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

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.

Download Full-text

The Role of Electrochemical Conditions in Near-Neutral pH SCC Initiation Mechanism(s)

2010 8th International Pipeline Conference, Volume 1 ◽

10.1115/ipc2010-31190 ◽

2010 ◽

Author(s):

Abdoulmajid Eslami ◽

Mohammadhassan Marvasti ◽

Weixing Chen ◽

Reg Eadie ◽

Richard Kania ◽

...

Keyword(s):

Cathodic Protection ◽

Pipeline Steel ◽

Synergistic Effects ◽

Co2 Concentration ◽

Initiation Mechanism ◽

Ph Values ◽

Neutral Ph ◽

Comprehensive Test ◽

Disbonded Coating

In order to improve our understanding of near-neutral pH SCC initiation mechanism(s), a comprehensive test setup was used to study the electrochemical conditions beneath the disbonded coatings in cracking environments. In this setup the synergistic effects of cyclic loading, coating disbondment, and cathodic protection were considered. Our previous results showed that there can be a significant variation in the pH of the localized environment under the disbonded coating of pipeline steel. The pH inside the disbondment can change significantly from near-neutral to high pH values, strongly depending on the level of cathodic protection and CO2 concentration. Both of these variables affected the electrochemical conditions on the steel surface and therefore the initiation mechanisms. This work highlights the role of electrochemical conditions in near-neutral pH SCC initiation mechanisms.

Download Full-text

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

4th International Pipeline Conference, Parts A and B ◽

10.1115/ipc2002-27234 ◽

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.

Download Full-text

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

Volume 1: Pipelines and Facilities Integrity ◽

10.1115/ipc2016-64627 ◽

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.

Download Full-text

Update of Understanding of Near-Neutral pH SCC Crack Growth Mechanisms and Development of Pipe-Online Software for Pipeline Integrity Management

Volume 1: Pipelines and Facilities Integrity ◽

10.1115/ipc2016-64626 ◽

2016 ◽

Cited By ~ 1

Author(s):

Weixing Chen ◽

Jiaxi Zhao ◽

Jenny Been ◽

Karina Chevil ◽

Greg Van Boven ◽

...

Keyword(s):

Service Life ◽

Crack Initiation ◽

Crack Growth ◽

Oil And Gas ◽

Stage Ii ◽

Variable Pressure ◽

Pipe Surface ◽

Neutral Ph ◽

Wide Range ◽

Online Software

This paper is aimed to introduce Pipe-Online Software that has been developed recently for crack growth and remaining service life prediction for pipelines experiencing near-neutral pH stress corrosion cracking and corrosion fatigue. The software was developed based on the latest understanding of the physical, chemical and mechanical processes involved during crack initiation, early crack growth and coalescence, and stage II crack growth. In each stage of cracking, governing equations were established based on extensive experimental simulations under realistic conditions found during pipeline operation in the field and vast amounts of field data collected, which include pipeline steel properties, crack geometries, field environmental conditions, Supervisory Control and Data Acquisition (SCADA) data of oil and gas pipelines. The model has considered a wide range of conditions that could lead to the crack initiation, crack dormancy and crack transition from a dormant state to active growth. It is concluded that the premature rupture caused by stress cracking at a service life of about 20–30 years commonly found during field operation could take place only when all the worst conditions responsible for crack initiation and growth have been realized concurrently at the site of rupture. This also explains the reason why over 95% of near-neutral pH cracks remain harmless, while about 1% of them become a threat to the integrity of pipeline steels. It has been found that crack initiation and early stage crack growth are primarily caused by the direct dissolution of steels at constrained areas. The rate of dissolution can be high at the pipe surface because of various galvanic effects, but decreases to a low value as the cracks approach a depth of ∼ 1.0 mm, leading to a state of dormancy as generally observed in the field. In stage II crack growth, the software has considered loading interactions occurring during oil and gas pipeline operations with underload-type variable pressure fluctuations. The software has provided predicted lifetimes that are comparable to the actual service lives found in the field. This forms a sharp contrast with the predictions made by existing methods that are generally conservative or inconsistent with the field observations.

Download Full-text

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

ACTA METALLURGICA SINICA ◽

10.3724/sp.j.1037.2012.00254 ◽

2012 ◽

Vol 48 (10) ◽

pp. 1267 ◽

Cited By ~ 5

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

Download Full-text

Near-neutral pH corrosion and stress corrosion crack initiation of a mill-scaled pipeline steel under the combined effect of oxygen and paint primer

Corrosion Science ◽

10.1016/j.corsci.2021.109511 ◽

2021 ◽

Vol 187 ◽

pp. 109511

Author(s):

Shidong Wang ◽

Lyndon Lamborn ◽

Weixing Chen

Keyword(s):

Crack Initiation ◽

Stress Corrosion ◽

Stress Corrosion Crack ◽

Pipeline Steel ◽

Combined Effect ◽

Neutral Ph ◽

Effect Of Oxygen

Download Full-text

Inspection of Buried Piping in Nuclear Power Plants: Field Observations and Challenges

Volume 1: Codes and Standards ◽

10.1115/pvp2012-78144 ◽

2012 ◽

Author(s):

Gabriel Ogundele ◽

Guylaine Goszczynski ◽

Darcy VanSligtenhorst

Keyword(s):

Cathodic Protection ◽

Nuclear Power ◽

Power Plants ◽

Nuclear Power Plants ◽

Cathodic Current ◽

Pipe Surface ◽

The Past ◽

Piping Systems ◽

External Corrosion ◽

Significant Attention

The issues over the integrity of buried piping in Nuclear Power Plants (NPPs) have received significant attention over the past few years. These piping systems have been in operation for over 30 years. Leaks from buried piping have the potential to raise safety, radiological, environmental, and financial concerns. Buried piping are subject to degradation mechanisms from the outside (soil side) as well as from the inside (fluid side) and they are primarily protected from external corrosion by applying coating on the pipe and then using cathodic protection to protect any bare areas or holidays in the coating. However, over a period of time the coating may lose its integrity and fail to provide the protection for which it was intended. As this happens, the amount of cathodic current needed for adequate protection increases. In some instances, the coating will disbond from the pipe and shield the cathodic protection from the pipe surface. Because of the economic, environmental, and safety consequences of a failure, NPPs embarked on inspection programs to determine the pipe’s condition and its suitability for continued service. This paper presents some of the observations made during the indirect and direct inspections of buried piping. In addition, the challenges encountered are reported.

Download Full-text