Environmentally friendlier space components

Products and components made out of stainless steel must be passivated prior to their final use: this means the formation of a stable oxide film that protects the material against corrosion. Nitric acid is commonly used in this process.

An innovative method replaces it by citric acid, an environmentally friendlier compound as it is biodegradable and does not emit NO_X vapours.

The process developed has been applied to stainless steels to be used in the manufacture of components in the space field, such as components for spacecraft, shuttles and ground-based structures. Its use is common in components for thermal protection systems, in securing elements, such as high resistance screws, and in the manufacture of fuel deposits and tanks, among others. The image shows the filling of the fuel tanks of the Gaia spacecraft (Source: ESA).

Citric passivation has equalled or exceeded the features of nitric passivation, achieving an excellent surface appearance avoiding yellowing it and without reducing mechanical properties.

TECNALIA has been leading these developments, while QUANTIS SAS has carried out their life-cycle analysis. ESA, the European Space Agency, has defined the specifications and has validated the new treatments.

The ideal treatment concentration, temperature and time conditions for each material has been established using design tools from characterisation experiments and advanced techniques as well as image analysis.

The tests performed have shown that the new passivation processes have not had a negative impact on the citric mechanical properties of the materials studied, such as embrittlement due to hydrogen or fatigue properties.

The next steps are optimising the process from an industrial point of view, minimising production costs (reducing treatment concentrations, temperatures and times) in an efficient, safe and environmentally friendly process: the use of toxic and corrosive substances, such as nitric acid, hydrofluoric acid and/or chromates is not required.

This development is funded by the European Space Agency through the GREENACID “Citric acid as green replacement for steels passivation” initiative.

Contact: Marta Brizuela