Cracking is a leading cause of pipeline failure and rupture in large-diameter transmission lines. Thus, developing and improving crack management programs is a strategic priority for pipeline operators. The most common cracking mechanism in Canada is Near Neutral pH Stress Corrosion Cracking (NNpH SCC). A key component of such a program is a growth model that captures the underlying growth mechanism; yet, there is disagreement on the nature of this cracking mechanism. Pipeline operators have deployed various growth models to differing degrees of success, including fatigue models, classical electrochemical SCC models, and hydrogen-enhanced corrosion fatigue models.

This paper presents experimental evidence and theoretical analysis suggesting a diffusible species, namely hydrogen, plays a pivotal role in NNpH SCC by enhancing the corrosion fatigue mechanism. Past work performed by the Chen group (University of Alberta – PRCI Project SCC 2-12) identified a critical frequency at which the crack growth rate reaches its maximum. We expand this body of work by examining the effect of loading frequency at multiple temperatures. First, we use a simple diffusion model to estimate the critical frequency at the targeted experimental temperatures calibrated with the known room temperature data. Next, we perform experimental testing across various loading frequencies to prove or disprove this prediction.

Experimentally, we fabricated specialized surface crack tension (SCT) specimens from X65 pipeline steel designed to simulate the geometry of features found in the field. These specimens are pre-cracked in the air and then immersed in an NNpH SCC electrolyte (C2 solution) at the test temperature for 12 days. Next, the samples are loaded under constant amplitude cyclic loading at various loading frequencies; the initial maximum and minimum stress intensities are held constant. The experimental results find a critical frequency that agrees with the theoretical calculations suggesting that hydrogen is the diffusible species playing a pivotal role in NNpH SCC growth.

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