Stress corrosion cracking crack initiation

Figure 10.7 Stress-corrosion cracking initially propagates perpendicularly to the crystalline grains of the steel subsequently it propagates longitudinally [4]...

A. Eslaini, B. Fang, R. Kania, B. Worthingham, J. Been, R. Eadie, W. Chen, Stress corrosion cracking initiation under the disbonded coating of pipeline steel in near-neutral pH environment, Corros. Sci. 52 (2010) 3750-3756. 

M. Elboujdaini, Y.-Z. Wang, R. W. Revie, R. N. Parkins, and M. T. Shehata, Stress corrosion crack initiation processes Pitting and microcrack coalescence. Paper No. 00379, CORROSION/2000, NACE International, Houston, TX, 2000. 

Observations on Stress Corrosion Cracking Initiation Sites in Carbon Steel Conference Corrosion 94, Paper No. 234 NACE, Houston, TX, USA, 1994, 13 S. [4 Van der Sluys, W. A. DeMiglio, D. S. Use of a Constant Delta Test Method in the Investigation of Fatigue Crack Growth in 288 deg C Water Environments. Conference Environment-Sensitive Fracture Evaluation and Comparison Test Methods, Gaithersburg, Maryland, USA, 26-28 Apr. 1982... 

Yamamoto, M. Kuniya, J. Uchida, S. Stress Corrosion Cracking Initiation Behaviour of XM-19 Stainless Steel Under Long-Term Uniaxial Constant Load Testing in High-Temperature Water NACE Corrosion 60 (2004) 7, S. 681-688... 

SCC occurs when the mechanical stress is high enough. According to several SCC models (Sec. 5.5), the macroscopic stress level is not necessarily the mechanical parameter that controls the cracking process. However, from an engineering point of view, it is accepted that the stress corrosion cracks initiate when the stress exceeds a threshold Oj, and propagate when the stress intensity factor is in excess of a threshold i-scc-... 

Stress corrosion cracking initiate and propagate without any outside evidence of corrosion. The failures can take place without any previous warning. They often initiate at pre-existing flaws or flaws formed during the service period of the component. The following are generally the sites for crack initiation. 

P.M. Scott, P. Combrade, On the mechanism of stress corrosion crack initiation and growth in alloy 600 exposed to PWR primary water, in Proc. Of the 11th Int. Symp. Environmental Degradation Materials Nuclear Power Systems — Water Reactors, Stevenson, WA, 10—14 August, 2003, 2003, pp. 29—38. ANS. 

Stress Corrosion Crocking. Stress corrosion cracking occurs from the combined action of corrosion and stress. The corrosion may be initiated by improper chemical cleaning, high dissolved oxygen levels, pH excursions in the boiler water, the presence of free hydroxide, and high levels of chlorides. Stresses are either residual in the metal or caused by thermal excursions. Rapid startup or shutdown can cause or further aggravate stresses. Tube failures occur near stressed areas such as welds, supports, or cold worked areas. 

Electrochemical noise A variety of related techniques are now available to monitor localized corrosion. No external polarization of the corroding metal is required, but the electrical noise on the corrosion potential of the metal is monitored and analyzed. Signatures characteristic of pit initiation, crevice corrosion and some forms of stress corrosion cracking is obtained. 

The arbitrary division of behaviour has been made because of the extreme behaviour of some chemicals that initiate small areas of attack on a well-passivated metal surface. The form of attack may manifest itself as stress-corrosion cracking, crevice attack or pitting. At certain temperatures and pressures, minute quantities of certain chemicals can result in this form of attack. Chloride ions, in particular, are responsible for many of the failures observed, and it can be present as an impurity in a large number of raw materials. This has led to the development of metals and alloys that can withstand pitting and crevice corrosion, but on the whole these are comparatively expensive. It has become important, therefore, to be able to predict the conditions where more conventional materials may be used. The effect of an increase in concentration on pitting corrosion follows a similar relationship to the Freundlich equation where... 

Any test (several such tests are used) in which time to failure of smooth specimens is determined is an overall measure of the incubation period to initiate a crack, the ability to resist the propagation of a stress corrosion crack and the ability to resist final mechanical fracture. Since this test does not indicate the relative merits of an alloy in each individual aspect of the... 

Clearly the variables that may influence or the threshold stress for initially plain specimens, and hence the susceptibility to stress-corrosion cracking, are P and rj, i.e. 

Fig. 8.3 Schematic representation of the stress corrosion cracking mechanism of the pit (after Pickering and Swann ). (a) Tubular pits initiated at solute-rich slip step. The pits may, but need not necessarily, follow the slip plane once they are initiated, (b) Ductile tearing along a plane containing the tubular pits. The stress is increased across the plane because of the reduced cross section and the stress raising effect...

Fig. 8.36 Minimum strains for initiating stress-corrosion cracks in annealed and in sensitised 316 during slow strain rate tests in S ppm chloride content water...

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