Failure Analysis of LMPH Module Inlet Elbow in Ethylene Cracker Convection Section

Literature Overview

The paper by Chen Binghua, published in Petrochemical Corrosion and Protection in 2023 (Vol. 40, No. 5, pp. 59-64), presents a detailed failure analysis of a 90-degree elbow at the inlet of the LMPH (Low Molecular Weight Polyolefin Hydrogen) module in the convection section of an ethylene cracker furnace at a Sinopec Shanghai Petrochemical facility. The elbow experienced corrosion cracking during operation, disrupting normal plant operations. The investigation employed macroscopic examination, chemical composition analysis, corrosion product analysis, and scanning electron microscopy to identify the root cause. The findings indicate that the failure was caused by sulfide stress corrosion cracking (SSCC) and chloride stress corrosion cracking (ClSCC), and the paper proposes material upgrade and welding process improvement as corrective measures.

Failure Investigation Methodology

Sample Preparation and Macroscopic Examination

The failure investigation followed a systematic approach consistent with ASTM E1911 and GB/T 21318 standards for corrosion failure analysis. The initial macroscopic examination revealed the following observations:

• The crack originated at the outer bend of the elbow, which is the region of maximum tensile stress due to the curvature geometry.
• The crack propagated in a transverse direction relative to the elbow axis, perpendicular to the primary tensile stress direction.
• The crack length extended approximately 60 percent of the elbow circumference, with a maximum depth reaching 80 percent of the wall thickness.
• The crack surface exhibited a characteristic intergranular morphology, indicative of stress corrosion cracking rather than mechanical fatigue or brittle fracture.

The location of crack initiation at the outer bend is significant because this is the region where the residual tensile stress from bending during fabrication is highest. In a 90-degree elbow formed by cold bending, the outer fibers experience tensile strain while the inner fibers experience compressive strain. This residual stress field, if not fully relieved by post-weld heat treatment, provides the driving force for stress corrosion cracking when exposed to a corrosive environment.

Chemical Composition Analysis

The chemical composition analysis revealed the base metal composition of the elbow material:

The base metal composition was within the specification limits for the designated material grade, which appears to be a 304 or 321 stainless steel based on the chromium and nickel content. However, the chemical analysis also revealed elevated sulfur content in the weld metal and heat-affected zone, suggesting that sulfur-bearing species were introduced during the welding process or accumulated from the process environment.

Corrosion Product Analysis and SEM Observation

The corrosion product analysis identified the presence of iron sulfide (FeS) and chromium sulfide (CrS) phases, confirming the presence of sulfide ions in the corrosive environment. The scanning electron microscopy examination revealed:

• Intergranular cracking with a network pattern following the grain boundaries.
• Secondary cracking perpendicular to the primary crack direction, characteristic of ClSCC.
• Sulfide inclusions at crack initiation sites, serving as preferential nucleation sites for SCC.
• A thin corrosion product film along the crack walls, indicating prolonged exposure to the corrosive environment.

The combination of intergranular cracking morphology, sulfide corrosion products, and the presence of chloride ions in the corrosion product layer led to the conclusion that both SSCC and ClSCC mechanisms were active simultaneously. This dual mechanism is particularly dangerous because the two types of cracking can interact synergistically, accelerating crack propagation beyond what either mechanism would cause independently.

Root Cause Analysis Using 5W2H Framework

Applying the 5W2H framework to systematically analyze the root cause:

• What: The elbow experienced intergranular stress corrosion cracking leading to loss of containment.
• Why: The combination of residual tensile stress from cold bending and welding, exposure to sulfide and chloride ions in the process environment, and insufficient material resistance to SCC.
• Where: The crack initiated at the outer bend of the elbow, in the region of maximum tensile residual stress.
• When: The failure occurred after a period of continuous operation, suggesting a time-dependent cracking mechanism rather than an instantaneous overload.
• Who: The failure affected the LMPH module, which is a critical component in the ethylene production process.
• How: The cracking mechanism involved the interaction of tensile stress with a corrosive environment containing sulfide and chloride ions.
• How much: The crack extended to 80 percent of wall thickness, representing a near-catastrophic failure that required immediate shutdown.

Corrective Measures and Engineering Recommendations

The paper proposes several corrective measures, which I will evaluate and expand upon from an engineering perspective:

Material Upgrade

The primary recommendation is to upgrade the material grade to improve resistance to both SSCC and ClSCC. The options include:

For the convection section of an ethylene cracker furnace, where temperatures can reach 600-700°C and the process environment contains both sulfide and chloride species, I recommend considering Alloy 825 or Alloy 625 for the elbow, or at minimum a 321H material with a controlled sulfur content below 0.005 percent. The cost premium for higher alloy materials must be balanced against the cost of unplanned shutdowns, which can exceed $1 million per day for a large ethylene plant.

Welding Process Improvement

The paper also recommends improving the welding process. Key improvements include:

1. Pre-welding heat treatment to relieve residual stresses from cold bending. The recommended procedure is solution annealing at 1050-1100°C followed by water quenching, or a stress relief treatment at 450-550°C for a minimum of 2 hours per inch of wall thickness.
2. Use of low-sulfur filler metals with sulfur content below 0.01 percent to minimize the introduction of sulfide inclusions in the weld metal.
3. Application of controlled welding parameters to minimize the heat-affected zone width and reduce the volume of material susceptible to sensitization. For 300-series stainless steels, the interpass temperature should be maintained below 150°C to prevent chromium carbide precipitation at grain boundaries.
4. Post-weld solution treatment to restore full corrosion resistance after welding. This is particularly important for the heat-affected zone, which may have undergone sensitization during the welding thermal cycle.
5. Consideration of weld overlay with a more corrosion-resistant alloy to provide a protective barrier at the weld joint, which is often the most vulnerable location for SCC initiation.

Additional Recommendations

Beyond the measures proposed in the paper, I recommend the following additional actions:

• Implement a periodic inspection program using ultrasonic thickness measurement and eddy current testing to monitor wall thickness and detect early-stage cracking. The inspection frequency should be based on RBI (Risk-Based Inspection) principles, with higher frequency for elbows in high-stress regions.
• Conduct a process chemistry review to identify sources of sulfide and chloride ions in the process environment. If possible, implement measures to reduce the concentration of these species, such as improved feedstock pretreatment or modified process conditions.
• Perform a metallurgical examination of the elbow manufacturing process to verify that the cold bending process did not introduce excessive strain hardening or microstructural changes that would reduce SCC resistance. The strain level at the outer bend should be kept below 10 percent to minimize the risk of strain-induced sensitization.
• Establish a monitoring program for hydrogen sulfide concentration in the furnace atmosphere. Elevated H2S levels, even at concentrations as low as 10 ppm, can initiate SCC in susceptible materials at elevated temperatures.

Summary and Lessons Learned

The failure analysis presented in this paper provides valuable lessons for the design, fabrication, and operation of stainless steel elbows in ethylene cracker furnaces. The root cause of the failure was a combination of material susceptibility, residual stress, and environmental exposure, which is a classic triad for stress corrosion cracking. The corrective measures proposed in the paper, particularly material upgrade and welding process improvement, are sound and should be implemented. However, I emphasize that a holistic approach is required, encompassing material selection, fabrication quality, process chemistry control, and inspection planning. The cost of preventing a failure through proper design and maintenance is always less than the cost of an unplanned shutdown, and this case study serves as a compelling reminder of that principle. Engineers should treat this analysis as a reference for similar applications and adapt the corrective measures to their specific operating conditions.