Surface treatments play an important, multi-functional role in satellite components. They are often critical in terms of optical properties, corrosion resistance, wear resistance, ability to withstand humidity, heat cycling and vibrations and also to reduce or eliminate degassing and particle emissions. Steiger Galvanotechnique SA has participated in the manufacture of many components for numerous telecommunication satellites and missions; as for instance: ISO, XMM, MERIS, STENTOR, METOP, EOS, ROSETTA, MXT CAMERA OF SVOM, EUCLID, CHEOPS.
Surface treatments for Space applications include:
- Alcaline anodisation of Titanium
- Chemical conversion coatings on aluminium alloys
- Black anodisation of aluminium alloys
- Electroless nickel on aluminium
- Gold plating
- Multicoatings
- 3D Additive Finishing – Innnosurf® advanced Coatings
Alcaline anodisation of titanium
Weighing less than steel, but stronger than most stainless steels, its superior strength-to weight ratio (particularly at high temperatures) and its resistance to several forms of corrosion have made titanium and its alloys significantly more attractive to the aero-space and aircraft designer.
A problem arising with Titanium is due to its severe galling tendency. An efficient way to reduce to galling problems is to apply an alcaline anodisation according to the AMS 2488c. By this process, a coating of a few μm of TiO² is formed by anodic conversion on the titanium surface. Steiger Galvanotechnique SA offers this coating under the trade name “Biodize®”.
More details on the characteristics of this coating are given in our Brochure for Medical Techniques. Today the largest application of this coating is for the surface treatment of medical implants.
Conversion coatings
The range of chromate conversion coatings are described in detail in various literature and standards (1–3). The coatings based on hexavalent chromium salts, for safety and environmental reasons, are increasingly substituted by chromite conversion coatings based on trivalent chromium salts.
The chromate conversion coatings are formed by a chemical reaction between the aluminium alloy surface and the hexavalent chromium in the solution bath. Cr +VI is reduced to Cr +III while the aluminium is oxidized to Al +III. During the course of the reaction the pH at the metal-liquid interface rises and causes the chromium and aluminium compounds to precipitate on the surface in the form of a gel containing chromates, chromium hydroxides, as well as aluminium oxides, oxyhydrates and bifluorides depending on the type of chromate conversion coating applied. The gel, once dry, achieves mechanical stability. The coating should not be heated above 80°C to prevent excessive dehydration of the layer. If overheated, the coating will disintegrate.
Different types of chromate conversion coatings are available under different trade names, for instance: Alodine 1000 and Alo-dine 1200s. The chromate conversion coating is chosen for two reasons, namely: corrosion protection and low electrical resistance. An increasing thickness of the coating improves the corrosion resistance but simultaneously also increases the electrical contact resistance. The thickness of the coating is influenced by the concentration of chromate in the solution, by the pH, and the immer-sion time. The Alodine 1000 coating is almost colorless, but the Alodine 1200s coatings are brass-colored; tending toward brown-ish for higher thicknesses.
The chromite conversion coatings based on trivalent chromium salts are available under different trade names; for instance: Surtec 650. Its appearance is a faint, but visible, iridescent blue. This conversion coating, as applied by Steiger, has been validated by prime users, namely Airbus Defence & Space and Thales Alenia Space. The validation criteria are the same as for chromate conversion coatings, including visual test, wipe test, peel test, layer weight, corrosion test, damp heat, ageing, thermal cycling and contact resistivity.
The physical properties of 3 types of conversion coatings are presented in the Table hereunder.