ABSTRACT

The objective of the research described in this paper was to evaluate the influence of the composition of the sparging gas on the electrochemical corrosion kinetics, hydrogen permeation, and crack growth behavior of a line pipe steel in a simulated near-neutral-pH cracking electrolyte. A series of potentiodynamic polarization experiments, Devanathan/Stachurski type hydrogen permeation experiments, and cyclic loading tests were performed with varying levels of O2 and CO2. Tests revealed that increasing the level of CO2 increased the corrosion rate at the free corrosion potential, increased hydrogen permeation, and accelerated crack growth during cyclic growth tests. Increasing the level of O2 had little effect on the corrosion rate at the free corrosion potential, decreased the rate of hydrogen permeation, and increased the crack growth rate in crack growth tests.

BACKGROUND

External stress corrosion cracking (SCC) continues to be an integrity concern for the natural gas pipeline industry. Two forms of cracking have been identified; namely high-pH SCC (also referred to as classical SCC) and near-neutral-pH SCC (also referred to as low-pH SCC). A characteristic of both forms of SCC is the presence of colonies of up to hundreds of longitudinal surface cracks in the body of the pipe that link up to form long shallow flaws. The fracture faces are covered with black magnetite or iron carbonate films. In the case of high-pH SCC, the cracking is intergranular and there is usually little evidence of general corrosion associated with the cracking. A concentrated carbonate-bicarbonate solution was identified as the most probable environment responsible for this form of cracking.(1, 2)

Near-neutral-pH SCC is transgranular and is associated with corrosion of the crack faces and, in some cases, with corrosion of the external surface of the pipe as well. This form of cracking occurs in near-neutral-pH (6 <pH <8) dilute CO2-containing electrolytes, and was first observed beneath polyethylene tape-coatings on TransCanada PipeLines Ltd?s. (TransCanada?s) System in the 1980?s.(3, 4) Other pipeline companies, such as NOVA Corporation, have also identified near-neutral-pH SCC on their pipelines.(5)

A research program was started at CC Technologies Laboratories, Inc. (CC Technologies) in 1992 to study the near-neutral-pH SCC phenomenon. Previous projects have focused on cracking in NS4, a simulated near-neutralpH cracking solution, with 5% CO2 cover gas. The previous studies did not attempt to examine other environments or cover gases or examine the relationship between crack growth and hydrogen permeation and electrochemical kinetics.

OBJECTIVE AND APPROACH

The objective of the research described in this paper was to evaluate the influence of the composition on the sparging gas on the electrochemical corrosion kinetics, hydrogen permeation, and crack growth behavior of a line pipe steel in a simulated near-neutral-pH cracking electrolyte. All experiments were performed on an X-65 line pipe steel with established susceptibility to near-neutral-pH SCC in a simulated cracking electrolyte known as NS4. This electrolyte was modeled after solutions found under the coating of pipelines that experienced near-neutral-pH SCC and its composition is given in Table 1. The cover gases studied were Argon, 0.5% to 25% CO2 (balance N2), and 1 to 20% O2 with 5% CO2 (balance N2).

The work was divided into three technical tasks, as described below.

Task 1 ? Potentiodynamic Polarization Testing

In Task 1, anodic and cathodic potentiodynamic polarization curves were obtained on X-65 line-pipe steel in NS4 electrolyte. The composition of the X-65 line pipe steel is given in Tab

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