Aerospace electroplating

Cadmium electroplating is used on aircraft for the corrosion protection of airframe components and fasteners manufactured from non–corrosion resisting high strength steels.

Cadmium plated coatings have good corrosion resistance and provide long–term protection. Chromate passivation treatments over the electrodeposited layer improves corrosione protection still further.

Cadmium is a sacrificial coating to steel, as its open–circuit potential is more negative than that of steel, suggesting that if the coating is damaged, cadmium will corrode first instead of the exposed steel.  

Cadmium is, in addition, self–lubricating and has a low coefficient of friction making it ideal when plating threaded components like fasteners, as only moderate torques are necessary to load the fastener.

On the other hand, cadmium plating does increase the risk of hydrogen embrittlement. De–embrittlement heat treatment is used to overcome this. There are also serious environmental and health and safety concerns over the toxicity of cadmium and its compounds to the extent where efforts worldwide have been focused on alternatives to cadmium for the sacrificial corrosion protection of steel components.

Alternative coatings to cadmium, which have been considered by the aerospace industry include zinc, zinc alloys, aluminium, aluminium alloys and aluminium pigmented metallic–ceramic coatings.

Zinc plating 

zinc plating adds volume and can cause the wedging of fasteners. In addition, zinc is substantially more electronegative than steel, and consequently, if its coatings are damaged, then rapid galvanic corrosion of zinc will shortly leave the exposed steel substrate unprotected.

Boeing, in the face of legislation, has been examining zinc alloy coatings of Zinc-Nickel alloys with 10% nickel and tin-zinc alloys with 20% zinc as alternatives.

Aluminium alloy coatings

Aluminium alloys are the best substitutes for cadmium and are the most likely to be galvanically compatible with the aircraft aluminium alloys, minimising the risk of bimetallic corrosion between steel components and the structural aluminium alloys with which they might come into contact.

Aluminium–based coatings electrodeposited from non–aqueous electrolytes are commercially available since the 1970s. Electroplated aluminium has the advantage that its coatings are very dense and have good adhesion.

Aluminium coatings applied by Physical Vapour Deposition (PVD), including Ion Vapour Deposition (IVD) have good adhesion. However, they tend to be highly porous, and consequently, glass beading is necessary to compact the structure, as well as improve the corrosion resistance and protection performance of the IVD coatings.

Aluminium–based coatings are assumed to be more active than cadmium, and hence hydrogen re–embrittlement is a concern.

Conclude we must that a fine blanacing act is required in the application and formulation of these coatings. They are performance critical, should be galvanically compatible with aircraft alloys, resistant to repassivation, and possess good barrier properties.