Fabio Scenini

Effect of Surface Preparation on Intergranular Stress Corrosion Cracking of Alloy 600 in Hydrogenated Steam

Abstract In the literature it is a common belief that electropolishing mitigates primary water stress corrosion cracking (PWSCC) because it removes superficial cold work. Here, it is shown that ele...

Influence of Pd and Ru additions on stress corrosion cracking of austenitic stainless steels

Abstract The influence of minor alloying platinum group metal additions on stress corrosion cracking (SCC) of austenitic stainless steels has been investigated in simulated pressurised water reactor environments. Crack propagation studies, employing tensile specimens precracked in acidified potassium tetrathionate and subsequently loaded in proving rings revealed that 1 wt-% ruthenium additions improved the resistance of 304 stainless steels to SCC. Analytical transmission electron microscopy was used to characterise crack tips and surface oxides after high temperature SCC tests. Enrichment of ruthenium and molybdenum within the dual oxide surface layers was observed in the ruthenium modified stainless steel, which is likely to have occurred during potassium tetrathionate pre-exposure and promoted intergranular SCC mitigation. Palladium additions (1 wt-%) showed no beneficial effects to SCC, and this was associated with the formation of second phases PdMn particles upon sensitisation, which decreased the availability of Pd.

Stress corrosion cracking of Ru doped 304 stainless steel in high temperature water

Abstract In pressurised water reactor primary water, there is generally growing concern that stress corrosion cracking (SCC) may occur in occluded locations where residual oxygen and other impurities may be trapped. For these critical components, the deployment of more SCC resistant materials is desirable. In this paper, the effect of 1 wt-%Ru additions on the SCC susceptibility of 304 austenitic stainless steels was investigated in high temperature water. Slow strain rate tensile tests were performed on standard and 1 wt-%Ru modified 304 stainless steels in both sensitised and cold worked conditions. Preliminary results showed that, although both ruthenium doped and standard 304 stainless steels exhibited intergranular SCC, the former was less susceptible as indicated by a greater strain to failure. The results obtained suggest an improved performance of the Ru doped 304 stainless steel towards SCC susceptibility in these environments.

Stress corrosion cracking of sensitized type 304 stainless steel in high-temperature water with anionic impurities contamination

This paper describes some results selected from a larger program that was aimed at understanding the stress corrosion cracking (SCC) initiation of Type 304 stainless steel (UNS S30400) in high-temperature deaerated water. Out of a large number of statically loaded samples, only a small minority of the tested samples underwent SCC. The occurrence of SCC indicates a synergism between sensitization, ionic impurities (mainly chloride and sulfate), and/or superficial defects and cold work. In fact, none of the nonsensitized materials initiated cracking (within the time scale of the tests), while only three sensitized samples underwent extensive SCC. The crack morphology of the fractured sample was predominantly inter-granular with some transgranular regions. Transmission electron microscopic samples containing crack tips were, in most respect, in line with the literature: a magnetite/spinel duplex layer on the crack surfaces, a Cr-rich oxide at the crack tip, and Ni enrichment at the metal/oxide interface and ...

The application of in situ analytical transmission electron microscopy to the study of preferential intergranular oxidation in Alloy 600.

In situ analytical transmission electron microscopy (TEM) can provide a unique perspective on dynamic reactions in a variety of environments, including liquids and gases. In this study, in situ analytical TEM techniques have been applied to examine the localised oxidation reactions that occur in a Ni-Cr-Fe alloy, Alloy 600, using a gas environmental cell at elevated temperatures. The initial stages of preferential intergranular oxidation, shown to be an important precursor phenomenon for intergranular stress corrosion cracking in pressurized water reactors (PWRs), have been successfully identified using the in situ approach. Furthermore, the detailed observations correspond to the ex situ results obtained from bulk specimens tested in hydrogenated steam and in high temperature PWR primary water. The excellent agreement between the in situ and ex situ oxidation studies demonstrates that this approach can be used to investigate the initial stages of preferential intergranular oxidation relevant to nuclear power systems.

The contribution of hydrogen evolution processes during corrosion of aluminium and aluminium alloys investigated by potentiodynamic polarisation coupled with real-time hydrogen measurement

Water reduction, which leads to the evolution of hydrogen, is a key cathodic process for corrosion of many metals of technological interest such as magnesium, aluminium, and zinc; and its understanding is critical for design of new alloys or protective treatments. In this work, real-time hydrogen evolution measurement was coupled with potentiodynamic measurements on high-purity aluminium and AA2024-T3 aluminium alloy. The results show that both materials exhibit superfluous hydrogen evolution during anodic polarisation and that the presence of cathodically active alloying elements enhances hydrogen evolution. Furthermore, it was observed for the first time that superfluous hydrogen evolution also occurs during cathodic polarisation. Both the anodic and cathodic behaviours can be rationalised by a model assuming that superfluous hydrogen evolution occurs locally where the naturally formed oxide is disrupted. Specifically, during anodic polarisation, oxide disruption is due to the combined presence of chloride ions and acidification, whereas during cathodic polarisation, such disruption is due to alkalinisation. Furthermore, the presence of cathodically active alloying elements enhances superfluous hydrogen evolution in response to either anodic or cathodic polarisation, and results in ‘cathodic activation’ of the dissolved regions.Corrosion of aluminium: the contribution of hydrogen evolutionAqueous corrosion is an electrochemical reaction resulting in materials degradation, involving simultaneous oxidation of metal and reduction of species in a wet environment. The overall corrosion rate depends on how fast the two processes proceed. Although the reduction of oxygen is the most important cathodic reaction, for more reactive materials such as Al and Mg the reduction of hydrogen is thermodynamically allowed and could contribute to the overall rate. Now a team led by Michele Curioni at University of Manchester look at the hydrogen evolution behaviour of high purity aluminium and AA2024T3 alloy utilising coupled real-time hydrogen evolution and potentiodynamic measurements. Superfluous hydrogen evolution was observed during both anodic and cathodic polarisation and associated in both cases to local oxide disruption. The derived understanding of corrosion could enable us to develop new protection treatments.

Effect of thermal ageing on creep and oxidation behaviour of Type 316H stainless steel

The UK has unique experience in operating high temperature civil nuclear power systems, known as advanced gas cooled reactors (AGRs). One of the primary challenges for extending the lifetime of the AGR power stations is to understand the interaction that occurs between the AGR CO2 environment and creep-fatigue cracking behaviour. This is one of the life limiting degradation mechanisms for steel components within the reactor pressure vessel. This paper addresses the effect of thermal aging on material internal state that controls both the creep deformation and oxidation behaviour of Type 316H stainless steels when they are exposed at a simulated AGR environment. Experimental results from creep tests are discussed with respect to a multi-scale self-consistent model, while experimental results from oxidation tests are considered with respect to the application of measured short term data to predict the long term oxidation behaviour. Finally, the interaction between oxidation and creep and its impact on high temperature structural integrity of AGR nuclear systems are discussed.

Effects of Material Composition on Corrosion Fatigue Crack Growth of Austenitic Stainless Steels in High Temperature Water

Laboratory studies on austenitic stainless steels in PWR primary coolant environments have shown that the ASME XI procedures used to assess fatigue crack growth of reactor components may not always be conservative. Recent work has shown that significant environmental enhancement of growth rates can occur in this environment, especially for some long rise time loading cycles. Although enhancements up to eighty times relative to air data have been observed, under some conditions retardation of the enhanced growth rates can also occur, leading to rates close to the ASME XI air line. Several factors appear to influence retardation, including temperature, water flow rate and material composition. The current study addresses the influence of material composition and it is shown that steels of high sulfur content (>0.02%) are much more prone to retardation than low sulfur (<0.01%) steels. Work aimed at elucidating possible mechanisms for this effect is described.

Development of a Microfluidic Setup to Study the Corrosion Product Deposition in Accelerated Flow Regions

CRUD (Chalk River Unidentified Deposit) forms in the water circuits of nuclear reactors due to corrosion of structural materials and the consequent release of species into the coolant. The deposition of CRUD is known to occur preferentially in regions of the primary circuit of pressurised water reactors (PWRs) where the water flow accelerates. In order to investigate this phenomenon, a micro-fluidic system, recreating plant conditions while using a simplified experimental set-up, was realised. A flow cell, comprising a stainless steel disc with a central micro-orifice, was used to create accelerated flow under representative operating conditions. By monitoring the pressure drop across the cell, the build-up rate (BUR) of CRUD within the micro-orifice was monitored in real time. By this setup, the conditions inducing deposition of CRUD under PWR conditions were emulated and CRUD deposition was induced in the accelerated flow region. Further effects associated with the presence of lithium hydroxide were investigated in real-time.Corrosion: Reproducing deposition in nuclear power plantsA simplified micro-fluidic system can successfully emulate corrosion products deposition in nuclear reactor water circuits. A team led by Fabio Scenini at the University of Manchester in the U.K. used a stainless steel disc with a micro-orifice and a micro-fluidic cell to build a system recreating the accelerated flows of an operating power plant. They monitored the pressure drop and build-up rate of corrosion products in real time, showing that more efficient setup reproduced corrosion seen under plant conditions, and that spallation of built-up oxide was a consequence of competition between its complex hydrodynamic and electrokinetic preferential deposition and its removal at high velocities. When adding lithium to the water, corrosion oxide formation was limited. Applying this methodology may help us better understand corrosion in nuclear reactors.

SCC Initiation in the Machined Austenitic Stainless Steel 316L in Simulated PWR Primary Water

Annealed and cold-worked stainless steel 316L samples with machined and polished surfaces were tested in simulated pressurized water reactor (PWR) primary water under slow strain rate tensile (SSRT) test conditions to investigate stress corrosion cracking (SCC) initiation. Roughness, residual stress and cross-sectional microstructure of the as-machined samples were characterized before SSRT tests. Plan view and cross-sectional examinations were performed after the test. Pre-test characterization indicated that a deformation layer was present on the machined surfaces. This deformation layer consisted of an ultrafine-grained layer on the top and deformation bands underneath. The thickness of the deformation layer on the annealed material was greater than that on the cold-worked material. Post-test characterization revealed that the SCC initiation behaviors of the as-machined and polished surfaces were different for both annealed and cold-worked materials. Machining increased SCC initiation susceptibility of the annealed material as many shallow cracks initiated along the machining marks in the machined surface, and it decreased the SCC initiation susceptibility of the cold-worked material as a reduced number of cracks were identified in the machined surface compared to the polished surface. The factors influencing SCC initiation are also discussed.