About a century ago, researchers first recognized a connection between the activity of environmental microorganisms and cases of anaerobic iron corrosion. Since then, such microbially influenced corrosion (MIC) has gained prominence and its technical and economic implications are now widely recognized. Under anoxic conditions (e.g., in oil and gas pipelines), sulfate-reducing bacteria (SRB) are commonly considered the main culprits of MIC. This perception largely stems from three recurrent observations. First, anoxic sulfate-rich environments (e.g., anoxic seawater) are particularly corrosive. Second, SRB and their characteristic corrosion product iron sulfide are ubiquitously associated with anaerobic corrosion damage, and third, no other physiological group produces comparably severe corrosion damage in laboratory-grown pure cultures. However, there remain many open questions as to the underlying mechanisms and their relative contributions to corrosion. On the one hand, SRB damage iron constructions indirectly through a corrosive chemical agent, hydrogen sulfide, formed by the organisms as a dissimilatory product from sulfate reduction with organic compounds or hydrogen ("chemical microbially influenced corrosion"; CMIC). On the other hand, certain SRB can also attack iron via withdrawal of electrons ("electrical microbially influenced corrosion"; EMIC), viz., directly by metabolic coupling. Corrosion of iron by SRB is typically associated with the formation of iron sulfides (FeS) which, paradoxically, may reduce corrosion in some cases while they increase it in others. This brief review traces the historical twists in the perception of SRB-induced corrosion, considering the presently most plausible explanations as well as possible early misconceptions in the understanding of severe corrosion in anoxic, sulfate-rich environments.

Corrosion of Iron by Sulfate-Reducing Bacteria: New Views of an Old Problem

Corrosion is a major cause of structural deterioration in marine and offshore structures. It affects the life of process equipment and pipelines, and can result in structural failure, leakage, product loss, environmental pollution and the loss of life. Pitting corrosion is regarded as one of the most hazardous forms of corrosion for marine and offshore structures. The total loss of the structure might be very small, but local rate of attack can be very large and can lead to early catastrophic failure. Pitting corrosion is a localized accelerated dissolution of metal that occurs as a result of a breakdown in the protective passive film on the metal surface. It has been studied for many years; however, the structural failure due to pit characteristics is still not fully understood. Accurate pit depth measurements, precise strength assessment techniques, risk analysis due to pitting, and the mathematical relationship of the environmental factors that causes pitting failure are also factors, which need further understanding. Hence this paper focuses on these issues. It reviews and analyses the current understanding of the pitting corrosion mechanism and investigates all possible factors that can cause pitting corrosion. Furthermore, different techniques employed by scientists and researchers to identify and model the pitting corrosion are also reviewed and analysed. Future work should involve an in-depth scientific study of the corrosion mechanism and an engineering predictive model is recommended in order to assess failure, and thereby attempt to increase the remaining life of offshore assets.

Modelling of pitting corrosion in marine and offshore steel structures – A technical review

The role of residual stress in neutral pH stress corrosion cracking of pipeline steels. Part I: Pitting and cracking occurrence

A new understanding of intergranular stress corrosion cracking resistance of pipeline steel through grain boundary character and crystallographic texture studies

Stress corrosion cracking behavior of X70 pipe steel in an acidic soil environment

36% were caused by external corrosion and 63% were caused by internal corrosion. For natural gas distribution pipeline accidents, only approximately 4% of the total accidents were caused by corrosion, and the majority of those were caused by external corrosion. The accidents reported in Table 1 are for major accidents that resulted in injury, fatality, or more than $50,000 in property damage. In addition to the reportable accidents, an average of 8000 corrosion leaks per year are repaired on natural gas transmission pipelines (Ref 7), and 1600 spills per year are repaired and cleaned up for liquid product pipelines. In a summary report for incidents between 1985 and 1994, corrosion accounted for 28.5% of pipeline incidents on natural gas transmission and gathering pipelines (Ref 8). In a summary report for incidents between 1986 and 1996, corrosion accounted for 25.1% of pipeline incidents on hazardous liquid pipelines (Ref 9). These values correspond very well to the statistics for 1994 to 1999 presented in Table 1. Given the implications of pipeline failures and the role that external corrosion plays in these failures, it is apparent that proper corrosion control can have a major impact on the safety, environmental preservation, and the economics of pipeline operation. The vast majority of underground pipelines are made of carbon steel, based on American Petroleum Institute API 5L specifications (Ref 10). Typically, maximum composition limits are specified for carbon, manganese, phosphorous, and sulfur. In some cases, other alloying elements are added to improve mechanical properties. Composition and tensile requirements for common line pipe steels are shown in Table 2. These steels have inadequate alloy additions to be considered corrosion resistant and undergo

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External Corrosion of Oil and Natural Gas Pipelines

This paper provides an overview of mechanisms for high-pH and near-neutral-pH stress corrosion cracking of underground pipelines. Characteristics and historical information on both forms of cracking are discussed. This information is then used to support proposed mechanisms for crack initiation and growth.Copyright © 1996 by ASME

Mechanisms of High-pH and Near-Neutral-pH SCC of Underground Pipelines

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....

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

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Materials Factors Influencing the Initiation of Near-Neutral pH SCC on Underground Pipelines

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.

John A. Beavers

Papers

External Corrosion of Oil and Natural Gas Pipelines

Mechanisms of High-pH and Near-Neutral-pH SCC of Underground Pipelines

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

Materials Factors Influencing the Initiation of Near-Neutral pH SCC on Underground Pipelines

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