John A. Beavers

Effects of Environmental and Metallurgical Factors on the Stress Corrosion Cracking of Carbon Steel in Fuel-Grade Ethanol

Abstract This paper presents the results of a research program to study the effects of metallurgical variables, ethanol-gasoline blends, dissolved oxygen, and inhibitors on the stress corrosion cracking (SCC) of carbon steel in ethanol. The study utilized both slow strain rate (SSR) and fatigue precracked compact tension (CT) tests to characterize the effect of metallurgical and environmental variables on SCC. Metallurgical factors, including steel grade within a range of pipeline grades, welds, and heat-affected zone, do not appear to have a significant effect on the degree or rate of SCC. In terms of environmental factors, it is shown that SCC does not occur even in a fully aerated condition if the ethanol-gasoline blends contain less than approximately 15 vol% ethanol; SCC susceptibility and crack growth rate are higher in 50 vol% ethanol gasoline blend (E-50) than in either lower or higher ethanol concentration blends; oxygen scavenging can be an effective method to prevent SCC; water content above 4....

Effect of Hydrostatic Testing on Ductile Tearing of X-65 Line Pipe Steel with Stress Corrosion Cracks

Abstract Hydrostatic testing is one method to confirm the integrity of pipelines containing colonies of stress corrosion cracks. Although this technique is widely used, a concern of the pipeline industry is the potential for ductile tearing damage of subcritical flaws. Objectives of the current study were to evaluate the influence of hydrostatic testing on the crack tip morphology and to determine the amount of ductile tearing that may occur for different hydrostatic testing conditions. Stress corrosion cracks were grown in compact tension specimens of X-65 line pipe steel in a near-neutral-pH stress corrosion cracking (SCC) environment. Simulated hydrostatic tests were performed at loads that corresponded to hoop stresses at and above the specified minimum yield strength (SMYS), resulting in applied J-integral values near to and above the J(Q) fracture toughness value of the material. Some specimens ruptured; some did not fail. Crack tip blunting occurred and the tearing extent was small and proportional...

Localized Corrosion of Carbon Steel and Its Implications on the Mechanism and Inhibition of Stress Corrosion Cracking in Fuel-Grade Ethanol

Abstract Stress corrosion cracking (SCC) of carbon steel has been observed in fuel-grade ethanol (FGE, C2H5OH). Electrochemical behavior of carbon steel and therefore the role of localized corrosion in SCC has not been studied adequately because of the difficulty in performing electrochemical experiments in resistive environments such as FGE. A microelectrode technique was used in this work to conduct electrochemical studies in simulated FGE (SFGE). It was found that oxygen likely played multiple roles: it participated in the passive film formation in addition to its more commonly observed role in aqueous environments of changing the open-circuit potential (OCP), and it may also participate in the oxidation of ethanol on steel surfaces. A model, based on this dual role of oxygen, was proposed to explain the SCC mechanism of carbon steel in SFGE. The model is supported by the inhibitor evaluation work, which also demonstrated that carbon steel SCC could be inhibited by adding inhibitors.

2013 Frank Newman Speller Award Lecture: Integrity Management of Natural Gas and Petroleum Pipelines Subject to Stress Corrosion Cracking

Stress corrosion cracking (SCC) can be a serious threat to the integrity of natural gas and petroleum pipelines. This paper describes how the pipeline industry responded to this threat by performin...

High-pH SCC: Temperature and Potential Dependence for Cracking in Field Environments

The objective of the work described in this paper was to establish the temperature and potential dependence for propagation of high-pH stress corrosion cracking (SCC) in field environments encountered on TransCanada PipeLine’s system in western Canada. Potentiodynamic and potentiostatic polarization techniques were used to identify the electrochemical potential range for performing SCC tests in the simulated field electrolytes. A slow strain rate technique was used for the SCC assessment. A standard 1N Na2CO3 – 1N NaHCO3 solution was included in the test program as a control. The more dilute simulated field electrolyte was a less potent cracking environment than the 1N Na2CO3 – 1N NaHCO3 solution in that the potential range for cracking was narrower and the maximum cracking velocity was lower at a given temperature. The center of the potential range for cracking with the simulated field electrolyte was consistently more negative than with the 1N Na2CO3 – 1N NaHCO3 solution. This may increase the likelihood that the pipe-to-soil potential of a cathodically protected pipeline lies in the cracking range.Copyright

Stress Corrosion Cracking and Localized Corrosion of Carbon Steel in Nitrate Solutions

The stress corrosion cracking (SCC) of carbon steel in nitrate-containing environments has been known for a long time and has become important in managing the integrity of aging underground radioac...

Development of a Probabilistic Model for Stress Corrosion Cracking of Underground Pipelines Using Bayesian Networks: A Concept

Stress corrosion cracking (SCC) continues to be a safety concern, mainly because it can remain undetected before a major pipeline failure occurs. SCC processes involve complex interactions between metallurgy, stress, external soil environment, and the electrolyte chemistry beneath disbonded coatings. For these reasons, assessing SCC failure probability at any given location on a pipeline is difficult.In an attempt to assess the SCC probability, a Bayesian network model was created. The model links events by cause-consequence connections. The strengths of these connections are adjusted using expert knowledge, analytical models, and data from the field. Bayesian network modeling was chosen because it takes into account the high degree of uncertainty in the input parameters. Other models have been developed to assess SCC: such as indexing methods, heuristics models, and mechanistic models. However, their main limitation is the uncertainty of the input parameters. One other strength of the Bayesian model is that calculations can be run in two directions: the forward direction from cause to consequence and the backward direction from observation to causative factors. In the forward direction, the model evaluates the probability of SCC failure using various input probabilities of factors that are important to SCC. In the backward direction, the model can evaluate the effect of any known occurrence of SCC failure on the probabilities of causative factors and thus condition the Bayesian network to evaluate the future failure probability.In this paper, we discuss a Bayesian network model for high-pH SCC. The conceptual framework, acquisition of data, and the inclusion of uncertainties are described. In addition, an example of the model application to high pH SCC is given. The effects of service and field conditions such as soil type, soil chemistry, coating type, surface preparation techniques, stresses, residual stress due to pipe manufacturing conditions, welds, dents, location such as proximity to rivers, wetting and drying cycles, etc. on the SCC probability can be assessed with the model. The model details shown in this publication will only cover the stress affect due to surface preparation, welds, dents, and manufacturing conditions and temperature effect. The effects of other factors and validation against field experience will be discussed in future publications.Copyright

Methanol-Induced Axial Stress Corrosion Cracking in a Northern Canadian Liquids Pipeline

Three in-service leaks on a crude oil pipeline operating in Canada were investigated to identify their metallurgical cause(s). The releases were found to be associated with cracks originating from the internal surface of the pipeline. Further similarities between the releases included: (1) the axial directionality of the cracks, (2) the short crack length, (3) the crack location adjacent to girth welds, (4) the circumferential location of the cracks and (5) the intergranular crack morphologies. A comprehensive metallurgical investigation concluded that the likely crack-initiating mechanism was methanol-induced stress corrosion cracking (SCC). While this SCC mechanism is extremely rare in buried petroleum pipelines, all other plausible causes were considered and eliminated. Methanol-induced SCC, similar to other forms of SCC, requires three contributing factors: (1) a susceptible material, (2) a corrosive environment and (3) sufficient tensile stresses.Although much research has been performed on the effects of ethanol on pipeline steels, published data on the effects of methanol is very scarce. A laboratory research program using slow strain rate (SSR) testing was initiated to determine if pipeline steels are susceptible to methanol-induced SCC and identify the conditions necessary to reproduce it in a laboratory environment. This paper outlines the major findings from this program.Copyright

Integrity Management of Ethanol Pipelines

Future biofuel mix may contain ethanol from cellulosic materials, butanol, and biodiesel from diverse sources. The anticipated variable mix introduces technical and business uncertainties to pipeline companies requiring a flexible risk management framework. The technical issues are outlined in this paper along with approaches to risk management. The bow tie approach is discussed as a method to assess overall risk and communicate it throughout an organization. Direct assessment methods and their applicability to biofuel pipelines are also discussed.Copyright

Interpretation of External Cracking on Underground Pipelines

When external cracks are discovered on underground pipelines, it is sometimes difficult to identify the cracking mechanism. This is especially true when the cracking coincides with local stress discontinuities such as dents, where significant stress fluctuations can occur. Additionally, cracking may initiate by one mechanism and another mechanism may subsequently dominate. For purposes of developing integrity programs for the affected pipeline segment, proper interpretation of the mechanism for crack formation is important. This paper describes the characteristics of, and methods to distinguish between, the common types of time-dependent cracking found on underground pipelines. It also discusses the relationships between the mechanisms for the environmentally assisted cracking phenomena.© 2006 ASME