Environmentally-assisted degradation

In our society metals are used in a wide variety of applications, ranging from enormous constructions in building and aerospace, to very small units in electronic devices.

Regardless of the application, they always have one common feature, namely that their durability is affected by their interaction with the environment. Often, this interaction, quoted as corrosion, has a detrimental influence on the mechanical, physical and esthetical properties. In the pursuit of increasing the sustainability of metals, a thorough knowledge of this electrochemical behaviour is crucial.

Fusion reactor materials

Leandro Tanure, in cooperation with SCK-CEN

The study and characterization of Plasma-Facing Materials (PFM) will largely determine the success of future nuclear fusion reactors. These reactors are proposed to replace conventional nuclear power plants (based on nuclear fission) once they are safer, more reliable and produce considerably less nuclear waste. 

Nuclear fusion reactions involving hydrogen isotopes require extremely high temperatures (millions °C) which are achieved in the plasma state. The main goal of this research is to understand how plasma exposure affects and changes the microstructure of the most promising PFM: tungsten and tungsten-based alloys.

During fusion loading conditions, PFM are challenged to keep their mechanical properties and microstructural features, as long as possible, under severe requirements such as high temperatures, thermal loads, thermally induced mechanical stresses and neutron irradiation. After plasma exposure experiments, the samples are characterized via Scanning Electron Microscope techniques and Electron Backscatter Diffraction (EBSD). Mechanical behavior is evaluated through nano-indentation and tensile tests.

This project is carried within the framework of Erasmus Mundus FUSION-DC program in close collaboration with the Belgian Nuclear Research Centre (SCK.CEN), Dutch Institute For Fundamental Energy Research (DIFFER) and the Eindhoven University of Technology (TU/e).

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 Inverse Pole Figure maps (obtained via EBSD) of a forged tungsten sample. (a) annealed at 1000 °C and (b) annealed at 1300 °C.
ND: Normal Direction. BA: Bar Axis. Color code with respect to ND.
Different colors represent a specific orientation of each small grain of the material.

Stress-corrosion cracking

Tim De Seranno

Stress corrosion cracking (SCC) is a process of metal degradation caused by corrosion and a simultaneous sustained tensile or torsional stress. In low pressure steam turbines, the first condensate, enriched with organic acids, can cause significant corrosion damage, including SCC. In this project, the mechanical degradation of steam turbine steel due to the presence of a corrosive environment is evaluated.

Information about crack initiation and propagation is obtained by means of scanning electron microscopy (SEM) in combination with electron backscatter diffraction (EBSD).

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SEM image (left) and phase map (right) of cracks interacting with titanium carbides.

The composition of the acidic aqueous environment is obtained from first condensate sampling in lab scale boiler experiments, performed at the Faculty of Bioscience Engineering (at the Particle and Interfacial Technology Group (PaInT) research group). This project is situated in the framework of the IMPROVED project, which is an INTERREG funded project that aims to build mobile units for water resource recovery in chemical industry.

Corrosion in sour conditions

Elien Wallaert

In the oil and gas industry, the presence of H2S gas might induce sulfide stress cracking (SSC). The interaction between steel and the H2S containing surrounding environment also results in the formation of a FexSy corrosion product. Moreover, hydrogen is also generated during this corrosion process and can be absorbed in the material.

It was investigated how the (Mo content of the) base material composition affects the generated corrosion products and their impact on the corrosion rate. The corrosion products that were generated in an aqueous solution saturated with H2S had a two-layered structure. The main composition of both layers was mackinawite (FeS1-x), however they contained different amounts of alloying elements, molybdenum and chromium. This double corrosion layer is illustrated in the schematic below.

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Schematic for the proposed mechanism of the double corrosion layer on steel in a sour environment

The upper layer, which was created via a precipitation mechanism, was enriched with Mo. While the inner layer was formed via a uniform corrosion process and was enriched with Mo and Cr. Investigation of the equilibrium phase diagrams for these environments showed the indeed Mo can undergo a deposition reaction, creating MoS2, while Cr will not undergo any deposition but remain in solution as Cr2+.

The presence of this double corrosion layer has a large influence on the amount of diffusible hydrogen in the materials. This amount decreased as a function of contact time with the H2S saturated solution, while the corrosion rate of the materials shows no significant reduction. Therefore, the corrosion products are assumed to act as a physical barrier against hydrogen uptake.

Hot-dip aluminized steel: characterization and evaluation

Babs Lemmens, in cooperation with VUB

Steel can be protected against the deteriorative effect of corrosion by applying a metallic coating, for which aluminium is widely used. The metallic coating is most often applied by hot-dip aluminizing. During the dipping of steel in Al interdiffusion occurs, giving rise to the formation of intermetallic phases (IMPs) which are hard and brittle.

In industrial aluminizing processes, Si is commonly added to the Al bath to reduce the thickness of these IMPs. Despite the abundant use of hot-dip aluminizing in industry, little is known about the microstructural build-up and characteristics, electrochemical behavior and mechanical performance of the IMPs formed.

In this research project, steel substrates were dipped in Al baths containing varying amounts of Si for changing times and temperatures. The microstructural build-up of these coatings was investigated using different state of the art electron microscopy techniques. Nanoscale investigations were performed to investigate the location of Si by using atom probe tomography.

The electrochemical behaviour of the hot dip aluminized steel was investigated on both a macroscopic as well as on a microscopic scale, using techniques as scanning vibrating electrode, salt spray testing and gold nanoplating. To fully understand the mechanical performance of Al coatings, standard tensile testing and micro-tensile testing were performed. Some of the results are illustrated in the figures below.

Various intermetallic phases for different %Si

Phase maps of hot-dip aluminzed steel dipped in pure Al (left), Al with 3% Si (middle), Al with 5% Si (top right) and Al with 10% Si (bottom right), obtained via electron backscatter diffraction.

 

APT measurement of IMPs

2-dimensional concentration plots of Al (a), Fe (b) and Si (c) of the intermetallic phases, obtained via atom probe tomography.

 

dipping T influence on deformation behaviour

 Evaluating the influence of dipping temperature on the deformation behavior via tensile testing and scanning electron microscopy analysis.

Microstructural characteristics of corrosion processes

Linsey Lapeire, in cooperation with VUB

In current literature, corrosion is often considered as a purely chemical interaction with nearly exclusive dependence on compositional effects, whilst ignoring the microstructural and crystallographic properties of the metal surface. In this research project it was demonstrated that there is an important correlation between the corrosion behaviour of the metals and the grain orientation, grain size and grain boundaries.

After a controlled variation and thorough characterization of the microstructure of a carefully selected metal, the resulting electrochemical behaviour was meticulously quantified. This was done in close collaboration with the research group SURF at VUB. In this way valuable knowledge was generated for the design of metals with improved corrosion resistance, which is illustrated in the following images.

EBSD and grain boundaries in Cu sample

Gold-nanoplating (right part of image), an in-house developed technique, in combination with electron backscatter diffraction (left and middle part of image) makes is possible to quantify the electrochemical behaviour of grains and grain boundaries.

Electrochemical measurement of Cu grains

The combination of scanning electrochemical microscopy and electron backscatter diffraction can distinguish differences in electrochemical activity of the surface.

Electrochemical measurement along line and several grains

With atomic force microscopy and electron backscatter diffraction it becomes possible to link the dissolution behaviour with specific crystallographic and microstructural features.