Corrosion in space

Corrosion in space is the corrosion of materials occurring in outer space. Instead of moisture and oxygen acting as the primary corrosion causes, the materials exposed to outer space are subjected to vacuum, bombardment by ultraviolet light and x-rays, high-energy charged particles (mostly electrons and protons from the solar wind). In the upper layers of the atmosphere (between 90–800 km), the atmospheric atoms, ions and free radicals, most notably atomic oxygen, play major role. The concentration of atomic oxygen depends on altitude and solar activity, as the bursts of ultraviolet radiation cause photodissociation of molecular oxygen.^{[citation needed]} Between 160 and 560 km, the atmosphere consists of about 90% atomic oxygen.^[1]

Materials

Corrosion in space has the highest impact on spacecraft with moving parts. Early satellites tended to develop problems with seizing bearings. Now the bearings are coated with a thin layer of gold.

Different materials resist corrosion in space differently. For example, aluminium is slowly eroded by atomic oxygen, while gold and platinum are highly corrosion-resistant. Gold-coated foils and thin layers of gold on exposed surfaces are therefore used to protect the spacecraft from the harsh environment. Thin layers of silicon dioxide deposited on the surfaces can also protect metals from the effects of atomic oxygen; eg, the Starshine 3 satellite aluminium front mirrors were protected that way. However, the protective layers are subject to erosion by micrometeorites.

Many plastics are considerably sensitive to atomic oxygen and ionizing radiation. Coatings resistant to atomic oxygen are a common protection method, especially for plastics. Silicone-based paints and coatings are frequently employed, due to their excellent resistance to radiation and atomic oxygen.^[2] However, the silicone durability is somewhat limited, as the surface exposed to atomic oxygen is converted to silica which is brittle and tends to crack.

Solving corrosion

The process of space corrosion is being actively investigated. One of the efforts aims to design a sensor based on zinc oxide, able to measure the amount of atomic oxygen in the vicinity of the spacecraft; the sensor relies on drop of electrical condictivity of zinc oxide as it absorbs further oxygen.^{[citation needed]}

Other problems

The outgassing of volatile silicones on low Earth orbit devices leads to presence of a cloud of contaminants around the spacecraft. Together with atomic oxygen bombardment, this may lead to gradual deposition of thin layers of carbon-containing silicon dioxide. Their poor transparency is a concern in case of optical systems and solar panels. Deposits of up to several micrometers were observed after 10 years of service on the solar panels of the Mir space station.^[3]

Other sources of problems for structures subjected to outer space are erosion and redeposition of the materials by sputtering caused by fast atoms and micrometeorites. Another major concern, though of non-corrosive kind, is material fatigue caused by cyclical heating and cooling and associated thermal expansion mechanical stresses.

References

1. ^ The Use of Silicone Adhesives in Space Applications
2. ^ Optimizing Atomic Oxygen Resistance on Coated Substrates Using TechOptimizer
3. ^ Banks, Bruce A.; De Groh, Kim K.; Rutledge, Sharon K.; Haytas, Christy A. (1999). "Consequences of atomic oxygen interaction with silicone and silicone contamination on surfaces in low earth orbit". Proc. SPIE 3784: 62. Bibcode 1999SPIE.3784...62B. 

External links

• Failure in spacecraft systems
• New Scientist premium article: Space is corrosive
• NASA Long Duration Exposition Facility: surface contamination in space