Near-Neutral pH SCC Initiation and Early Propagation of X70 Pipeline Steel

2002 ◽  
Author(s):  
Baotong Lu ◽  
Jingli Luo ◽  
Brian McCrady
Keyword(s):  
Crack Initiation ◽  
Applied Stress ◽  
Stress Level ◽  
Pipeline Steel ◽  
Surface Condition ◽  
Test Duration ◽  
Pipe Surface ◽  
X70 Pipeline Steel ◽  
Neutral Ph ◽  
Dog Bone

The near-neutral pH SCC initiation and propagation behavior of X70 pipeline steel was investigated using dog-bone specimens with two different surface conditions, the original external pipe surface and a ground surface. The tests were conducted in NS4 solution de-aerated with N2/5%CO2, under low frequency cyclic load (v = 0.1Hz, R = σmin/σmax = 0.5). The fractography shows that the transgranular crack morphology of near-neutral pH SCC is reproduced under the current test condition. In line with the crack initiation mechanisms, the cracks observed are divided into two types, pit cracks and non-pit cracks. Non-pit crack initiation is found to be the dominative mechanism for the original external pipe surface but the contribution of pit cracks depends on the test duration. The test results indicate that the crack initiation rate increases and the crack initiation life decreases with increasing applied stress level. The crack propagation rate is essentially independent of the applied stress level. Finally, the effects of surface condition and non-metallic inclusions on the SCC mechanism of pipeline steels are discussed.

Stress Corrosion Crack Initiation in X-65 Pipeline Steel Under Disbonded Coating With Cathodic Protection

2008 ◽  
Author(s):  
Bingyan Fang ◽  
Abdoulmajid Eslami ◽  
Richard Kania ◽  
Robert Worthingham ◽  
Jenny Been ◽  
...  
Keyword(s):  
Crack Initiation ◽  
Cathodic Protection ◽  
Pipeline Steel ◽  
Seasonal Fluctuation ◽  
General Corrosion ◽  
Cathodic Current ◽  
Pipe Surface ◽  
Neutral Ph ◽  
Wide Range ◽  
Disbonded Coating

A novel testing setup has been used in this study to simulate crack initiation in X65 pipeline steel exposed to near-neutral pH soil environment. This test setup was designed to simulate synergistic interactions of cathodic current with soil environments underneath the disbonded coating on the pipe surface. It was found from the simulations that the local environment underneath the disbonded coating can be very acidic or alkaline, instead of near-neutral pH as commonly believed, depending on seasonal fluctuation in CO2 level and cathodic current. There exists a wide range of corrosion conditions on the steel surface up the gradient of cathodic current underneath the disbonded coating. General corrosion was found to increase as CP current diminishes. Pitting corrosion in terms of number of pits and size of pits was found to be the most severe at locations where cathodic protection was nearly diminished. These locations had also developed some crack like-defects, which were usually elongated in a direction perpendicular to the loading axis and appeared to be formed from a linkage of neighboring pits and by enhanced corrosion at stress raisers.

Retarding Crack Growth by Static Pressure Hold for Pipeline Steel Exposed to a Near-Neutral pH Environment

2016 ◽  
Author(s):  
Mengshan Yu ◽  
Weixing Chen ◽  
Karina Chevil ◽  
Greg Van Boven ◽  
Jenny Been
Keyword(s):  
Growth Rate ◽  
Cyclic Loading ◽  
Crack Growth Rate ◽  
Crack Initiation ◽  
Crack Growth ◽  
Growth Curve ◽  
Pipeline Steel ◽  
Early Stage ◽  
Pipe Surface ◽  
Neutral Ph

From extensive investigations for over 30 years since the discovery of near-neutral pH stress corrosion cracking (NNpHSCC), the physical processes of crack initiation and growth have been determined, despite that some details in various aspects of crack initiation and growth are still to be understood. The growth curve is a function of crack growth by direct dissolution of steels at localized areas on pipe surface during initiation or at the tip of a crack during early stage of crack growth (the dissolution growth curve), and by a process involving the interaction of fatigue and hydrogen embrittlement (corrosion fatigue, the hydrogen enhanced fatigue growth curve) in Stage II after crack initiation and early stage of crack growth. For the latter case, recent research shows that crack growth rate can be substantially enhanced by variable amplitude cyclic loading. One of the most severe scenarios of cyclic loading in terms of crack growth rate is the underload type of pressure fluctuations that is often found within 30 km downstream of a compressor station. This investigation is aimed to evaluate pressure scenarios that could reduce or retard crack growth during pipeline operation. Specifically, the effect of pressure holds was investigated. Different periods of static hold were performed to an X65 pipeline steel exposed to a near-neutral pH solution. It was found that a static hold at the maximum load for one hour immediately after a large depressurization-repressurization cycle (underload cycle) yielded the lowest crack growth rate, which was about one third of that of constant amplitude fatigue without the static hold. Static holds for a period shorter or longer than one hour have yielded higher crack growth rates. This observation can be applied to field pipelines during operations to retard crack propagation.

STRESS CORROSION CRACK INITIATION BEHAVIOR FOR THE X70 PIPELINE STEEL BENEATH A DISBONDED COATING

2012 ◽  
Vol 48 (10) ◽  
pp. 1267 ◽  
Author(s):  
Zhiying WANG ◽  
Jianqiu WANG ◽  
En-hou HAN ◽  
Wei KE ◽  
Maocheng YAN ◽  
...  
Keyword(s):  
Crack Initiation ◽  
Stress Corrosion ◽  
Stress Corrosion Crack ◽  
Pipeline Steel ◽  
X70 Pipeline Steel ◽  
Disbonded Coating

Threshold for Stress Corrosion Cracking Initiation of Alloy X750 under Long-Term Uniaxial Constant Load Test in High-Temperature Water

CORROSION ◽  
2004 ◽  
Vol 60 (3) ◽  
pp. 229-236 ◽  
Author(s):  
M. Yamamoto ◽  
J. Kuniya ◽  
S. Uchida
Keyword(s):  
High Temperature ◽  
Crack Initiation ◽  
Stress Corrosion ◽  
Applied Stress ◽  
Stress Level ◽  
Stress Corrosion Cracking ◽  
Pure Water ◽  
Constant Load ◽  
Corrosion Cracking ◽  
Load Test

Abstract Uniaxial constant load (UCL) tests of the nickel-based alloy X750 (UNS N07550) were performed in high-temperature pure water (288°C, 8 ppm dissolved oxygen [DO]) to investigate stress corrosion cracking (SCC) fracture time and the crack initiation process. The SCC fracture was initiated at a stress level below the 0.2% offset yield stress and many small cracks were observed in the middle of the nonfractured test specimens. The distribution of the crack length for each observation time is shown by Weibull probability distributions. Crack initiation and propagation process had different behavior depending on the applied stress level and the stress intensity factor at the crack tip. SCC initiation at the minimum applied stress is discussed with respect to the grain size, which depended on the size of an initial crack.

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