Failure Analysis of a Tee Fitting in a Hydrotreating Pump Inlet Line

Literature Overview

This paper by Sima Liangming, published in Petroleum and Chemical Industry Corrosion and Protection (Vol. 29, No. 4, 2012), reports a detailed failure investigation of a stainless steel tee fitting located at the liquefied gas pump inlet of a hydrotreating unit. The study employed macroscopic examination, chemical composition analysis, metallographic analysis, and scanning electron microscopy (SEM) to determine the root cause of the failure. The conclusion identifies a synergistic chloride-ion and hydrogen sulfide stress corrosion cracking mechanism under wet conditions, exacerbated by manufacturing defects including sigma phase precipitation and the absence of post-fabrication solution treatment.

Core Findings and Failure Mechanism

The investigation revealed a multi-factorial failure scenario. The stainless steel tee fitting failed due to stress corrosion cracking (SCC) caused by the combined action of chloride ions and hydrogen sulfide in a wet environment. The critical finding is that the material's corrosion resistance was significantly degraded during manufacturing because of sigma phase (σ-phase) precipitation in the microstructure, which occurred due to non-compliant fabrication practices. Furthermore, the fitting was not subjected to solution annealing after manufacturing, leaving the detrimental intermetallic phase intact and reducing the material's inherent resistance to pitting and stress corrosion.

The degradation pathway can be summarized as follows:

1. Non-compliant manufacturing led to sigma phase precipitation in the austenitic stainless steel matrix.
2. Absence of post-manufacturing solution treatment failed to dissolve the sigma phase.
3. The sigma phase, being chromium-depleted at its boundaries, created preferential sites for pitting initiation.
4. Chloride ions in the service fluid attacked these vulnerable sites, forming pit nuclei.
5. Pits acted as stress concentrators, initiating crack nucleation under operating stresses.
6. Hydrogen sulfide accelerated the stress corrosion cracking rate, and chloride ions further enhanced the H₂S-induced SCC kinetics.
7. Progressive crack growth eventually led to through-wall failure.

Key Technical Parameters and Service Conditions

Engineering Practice Implications

This case study carries profound lessons for engineering practice in the oil and gas sector. First, the dramatic increase in H₂S concentration from below 1 g/L to 7–10 g/L represents a significant change in service conditions that would have been captured by proper process monitoring and materials requalification. Second, the chloride ion concentration reaching 124 mg/L far exceeds typical thresholds for austenitic stainless steels in wet H₂S service, where even 50–100 mg/L can trigger pitting in susceptible microstructures.

From a quality control perspective, this failure highlights the critical importance of enforcing manufacturing process specifications for stainless steel fittings. The absence of solution annealing after fabrication is a fundamental deviation from standards such as ASTM A403, which mandates heat treatment to restore the single-phase austenitic structure and dissolve deleterious carbides or intermetallic phases. The presence of sigma phase—a Cr-rich, Fe-rich intermetallic compound that forms preferentially in the 450–870°C range over prolonged exposure—demonstrates that the fitting likely experienced a thermal history during welding or forming that promoted phase transformation without subsequent corrective annealing.

Defect Analysis and Countermeasures

Study Insights and Reflections

After nearly four decades in metallurgy and welding engineering, I find this case particularly instructive because it demonstrates how a single manufacturing deviation—omitting a solution anneal—can cascade into catastrophic in-service failure. The sigma phase, which I have seen many times in improperly heat-treated austenitic stainless steels, is a silent killer of corrosion resistance. Its formation is often subtle in macroscopic examination but devastating when chloride ions are present.

The synergistic interaction between chloride SCC and H₂S-induced SCC deserves particular attention. While each mechanism individually can be managed with appropriate material selection, their combined action creates a failure environment that is more aggressive than the sum of its parts. The chloride pits serve as crack initiation sites, and the H₂S accelerates crack propagation through hydrogen-assisted mechanisms. This underscores the importance of considering multi-mechanism degradation in design reviews for sour service applications.

From a standards compliance standpoint, this failure would likely have been prevented by adherence to ASTM A403 (which governs butt-welding fittings of wrought austenitic castor stainless steel), which requires solution annealing to achieve the specified microstructure. Additionally, API 5L and NACE MR0175/ISO 15156 provide guidance on material selection for sour service, including hardness limits and corrosion resistance requirements. The engineering lesson is clear: quality control at the manufacturing stage is not merely a regulatory formality but a fundamental safeguard against in-service degradation.