Fengmei Song

Predicting Carbon Dioxide Corrosion of Bare Steel under an Aqueous Boundary Layer

Abstract The corrosion of bare steel under an aqueous boundary layer with dissolved carbon dioxide (CO2) was modeled to investigate the effect of CO2. The model incorporated the coupled effect of CO2 diffusion, hydration, local ionic equilibria, ferrous carbonate (FeCO3) precipitation, and steel corrosion. The model was verified against published experimental data under both FeCO3-saturated and unsaturated boundary layers. Good agreement was shown under a variety of conditions. For saturated boundary layers, the results show that the corrosion rate in carbonic acid (H2CO3) is greater than in hydrochloric acid (HCl)for a given pH and that H2CO3 reduction is the cause for the increase of corrosion rate in H2CO3. Increasing temperature was found to increase corrosion rate substantially. This work provides further understanding of the CO2 corrosion mechanism and is a reliable, convenient, and practical tool for predicting the rate of CO2 corrosion.

Predicting Corrosion and Current Flow within a Disc Crevice on Coated Steels

Abstract The objective of this work was to develop a model to describe pipeline corrosion within a disc-shaped crevice in the presence of oxygen (O2). The model results from modification of the pip...

Steel corrosion under a disbonded coating with a holiday-Part 2: Corrosion behavior

Abstract Corrosion of a pipe surface in a channel crevice has been modeled assuming oxygen diffusion through a holiday in a permeable coating. The crevice was thin and narrow with a finite length. ...

Predicting the Effect of Soil Seasonal Change on Stress Corrosion Cracking Susceptibility of Buried Pipelines at High pH

Abstract Soil seasonal change is believed to be one of the major factors that contribute to high-pH stress corrosion cracking (SCC) of underground pipelines, although the specific role and mechanisms of the soil seasonal change in the SCC susceptibility are not clearly understood. This study is directed at investigating this role and mechanisms of the soil seasonal change through mathematical modeling. The temporal and spatial variation, with seasonal changes in the soil, of the solution chemistry and the pipe steel potential and corrosion rate in a coating disbonded region is explored. With the pipe potential at the holiday varied by soil seasonal wet and dry cycles, the model results suggest that the susceptible conditions for high-pH SCC could be formed during soil wet seasons and seasonal transitions. Depending on the level of drought and the effectiveness of cathodic protection (CP) during dry seasons, the susceptible conditions of the high-pH SCC may or may not be possible to form.

Effect of ferrous ion oxidation on corrosion of active iron under an aerated solution layer

Abstract A new model was developed to study the effect of ferrous ion oxidation on the corrosion of active iron. When ferrous hydroxide precipitate is present at the iron surface as the result of c...

Predicting carbon dioxide corrosion of bare steel under an aqueous boundary layer with oxygen and cathodic protection

Abstract Carbon dioxide (CO2) corrosion of bare steel under an aerated aqueous boundary layer with imposed cathodic protection (CP) was modeled. The results show that an imposed CP current decreases the corrosion rate because it changes the steel potential to a more negative value while consuming the oxygen diffusing to the pipe surface as well as the hydrogen ions that dissociate from carbonic acid (H2CO3). This decrease of corrosion rate is greatest near the free corrosion potential and becomes less as the potential is shifted to a more negative value. This change of corrosion rate with the steel potential follows the Tafel relationship between them. At very negative potentials, the reduction of hydrogen ions and H2CO3 to hydrogen is increased and may lead to hydrogen-induced cracking.

Predicting Near-Neutral pH SCC Conditions Under a Disbonded Coating on Pipelines

Cathodic protection (CP) shielding and the presence of carbon dioxide (CO2 ) in the soil environment are necessary for the occurrence of near-neutral pH stress corrosion cracking (NNPHSCC). Quantitative understanding of the relationship between external conditions, coating deterioration and NNPHSCC is emerging but needs further improvements in modeling and experimental tool. This paper is aimed at understanding the environments initiating the NNPHSCC. New experimental results are presented on crevice chemistry due to the degradation of the mastic of a commercial high-density polyethylene (HDPE) coating and due to CO2 penetration into a disbonded crevice through the coating. Also presented are the results obtained from a comprehensive model, TECTRAN, on the effect of CO2 penetration into a crevice through the holiday alone and through both the holiday and the coating. The experimental results show that as the coating mastic degrades in the soil solution, the solution pH decreases within a few days from about 9 to a steady-state value of about 7.5. The Co2 diffusion through a 0.3 mm commercial HDPE coating is rapid, with a decrease of the soil solution pH from 9 to 5 within a matter of days (external CO2 pressure is 1 atm). The model results show that the presence of CO2 in the soil (0.05 atm partial pressure) can reduce the crevice solution pH to near neutral due mainly to its penetration through the coating, confirming previous hypotheses regarding its role in initiating NNPHSCC.Copyright

A Large-Scale Model Developed for Predicting the Corroding Conditions of a Carrier Pipe in a Casing

The corroding conditions of a cased carrier pipe segment are difficult to measure in the field. It is hard to know whether or how well the cased pipe segment is cathodically protected. The pipeline industry needs a tool that can predict the in-situ corrosion potential of the carrier pipe in the casing annulus in order to determine the level of cathodic protection (CP). This work reports on a large-scale, three-dimensional computer model developed to make such a prediction. The casing wall may be treated as bare or coated on both or either of the external and internal surfaces. The carrier pipe is coated, and the coating quality inside the annulus may or may not be the same as the outside of the casing segment. The casing annulus is full of electrolytes. The model is used to investigate the effect of several factors on the level of CP imposed on the carrier pipe in the annulus, including the coating quality on the carrier pipe, the casing wall native potentials (both external and internal), the presence of a coating on a casing wall surface, and a metallic contact (between the casing and the carrier pipe) with the contact resistance being a variable. The effect of the voltage variation inside the pipe metal wall is also investigated.Copyright

Evaluation of Wireless Tools for Determining Contact Status Between a Casing and the Carrier Pipe

For more than half a century, cased crossings have been used to support oil and gas steel pipes for crossing highways, railways, or rivers. Leaks or ruptures of cased crossings have occurred that resulted in casualties and property damage. Unlike uncased pipes buried in soils, which can be assessed for external corrosion directly and indirectly, the presence of a steel casing wall makes it challenging, even if possible, to conduct these assessments. The effect of the casing wall on the external corrosion of the carrier pipe inside the annulus is not well understood; it is unclear what tools can be used to effectively inspect the cased pipes. Empirical experience from the pipeline industry has shown that a few tools can be used to detect the contact status between a casing and the carrier pipe, which can help assess the severity of the carrier pipe external corrosion. Unfortunately, the criteria used to detect a contact status vary by user for the same tool and thus, the effectiveness of these criteria is uncertain. This paper reports results from a recent study aimed at evaluating the effectiveness of AC current attenuation, AC voltage gradient, and close interval (potential) survey as tools for detecting the contact status between a casing and the carrier pipe.Copyright

A Probabilistic Model for Predicting Internal Corrosion Threat in Dry Natural Gas Pipelines

A probabilistic model is developed in this work to predict the internal corrosion (IC) threat due to water condensation in dry natural gas pipelines. The model involves the understanding of tariff limits (TLs) for water and other corrosive species in natural gas; a consensus definition of an extremely unlikely condition for IC threat; a statistical analysis of field operating temperature, pressure, and water content (WC) data from a number of operators; and a known but modified relation of the saturated WC vs. operating temperature and pressure. By setting the limit of the probability of water condensation at 2% of the time that the pipe surface is wet, the maximum WC allowed in the natural gas can be determined for any given temperature and pressure. Practical operating charts have been developed for guiding pipeline operators to understand and minimize IC threats in dry gas (DG) pipelines. This paper presents the probabilistic modeling approach and discusses some model results.Copyright