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Aluminium alloy plating techniques

The high performance aluminium alloys used in the aeronautical industry are very sensitive to corrosion. To prevent the development of this kind of damage, chemical or electrochemical surface treatments are currently employed before applying an organic coating.

The objectives of the aircraft manufacturers were to develop new surface treatments in order:
to solve the environmental and health care problems associated with surface treatment baths which contain hexavalent chromium based products;
to improve the performances of plate materials in the fields of fatigue strength and/or corrosion resistance by comparison with the conventional processes.

The following 5 different coating process combinations were selected and then optimized:
magnetron sputtering of a pure aluminium or an aluminiium zinc alloy coating;
evaporation of a pure aluminium coating;
deposition of an aluminium silicon alloy coating by electric arc;
sputtering of a silicon coating on a thin oxide layer formed by sulphuric acid anodizing;
aluminium oxide elaborated by etching and anodizing in alkaline baths.

An in depth characterization of the coatings was undertaken in order to improve knowledge of their properties in the fields of corrosion resistance, paint adherence, fatigue behaviour and stress corrosion cracking. The behaviour of the coatings was shown to be highly satisfactory.


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