Session: 05-03-02 Crack Propagation

Paper Number: 87794

87794 - Temperature and Loading Frequency Effects in the Mechanism for Nnph Scc of Pipeline Steel 

Cracking is a leading cause of pipeline failure and rupture in large-diameter transmission lines. Thus, it is a strategic priority to develop and improve crack management programs 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 with 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 fabricate 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 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.

Presenting Author: Greg Nelson University of Alberta

Presenting Author Biography: Greg Nelson worked in the Alberta oil &amp; gas industry as a consultant for pipeline integrity, failure analysis, and fixed equipment integrity after graduating with an M.Sc in 2013. Greg&#39;s background includes both field and office experience. Greg&#39;s pipeline integrity work focused on small diameter pipelines, station piping, ILI of these lines, generation of dig packages, engineering assessments of corrosion and dent features, correlating NDE to ILI, and regulatory compliance. In December 2017, APEGA approved Greg&#39;s registration as a P.Eng. in Alberta.<br/><br/> In January 2018, Greg enrolled in a Ph.D. program studying stress corrosion cracking of pipeline steels. In May 2019, Greg received the Vanier Canada Graduate Scholarship to support his research. His work is part of a more extensive research program funded by PRCI, government, and pipeline operators. He also assists Dr. Weixing Chen with supporting the technical needs of local industry collaborators. Program completion is expected in Summer to Fall of 2022