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Lemon juice: new ingredient for space designers?

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Posted September 19, 2014

Corrosion resistance and high strength put stainless steel high on the list of essential materials for satellite and rocket designers. Now ESA plans to investigate an alternative, environmental-friendly method of readying this important metal.

Citric acid – found in a dilute form in lemon juice – is being investigated as a greener alternative means of passivating stainless steel items for the space business, compared to the current standard of nitric acid. Copyright Wikimedia Commons–André Karwath

Citric acid – found in a dilute form in lemon juice – is being investigated as a greener alternative means of passivating stainless steel items for the space business, compared to the current standard of nitric acid. Copyright Wikimedia Commons–André Karwath

Back in 1913, metallurgist Harry Brearley glanced at a pile of experimental steel alloys, rejected for not being hard enough, and noticed one specimen that gleamed as bright as new, rather than rusting like all the rest.

He had just discovered stainless steel. Adding chromium to steel in the correct ratio prevents it from tarnishing in air. Instead of forming iron oxide in reaction to air and water, stainless steel builds an invisible protective layer of chromium oxide instead.

Monument to metallurgist Harry Brearley, who in 1913 discovered that chromium added to steel gave it enhanced corrosion protection, which became known as 'stainless steel'. The chromium in the steel reacts with oxygen in the air to form an invisible protective and self-healing chromium oxide layer in place of the more usual iron oxide, better known as rust. Copyright Wikimedia Commons–David Morris

Monument to metallurgist Harry Brearley, who in 1913 discovered that chromium added to steel gave it enhanced corrosion protection, which became known as ‘stainless steel’. The chromium in the steel reacts with oxygen in the air to form an invisible protective and self-healing chromium oxide layer in place of the more usual iron oxide, better known as rust. Copyright Wikimedia Commons–David Morris

A century on and stainless steel is used to make everything from cutlery to architecture to aircraft – as well as rockets and satellites.

Filling of the Gaia spacecraft fuel tanks at the Europe's Spaceport in Kourou, French Guiana. A global space astrometry mission, Gaia will make the largest, most precise three-dimensional map of our Galaxy by surveying more than a thousand million stars. Gaia is scheduled for launch on 19 December 2013. Copyright ESA-CNES-Arianespace / Optique Vidéo du CSG - S. Martin

Filling of the Gaia spacecraft fuel tanks at the Europe’s Spaceport in Kourou, French Guiana. A global space astrometry mission, Gaia will make the largest, most precise three-dimensional map of our Galaxy by surveying more than a thousand million stars. Gaia is scheduled for launch on 19 December 2013. Copyright ESA-CNES-Arianespace / Optique Vidéo du CSG – S. Martin

For the space business, stainless steel is commonly used in the storage and handling of propellants and waste, components of thermal protection systems and fasteners such as high-strength bolts.

Before they are put to use, however, stainless steel parts must first be ‘passivated’ – stripping off their surface layer to remove any imperfections or contamination left over from the machining process, which might otherwise start to rust.

Traditionally this is done by bathing the parts in nitric acid, but this has environmental and safety disadvantages. It involves special handling and produces nitrogen oxides, which are greenhouse gases and potentially harmful to workers.

Citric acid – found in a dilute form in lemon juice – has been put forward as a greener passivation alternative, being adopted in a variety of sectors, including the medical, automotive and aerospace industries.

But does this alternative option satisfy the unique requirements of the space industry? A new study by ESA’s Clean Space initiative, tasked with reducing the space industry’s environmental impact on Earth and space, aims to find out.

The study will involve passivating stainless steel alloys and components typically used in the space sector, assessing their manufacturing process, then subjecting them to a gamut of environmental tests – including stress corrosion cracking, atmospheric and laboratory corrosion testing, adhesion testing and mechanical fatigue – along with a complete metallurgical analysis.

Stainless steel water-on-water-off or 'woov' valve employed aboard the Columbus module of the International Space Station. Copyright ESA

Stainless steel water-on-water-off or ‘woov’ valve employed aboard the Columbus module of the International Space Station. Copyright ESA

Bidders are welcome on the study contract. For more information, check the invitation package, accessible here.

Source: ESA

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