Cracking Analysis of Desalination Water Stainless Steel Pipes in Olefin Units

Literature Overview

This case study investigates the cracking failure of stainless steel pipes carrying desalination water in an olefin production unit. Desalination water systems in petrochemical complexes operate under conditions that can promote stress corrosion cracking (SCC), chloride-induced pitting, and crevice corrosion. The failure occurred after approximately 18 months of service, with no prior indication from routine inspection. The analysis employs a systematic approach combining visual examination, metallographic analysis, scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and residual stress measurement to identify the root cause and develop preventive measures.

Service Conditions and Failure Description

Operating Parameters

Failure Mode Description

The cracking was first detected during a scheduled shutdown when a small leak was observed at a weld joint in the desalination water supply line. Upon inspection, multiple cracks were found in the heat-affected zone (HAZ) of several butt-welded joints, with crack lengths ranging from 2-15 mm and depths of 0.3-1.2 mm. The cracks propagated primarily in the transverse direction relative to the weld axis, with some branching patterns observed near the crack tips.

Root Cause Analysis

Systematic Investigation Approach

The investigation followed a structured methodology:

1. Visual and macroscopic examination: Identified crack location, orientation, and extent.
2. Metallographic analysis: Prepared cross-sections of cracked regions for optical microscopy examination.
3. Fracture surface analysis: SEM examination of crack surfaces to identify fracture mode.
4. Chemical composition verification: Confirmed material grade and composition.
5. Residual stress measurement: X-ray diffraction method to quantify weld residual stresses.
6. Environmental analysis: Characterized fluid chemistry at time of failure.

Key Findings

Finding 1: Stress Corrosion Cracking (SCC)

The fracture surface examination revealed characteristic intergranular cracking with branching patterns, consistent with chloride-induced stress corrosion cracking (Cl-SCC). The presence of chloride ions above 100 ppm in the desalination water, combined with tensile residual stresses in the weld HAZ and the elevated temperature range (particularly during summer peak temperatures exceeding 70°C), created the necessary conditions for Cl-SCC initiation and propagation.

Finding 2: Inadequate Weld Heat Input Control

Metallographic examination of the HAZ revealed a wide sensitization zone (2-3 mm) with extensive chromium carbide precipitation along grain boundaries. The weld was performed with excessive heat input (estimated 2.5-3.0 kJ/mm), resulting in prolonged residence time in the sensitization temperature range (450-850°C). This created a chromium-depleted zone susceptible to intergranular corrosion.

Finding 3: Residual Stress Contribution

X-ray diffraction measurements confirmed high tensile residual stresses in the HAZ region, with values of 280-350 MPa in the transverse direction. These stresses exceeded the threshold stress for Cl-SCC initiation in sensitized 304 stainless steel (typically 100-150 MPa at 60°C with 300 ppm Cl-).

Root Cause Summary

The cracking was caused by the combined effect of:

• Chloride concentration in service fluid exceeding the critical threshold for 304 stainless steel
• Weld sensitization due to excessive heat input creating chromium-depleted grain boundaries
• High tensile residual stresses in the weld HAZ providing the driving force for crack propagation
• Elevated operating temperatures accelerating the SCC kinetics

Countermeasures and Recommendations

Material Selection

Welding Process Optimization

For 304 or 316L stainless steel piping in chloride-containing service:

1. Heat input control: Limit heat input to 1.0-1.5 kJ/mm for wall thickness ≤ 3 mm, using GTAW (TIG) with pulsed current.
2. Interpass temperature: Maintain below 150°C between passes to prevent sensitization.
3. Fill metal selection: Use low-carbon (L-grade) or stabilized (321/347) fill metals to minimize sensitization.
4. Post-weld treatment: Implement solution heat treatment at 1050°C followed by water quench for critical applications, or pickling and passivation for less critical service.

Operational Measures

• Maintain chloride concentration below 100 ppm through enhanced water treatment
• Implement continuous monitoring of water quality parameters (Cl-, pH, dissolved oxygen)
• Establish a periodic inspection program using eddy current testing for weld integrity
• Install corrosion coupons and electrochemical monitoring probes
• Control operating temperature below 50°C where possible

Study Insights and Implications

This case study illustrates a fundamental principle in stainless steel engineering: material selection must be based on the worst-case service environment, not the design average. The desalination water system was designed with 304 stainless steel based on assumed chloride levels below 200 ppm, but operational variations allowed chloride concentrations to exceed 300 ppm during certain periods. Combined with inadequate weld sensitization control, this created a failure scenario that was entirely preventable through proper engineering judgment.

The investigation methodology—combining metallurgical analysis, fracture mechanics, and environmental characterization—demonstrates the systematic approach required for reliable root cause analysis. For practitioners, the key takeaway is that every stainless steel welding joint in chloride-containing service should be treated as a potential Cl-SCC risk point requiring careful heat input control, appropriate material selection, and ongoing monitoring.