A. Eslami

Corrosion of X-65 Pipeline Steel Under a Simulated Cathodic Protection Shielding Coating Disbondment

Pipeline corrosion is a common problem for polyethylene (PE) tape-coated oil and gas transmission pipes buried under ground. It is often seen under cathodic protection (CP) shielding coating disbondments, where a wrinkle filled with ground water could be formed. This study investigates corrosion of X-65 pipeline steel under a simulated CP shielding coating disbondment with a gap size of 1 cm. Results showed that the corrosion rate of pipeline steel under the CP shielding coating disbondments is a function of carbon dioxide (CO2) and level of CP at the holiday of the coating disbondment, and could be significantly different to that in bulk environments.

Developing Cathodic Protection Based on Disbondment Geometry

Coating disbondment on pipelines is a common phenomenon that leads to exposure of the pipeline metal to ground water solutions, promoting a corrosive environment which is associated with stress corrosion cracking (SCC). This investigation tracks the corrosion behavior of X-65 steel at different coating disbondments at Open Circuit Potential (OCP) conditions through weight loss tests, as well as SEM analysis. For the weight loss test, X-65 coupons were placed into a vertical coupon holder with varying gap sizes between the coupons and the shielding (2 mm 5 mm, and 10 mm) simulating the coating disbondment. The results of the tests suggest that the corrosion rate inside the gap is dependent on the mass transfer rate of CO2. For larger gaps, the corrosion rate inside the gap decreases almost linearly with position (highest at the top of the disbondment) and is dependent on the iron carbonate film deposition. For smaller gaps (≤5 mm) a galvanic effect was observed between the coupons at the top of the disbondment (anodic) which showed a high degree of general corrosion, and the coupons at the bottom of the disbondment (cathodic) which had very low corrosion rates. The experimental observations were related to the corrosion scenarios found in the field. Cathodic protection strategies with respect to the geometry of disbondment are also discussed.Copyright

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

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.Copyright

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.Copyright

Effect of plasma nitriding parameters on corrosion performance of 17-4 PH stainless steel

ABSTRACT This study investigates the effect of plasma nitriding parameters on corrosion susceptibility of 17-4 PH stainless steel in 3.5 wt-% NaCl solution. In this regard, 17-4 PH stainless steel was plasma nitrided at 400°C for 5 and 10 h, 450°C for 5 h and 500°C for 5 h. Cross-sectional images after nitriding process showed that a uniform nitrided layer has been formed on steel substrate. Depending on the temperature and time of the nitriding process, different phases were formed in the nitrided layer. This affected general corrosion and pitting corrosion performance of 17-4 PH stainless steel in 3.5 wt-% NaCl solution. While precipitation of chromium nitrides for nitrided specimens at 450°C and higher increased the susceptibility to pitting and general corrosion, formation of expanded martensite (EM) in nitriding at 400°C improved the pitting corrosion resistance of 17-4 PH stainless steel. This is believed to be due to the release of nitrogen atoms from EM phase to form ammonium ions and increase the pH of the solution, supressing pit growth.

Effect of oxygen on near-neutral pH stress corrosion crack initiation under a simulated tape coating disbondment

A test setup that enables investigating the synergistic effects of tape coating disbondment, cathodic protection (CP) and low-frequency cyclic loading is used to determine the effect of oxygen on near-neutral pH stress corrosion crack initiation. After low-frequency corrosion fatigue tests, results show that surface micro-cracks initiate under the simulated tape coating disbondments at regions with insufficient CP potentials, not more remote locations completely free from CP as commonly believed. The presence of 1% oxygen in the composition of the purging gas has binary effects on the crack initiation under the simulated tape coating disbondment. On one hand it increases the size of the surface micro-cracks initiated on the steel surface at locations with some but insufficient CP potentials; and on the other, it reduces the number of surface micro-cracks at regions with no CP because of an extension of the general corrosion region deep inside the simulated coating disbondment.

Growth of Surface-Type Stress Corrosion Cracks in Near-Neutral pH Environments Under Disbonded Coatings

Near-neutral pH stress corrosion cracking (NNPHSCC), which occurs when ground water penetrates under the pipe coating, causes longitudinal cracks to develop on the surface of pipelines. Such cracks grow over time and can ultimately lead to pipeline failure. NNPHSCC is currently managed by in-line inspection or hydrostatic testing for oil and gas pipelines respectively. These procedures are enormously expensive and have to be repeated at predetermined intervals. Re-inspection intervals are currently determined by empirical models, which have been found rather imprecise. A major flaw in currently applied models is that they assume that once a NNPHSCC crack is formed, it grows at a constant rate that is independent of pipeline operating variables and both pre- and in-service history of the pipeline material. This is not necessarily true as pipeline history, the nature of the service environment and operating factors, among several other factors, have a strong influence on the rate of NNPHSCC crack propagation. Most existing models also treat NNPHSCC cracks as long through thickness cracks rather than surface type cracks typically observed in the field. This research proposes to provide an empirical model that more accurately predicts the growth rate of near-neutral pH SCC cracks in near-neutral pH environments by studying the growth rate of surface type flaws while also accounting for the influence of operating factors, environmental factors, coating disbondment and cathodic protection on the rate of crack propagation. This paper reports some preliminary test results obtained using a long specimen with three semi elliptical surface flaws located in three reduced sections to simulate field observed NNPHSCC cracks. Preliminary results suggest that: 1) crack grows much faster at the open mouth, which was attributed to hydrogen effects; 2) crack dormancy can occur under certain combined mechanical factors; 3) although the benign mechanical loading cannot lead to a direct crack growth (crack dormancy), it causes damage to the crack tip, which makes the crack more susceptible to crack growth upon a more aggressive condition is encountered.Copyright