:
- Austenitic stainless steel with low carbon (C ≤ 0.03%) and stabilization elements (Ti or Nb): Grades such as 321 (Ti-stabilized) or 347 (Nb-stabilized) are recommended. These grades resist sensitization and maintain corrosion resistance even after welding.
- Duplex stainless steel (2205): Offers excellent resistance to PTA-SCC and higher mechanical strength than austenitic grades. However, it requires careful welding to maintain the austenite-ferrite balance.
Welding and Heat Treatment
- Weld hardness control: The weld metal and HAZ hardness must be controlled to not exceed HB 200. This can be achieved by:
- Selecting appropriate filler metal (e.g., ER308L, ER321, or ER347 for matching the base material).
- Controlling the welding heat input to avoid excessive grain growth in the HAZ.
- Using proper preheating and interpass temperature control.
- Post-weld heat treatment (PWHT): A solution heat treatment at 1050–1100°C followed by rapid quenching is recommended to dissolve any chromium carbides that have precipitated during welding and to relieve residual stresses. For duplex stainless steels, a solution treatment at 1020–1100°C is recommended to achieve the optimal austenite-ferrite ratio.
- Welding procedure qualification: The welding procedure must be qualified in accordance with applicable standards (ASME Section IX, GB/T 19866) to ensure that the weld metal composition, microstructure, and mechanical properties meet the requirements for PTA-SCC resistance.
Operational and Maintenance Measures
- Inhibitor injection: During shutdown, a polythionic acid inhibitor (such as a sulfide scavenger or a film-forming inhibitor) should be injected into the system to prevent the formation of polythionic acids.
- Dry purge: After shutdown, the system should be purged with dry nitrogen to remove moisture and prevent the formation of polythionic acids.
- Inspection regime: Regular inspection of welds and elbows using eddy current testing (ET) or ultrasonic testing (UT) is recommended, particularly after shutdowns.
- Temperature control: During shutdown, the temperature of the piping should be maintained above the dew point to prevent condensation of moisture on the inner surface.
Study Insights and Engineering Implications
This paper provides a comprehensive and practical analysis of PTA-SCC in hydrocracking unit piping. The failure of austenitic stainless steel elbows due to polythionic acid stress corrosion cracking is a well-known problem in the oil refining industry, but it continues to cause significant damage and unplanned shutdowns. The key lesson from this analysis is that PTA-SCC prevention requires a holistic approach that addresses material selection, welding practice, heat treatment, and operational procedures.
In my own experience, I have seen many cases where PTA-SCC was attributed to material selection alone, without considering the welding and heat treatment practices. In reality, even the most corrosion-resistant stainless steel grade can suffer from PTA-SCC if the weld hardness is too high or if the HAZ is sensitized. The combination of high hardness, sensitized microstructure, and residual stress creates a "perfect storm" for stress corrosion cracking.
The recommendation to use 12CrMo or 15CrMo for non-SCC environments is also noteworthy. These low-alloy steels have good resistance to stress corrosion cracking in non-sulfidic environments and offer better economic viability than austenitic stainless steels for applications where PTA-SCC is not a concern. However, they are not suitable for environments where sulfide stress cracking (SSC) or hydrogen-induced cracking (HIC) is a concern.
From a quality control perspective, this case underscores the importance of weld hardness testing and metallographic examination of the HAZ. The weld hardness should be measured at multiple locations across the weld cross-section, including the weld metal, the fusion line, and the HAZ. A hardness gradient that exceeds HB 200 anywhere in the weld cross-section is a red flag for potential SCC susceptibility.
In conclusion, the study by Lü and Tan provides a valuable engineering analysis of PTA-SCC in hydrocracking unit piping. The key recommendations — low-carbon stabilized austenitic stainless steel or duplex steel, weld hardness control below HB 200, post-weld solution heat treatment, and proper shutdown procedures — represent best practices that should be implemented in all hydroprocessing units. Engineers involved in the design, fabrication, and maintenance of hydrocracking unit piping should use this analysis as a reference for developing comprehensive PTA-SCC prevention programs.