Crack Failure Analysis of Cracker Injection Gas Line Elbows

Literature Overview

The paper by Li Xiwei, published in Petrochemical Corrosion and Protection (2018, Vol. 35, No. 6, pp. 62-64), presents a failure investigation of a cracked elbow in a cracker injection gas line at Daqing Petrochemical Branch of China National Petroleum Corporation. The investigation employed scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and metallographic analysis to determine the root cause of the failure. The findings identified stress corrosion cracking (SCC) at the weld fusion line on the outer wall of the elbow as the primary failure mechanism, with trace chloride ions identified as the environmental factor.

Failure Analysis Methodology

The investigation followed a rigorous analytical protocol appropriate for SCC failure analysis:

Characterization Results

Stress Corrosion Cracking Mechanism

The failure mechanism in this case is a classic example of chloride-induced stress corrosion cracking (Cl-SCC) in a welded carbon steel or low-alloy steel piping system. The three essential elements of the SCC triangle—susceptible material, tensile stress, and corrosive environment—were all present:

1. Susceptible material: The base material and heat-affected zone of the welded elbow were susceptible to Cl-SCC. The weld fusion line is particularly susceptible due to the segregation of impurities and the presence of residual tensile stresses.
2. Tensile stress: Residual welding stresses at the fusion line provided the tensile stress component. Additionally, the operating stresses from internal pressure and thermal cycling contributed to the overall stress state.
3. Corrosive environment: Trace chloride ions present in the injection gas or as contamination in the system provided the aggressive environmental species.

Crack Initiation and Propagation

The SEM analysis revealed that cracks initiated at the weld fusion line on the outer wall of the elbow. This location is not surprising, as the outer wall of an elbow is subject to higher tensile stresses due to the bending geometry, and the weld fusion line is a region of elevated residual stress and potential microstructural susceptibility. The cracks propagated in an intergranular manner, which is characteristic of Cl-SCC, and ultimately led to a through-wall crack that caused the failure.

Engineering Countermeasures

The paper identifies several corrective measures to eliminate the hazard:

1. Chloride control: Implement rigorous control of chloride contamination in the injection gas system, including upstream purification and periodic monitoring of chloride levels.
2. Stress relief: Apply post-weld heat treatment (PWHT) to reduce residual welding stresses below the SCC threshold.
3. Material upgrade: Consider replacing the carbon steel elbow with a material that is resistant to Cl-SCC, such as a 300-series stainless steel (304L, 316L) or a duplex stainless steel (2205).
4. Cathodic protection: If the system is susceptible to external corrosion, consider cathodic protection to mitigate external SCC risk.
5. Inspection program: Implement a comprehensive inspection program including periodic dye penetrant testing (PT) or magnetic particle testing (MT) of welds, and UT or PAUT for crack detection.

Study Insights and Reflections

This case is a textbook example of Cl-SCC in a process piping system, and it reinforces several fundamental principles that engineers must understand:

• The weld fusion line is a critical location for SCC initiation: The combination of residual stress, microstructural susceptibility, and potential impurity segregation at the fusion line makes this location highly vulnerable to SCC. Engineers must recognize that even a properly fabricated weld can be susceptible to SCC if the environmental conditions are aggressive enough.
• Trace chloride levels can be sufficient to cause SCC: The EDS analysis revealed only trace levels of Cl-, which underscores the fact that SCC can occur at surprisingly low chloride concentrations. The threshold for Cl-SCC in carbon steel is typically in the range of 10-100 ppm, and in some cases even lower. Engineers must not assume that "low" chloride levels are safe.
• The outer wall of elbows is a high-risk location: The combination of geometric stress concentration and the weld fusion line makes the outer wall of elbows a particularly vulnerable location for SCC. This location should be prioritized in inspection programs for systems that are susceptible to Cl-SCC.

This case also highlights the importance of a systematic approach to failure analysis. The combination of SEM, EDS, and metallographic analysis provided a comprehensive picture of the failure mechanism that would not have been achievable with any single technique. This multi-technique approach should be the standard practice for all significant piping failure investigations.