Анализ причин возникновения и развития стресс-коррозионных дефектов в процессе длительной эксплуатации подземных трубопроводов

However, the time required for crack initiation is strongly dependent on a wide variety of parameters, such as surface finish. The presence of flaws that concentrate stress or crevices that alter the environment may dramatically change the threshold stress or the crack-initiation time. The entire crack-initiation process is presently not well understood. Tests on statically loaded precracked samples are usually conducted with either a constant applied load or with a fixed crack opening displacement, and the actual rate or velocity of crack propagation, da/dt, is measured. The magnitude of the stress distribution at the crack tip (the mechanical driving force for crack propagation) is quantified by the stress-intensity factor, K, for the specific crack and loading geometry.

As a result, the crack-propagation rate, da/dt, is plotted versus K, as illustrated in Fig. 2. These tests can be configured such that K increases with crack length (constant applied load), decreases with increasing crack length (constant crack mouth opening displacement), or is approximately constant as the crack length changes (special tapered samples). Each type of test has its advantages and disadvantages. However, in service, most SCC failures occur under constant-load conditions, so that the stress intensity increases as the crack propagates. As a result, it is usually assumed in SCC discussions that the stress intensity is increasing with increasing crack length.

Typically, three regions of crack-propagation rate versus stress-intensity level are found during crack-propagation experiments. These are identified according to increasing stress-intensity factor as stage 1,2, or 3 crack propagation (Fig. 2). No crack propagation is observed below some threshold stress-intensity level, K_ISCC. This threshold stress level is determined not only by the alloy but also by the environment and metallurgical condition of the alloy, and, presumably, this level corresponds to the minimum required stress level for synergistic interaction with the environment. At low stress-intensity levels (stage 1), the crack-propagation rate increases rapidly with the stress-intensity factor. At intermediate stress-intensity levels (stage 2), the crack-propagation rate approaches some constant velocity that is virtually independent of the mechanical driving force. This plateau velocity is characteristic of the alloy/environment combination and is the result of rate-limiting environmental processes such as mass transport of environmental species up the crack to the crack tip. In stage 3, the rate of crack propagation exceeds the plateau velocity as the stress-intensity level approaches the critical stress-intensity level for mechanical fracture in an inert environment, K_IC.

Slow-Strain-Rate Testing. Stress-corrosion tests can also be conducted by slowly increasing the load or strain on either precracked or smooth samples. These tests are called constant-extension-rate tests, slow-strain-rate tests, or straining electrode tests. Usually, a tensile machine pulls a smooth sample that is exposed to the corrosive environment at a low crosshead speed (10^-5 to 10^-9 m/s). The strain to failure in the corrosive environment and the strain to failure in an inert environment can then be plotted against the strain rate, as shown in Fig. 3(a), or the ratio of these measurements can be plotted as shown in Fig. 3(b). The ratio of other tensile-property measurements, such as reduction in area and ultimate tensile strength, may be plotted. Frequently, this type of test is used to evaluate the influence of metallurgical variables, such as heat treatment, on SCC resistance. This type of experiment yields rapid comparisons. However, since the mechanical properties of the samples also vary with the metallurgical condition, such evaluations can become difficult.

As a result, it was proposed (Ref 13) that the environment-dependent property be plotted versus the inert-environment or environment-independent value of this property, as shown in Fig. 4. In this manner, the strength of the material in the environment, or the "situation-dependent strength", and the extent of any environmental effect can be visualized simultaneously. However, the application of these data to the prediction of actual inservice lifetimes is difficult and unreliable.

Overview of SCC Mechanisms

Many different mechanisms have been proposed to explain the synergistic stress-corrosion interactions that occur at the crack tip, and there may be more than one process that causes SCC. The proposed mechanisms can be classed into two basic categories: anodic mechanisms and cathodic mechanisms. That is, during corrosion, both anodic and cathodic reactions must occur, and the phenomena that result in crack propagation may be associated with either type. The most obvious anodic mechanism is that of simple active dissolution and removal of material from the crack tip. The most obvious cathodic mechanism is hydrogen evolution, absorption, diffusion, and embrittlement. However, a specific mechanism must be able to explain the actual crack-propagation rates, the fractographic evidence, and the mechanism of formation or nucleation of cracks.

Mechanical fracture includes normal fracture processes that are assumed to be stimulated or induced by one of the following interactions between the material and the environment:

· Adsorption of environmental species

· Surface reactions

· Reactions in the metal ahead of the crack tip

· Surface films

All of the proposed mechanical fracture mechanisms contain one or more of these processes as an essential step in the SCC process. Specific mechanisms differ in the processes assumed to be responsible for crack propagation and the way that environmental reactions combine to result in the actual fracture process.

Controlling Parameters

The mechanisms that have been proposed for SCC require that certain processes or events occur in sequence for sustained crack propagation to be possible. These requirements explain the plateau region in which the rate of crack propagation is independent of the applied mechanical stress. That is, a sequence of chemical reactions and processes is required, and the rate-limiting step in this sequence of events determines the limiting rate or plateau velocity of crack propagation (until mechanical overload fracture starts contributing to the fracture process in stage 3). Figure 5 illustrates a crack tip in which crack propagation results from reactions in metal ahead of the propagating crack. This example was chosen because it maximizes the number of possible rate-limiting steps. Close examination of Fig. 5 reveals that potential rate-determining steps include:

· Mass transport along the crack to or away from the crack tip

· Reactions in the solution near the crack

· Surface adsorption at or near the crack tip

· Surface diffusion

· Surface reactions

· Absorption into the bulk

· Bulk diffusion to the plastic zone ahead of the advancing crack

· Chemical reactions in the bulk

· The rate of interatomic bond rupture

Changes in the environment that modify the rate-determining step will have a dramatic influence on the rate of crack propagation, while alterations to factors not involved in the rate-determining step or steps will have little influence, if any. However, significantly retarding the rate of any one of the required steps in the sequence could make that step the rate-determining one. In aqueous solutions, the rate of adsorption and surface reactions is usually very fast compared to the rate of mass transport along the crack to the crack tip. As a result, bulk transport into this region or reactions in this region are frequently believed to be responsible for determining the steady-state crack-propagation rate or plateau velocity. In gaseous environments, surface reactions, surface diffusion, and adsorption may be rate limiting, as well as the rate of bulk transport to the crack tip. Several different environmental parameters are known to influence the rate of crack growth in aqueous solutions. These include, but are not limited to:

· Temperature

· Pressure

· Solute species

· Solute concentration and activity . pH

· Electrochemical potential

· Solution viscosity

· Stirring or mixing

Altering any of these parameters may affect the rate of the rate-controlling steps, either accelerating or reducing the rate of crack propagation. Also, it may be possible to arrest or stimulate crack propagation by altering the rate of an environmental reaction. It is well known and generally accepted that the environment at occluded sites, such as crack tips, can differ significantly from the bulk solution. If an alteration to the bulk environment allows the formation of a critical SCC environment at crack nuclei, then crack propagation will result. If the bulk environment cannot maintain this local crack-tip environment, then crack propagation will stop. As a result, slight changes to the environment may have a dramatic influence on crack propagation, while dramatic changes may have only a slight influence. In addition to the environmental parameters listed above, stress-corrosion crack-propagation rates are influenced by:

· The magnitude of the applied stress or the stress-intensity factor

· The stress state, which includes (1) plane stress and (2) plane strain *

· The loading mode at the crack tip (tension or torsion, for example)

· Alloy composition, which includes (1) nominal composition, (2) exact composition (all constituents), and (3) impurity or tramp element composition

· Metallurgical condition, which includes (1) strength level, (2) second phases present in the matrix and at the grain boundaries, (3) composition of phases, (4) grain size, (5) grain-boundary segregation, and (6) residual stresses

· Crack geometry, which includes (1) length, width, and aspect ratio, and (2) crack opening and crack-tip closure

Important Fracture Features

Stress-corrosion cracks can initiate and propagate with little outside evidence of corrosion and with no warning as catastrophic failure approaches. The cracks frequently initiate at surface flaws that either preexist or are formed during service by corrosion, wear, or other processes. The cracks then grow with little macroscopic evidence of mechanical deformation in metals and alloys that are normally quite ductile. Crack propagation can be either intergranular or trans- granular; sometimes, both types are observed on the same fracture surface.

Crack openings and the deformation associated with crack propagation may be so small that the cracks are virtually invisible except in special nondestructive examinations. As the stress intensity increases, the plastic deformation associated with crack propagation increases and the crack opening increases. When the final fracture region is approached, plastic deformation can be appreciable, because corrosionresistant alloys are frequently quite ductile.

Phenomenology of Crack-Initiation Processes

Crack Initiation at Surface Discontinuities

Stress-corrosion cracking frequently initiates at preexisting or corrosion-induced surface features. These features may include grooves, laps, or burrs caused by fabrication processes. Examples of such features are shown in Fig. 6; these were produced during grinding in the preparation of a joint for welding. The feature shown in Fig. 6(a) is a lap, which is subsequently recrystallized during welding and could then act as a crevice at which deleterious anions or cations concentrate. The highly sensitized recrystallized material could also more readily become the site of crack initiation by intergranular corrosion. A coldworked layer and surface burrs, shown in Fig. 6(b), can also assist crack initiation.

Crack Initiation at Corrosion Pits. Stresscorrosion cracks can also initiate at pits that form during exposure to the service environment (Fig. 7) or during cleaning operations, such as pickling of type 304 stainless steel before fabrication. Pits can form at inclusions that intersect the free surface or by a breakdown in the protective film. In electrochemical terms, pits form when the potential exceeds the pitting potential. It has been shown that the SCC potential and pitting potential were identical for steel in nitrite solutions.

The transition between pitting and cracking depends on the same parameters that control SCC, that is, the electrochemistry at the base of the pit, pit geometry, chemistry of the material, and stress or strain rate at the base of the pit. Adetailed description of the relationship between these parameters and crack initiation has not been developed because of the difficulty in measuring crack initiation. Methods for measuring short surface cracks are under development, but are limited to detecting cracks that are beyond the initiation stage. The geometry of apit is important in determining the stress and strain rate at its base. Generally, the aspect ratio between the penetration and the lateral corrosion of a pit must be greater than about 10 before a pit acts as a crackinitiation site. A penetration to lateral corrosion ratio of 1 corresponds to uniform corrosion, and a ratio of about 1000 is generally observed for a growing stress-corrosion crack. As in the ease of a growing crack, the pit walls must exhibit some passive-film-forming capability in order for the corrosion ratio to exceed 1. A change in the corrosive environment and potential within a pit may also be necessary for the pit to act as a crack initiator. Pits can act as occluded cells similar to cracks and crevices, although in general their volume is not as restricted. There are a number of examples in which stresscorrosion cracks initiated at the base of a pit by intergranylar corrosion. In these circumstances, the grain-boundary chemistry and the pit chemistry were such that intergranular corrosion was favored. Crack propagation was also by intergranular SCC in these cases. Although the local stresses and strain rates at the base of the pit play a role in SCC initiation, there are examples of preexisting pits that did not initiate stress-corrosion cracks.

This observation has led to the conclusion mat the electrochemistry of the pit is more important than the local stress or strain rate. A preexisting pit may not develop the same local electrochemistry as one grown during service, because the development of a concentration cell depends on the presence of an actively corroding tip that establishes the anion and cation current flows. Similarly, an inability to reinitiate crack growth in samples in which active growth was occurring before the samples were removed from solution, rinsed, dried, and reinserted into solution also suggests that the local chemistry is very important. Crack Initiation by Intergranular Corrosion or Slip Dissolution.

Stress-corrosion crack initiation can also occur in the absence of pitting by intergranular or slip-dissolution processes. Intergranular corrosion-initiated SCC requires that the local grain-boundary chemistry differ from the bulk chemistry. This condition occurs in sensitized austenitic stainless steels or with the segregation of impurities such as phosphorus, sulfur, or silicon in a variety of materials. Slip-dissolu tion-initiated SCC results from local corrosion at emerging slip planes and occurs primarily in lowstacking-fault materials.

The processes of crack initiation and propagation by the slip-dissolution process are in fact very similar. Crack initiation can be thought of as occurring in several stages, including the first few atomic layers that fail due to the transition from shortcrack to long-crack behavior. Evidence obtained by electrochemical and acoustic-emission monitoring of crack initiation in austenitic stainless steel suggests that the process occurs by the initiation of multiple short cracks prior to the propagation of a single dominant crack.

Retardation of the initiated cracks occurs because of variations in grain-boundary sensitization, crack angle, electrochemistry, and so on, with cracks being arrested after they grow about one grain diameter. Details of the conditions required for a single dominant crack to propagate have not been fully evaluated, but most likely it is a statistical process where there is a finite probability for a crack to find a path that does not retard its growth. Another concept suggested by Parkins for gas pipeline SCC initiation involves crack coalescence. It has been suggested that crack initiation in gas pipeline steels occurs by the following steps:

· Penetration of ground water to the pipeline surface and generation of a critical SCC environment

· Initially rapid formation of multiple cracks, the velocity decreasing with time to a constant value

· A period of constant crack-growth velocity

· Conventional stages 1,2, and 3 as described by single-crack behavior, during which time crack coalescence leads to final fracture

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