The majority of metal fittings used in homebuilt aircraft are made from plain or low alloy steels. Today the most commonly used grades of steel used in the UK will probably be S510, S514/5 and 4130N. Steel parts which are left unprotected will corrode very quickly, (it is a common misconception that parts made from S514/5 will corrode less readily than parts produced from S510). Protection from corrosive elements and the atmosphere (water vapour in particular) is therefore required. Cadmium plating has for many years proved to be the most effective form of plating used on aircraft components, in particular those operating in a marine environment. Cadmium is a ductile metal and does not suffer from brittleness when electro-deposited. This ductility is of great benefit in protecting threaded fasteners. Cadmium plating is also used to provide a barrier between dissimilar metals in contact, to prevent, or reduce the rate and effect of electrolytic or galvanic corrosion. Typical examples of where this occurs very readily are between steels and aluminium alloys. Cadmium plating imparts a corrosion resistant surface to steel parts. It may also be used on some brass and bronze bushes which are subsequently fitted into an aluminium alloy component. In this situation it is acting as a barrier.
The thickness of the plating should be specified. The now obsolete DTD Spec. 904C required an average film thickness of 0.0004". After plating, the part is given additional corrosion protection in the form of a chromate conversion coating loosely termed 'passivation'. It is this coating which gives the familiar appearance to a cadmium plated part. The chromate coating is typically a yellow iridescent colour or an opaque olive green/brown colour which may give the appearance of weathered bronze. The latter type of coating generally gives the best corrosion protection and is generally found on military hardware.
Unless parts are exposed to the elements it is not essential to add further protection in the form of paint. However, there have been cases of cadmium plated parts developing a powdery white deposit in the presence of fumes given off by the acid hardener used in Aerolite adhesive. This will typically occur where air cannot circulate, for instance in closed box sections e.g. the space between the spar and the ply covered leading edge. It is therefore strongly recommended that all cadmium plated parts are painted.
Paint does not adhere particularly well to cadmium and for this reason parts manufactured by Aviation Metalcraft are given a micro textured surface prior to plating. This not only provides better mechanical adhesion of the paint but also gives an excellent finish to the plated part.
It is sometimes necessary to carry out a heat treatment process after plating to eliminate the effects of hydrogen embrittlement. This takes the form of heating the part at a temperature of between 190 deg.C and 220 deg.C. The decision as to whether dembrittlement is necessary is largely dependent on the tensile strength of the steel. BS1706: 1990 recommends that up to a UTS OF 1050 MPa (68 Ton/ in2 ) no treatment is necessary. For steels with a UTS above 1050 MPa heat treatment is necessary.
Due to recent changes in the law relating to the disposal of toxic heavy metal waste, many smaller plating companies which previously carried out cadmium plating have been faced with large clean up bills. This has forced all but the larger companies to abandon the process. The alternative offered being zinc plate and passivate. As the largest use of cadmium plating is still protecting aircraft components the companies still offering the service are typically the larger CAA approved organisations.
Because of its toxicity it is essential that cadmium is not employed as a coating which will come into contact with food or beverages.
The chromate conversion coating can cause skin dermatitis, so skin contact should be avoided as much as possible. In any event it is wise to wash hands thoroughly after handling cadmium plated parts especially before eating.
Cadmium plated parts are subject to attack by certain organic materials e.g. cardboard, wood and certain electrical insulators which emit reactive vapours ( typically of an acidic nature). Parts to be stored for a long period should be wrapped in an acid free tissue paper (available from art shops) and placed in a sealed container with a suitable desiccant.
The section on safety is taken largely from BS1706:1990 method for specifying Electroplated Coatings of Zinc and Cadmium on Iron and Steel.
Further references:-
Materials of Aircraft Construction, F.T. Hill, Pitman 1942
Aircraft Maintenance, Brimm and Bogess, Pitman 1950
Metal Aircraft Construction, Langley, Pitman 1942
Aircraft and Missile Design Handbook, C.A. Overbury, Macmillan 1960
Standard DTD904C (obsolete)
Standard BS1760:1990
Standard DEF 03-19
