pipeline hydrogen embrittlement HISC diffusion corrosion mechanical considerations

Real-Life Behaviour of Thin Film Flexible Solar Cells

In this case study, the lifetime of a thin film flexible solar cells is assessed. The multilayered solar cell will be applied in curved body parts for vehicles. The long term mechanical and corrosion resistant properties are focus of the study. Below a short introduction.

For sake of corrosion prevention of the subsequent functional layers, on one hand crystallinity and thickness of the transparent surface polymer must be as high as possible. As such, the permeation rate of Oxygen and Water through the surface layer is kept low. On the other hand, adhesion of the polymer to the substrate decreases as function of thickness. The robustness of the interface is of major importance in restraining the mass solubility and temperature driven swelling stresses and mechanical stresses at the surface.

To complicate things even more, the impact resistance / fracture toughness of the surface polymer decrease as function of the degree of crystallinity.

In this study it is found that for each material configuration (inorganic coating and several substrates, possibly in series) there is an optimal thickness with regard to overall long term permeation, corrosion resistance and mechanical properties in real-life circumstances. The analysis and simulation was carried out using CheFEM software (diffusion & corrosion simulation) and FEM stress & strain simulation tools.

thin film solar cell fem analysis

Figure: deployed model of the subsequent layers of the polymer based photovoltaic.
From right to left: transparant polymer substrate (PEN/ETFE/PI), ITO anode,
PEDOT PSS, BHJ active layer and Aluminium Cathode.

Retrofitting for Hydrogen Transport using Epoxy Coating?

In the near future, natural gas pipelines (type X-52, X-60, X-65 and X-70) may also be used for transport of Hydrogen or even Carbon Dioxide. In case of Hydrogen conveyance, Hydrogen might be mixed with Natural Gas (parallel gas transport, using a membrane to seperate the gases at the outlet) or transported solely. Discarded natural gas pipelines may also be completely retrofitted for Hydrogen or Carbon Dioxide transport.

With regard to Hydrogen, a service life concern could be HISC (Hydrogen Initiated Stress Cracking) of steel. Hydrogen embrittlement or HISC results from combining of diffusing Hydrogen atoms into molecular Hydrogen - or the formation of molecular Methane - in internal metal voids of nanoscopic size. The generated pressure - in combination with intrinsic circumferential stress in the material - can exceed the restrain pressure of steel, especially near the loading surface. Whether HISC is an issue is largely dependent on the sort of steel, internal pressure and temperature. If HISC takes place, the strength of the pipeline will reduce, Hydrogen will escape and in a worst case, even buckling could occur. To enhance the lifetime of these pipelines or to improve the reliability of these pipelines, nowadays several organic and inorganic material combinations (in a composite or multilayer configuration) are being researched. One suggestion for large diameter X-60 pipelines is the application of an internal Fusion Bonded Epoxy (FBE) coating.

In this case study a diffusion-chemical-mechanical simulation will be carried out for a retrofitted Natural Gas pipeline, in order to transport Hydrogen. The material under evaluation consists of (1) an inside Fusion Bonded Epoxy coating, (2) structural high strength steel and (3) an outside PE (Polyethylene) weathering coating. Key issues are:

Will HISC reduce by application of an internal Epoxy (eventually reinforced) coating?

What is the durability of the coating under internal pressure, interfacial adhesion and if a metal crack due to HISC would appear under the coating (severe conditions or long times)?

Figure: Hydrogen concentration profile of a retrofitted X-60 pipeline (subsequently: 1 micrometer coating, 10 millimeter steel, 3 millimeter PE coating).

Advanced Chemical Resistance Analysis of (Nano) Composites

Proper mechanical and corrosion resistance properties of polymer based materials is of major importance in many applications. Traditional chemical resistance guides (e.g. Salt Water - max Temp 100 degr. C.) can provide valuable information with this regard. However, the analysis gets complex if the polymer is reinforced with filler material and is sequenced in a composite laminate. Because of the discontinuous nature of diffusion rate, solubilities and restraints in the different layers, swelling strains develop between each layer.

This might lead to composite failure, initiated by one of the layers or the interface (in the animation below the surface layer fails). And what if nano materials are added to the polymer in question? Even this can be simulated rigorously using the CheFEM tool!

In this case study different laminate are assessed for its intrinsic chemical resistance - including a composite laminate with a surface layer filled with nano particles. Do not hesitate to contact us for more information.