Initiation of SCC in nickel base alloys in primary PWR environment: studies at Studsvik since mid 1980s

Abstract

Investigations on initiation of primary water stress corrosion cracking (PWSCC) in alloys 600 and 690 in simulated primary pressurised water reactor (PWR) environments, carried out since 1985, are reviewed. A large number of tests, mostly on reverse U bend specimens, some from steam generator tubes from operating PWRs, have been conducted for periods up to 33 000 h (4 years). Most exposures were at 365 or 330°C in either high purity water with hydrogen addition or simulated primary water (representing beginning of cycle conditions). Parameters investigated include: material (composition, heat treatment (mill annealed or thermally treated), carbide distribution, yield strength, grain size, etc.), environmental (hydrogen content, boron–lithium–pH, zinc) and experimental environment control techniques. Although the PWSCC mechanism has yet to be fully explained, these studies provide an overview of important parameters for crack initiation. The present review aims to survey the evidence for PWSCC initiation in nickel base alloys. It is concluded that, within the normal range for operating PWRs, the influence of dissolved hydrogen on initiation is small. However, a weak maximum in crack growth rate is observed at 15–25 ml H₂/kg H₂O; a corresponding minimum in crack initiation time has not been confirmed but cannot be excluded. It is concluded that hydrogen must be reduced to well below 10 ml H₂/kg H₂O to achieve significant benefits. Increasing the Li content from 2–2·5 to 3·5 ppm is shown to reduce crack initiation time by 30–50%. Lithium in the range 2·2–7·5 ppm has little effect on crack growth rate at ～1200 ppm B. Boron appears to have little influence on crack initiation or growth.     

Characterisation of oxide films formed on Alloy 600 in simulated PWR primary water

Abstract

Long term exposure of austenitic nickel based alloys in the primary water of pressurised water reactors leads to the development of an oxide film that is generally considered to influence the stress corrosion cracking behaviour of the alloys. The structure, composition and thickness of the film depend on the chemical composition of the alloy and the exposure conditions. Previous laboratory tests have indicated that zinc can lower the susceptibility to initiation of primary water stress corrosion cracking of Alloy 600. In the present work, oxide films have been grown on polished samples of Alloy 600 (15 × 20 × 1 mm) under simulated beginning of cycle primary water at 303°C for periods between 485 and 725 h and their morphology, chemical composition and corrosion behaviour examined. Layers oxidised under stable chemical conditions were less porous and showed higher corrosion resistance. Samples oxidised for longer times showed higher transpassive potentials. Initial results from electrochemical data showed an influence of zinc on the corrosion susceptibility of Alloy 600 that requires further evaluation.     

Corrosion monitoring application in Taiwan's nuclear power plants

Abstract

Corrosion monitoring methods have been applied extensively for system condition diagnosis, material degradation resistance evaluations, and water chemistry optimisation in Taiwan's boiling water and pressurised water reactor nuclear power plants. The online corrosion monitoring systems developed and currently applied in Taiwan are reviewed. Examples of applications discussed include electrochemical potential monitoring, online crack growth rate measurements, electrochemical noise monitoring, linear polarisation resistance monitoring, and potential drop pipe thinning monitoring. Online corrosion monitors have proven useful tools within the nuclear power plant monitoring systems currently installed and have achieved their specific objectives in the context of Taiwan's power plants.     

Monitoring of stress corrosion cracking of sensitised 304H stainless steel in nuclear applications by electrochemical methods and acoustic emission

Abstract

A hybrid monitoring technique for stress corrosion cracking (SCC) has been developed that employs simultaneously localised corrosion monitoring, electrochemical noise and acoustic emission (AE) techniques. The application of the hybrid technique for detection of SCC initiation and propagation in sensitised 304H stainless steel in dilute tetrathionate solutions at ambient temperature is reported. Initial result shows that SCC initiation and its early stage propagation can be detected by the localised corrosion monitoring and electrochemical noise methods. The dimensions of the crack can be estimated from the charge values derived from the detected transients. The locations of AE events determined using two sensors are in good agreement with the locations of cracks observed in the specimen. The AE technique is sensitive to rapid crack propagation, but does not appear to be sensitive to SCC initiation and early stage propagation for the present material environment load combination. It is postulated that AE is sensitive to SCC propagation involving a relatively large volume of plastic deformation. On the basis of test results and on information from the literature, it is suggested that in this material environment system SCC cracks initiate via slow anodic dissolution at the chromium depleted grain boundaries. Subsequently, elemental sulphur adsorbed on the surface around the crack tip catalyses the entry of hydrogen atoms produced by the hydrogen reduction reaction into the steel matrix ahead of the crack tip; this hydrogen accumulates gradually over a relatively long period of time and preferentially at carbide/matrix interfaces, eventually causing hydrogen induced brittle fracture along grain boundaries.     

Stress corrosion cracking of oil and gas pipelines in near neutral pH environment: review of recent research

Abstract

Since the discovery of transgranular stress corrosion cracking (SCC) on a Canadian gas transmission line in 1985, much research has been conducted in the past 20 years. Findings of the effects of operating conditions, metallurgical and the environmental factors have been useful in preventing and mitigating failures. Several overviews of this problem can be found in the literature and the purpose of this update is to review the research results produced since the turn of the century. The recent report of SCC under static stressing conditions confirms that the cracking is indeed a true SCC process, although the rate of which is low without dynamic loading. In contrast to the high pH pipeline stress corrosion cracking in the carbonate–bicarbonate solution, this forms of cracking in dilute near neutral environment takes much longer time to initiate. Once initiated, the crack growth rate is highly sensitive to the loading rate of the applied mechanical force.     

Grain boundary control for improved intergranular stress corrosion cracking resistance in austenitic stainless steels: new approach

Abstract

The present paper provides an overview of a new approach which has focused on the behaviour of special grain boundaries in sensitised austenitic stainless steel. The aim of the work was to develop a general model for stress corrosion cracking, which would ultimately be capable of predicting the effects of the degree of sensitisation, the connectivity of special boundaries and the influence of stress gradients, such as those developed from surface preparation (machining or peening) or due to the stress concentration effect of pit formation. Experimental work using electron backscatter diffraction analysis and in situ high resolution computed X-ray tomography has correlated cracking with the microstructure in a type 304 austenitic stainless steel. In situ three-dimensional observations demonstrated that annealing twins cause local crack arrest and diversion, leaving non-fractured ligaments in the wake of the cracking path. The mechanical effects of the deformation and failure of these bridges have been modelled, demonstrating that special grain boundaries cause crack tip shielding. Increasing the fraction of special boundaries and decreasing grain size are both predicted to increase stress corrosion cracking resistance. Experimental observations using room temperature intergranular stress corrosion tests and high temperature autoclave tests confirm these predictions for thermomechanically processed microstructures. The effects of applied stress and stress gradients are also predicted by the model, which may be extended to include the kinetics of crack growth, clustering of grain boundary types and variation of the degree of sensitisation.