Pipeline External Stress Corrosion Cracking:

Oil and gas buried pipelines are externally protected by a combination of coatings/wrappings and cathodic protection. When the coating or wrapping fails, water can accumulate between the coating/wrapping and the pipeline steel surface. The composition of such accumulated water can result in external stress corrosion cracking.

An oil transportation pipeline observed with oil leaking. Visual inspections revealed macrocracks and shallow pits on the external surface of the corroded area (Figure 1). Optical observation indicated that cracks were started from the bottom of the pits and extended into the wall thickness as shown in Figure 2. There was a main singular crack and some branched cracks at the end. The crack features are characteristics of a stress corrosion cracking.

Figure 1: Macroscopic Observation of the cracking Area:

Figure 2: (a) Initiating of cracks from pitting on the surface of the carbon steel pipe and its branched development (unetched), (b) intergranular crack growth in the microstructure (etched)

Description of Damage Mechanisms:

The external pipeline corrosion damage mechanism is due to 1. High pH SCC and 2. Near neutral pH SCC.

High-pH SCC:

In high-pH SCC case, the external cracks more often initiate and progress intergranular. Fig 2 (b).

A concentrated carbonate-bicarbonate (CO3-HCO3) solution has been identified as the most probable environment responsible for high pH SCC. Such an environment can develop as a result of the interaction between the hydroxyl ions produced by the cathode reaction and carbon dioxide (CO2) in the soil. The pH of this electrolyte depends on the relative concentration of carbonate and bicarbonate, and the cracking range is between pH 8 and 11. The presence of multiple black thumbnail-like flaws on the fracture surface normally is an indicator that SCC caused the failure. Analysis of liquid trapped in the dis-bonded area or in the crack itself from several pipeline failures revealed a carbonate-bicarbonate solution with a pH of 8 to 11.

The reason for the cracks remaining at the grain boundaries is related to the chemical heterogeneity at the grain boundaries. The grain boundaries are preferentially attacked or etched by exposure to the carbonate-bicarbonate solution at potentials in the intergranular cracking range.

Near-neutral pH SCC:

In low-pH SCC, cracks extend trans granular. Fig 2(a)

In the case of near-neutral pH SCC, the exact mechanism is not clear. The fact that the cracks contain corrosion products suggests that dissolution occurs within the cracks, but this does not appear adequate to account for the observed crack growth rates. Hydrogen evolution occurs during SCC of pipeline steels in dilute aqueous solutions. The evolved hydrogen diffuses into the steel around the crack tip. Lower pH and lower electrochemical potentials facilitate evolution and enrichment of hydrogen. These increase the SCC growth rate.

In near-neutral pH solutions, hydrogen enhances anodic dissolution at or near the free corrosion potential. However, at lower pH values or below a certain applied cathodic potential, the cracking process is dominated by hydrogen-assisted cracking.

Affected Materials:

All ferritic pipeline steels are susceptible to external high-pH and near-neutral pH stress corrosion cracking (SCC)

Affected Units of Equipment:

All buried pipelines with an external coating/wrapping under cathodic protection are susceptible.

Appearance or Morphology of Damage:

High-pH SCC:

Cracking is primarily intergranular (between the steel grains), forming narrow, tight cracks with almost no evidence of secondary corrosion of the crack wall. Coalescence of cracks is essential for crack growth.

Near-Neutral pH SCC:

Near-neutral pH SCC is primarily transgranular (across the steel grains), developing wide cracks with evidence of substantial corrosion of crack side wall. Cracking is not continuous and is found in locations where environmental conditions change.

Prevention/Mitigation:

• Use a coating system that is resistant to disbondment and has the ability to pass cathodic protection (CP) current such as fusion-bonded epoxy (FBE).
• Utilizing a high level of negative potential in cathodic protection provides condition for production of carbonate and bicarbonate which induced SCC. Therefore, controlling of the potential can be effective in reduction of these failures.

Inspection and Monitoring:

Pipeline external SCC can be detected through the following:

Inline Inspection (ILI): ILI continues to improve as new methods of detecting and sizing SCC have been developed. Ultrasonic Shear Wave Ultrasonic Crack Detection (UTCD) and Electro Magnetic Acoustic Transducers (EMAT) are the proven ILI technologies for SCC sizing and detection. UTCD tool is the preferred technology for SCC inspection of liquid pipelines while EMAT is the preferred method for Gas and NGL pipelines.