Citric Acid as a Green Alternative for Passivating Stainless Steels

On Tuesday 19th June, ESR Technology presented at the European Space Agency’s Space Engineering and Technology Final Presentation Days. Dr. Anthony Kent presented to ESA and industry the outcome of two years’ work on the use of citric acid as a green alternative to nitric acid in the passivation of stainless steels. The purpose of this work was to demonstrate that citric acid could be used to achieve the same corrosion resistance for space components as the more commonly used nitric acid process, but with significantly lower environmental impacts.

ESR used a novel electrochemical analysis technique to develop a passivation process using citric acid. ESR were able to optimise the process parameters quickly and effectively to establish the best performing citric acid processes. Drawing on the extensive surface engineering and materials expertise at ESR, the selected process was then subjected to a rigorous test campaign including mechanical testing, detailed metallurgical examination and standard corrosion testing. All tests confirmed that the developed processes performed at least as well as the best performing nitric acid process (and in some cases better) and that there was no significant change to the material characteristics.

This work was carried out under European Space Agency Contract No. 4000114892/15/NL/KML and was fully funded by ESA. However the views expressed in this article can in no way be taken to reflect the official opinion of the European Space Agency.

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