STRESS CORROSION:Stress corrosion cracking (SCC) is the formation of brittle cracks in a normally sound material through the simultaneous action of a tensile stress and a corrosive environment. In most cases, SCC has been associated with the process of active path corrosion (APC) whereby the corrosive attack or anodic dissolution initiates at specific localized sites and is focused along specific paths within the material. In some cases, these are along grain boundaries, in other cases, the path is along specific crystallographic within the grains. Quite often, SCC is strongly affected by alloy composition, the concentration of specific corrodent species, and, to a lesser degree, the stress intensity. In some cases, this latter point may make the use of test methods based on fracture mechanics concepts difficult to utilize effectively due to excessive crack branching and tendencies for nonplanar propagation of cracks.

Furthermore, corrosion film characteristics (i.e., passivation) and local anodic attack (i.e., depassivation) serve as controlling factors in SCC crack initiation and growth. Therefore, localized corrosion can promote SCC making exposure geometry and specimen design important factors. In many cases, mechanical straining or electrochemical inducements such as crevices or controlled potential are utilized to overcome the problems and uncertainties of SCC initiation so that the inherent resistance of the material to SCC can be obtained at reasonable test duration (see Table 1).

Table 1 - Applied Potentials for SCC in Steel Exposed to Various Service Environments

Environment Potential rate (mV, SCE)Nitrate -250 to +1200Liquid ammonia -400 to > +1500Carbonate -650 to -550Hydroxide -1100 to -850 and +350 to +500

THE EFFECTS OF STRESS ON THE SEVERITY OF AXIAL SCC

The great majority of SCC failures in the Canadian system occurred as axial cracking, driven by the hoop stress caused by the operating pressure of oil or gas pipelines. In the NEB Report on Pipeline SCC [1], it is shown in Figure 1 that SCC colonies could form on pipe sections where the operating stress was as low as 64% of the specified minimum yield stress (SMYS). It should be pointed out that the operating pressure for this particular pipeline is generally quite stable, with most of the pressure fluctuation events being associated with R-values of 0.9 or greater (R=minimum pressure / maximum pressure) and only infrequent excursions to lower R-values. For such loading conditions (i.e., maximum stress 64% SMYS and R=0.9), it has been difficult to initiate stress corrosion cracks under laboratory test conditions. In fact, it was reported that even for a lower R-value of 0.85, cracks could only be grown in the laboratory at a stress level of 72% SMYS or higher [10].

Figure 1: Variation of SCC severity as a function of operating stress for one gas pipeline [1].

There could be a number of reasons for this discrepancy. For example, the presence of residual stress on the pipe, a result of pipe-making processes, can be quite significant and thus the true stress in the sites where SCC initiated can be much above the nominal applied stress level. The stress concentration effect of pipe surface features such as welds and corrosion pits are well known to act as stress raisers. In fact, a significant portion of SCC failures have been associated with the welds, as shown in Figure 2.

Figure 2 Distribution of axial SCC ruptures as function of pipe surface features

(based on data in [1] )

It should be noted that corrosion grooves, or "linear corrosion" as it is known, forms on pipe surface when the tape coating wrinkles to form long and narrow pockets of disbandment and the subsequent corrosion takes on the appearance of the coating wrinkles.

In the case of DSAW, finite element calculations revealed that the stress level in the vicinity of the weld toe can be significantly higher than that in the pipe body. Figure 3 shows the stress profile for a typical seam weld geometry. Only half of the pipe wall thickness is indicated in the figure, as the weld was assumed to be symmetrical across the mid-wall line. Under an applied stress of 340 MPa (77% of SMYS for the pipe in question), there is a zone a few mm width in

Figure 3 Results of finite element calculation of stress levels in the vicinity of a weld.

which the actual local stress is close to the SMYS of the base steel. At the very toe, the stress is above the actual yield point of the material.

In one series of measurements of residual stress in pipes retrieved from service, tensile residual stresses in the range of 20% SMYS were often found to exist in the pipe wall up to a depth of about 1 mm [11], and the level of residual stress varied as a function of distance from the pipe surface. Thus if the nominal operating stress is at 72% of SMYS, the total net stress could be at 92% SMYS in the metal at this depth under the surface; such a stress level is conceivably high enough for crack initiation for many SCC systems.

EFFECTS OF STRESS FLUCTUATION

As in the case of pipeline SCC in carbonate-bicarbonate environment, the severity of transgranular SCC is not only affected by stress level per se, but also the degree of stress fluctuation. In a CANMET study on crack initiation [6], detectable cracks could be produced when stress was applied in a cyclic wave with a maximum of 90% SMYS and R=0.6. The cracking severity was much increased when the R-value was reduced to 0.4, under the same environmental conditions, maximum stress, load frequency and wave form. While these R-values are not typical of many gas transmission pipelines, the results do show the effects of R-values.

Laboratory results on the growth of deep SCC cracks demonstrate dramatic effects of pressure fluctuation. Figure 4 shows typical growth behavior of cracks in a full-scale test. In such tests, sections of full-size pipes containing sharp fatigue pre-cracks were buried in soil,

Figure 4 Typical growth behavior of cracks during full-scale tests showing the effect of pressure fluctuation. ("P"- pressure in psi, "S" - static hold period (min.) and "Dyn" - Dynamic load period (min.))

It should be noted that corrosion grooves, or "linear corrosion" as it is known, forms on pipe surface when the tape coating wrinkles to form long and narrow pockets of disbandment and the subsequent corrosion takes on the appearance of the coating wrinkles.

EFFECTS OF COMPRESSIVE RESIDUAL STRESS INTRODUCED BY HYDRROSTATIC TESTING

Hydrostatic testing is the primary operational measure for eliminating major axial defects in pipelines. Since hydrostatic tests can be performed at pressure levels of 125% to 140% of the maximum operating pressure, the critical defect size at hydrotest pressure is smaller than that associated with normal service conditions. Because of this difference, hydrostatic testing provides a safety margin against subsequent service failure. In order to evaluate quantitatively the effects of hydrotesting on SCC growth behaviour, two independent test programs were carried out, one using pre-cracked CT-type specimens [7] and the other using an X-52 full-scale pipe [20]. In both cases, SCC growth was started by applying cyclic loading and a high load excursion was applied to simulate a field hydrotest event. Following the excursion, the SCC growth rate was measured again for some time until reliable, consistent growth rate data could be obtained. Figure 8 [20] shows a comparison of the crack growth rates for fifteen cracks before and after a hydrotest performed on a full-scale pipe. The highest pressure reached during the hydrotest equaled 108% of the yield stress of the line pipe. All cracks showed detectable reduction in growth rate after the

hydrotest. Before the first hydrotest, three cracks showed growth rates in the order of 2.0*10-3 mm/day or about 0.73 mm per year. The highest growth rates of all 15 cracks, of depths generally between 35 to 50% of the wall thickness of the pipe, was about 0.8*10-3 mm per day after the test. In fact, two cracks became practically dormant, and their growth rates were not measurable by the crack detection [DCPD] system. It has been argued that hydrotesting could significantly increase the crack tip radius, thus reducing the effective mechanical driving force for subsequent SCC growth. However, in the full-scale study, metallographic examination suggested this is not the case. Most of the nine cracks examined metallographically following the test program had a crack tip opening of a few microns, usually less than 5 microns. Therefore, the crack was essentially a sharp one for

Figure 8 Effects of Hydrostatic Testing on SCC Growth Rates [20]

practical purposes. Again, the effect of hydrogen or the corrosion environment on the behaviour of a crack during and after the overload remains unclear. In one recently reported study using A537 steel (yield strength 380 MPa) [21], the behaviours of a fatigue crack during and after a single overload in air, in a 3.5% NaCl solution at the free corrosion potential, and in the same solution but under cathodic polarization were all different. Whereas the instantaneous crack extension upon the overload was significantly greater when the steel was under cathodic polarization, the overall overload retardation zone was much smaller when the steel was tested in the salt solution than in air. The embrittling effect of hydrogen was surmised by the authors to be the reason for this observation.

In the case of linepipe steel in near-neutral pH environment, the retarding effects of hydrotesting on SCC growth may be a result of the creation of compressive residual stress in front of the crack tip. It is well-known that a compressive region is generated at a crack tip by overloading; the compressive stress can be as large as the yield stress [22].

CONCLUSION

The following conclusions can be drawn from the preceding discussions:

1. Depending on the surface geometry of the pipe, the net total stress available for the initiation and growth of stress corrosion cracks may be considerably greater than the nominal operating stress as the presence of residual stress and stress raisers contribute to the local stress.

2. In the laboratory tests carried out using cyclic loading with the maximum load below the yield stress of the steels, stress fluctuation is required for crack initiation and growth. The crack growth rates are found to increase with the time rate of J on a log-log plot.

3. When a linepipe steel is stressed close to its yield point in a susceptible environment, cracks may develop with very minor pressure fluctuation. In these cases, low-temperature creep can be a factor in generating the necessary plastic straining and the presence of hydrogen in the steel may facilitate this creep process.

4. Hydrostatic testing retards subsequent crack growth. It is probable that compressive residual stress plays a key role in the retardation. Hydrogen effects may also be involved.