Stress Corrosion Cracking of Pipelines

1

Introduction

Statistically, pipelines provide the safest and most economical form of transportation of crude oil, natural gas, and other petrochemical commodities compared to truck, rail cars, and tankers [Cheng, 2010]. There are about 2 million kilometers of transmission pipelines worldwide. These include natural gas, oil, condensates, petroleum gas, and other refined petroleum products, as well as carbon dioxide (CO2) and hydrogen. The pipelines could be very large in diameter (e.g., a Russian pipeline system has a diameter of up to 1422 mm) and can be over several thousand kilometers in length [Hopkins, 2007]. Most pipelines are buried or under the sea, but some operate aboveground.

Liquids and gases have been transported by pipelines for thousands of years. Ancient Chinese and Egyptians used pipes to transport water, hydrocarbons, and even natural gases [Hopkins, 2007]. Most of the current pipeline industry was developed to transport oil, bringing considerable profits to energy producers and pipeline operators, and development is driven by expanding energy demands. Tens of thousands of kilometers of new pipelines are constructed every year. Pipelines have become one of the most environmentally friendly and safest means of oil and natural gas transportation and contribute to strong national economies. As a consequence, they have been integrated into the components of national security in most countries.

More than 90% of pipelines are made of steel, primarily carbon steel, with aluminum, fiberglass, composite, polyethylene, and other types making up the remaining 10% [Alberta Energy and Utilities Board, 2007]. Requirements for higher capacities and operating pressure and additional economic benefits have led to a demand for higher-strength pipeline materials, especially high-strength steels, as well as new techniques for welding, construction, inspection, and pipeline integrity and maintenance programs.

1.1 PIPELINES AS ENERGY HIGHWAYS

Human beings need energy to survive. For today and tomorrow, fossil fuels, including oil and gas, are the predominant forms of energy consumed worldwide. In fact, even if the use of renewable energies doubles or triples over the next 25 years, the world is likely to still depend on fossil fuels for at least 50 percent of its energy needs [Chevron, 2012]. The International Energy Agency estimated in 2010 that the world oil supply rises by 85 million barrels per day and forecast that the global demand would average nearly 88 million barrels per day in 2011 [Whipple, 2010], which demonstrates a clear relationship between oil consumption and a country's economic situation.

Oil and gas are usually found in very remote regions that are different from the locations where they are processed and consumed. Pipelines provide the necessary transportation function for this form of energy. Pipelines are regarded as energy highways of the global oil and gas industry, and their impact on the energy industry and the general economy therefore cannot be underestimated. In North America, a total length of over 800,000 kms of transmission pipeline network transports 97% of Canadian crude oil and natural gas from the producing regions to markets throughout Canada and the United States. Statistics show [Canadian Energy Pipeline Association, 2007] that Canadian pipelines transport approximately 2.65 million barrels of crude oil and equivalent and 17.1 billion cubic feet (bcf) of natural gas daily. Moreover, virtually all oil and gas exports—worth $60 billion in 2009—are carried by pipelines [Canadian Energy Pipeline Association, 2012]. With an asset value of approximately $20 billion, the Canadian pipelines are anticipated to double in size by 2015 to meet the oil and gas production increases that are forecast. Among the world's nations, the United States and Canada have the largest networks of energy pipelines for both oil and natural gas.

Oil pipeline networks are classified into crude oil lines and refined product lines, and the crude oil lines are subdivided into gathering lines and trunk lines. Gathering lines are small pipelines, from 2 to 8 in. in diameter, and are used where crude oil is found deep within the Earth where it is impractical to use larger diameters [Alberta Energy and Utilities Board, 2007]. It is estimated that there are between 48,000 and 64,000 kms of small gathering lines in the United States. These small lines gather oil from many wells, both onshore and offshore, and connect to larger trunk lines ranging from 8 to 24 in. in diameter. Trunk lines include a few very large lines, such as the TransAlaska Pipeline System, which is 48 in. in diameter [Alberta Energy and Utilities Board, 2007]. There are approximately 89,000 km of crude oil trunk lines in the United States.

Gas gathering lines connect individual gas wells to field gas-treating and processing facilities or to branches of larger gathering systems. Most gas wells flow naturally with sufficient pressure to supply the energy needed to force the gas through the gathering line to the processing plant. Like crude oil trunk lines, gas transmission systems can cover large geographical areas and be several hundreds or thousands of miles long. One of the largest natural gas supplies is in western Siberia. A large-diameter pipeline system moves gas from that area, including a pipeline almost 4600 km long, to export gas to Western Europe [Hopkins, 2007]. These trunk lines, which have diameters ranging from 40 to 55 in., constitute an impressive pipeline network. Compared to crude oil pipelines, gas transmission lines operate at relative high pressures.

Oil and gas pipeline systems are remarkable for their efficiency and low cost. Compared to other conventional means of transportation, such as rail and trucks, pipelines provide a very cheap way to transport oil. For example, for every 1000 barrel-miles of transportation of petroleum, the cost by pipeline is between 4 and 12 cents, whereas those by rail and truck are 12 to 60 cents and 52 to 75 cents, respectively [Kennedy, 1993]. Oil and gas pipelines are also energy-efficient, consuming about 0.4% of the energy content of the crude oil or gas transported per 1000 km [Marcus, 2009].

1.2 PIPELINE SAFETY AND INTEGRITY MANAGEMENT

Pipeline integrity is maintained by coating and cathodic protection (CP) as well as by comprehensive pipeline safety maintenance programs generally called pipeline integrity management (PIM) programs. A PIM is a process to develop, implement, measure, and manage the integrity of a pipeline through assessment, mitigation, and prevention of risks to ensure safe, environmentally responsible, and reliable service [Nelson, 2002]. Integrity management of pipeline systems is essential to the safe and efficient transport of oil and natural gas on the basis of safety assessment and lifetime prediction. Attempts to define pipeline performance, structural strength, and lifetime spawn a number of specialized fields, including corrosion, materials science, fracture mechanics, nondestructive evaluation, electrochemistry, environmental science, and mathematical modeling on both microscopic and macroscopic scales.

The goal of a PIM program is to ensure that the risk is as low as is reasonably practicable [Nelson, 2002]. An integrity management program (IMP) is usually valid for two or three years and is then updated to include new or modified processes, developed during implementation of the PIM, through multiple time-driven integrity plans. A PIM program supports monitoring, inspection, and maintenance programs to reduce greatly the risk of failures that could cause disastrous consequences to human life, the environment, and business operations.

1.3 PIPELINE STRESS CORROSION CRACKING

A number of factors contribute to pipeline failures. Although corrosion is identified as the most common cause of oil and gas transmission pipeline failure [U.S. Department of Transportation, 2005], stress corrosion cracking has been identified as leading to a number of pipeline leaking and/or rupture events, with catastrophic consequences [National Energy Board, 1996].

Stress corrosion cracking (SCC) is a term used to describe service failure in engineering materials that occurs by slow, environmentally induced crack propagation. The crack propagation observed is the result of the combined and synergistic interactions of mechanical stress and corrosion reactions [Jones, 1992]. For a certain material, SCC occurrence depends on both an aggressive environment and a stress, especially a tensile stress. During the operation of pipelines in the field, line pipe steels are exposed to electrolytes trapped under disbonded coating, where solution chemistry or electrochemistry is developed to support pipeline SCC [Fu and Cheng, 2010]. The stress is due primarily to the internal operating pressure or pressure fluctuation of natural gas or liquid petroleum [Zheng et al., 1997]. Moreover, soil movement–induced longitudinal stress and strain contribute to the initiation and propagation of stress corrosion cracks in pipelines [Canadian Energy Pipeline Association, 1998]. A wide variety of factors experienced by pipelines during their operation have been demonstrated to affect and contribute to SCC at somewhat different levels, such as the steel metallurgy (chemical composition, grade, microstructure, heat treatment, alloying elements, impurities, and welding), environmental parameters (soil chemistry, conductivity, seasonal dry–wet cycle, temperature, humidity, CO2 and gas conditions, and microorganisms), coatings, and CP (type, properties, failure mode, coating compatibility with CP, and CP potential/current), stressing condition (pressure, pressure fluctuation, residual stress, longitudinal stress, local stress–strain concentration), and corrosion reaction (corrosion pits, geometry of pits, hydrogen evolution, passivity and passive film formation, active dissolution, and mass transport) [Parkins, 2000].

Pipeline SCC incidents throughout North America and the world, including in Australia, Russia, Iran, Saudi Arabic, Brazil, and Argentina, have highlighted threats to pipelines from this problem. In Canada, two major ruptures and fires on the TransCanada Pipeline System in 1995, together with further evidence of the more widespread nature of SCC, led to the initiation of a national inquiry. This was the first comprehensive inquiry in the world on pipeline SCC and has been far-reaching across Canadian pipelines and extended to other countries [National Energy Board, 1996].

In the United States, the Williams 26-in. pipeline ruptured near Toledo, Washington in 2003, resulting in the shutdown of its trunk line from Canada to Oregon [Williams Pipeline, 2003]. This pipeline had also failed in 1992, 1994, and 1999, failures all attributed to SCC. With the occurrence of SCC-caused failures of gas and liquid pipelines, an advisory bulletin was issued in 2003 to remind owners and operators of gas transmission and hazardous liquid pipelines to consider SCC as a risk factor when developing and implementing integrity management plans [Baker, 2005]. It was commented that SCC is a serious pipeline integrity issue of concern to operators of pipelines within the United States. When comparing the pipeline SCC statistics in the United States and Canada, it was pointed out that the fact that SCC represents only 1.5 percent of reportable incidents in the United States versus 17 percent in Canada is due to the far greater occurrence of third party damage in the United States.

Research on pipeline SCC could be tracked back to the 1980s, and still has global interest. Management of SCC in the modern pipeline industry has been integrated with companies' integrity management programs. Our understanding of this important problem has evolved to the stage that comprehensive reviews describing the scientific, technical, and practical aspects of SCC in pipelines are common, all of which have facilitated the development of this book.

In addition to SCC fundamentals, such as the metallurgical, environmental, and mechanical aspects of SCC and the correlation with various hydrogen damage and corrosion fatigue, this book covers a wide spectrum of topics. Specifically, it includes the primary characteristics of and factors contributing to pipeline SCC and reports on progress to date on the investigation and understanding of SCC in pipelines occurring in nearly neutral–pH, high-pH, and acidic soil environments. The pipeline weld poses a sensitive region to SCC. Consequently, welding metallurgy is included, and the implications of corrosion and SCC are discussed. As advanced pipeline materials, high-strength steels distinguish themselves from conventional pipeline steels with unique metallurgical, mechanical, and metallurgical microelectrochemical characteristics. All of them contribute to the occurrence of hydrogen damage, corrosion, and SCC. The corrosion- and SCC-preventive strain-based design of high-strength steel pipelines is discussed based on the latest research in this area. Finally, industrial experience in the management of pipeline SCC, including prevention, monitoring, and mitigation as well as its integration with pipeline IMP, is incorporated.

REFERENCES

Alberta Energy and Utilities Board (2007) Pipeline Performance in Alberta, 1990–2005, Report 2007-A, AEUB, Calgary, Alberta, Canada.

Baker, M, Jr. (2005) Final Report on Stress Corrosion Cracking Study, Integrity Management Program Delivery Order DTRS56-02-D-70036, Office of Pipeline Safety, U.S. Department of Transportation, Washington, DC.

Canadian Energy Pipeline Association (1998) Stress Corrosion Cracking Recommended Practices: Addendum on Circumferential SCC, CEPA, Calgary, Alberta, Canada.

Canadian Energy Pipeline Association (2007) CEPA Statistics, CEPA, Calgary, Alberta, Canada.

Canadian Energy Pipeline Association (2012) http://www.cepa.com/about-cepa/industry-information/factoids.

Cheng, Frank Y (2010) Pipeline engineering, in Pipeline Engineering, Y.F. Cheng, Editor, Encyclopedia of Life Support Systems (EOLSS), developed under the auspices of UNESCO, EOLSS Publishers, Oxford, UK.

Chevron (2012) Energy supply and demands, http://www.chevron.com/globalissues/energysupplydemand/.

Fu, AQ, Cheng, YF (2010) Electrochemical polarization behavior of X70 steel in thin carbonate/bicarbonate solution layers trapped under a disbonded coating and its implication on pipeline SCC, Corros. Sci. 52, 2511–2518.

Hopkins, P (2007) Oil and Gas Pipelines: Yesterday and Today, Pipeline Systems Division, American Society of Mechanical Engineers, New York.

Jones, RH (1992) Stress Corrosion Cracking: Materials Performance and Evaluation, ASM, Materials Park, OH.

Kennedy, JL (1993) Oil and Gas Pipeline Fundamentals, 2nd ed., PennWell, Tulsa, OK.

Marcus, S (2009) Oil and gas pipeline in Canada, J. Oil Gas 2, 15.

National Energy Board (1996) Stress Corrosion Cracking on Canadian Oil and Gas Pipelines, MH-2-95, NEB, Calgary, Alberta, Canada.

Nelson, BR (2002) Pipeline integrity: program development, risk assessment and data management, 11th Annual GIS for Oil and Gas Conference, Houston, TX.

Parkins, RN (2000) A review of stress corrosion cracking of high pressure gas pipelines, Corrosion 2000, Paper 363, NACE, Houston, TX.

U.S. Department of Transportation (2005) Pipeline Accident Brief, Research and Special Programs Administration, Office of Pipeline Safety, U.S. DOT, Washington, DC.

Whipple, T (2010) Peak oil review, Energy Bull., Aug. 16.

Williams Pipeline, Gas pipeline SCC: catastrophic ruptures, 1 May and 13 December 2003, http://www.corrosion-doctors.org/Pipeline/Williams-explosion.htm.

Zheng, W, MacLeod, FA, Revie, RW, Tyson, WR, Shen, G, Shehata, M, Ray, G, Kiff, D, McKinnon, J (1997) Growth of Stress Corrosion Cracks in Pipelines in Near-Neutral pH Environment: The CANMET Full-Scale Tests Final Report to the CANMET/Industry Consortium, CANMET/MTL, Ottawa, Ontario, Canada.

2

Fundamentals of Stress Corrosion Cracking

2.1 DEFINITION OF STRESS CORROSION CRACKING

Cracking is possibly the most common mode of material failure. It may be the most dangerous failure mechanism since fracture can occur instantaneously and without advance warning. Harsh environments can compound the cracking problem and accelerate the rate of failure. Environmentally assisted cracking (EAC) is a generic term used to describe various mechanisms for this phenomenon. It can generally be classified into three different forms: stress corrosion cracking (SCC), corrosion fatigue (CF), and hydrogen-induced cracking (HIC). The three forms can appear to be very similar in nature despite some fundamental differences.

SCC is defined slightly differently in various sources. For example, Jones [1992] defines SCC as a term used to describe service failures in engineering materials that occur by slow, environmentally induced crack propagation. The crack propagation observed is the result of the combined and synergistic interaction of mechanical stress and corrosion reactions. According to Corrosion Doctors [online source 1], SCC is a cracking process of metals that requires the simultaneous action of a corrodent and sustainable tensile stress. In Wikipedia, online source SCC is defined as the unexpected sudden failure of normally ductile metals subjected to constant tensile stress in a corrosive environment. Finally, SCC could be defined as cracking due to a process involving conjoint corrosion and straining of a metal due to residual or applied stresses [UK National Physical Laboratory, 1982].

Despite the difference in definitions, there is a common requirement for the occurrence of SCC. Three essential factors must happen simultaneously: a susceptible material, a corrosive environment, and sufficient tensile stress, as described in Fig. 2.1. Consequently, SCC is relatively rare, although failures can be very costly and destructive when they do occur.

Figure 2.1 Three essential factors required for the occurrence of SCC.

The stresses that cause SCC are either generated as a result of the use of metallic components in service or as residual stress introduced during manufacturing, such as welding and bending. The stress required to cause SCC is small, usually below the macroscopic yield strength of a metal. However, stress concentration may develop locally since stress corrosion cracks frequently initiate at surface flaws that either preexist or are formed during service by corrosion, wear, or other processes. Moreover, the stress must be tensile in nature, and comprehensive stresses can be used to prevent SCC.

The corrosive environment may be a permanent service environment, such as seawater for an offshore platform structure, or temporary environments caused by a particular operation, such as electrolyte residue after cleaning rust from a metallic structure. Moreover, environments that cause SCC are usually aqueous and can be either condensed layers of moisture or bulk solutions. Cracking will not normally occur when there is a significant corrosion rate, and stress corrosion cracks can initiate and propagate with little outside evidence of corrosion. Generally, SCC is observed in metal–environment combinations that result in the formation of a film on a metal surface [Jones, 1992]. Thus, SCC is of great concern in corrosion-resistant metals exposed to aggressive environments.

There are two types of SCC modes in metals: intergranular or transgranular. In the former mode, cracks grow along the grain boundaries, whereas cracks following the latter mode grow across the grains.

2.2 SPECIFIC METAL–ENVIRONMENT COMBINATIONS

SCC is not an inevitable process. For most metals in most environments it will not occur. It can therefore identify specific combinations of metal and environment that are subject to the problem. Table 2.1 lists some combinations of metal and environment that are most commonly associated with SCC. Some typical combinations that result in SCC are examined in detail as below.

1. Brass in ammonia-containing environments. This was first identified as an SCC problem when the brass cartridge cases used by the British Army in India were found to suffer from cracking, where the ammonia comes from the decay of organic material [Wikipedia, online source]. Since it usually occurred during the rainy season, or the stress corrosion cracks resembled those in seasoned wood, it is also called seasonal cracking. The cracking is intergranular.

2. Chloride SCC of stainless steel. Austenitic stainless steels suffer from SCC in hot solutions containing chloride, where a high chloride concentration is required. Even when the average amount of chloride in the environment is low, chloride can concentrate at heated surfaces, or by pitting or crevice corrosion, to cause cracking. The temperature usually needs to be above 70°C, although SCC can occur at lower temperatures in some situations, notably in connection with more acid solutions. Cracking continues at low stresses and commonly occurs as a result of residual stresses from welding or fabrication. Cracking is normally transgranular, although it may switch to an intergranular path as a result of sensitization of the steel [Cassagne, 2007].

3. Carbon steels in passivating environments. Carbon and low-alloy steels can suffer from SCC in a wide range of environments that tend to form a protective passive or oxide film. The environments that would passivate carbon steels have been found to cause SCC, including strong caustic solutions, phosphates, nitrates, carbonates, and high-temperature water. The problems are important for both economic and safety reasons, due to the extensive use of carbon steels. For example, caustic cracking of steam-generating boilers was a serious problem in the late-nineteenth century, where the necessary strong caustic solution was produced by evaporation of the very dilute solution inside the boiler as it escaped through leaks in the riveted seams [Brown, 1972]. More recently, gas transmission pipelines have cracked in carbonate–bicarbonate solutions trapped under disbonded coatings in the presence of nonappropriate CP, resulting potentially in pipeline leaking, rupture, and even more devastating consequences [National Energy Board, 1996].

4. Hydrogen embrittlement and HIC of high-strength steels. All steels are affected by hydrogen, as evidenced by the influence of hydrogen on SCC growth and the occurrence of HIC under the influence of very high hydrogen concentrations [Bernstein and Thompson, 1980]. However, hydrogen embrittlement (HE) under static load is experienced only in steels of relatively high strength. There is no clearly defined limit for the strength level above which HE will be experienced, since this problem will be a function of the amount of hydrogen in the steel, the stress applied, the severity of the stress concentration, and the composition and microstructure of the steel. Hydrogen may be introduced into the steel by a number of routes, including welding, pickling, electroplating, exposure to hydrogen-containing gases, and corrosion in service. Hydrogen penetrating steel may be released or may get trapped inside the metal.

Table 2.1 Some Combinations of Metals and Environments That Result in SCC

Since the first SCC documented in a boiler explosion, which was attributed to carbon steel caustic cracking, in the UK in 1865 [Galvele, 1999], the scope of environmentally induced cracking has increased considerably. Moreover, not only metals show SCC; it has been found [Wiederhorn and Bolz, 1970] that glasses experience cracking in the presence of water and that SCC can occur on ceramics and polymeric materials in corrosive environments [Jones, 1992].

2.3 METALLURGICAL ASPECTS OF SCC

The exact alloy composition, microstructure, and heat treatment can have marked effects on the SCC performance of a metal. Actually, stress corrosion cracks are usually initiated at the metallurgical defects contained in steels, which also affect the crack propagation mechanism and kinetics.

2.3.1 Effect of Strength of Materials on SCC

There is a common misunderstanding regarding how the strength of metals correlates with SCC (i.e., how an increase in strength of materials increases susceptibility to SCC). Actually, there have been few general rules governing the influence of material strength on SCC susceptibility [UK National Physical Laboratory, 1982]. For example, for the hydrogen embrittlement process, high strength normally increases the material susceptibility. However, the SCC process that replies on plastic strain at the crack tip will be easier for lower-strength materials.

2.3.2 Effect of Alloying Composition on SCC

Quite small changes in the composition of an alloy can have a marked influence on SCC behavior [UK National Physical Laboratory, 1982]. The effects of alloying additions are not necessarily consistent from one environment to another. For example, a high molybdenum content improves the resistance of a low-alloy steel to SCC in carbonate–bicarbonate solutions, but makes it more susceptible to caustic cracking [UK National Physical Laboratory, 1982].

2.3.3 Effect of Heat Treatment on SCC

Changes in the heat-treatment processes of an alloy can change its sensitivity to SCC, the mode of fracture, and even the fracture mechanism. Austenitic stainless steels suffer from transgranular SCC in chloride solutions. However, in steels heat-treated appropriately, SCC cracks could become intergranular [Alyousif and Nishimura, 2008]. Furthermore, if the same alloy is rolled, a certain amount of strain-induced martensite will be formed. This, combined with the high strength of work-hardened material, leads to a susceptibility to hydrogen embrittlement.

2.3.4 Grain Boundary Precipitation

A typical example of grain boundary precipitation is sensitization of austenitic stainless steels, where carbide precipitates at the grain boundaries, causing a depletion of chromium adjacent to the boundaries [Callister and Rethwisch, 2009]. Sensitization is an important metallurgical phenomenon occurring in austenitic stainless steels. It usually occurs in the temperature range 500 to 850°C, with the rate of precipitation controlled by the chromium diffusion. The chromium depletion zone can have a minimum chromium concentration of 8 to 10%, with a width of 10 nm to hundreds of nanometers. Due to the different chromium contents between grain boundaries and the grains, there exists a galvanic effect, where the grain boundaries serve as an anode and the grains as a cathode. A preferential dissolution of grain boundaries would result in the propagation of cracks along the grain boundaries, causing intergranular SCC. SCC susceptibility and crack growth rate can be described as the degree of sensitization, which is expressed as the width of the chromium depletion zone and the chromium concentration. The most common method of reducing the possibility of developing a sensitized microstructure is to reduce the carbon concentration in the steel or to control the thermal history of the material.

In addition to stainless steels, chromium carbide precipitation and chromium depletion can also occur in nickel-based alloys, such as alloy 600, but it does not connect to intergranular SCC of the alloy. Intermetallic precipitate can also form in aluminum alloys, resulting in intergranular SCC [Speidel, 1975], where the galvanic effect between the precipitates and the aluminum alloy matrix is important for cracking initiation and propagation. Sometimes, the precipitate is anodic to the matrix; in others, it is cathodic. In general, hydrogen evolution always accompanies the anodic reactions that occur on aluminum alloys. Since the passive layer on aluminum alloys is solvable in high-pH solutions, cathodic polarization that causes reduction of oxygen or water and the generation of hydroxyl ions stimulates dissolution of aluminum-passive film as well as hydrogen evolution. Thus, it is difficult to distinguish between anodic dissolution and hydrogen embrittlement for aluminum alloys in aqueous solutions.

2.3.5 Grain Boundary Segregation

Carbon and/or nitrogen segregation in grain boundaries plays an important role in the SCC of carbon steels. It was found that approximately 0.01% carbon is required to cause SCC of steels in nitrate or caustic environments [Parkins, 1990]. Actually, grain boundary enrichment of impurities can contribute to the intergranular SCC of iron-based alloys, austenitic stainless steels, and nickel-based alloys. The extent of their effect depends on the electrochemical potential, corrosive environment, and the type, shape, and concentration of impurities on the grain boundary.

Although segregation of substitutional elements to grain boundaries can strongly affect SCC of steels, it does not occur at all potentials or in all solutions. For example, phosphorus segregation promotes intergranular SCC of low-alloy steels in caustic or water environments at relatively oxidizing potentials [Burstein and Woodward, 1981; Bandyopadhyay and Briant, 1983]. However, phosphorus does not affect SCC at low potentials in caustic solutions [Bandyopadhyay and Briant, 1983] and has only modest effects in carbonate–bicarbonate solutions [Stenzel et al., 1986].

2.4 ELECTROCHEMISTRY OF SCC

A complete description of SCC must treat both the thermodynamic requirements and kinetic aspects of cracking. While knowledge of the thermodynamic conditions helps to determine whether cracking is feasible, kinetic information describes the rate at which cracks propagate.

2.4.1 SCC Thermodynamics

The thermodynamic conditions for anodic dissolution–based SCC include two aspects: Dissolution or oxidation of the metal in the electrolyte must be thermodynamically possible, and a protective film formed on the crack wall must be thermodynamically stable.

The first