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Guidance for Selecting SCC Direct Assessment Locations and Estimation of Re-Inspection Intervals
Abstract
A significant amount of research and development has been carried out on the mechanism of the stress corrosion cracking of underground pipelines. This paper describes the results of a study, co-funded by PRCI, the US DOT, and pipelines companies, to bring together the results of these various studies in the form of a set of guidelines that will assist companies in identifying the most likely SCC locations on their systems and in predicting how frequently inspection or other mitigation is required. The guidelines have been developed along mechanistic lines, and are divided into four “steps” representing: susceptibility to SCC, crack initiation, early-stage growth and dormancy, and crack growth to failure. For each step, a series of Research Guidelines has been derived from the results of individual research papers or studies. These Research Guidelines may or may not be easily validated against field data. The SCC Guidelines were then developed based on one or more Research Guidelines. Wherever possible, the SCC Guidelines have been validated against field data, but in some cases currently un-testable SCC Guidelines were defined because they offer a potentially unique opportunity to identify where and when SCC might occur.
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Environmentally assisted cracking (EAC) in dilute bicarbonate, near-neutral pH soil solutions occurs at the free corrosion potential underneath coatings that shield the pipe from cathodic protection. The transgranular quasi-cleavage crack appearance is an indication of a role for hydrogen in the crack growth mechanism. The mechanical driving forces for crack growth can be described by the stress intensity factor (K) and stress intensity factor range (AK = Kmax -Kmin). Threshold values for K and AK vary with the type of steel involved microstructure, and environment. Cyclic plastic deformation is critical for crack growth. No crack growth is observed under constant load conditions up to 110% of the specified minimum yield strength. Mechanical overloads, crack deflection, and crack closure phenomena reduce the effective ?K driving force. Work hardening has a similar effect by reducing the strain at the crack tip. Neighboring cracks in dense crack colonies can shield each other from the applied stress, thereby decreasing crack growth. In contrast, residual tensile stresses, and stress intensifiers increase the local K and AK, resulting in faster crack growth.
Electrochemical impedance spectroscopy, corrosion potential measurements, and surface analysis by scanning electron microscopy/energy-dispersive x-ray spectroscopy (SEM/EDX) and Raman spectroscopy were used to investigate the localized dissolution of millscale-covered pipeline steel surfaces. The porous millscale originally present on the pipe surface exerts an influence on the corrosion of the pipeline and may contribute to the eventual onset of stress corrosion cracking (SCC). Three regions in the corrosion potential-time plot were observed after exposure to an aqueous environment, corresponding to the initial attempts at breakdown of the millscale, coupling of the dissolution of the underlying steel to reductive dissolution of the millscale, and active corrosion of the steel at the base of pores in the film supported by water reduction either on the metal or on the millscale surface. The corrosion rate increases as the dissolved carbon dioxide (CO2) concentration increases. Changes in the solution resistance, polarization resistance, and pore resistance are related to the corrosion kinetics and growth of the pores. The porous structure of the millscale increases the possibility of local separation of anodic and cathodic sites, which would promote localized corrosion at the base of pores. These stress-raising pits eventually could act as precursor sites for the initiation of SCC.