Wenyue Zheng

Hydrogen embrittlement susceptibility of galvanized 4135 steel in cement environment

Abstract Hydrogen embrittlement (HE) in SAE 4135 steel in a simulated cement solution was studied in terms of environmental susceptibility and metallurgical influences. Slow strain rate testing (SSRT) showed that the steel is susceptible to hydrogen embrittlement when its potential is less noble than −1.0V (SCE) for all three tempering temperatures studied, i.e. 480, 580 and 650°C. At a given cathodic potential, the degree of embrittlement decreased with increasing tempering temperature, accompanied by a change in fracture mode from a mostly intergranular fracture (i.e. along the prior austenite grain boundary) to a predominantly transgranular type with the crack path following the interface of martensitic laths. The likelihood for hydrogen evolution in a cured Portland cement was highlighted with measurement of the corrosion potential in simulated solutions with and without aeration. The corrosion behaviour was found to be similar in wet cement where the free corrosion potential of galvanized steel stabilized at a level well below the hydrogen line under de-aerated conditions.

Assessment of Candidate Fuel Cladding Alloys for the Canadian Supercritical Water-Cooled Reactor Concept

Selecting and qualifying a fuel cladding material for the Canadian supercritical water-cooled reactor (SCWR) concept remains the most significant materials challenge to be overcome. The peak cladding temperature in the Canadian SCWR concept is predicted to be as high as 800°C. While advanced materials show promise for future deployment, currently, the best options available are austenitic stainless steels and nickel-based alloys. Many of these alloys were extensively studied for use as fuel cladding materials in the 1960s, as part of programs to develop nuclear superheated steam reactors. After extensive out-of-pile testing and consideration of the existing data, five alloys (347 SS, 310 SS, Alloy 800H, Alloy 625, and Alloy 214) were selected for more detailed assessment using a combination of literature surveys and targeted testing to fill in major knowledge gaps. Wherever possible, performance criteria were developed for key materials properties. This paper summarizes the methodology used for the assessment and presents the key results, which show that 310 SS, Alloy 800H, and Alloy 625 would all be expected to give acceptable performance in the Canadian SCWR concept.

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.

Corrosion Protection of Joining Areas in Magnesium Die-Cast and Sheet Products

The common joining techniques for Mg sheet and die cast alloys such as riveting, friction-stir welding and adhesive bonding all introduce additional challenges for protection in the joining areas. First of all, the sheet products are prone to high rate of corrosion due to surface contamination. Introduction of iron-rich contaminants can be encountered from the friction-stir welding process. Although powder coating on top of conversion pre-treatments is practical for mitigating corrosion, the lap-shear adhesion of such surfaces can be negatively impacted when the adhesively bonded joints are exposed to a corrosion environment. Anodized surfaces are better in terms of their ability to retain adhesion strength, but their resistance to galvanic corrosion is an issue. Development of galvanically compatible coatings for steel rivets will benefit the adaptation of riveting as a joining technology for magnesium.

Near-Neutral pH SCC of Two Line Pipe Steels Under Quasi-Static Stressing Conditions

Most published SCC results for the near-neutral pH condition were produced under cyclic loading. However, the presence of stress corrosion cracks in pipeline systems involving very small pressure fluctuations suggests the cracking should initiate and grow without large dynamic loads. This study was designed to investigate this issue. A Grade 448 (X-65) line pipe steel and a prototype Grade 550 (X-80) steel were evaluated in near-neutral pH solutions. The maximum stress applied was at 95% of the respective yield strengths and the R values applied were between 0.98 and 1.0. Two solutions were used for each steel: NS4 and NS4/clay mixture. The solutions were purged with a gas mixture of 95%N2 and 5%CO2 . Recognizing that the crack propagation rate can be very slow under such near-static conditions, relatively long-term tests were carried out. The durations of the three tests using the prototype Grade 550 (X-80) steel were 110 days, 54 days and 26 days, and the duration for the X-65 steel was 110 days. After 110 days, the majority of the cracks in the Grade 550 (X-80) steel were in the range of 5 to 30 micrometers (μm) deep, giving an average crack propagation rate of 2*10−9 mm/s. Tests at short durations revealed that only a few cracks were detectable after 26 days and that several more cracks were produced after 54 days. So majority of the cracks in the 110-day were likely produced after 54 days of testing. The NS4/clay mixture was found to be less aggressive than the NS4 solution for both steels studied. The cracks in the prototype Grade 550 (X-80) steel were deeper and more numerous in comparison with the X-65 steel. Possible reasons for this observation are also explored in terms of the presence of martensite-austenite (MA) phase in the Grade 550 (X-80) steel.Copyright

TEM Study of Supercritical Water Corrosion in 310S and 800H Alloys

Corrosion resistance is one of the key factors in materials selection for Gen IV supercritical water-cooled reactor (SCWR) concept; especially when it comes to selection for fuel cladding. Amongst the candidate materials, 310S austenitic stainless steel and INCOLOY® 800H are deemed very promising. A summary of the previous corrosion studies on candidate alloys can be found elsewhere [1,2]. In this work TEM study of FIB sectioned test coupons is presented in our effort to understand the microstructural evolution of these alloys upon SCW exposure. Characterization of the sigma phase as a high temperature Cr-Fe intermetallic that is brittle in nature and potentially detrimental to the mechanical properties [3] is discussed. Coupons (20x10x2 mm) were cut from commercial austenitic steel 310S and 800H plates. The coupons were placed in a static autoclave containing deionized water and exposed to SCW condition (625°C, 25MPa) for 250 hours. Cross sectional transmission electron microscopy (TEM) samples were prepared using the FEI Helios NanoLabTM DualBeamTM FIB microscope from the top surface of the coupons [4]. A protective layer of Pt was first deposited on the surface to protect the corrosion layer. TEM samples were observed using the FEI Tecnai OsirisTM TEM equipped with Super X field emission gun (FIG) and ChemiSTEMTM X-ray detection technology operating at 200kV. EDX analyses were done in STEM High angle annular dark field (HAADF) imaging mode. The Esprit software was used for qualitative and quantitative elemental mapping. Conventional bright field/ dark field imaging and electron diffraction techniques were used to characterize different corrosion regions and for phase identification. Fig 1 shows EDX elemental maps from the top region of a TEM specimen prepared from the 310S coupon. The top layer contains very fine grains of CrO2 with average grain size of ~50nm. A layer featuring recrystallized γ grains below the oxide layer is highlighted in Figure 2 and is also visible in the 800H sample in Fig 3. The average thickness of top chromium oxide layer for 310S and 800H samples were 270nm and 240nm respectively; and the thickness of the sublayer Cr-depleted recrystallized γ regions were 760nm and 800nm respectively. A selected area diffraction pattern (SAD) taken from this region showed superlattice spots coming from a σ grain grown coherently on the γ (Fig 2). An orientation relationship between the γ and σ was evident in both alloys. EDX mapping along with SAD analysis revealed that in the base metal in vicinity of the recrystallized region, the γ grains contained a network of very fine coherent σ nuclei. An example is shown in Fig 4 where an SAD from a large γ grains (region 1) below the recrystallized region 2 shows superlattice spots from the σ. The dark field image in Figure 4 acquired from, (0 0 2) reflection of σ reveals a network of σ within the γ matrix in the 800H. Similar phenomenon was observed in 310S. In summary, FIB/TEM provided a method for studying the microstructural changes after SCW exposure. This study sheds light on the nucleation and the evolution of σ phase during SCW exposure that may play an important role in corrosion resistance of SCWR candidate alloys.

Materials Research in Support of SCWR Development: Current Areas, Gaps and Needs

International efforts on materials selection and development related to supercritical water-cooled reactors (SCWRs) have produced a considerable amount of data in the open literature [1], The majority of these data are on aspects of materials properties such as corrosion, stress corrosion cracking, creep, irradiation damage as well as microstructural degradation under various exposure conditions. These prior efforts are helping guide the current selection of candidate alloys for further, longer-term evaluation. As continuing research on the SCWR advances, gaps and limitations in the published data are being identified. In terms of corrosion properties, these gaps can be seen in several areas, including: 1) the test environment, 2) the physical and chemical severity of the tests conducted as compared with likely reactor service/operating condition, and 3) test methods used. While some of these gaps can be filled readily by the current research projects, in particular those occurring in Generation IV International Forum (GIF) member countries, others require further advances in our understanding of material-environment interactions involving supercritical water. Gaps in advanced test facilities for future research are also becoming evident. Future needs for materials development and suggestions for expanded international collaborations to link with groups working on materials for advanced fossil-fired supercritical water power plants, as well as other GEN IV and fusion reactor designs are summarized.Copyright

Corrosion Protection of Structural Magnesium Alloys: Recent Development

Magnesium is one of the viable materials for significantly reducing the weight of automotive component. However, wide-scale use of magnesium alloy is complicated by a number performance factors related to the intrinsic properties of these alloys such as corrosion resistance. The latest developments in coating technology and alloy chemistry design have removed much of the barrier and a number of chromate-free processes are commercially available for the parts that require a coating or coatings. As part of the Structural Cast Magnesium Development (SCMD) project, several types of newly selected coatings were evaluated for protection against general and galvanic corrosion. The latest results are presented in this paper. Galvanic corrosion is a major concern for the parts that are joined with other metals, especially when there is salt-containing road poultice on the surface. While the use of compatible Al alloy isolators can significantly mitigate the problem, elimination of electrical contact or the electrochemical driving force between the dissimilar metals is essential for complete prevention of the problem. In the test in which a thin mylar film was inserted between a steel washer and the Mg surface, no visible galvanic corrosion was found after 40 cycles of GM9540P testing. Anodizing coating alone provides good resistance to general corrosion, but it does not seem be effective against galvanic corrosion. The latest experimental work suggests that application of coatings can also beneficially affect the resistance of Mg alloy to stress corrosion cracking (SCC). When treated with Alodine 5200 and coated with a powder coat, samples of both AM50 and AE44 alloys showed drastic improvement in failure life, i.e., by at least 200%. Although the test conditions used are far more severe than that likely to be encountered during an automotive lifetime, these results have mechanistic implications for all applications where a high loading stress is present.

Environmentally assisted cracking

Environmentally assisted cracking (EAC) is a complex phenomenon driven by the synergistic interaction of mechanical, chemical and metallurgical factors. The complex interplay between causative factors makes experimental measurements difficult, and the state of knowledge on EAC under supercritical water-cooled reactor (SCWR) conditions is not as well advanced as that of general corrosion. This chapter discusses the effects of the three key causative factors (environment, material, and mechanical) on the occurrence of EAC in supercritical water, focussing on candidate SCWR alloys and expected SCWR in-core conditions. Possible differences in mechanisms in the near-critical and higher temperature regimes are highlighted.

Effect of Thermal Pretreatment on the Corrosion of Stainless Steel in Flowing Supercritical Water

The effect of high-temperature microstructure degradation (thermal ageing) on the corrosion resistance of austenitic stainless steels in supercritical water (SCW) was evaluated in this study. Mill-annealed (MA) and thermally treated (TT) samples of Type 316L and Type 310S stainless steel were exposed in 25 MPa SCW at 550°C with 8 ppm dissolved oxygen in a flowing autoclave testing loop. The thermal treatments applied to Type 316L (815°C for 1000 hr + water quench) and Type 310S (800°C for 1000 hr + air cool) were successful in precipitating the expected intermetallic phases in each alloy, both within the grains and on the grain boundaries. It was found that a prolonged time at relatively high temperature was sufficient to suppress significant compositional variation across the various intermetallic phase boundaries. This paper presents the results of the gravimetric analysis and oxide scale characterization using scanning electron microscopy (SEM) coupled with X-ray energy-dispersive spectroscopy (EDS). The role played by the fine precipitate structure on formation of the oxide scale, and thus corrosion resistance, is discussed. The combined role of dissolved oxygen and flow (revealed by examining the differences between Type 316L samples exposed in a static autoclave and in the flowing autoclave loop) is also addressed. It was concluded that formation of intermetallic phase precipitates during high-temperature exposure is not likely to have a major effect on the apparent corrosion resistance because of the discontinuous nature of the precipitation.