Predicting Crack Growth Rates in Pipelines for Prioritization of Stress Corrosion Cracking (SCC) Locations For Direct Examination, 18-R9639Printer Friendly Version
Inclusive Dates: 07/01/06 Current
Background - Stress corrosion cracking (SCC) is a dangerous failure mode resulting from synergistic actions of mechanical stress, susceptible material, and specific corrosive environment (Figure 1). SCC failure can lead to problems such as catastrophic burst of a gas line (Figure 2). Because the crack is normally extremely narrow (measured in micrometers) relative to its depth (measured in millimeters) as shown in Figure 3, SCC is difficult to detect and predict, and it grows relatively fast after initation. There is a strong need in the pipeline industry for a tool that can help with determining possible SCC locations and estimating the crack growth rate useful for risk ranking of the locations. Because buried pipelines are protected by barrier coatings and cathodic polarization, SCC always occurs underneath a disbonded coating where the soil ground water can accumulate through a defect in the coating, and its chemistry varies by corrosion and the external polarization. Under tensile stress and with susceptible microstructure in pipe steel, such solution chemistry would lead to crack initiation and propagation. To predict SCC susceptibility and severity requires the link from known soil chemistry=>chemistry in coating-disbonded region modified by corrosion=>chemistry and corrosion in crack and stress in pipe steel=>crack growth rate. The first of these chain links has been established in a recently conducted project [Song, F.M. and N. Sridhar, "Determining Integrity Reassessment Intervals through Corrosion Rate Modeling and Monitoring," Contract No. DTRS56-04-T-0002, completed January 2006].
Approach - The goal of this project is to develop an intermediate modeling tool, as proof of concept, that would allow for prediction of crack growth rate from known chemistry in the disbonded region. A complete prediction of crack susceptibility and growth rate from known bulk soil chemistry will eventually be realized through external funding resources. Three tasks were proposed in this project, respectively: model development (Task 1) with the need of solving a set of partial differential equations, validation of the model (Task 2) with the use of laboratory and field data, and simplification of the model (Task 3) to demonstrate its applicability to the pipeline industry.
Accomplishments - (a) Literature Review: A comprehensive review of literature on pipeline SCC was conducted to attain a better understanding of the general SCC mechanisms of carbon steel. High pH SCC is intergranular due to alkaline solution pH, ferrite grains being anodic to carbide at grain boundaries, and stress concentration at the atom-disoriented grain boundaries. Very little work has been conducted to understand solution chemistry inside cracks. The limited tests done by Parkins et al. [Parkins, R.N. (1984), I.H. Craig, and J. Congleton, "Current and Potential Measurement along Simulated Cracks," Corrosion Science 24:8, pp. 709 - 730] indicated roughly the same potential and chemistry inside the crack as in the bulk. In contrast, others [Sandoz, G. (1970), C.T. Fuji, and B.F. Brown, "Solution Chemistry within Stress Corrosion Cracks in Alloy Steels," Corrosion Science, 10, pp. 839 - 845] showed low pH near the crack tip regardless of bulk solution pH. These results, although still controversial, will be assessed by the model. (b) Model Development: Well established fundamental equations applicable to corrosion systems are the bases of the current model, which has been successfully utilized in a recent project [see Song and Sridhar]. To advance this modeling work to SCC, the following items must be taken into account: (1) passivity of the steel surface at crack walls, (2) more complex chemistry involving buffering carbonate/bicarbonate ions, (3) more complex geometry of the system, (4) growth of the crack tip (moving boundary condition), and (5) effect of stress and strain near the crack tip, characterized by stress intensity factor, on dissolution at the crack tip. The SCC model under development is progressing to address the above issues.