Modeling the Interactions of Hydrogen, Stress and Dissolution in Near-Neutral pH Stress Corrosion Cracking of Pipelines

Author(s):  
Frank Y. Cheng

A thermodynamic model was developed to determine the interactions of hydrogen, stress and anodic dissolution at the crack-tip during near-neutral pH stress corrosion cracking in pipelines. By analyzing the free-energy of the steel in the presence and absence of hydrogen and stress, it is demonstrated that a synergism of hydrogen and stress promotes the cracking of the steel. The enhanced hydrogen concentration in the stressed steel significantly accelerates the crack growth. The quantitative prediction of the crack growth rate in near-neutral pH environment is based on the determination of the effect of hydrogen on the anodic dissolution rate in the absence of stress, the effect of stress on the anodic dissolution rate in the absence of hydrogen, the synergistic effect of hydrogen and stress on the anodic dissolution rate at the crack-tip and the effect of the variation of hydrogen concentration on the anodic dissolution rate.

Author(s):  
Edgar I. Cote ◽  
James Ferguson ◽  
Nauman Tehsin

Pipelines are subjected to both residual and applied tensile stresses, and can form near-neutral pH SCC (transgranular stress corrosion cracking) if the pipeline is exposed to a conducive environment and is made from a material that is susceptible to SCC. This transgranular SCC is an ongoing integrity concern for pipeline operators. As part of an SCC Integrity Management Program (IMP), it is necessary to perform integrity assessments and prioritize segments of the pipelines to manage the SCC threat. Ultrasonic crack detection in-line inspection tools have proven capable of locating SCC, but reliability of these tools is not absolute and the reduced probability of detection of subcritical flaws limits options for proactive management. Hydrostatic retesting is a very effective program for removing near-critical axial defects, such as SCC, but does not provide useful information as to the location of SCC along the pipeline. NACE Standard RP0204-2004 (SCC Direct Assessment Methodology or SCCDA) outlines factors to consider and methodologies to employ to predict where the SCC is likely to occur, but the standard acknowledges that there are no well-established methods for predicting the presence of SCC with a high degree of certainty. The trend in probabilistic modelling has been to focus on establishing deterministic relationships between environmental factors, tensile stress and SCC formation, and growth; these models have achieved varying degrees of success. The Statistical Predictive Model (SPM) was previously developed to predict the likelihood of occurrence of near-neutral pH Stress Corrosion Cracking (SCC) for the NPS 10 Alberta Products Pipeline (APPL). SPM Phase 5 uses selected predictor variables representing tensile stress, environmental, pipe-related, corrosion control and operational relevant factors to determine the Probability of Occurrence of SCC. Regression techniques were used to create multi-variable logistic regression models. The results for each model are checked at locations where SCC is known to be present or absent to assess predictive accuracy, then used to prioritize susceptible segments for field excavation. The relative strength of individual predictor variables provides insight into the mechanism of near-neutral pH SCC crack initiation.


Subcritical crack growth can occur under steady or varying loads. In the former it is precipitated by specific environmental conditions that encourage the operation of time-dependent processes controlling crack advance. These include aggressive environments leading to stress corrosion cracking, or elevated temperature conditions leading to creep cavitation. The result is a time-dependent maintenance of a sharp crack profile during crack extension. Under varying loads such a sharp profile is readily achieved by plastic deformation on load reduction. Net crack advance in fatigue therefore occurs in each load cycle by this blunting-resharpening process, and empirical crack growth laws reflect this physical basis. Parameters such as K and J, which define crack tip deformation, are useful for correlating fatigue crack growth. In that they define crack tip stress-strain fields under load, they also partly describe crack advance for steady load creep and stress corrosion cracking. In particular they can define a threshold state for crack extension by all three processes. Under varying loads, if fatigue conditions are combined with an aggressive or high-temperature environment the description of crack growth can be complex. These areas of corrosion fatigue and creep fatigue are of considerable current practical interest.


2013 ◽  
Vol 10 (05) ◽  
pp. 1350033
Author(s):  
IGOR TELICHEV ◽  
OLEG VINOGRADOV

An Inverse Broyden's Method was applied in the context of a molecular statics approach for the analysis of the contribution of hydrogen to the near-neutral pH stress corrosion cracking. A 3D crystal structure was tested numerically. It is shown that the Inverse Broyden's Method provides a computationally efficient technique to evaluate the effect of hydrogen on the material degradation. Simulation results demonstrated that atoms of hydrogen placed near the crack tip produced a strong effect on deformation and crack propagation in bcc iron leading to a 15–20% loss in a residual strength of numerically tested samples.


Author(s):  
Zhanpeng Lu ◽  
He Xue ◽  
Tetsuo Shoji

Crack kinking or branching has been observed in laboratory stress corrosion cracking tests and in some components suffering from stress corrosion cracking in nuclear power plant coolants. There are several types of crack branching: i.e., macroscopic multiple branching cracks, local crack branching or the combination of both. Crack branching affects the crack tip stress/strain distribution in terms of stress intensity factor and crack tip strain rate, and consequently affects crack growth behavior. The crack tip mechanical fields in some typical crack branching systems are quantified using empirical, analytical and numerical simulation methods. The effect of crack branching is less significant in contoured double cantilever beam specimens than in compact tension specimens for the same size and configuration of branched cracks. The applications of the analysis results to some observed crack branching phenomena of austenitic alloys in high temperature water environments are discussed based on the theoretical crack growth rate formulation.


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