Analysis of Leakage Causes in Ethylene Feed Line Pipe Elbows

Literature Overview

This paper, published in Physical Testing and Chemical Analysis (Physics Part) in 2020, presents a detailed failure investigation of feed line pipe elbows in an ethylene plant at Dushanzi Petrochemical Company. The elbows experienced multiple leakage incidents during operation, prompting a systematic root-cause analysis involving macroscopic examination, chemical composition analysis, hardness testing, metallographic inspection, scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), and sulfur content analysis of the naphtha feedstock. The authors conclude that high-temperature sulfide corrosion, combined with mechanical erosion from flowing materials, was the primary failure mechanism.

Core Technical Findings

The investigation revealed a synergistic failure mechanism rather than a single-mode degradation. The key findings are summarized below:

The authors identify a self-accelerating cycle: high-temperature sulfide corrosion produces a corrosion product film; material flow mechanically strips this film; the removal of the protective film exposes fresh metal to aggressive sulfide species; renewed corrosion then forms a new film that is again stripped by flow. This erosion-corrosion interaction concentrates at the back-bend region of the elbow, where the fluid impingement is most severe due to the geometry-induced flow pattern.

Technical Interpretation of the Failure Mechanism

From a metallurgical perspective, this case is a textbook example of sulfide stress corrosion cracking (SSCC) transitioning into sulfide stress corrosion and erosion-corrosion in carbon and low-alloy steel piping. At elevated temperatures, the Fe-H₂S reaction kinetics accelerate significantly. The critical threshold for sulfide corrosion in carbon steel is generally accepted as approximately 60 °C in the presence of wet H₂S, with corrosion rates increasing exponentially above this temperature. The back-bend region of an elbow is inherently the most vulnerable location because centrifugal forces drive the denser fluid phase (and any entrained liquid or corrosion products) toward the outer bend wall, creating a zone of high shear stress on the pipe wall.

The hardness softening observed near the corrosion site is particularly concerning. In carbon and low-alloy steels such as ASTM A106 Gr. B or A53, the normal hardness range is 120-200 HV. Localized softening below 150 HV indicates either prior overheating during fabrication or microstructural coarsening caused by prolonged thermal exposure, both of which reduce resistance to sulfide attack. The NACE MR0175/ISO 15156 hardness limit of 22 HRC (approximately 250 HV) is a critical control parameter for sour service components, and the observed softening suggests that the material may have undergone a low-temperature service exposure that promoted grain boundary attack.

Engineering Practice Implications

For engineers responsible for piping integrity management in ethylene and petrochemical facilities, this case study offers several actionable lessons. First, material selection for elbows in sour-service feed lines must account for the combined effect of temperature, H₂S partial pressure, and flow velocity. Carbon steel elbows in hot naphtha service with elevated sulfur content should be replaced with corrosion-resistant alloy (CRA) materials such as duplex stainless steel (UNS S31803/S32205) or nickel-based alloys (UNS N08825/Alloy C-276) where the economics justify the upgrade. Second, the inspection interval for elbows in known sour-service locations should be reduced, and ultrasonic thickness mapping should be performed at the back-bend region specifically, not merely at the centerline. Third, the sulfur content of the feed naphtha must be monitored continuously, and alarm thresholds should be established based on the NACE MR0175/ISO 15156 material selection table.

Key Questions and Reflections

The paper raises an important question that deserves further investigation: whether the initial fabrication quality of the elbows contributed to the accelerated failure. If the elbows were produced by hot bending without proper heat treatment, residual stresses and localized microstructural changes at the bend apex could have created preferential initiation sites for sulfide attack. The authors do not address the manufacturing history of the elbows in detail, which represents a gap in the analysis. Additionally, the role of the corrosion product film as a potential barrier to further corrosion is discussed only qualitatively; a quantitative assessment of film stability under varying flow conditions would strengthen the failure model considerably.

Study Insights

This failure analysis underscores a fundamental principle in piping integrity management: corrosion in process piping is rarely a single-mode phenomenon. The interaction between chemical attack and mechanical erosion creates failure modes that are significantly more aggressive than either mechanism alone. Engineers must adopt a multi-parameter approach to risk assessment, considering not only material grade and environmental conditions but also flow geometry, velocity, and the mechanical properties of the corrosion product layer. The back-bend region of elbows should be treated as a critical inspection zone in any piping integrity program, particularly in sour-service applications.