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Chloride Stress Corrosion Cracking of Buried Stainless Steel Pipeline

ENVIRONMENT:

Eastern U.S. soil with high chloride content.

EQUIPMENT:

Buried 28" diameter pipeline
MATERIAL: 304L Stainless steel
OPERATING TEMPERATURE: Nominal 150F with excursions to +200F

SERVICE TIME:

7 Years

FAILURE:

Chloride Stress Corrosion Cracking

 

A buried pipeline carrying carbon dioxide feedstock experienced several failures during its short lifetime. The pipeline was buried four feet deep running parallel to a roadway which is frequently deiced with deicing salts during the winter months. Several weld repairs had already been made on the pipeline due to leaks at flanged joints and distorted sections of the pipeline caused by thermal expansion.

Chloride promoted Stress Corrosion Cracking occurs when three conditions are satisfied.

1) The material of construction must be exposed to tensile stresses. Thermal stresses due to varying operating temperatures, ineffective expansion bellows, or residual stresses from welding have all been found capable of promoting stress corrosion cracking. This pipeline showed evidence of exposure to all three sources of stresses.

2) There must be a sufficient quantity of chloride ions present. The 150 to 260 ppm levels found in the soil beside the pipeline are sufficient.

3) The temperature of the part must be above 120F.

A macro of the exterior surface and a cross-section of the pipe illustrate typical stress corrosion cracking.

 
Figure 1. Piping exterior surface (0.7X Original Magnification) Figure 2. Branched cracking in pipe cross section, characteristic of stress corrosion cracking (50X Original Magnification with Oxalic Etch)

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