Passivating

Passivation is a general term for the chemical treatment of the surface of the diecasting to improve its corrosion resistance. There are a large number of proprietary finishes available, many of which are based on chromates. One major advantage of this type of finish is the very low and even coating thickness, usually well below 5 microns, which means that the dimensional precision of the component is not significantly affected.

The appearance of these finishes varies from colourless through gold to bronze and olive, and there are also some black varieties available. The colour often tends to vary according to the actual zinc alloy to which it is applied and to the processing variables. Their effectiveness in combating corrosion varies from moderate to very good depending on the actual finishing chemicals chosen and the processing conditions. It is possible to achieve salt spray resistances (ASTM B117) up to about 400 hours with 240 hours being readily attainable. However the colourless and black versions are generally not capable of achieving this level of performance.

Because the surface appearance of passivation treatments is affected by the composition of the underlying metal and is usually brighter on non-alloyed zinc it is quite common to electroplate zinc alloy diecastings with a thin layer of zinc before passivating them. The value of this procedure is questionable, particularly so for ZP3 diecastings because some of the available finishes can produce good bright finishes directly on this alloy.

None of the simple chemical dip treatments presently available will withstand a temperature of 120 Celsius without a serious degradation in their subsequent corrosion protective abilities. However the addition of specially formulated lacquers, of which there are several proprietary examples available and which are applied by dipping in the same way as the passivation, can improve the temperature resistance markedly.

Straightforward chemical passivation treatments are generally fairly trouble free and make few demands on the quality of the casting. However points to be born in mind by the designer are the necessity for the chemical solutions to access the surfaces required to be finished and drainage of those liquids and wash water from the component. Castings can be processed en masse or they may be racked, in the former case a rotating barrel overcomes most problems of drainage but is only really suitable for quite small components. Treatment of larger pieces in a basket requires consideration of how the components will be oriented to prevent air pockets and give free drainage. Rack finishing is more suitable for some parts but it is usually more time consuming and therefore more costly. If no convenient and effective racking points present themselves on the casting it will be necessary to provide features for this purpose. This should be done in consultation with the diecaster and the finisher and should then involve minimal extra cost.

The addition of a lacquer at the end of the process accentuates the demands for good component drainage because of the higher solution viscosity.

The question of which actual passivation or passivation plus lacquer finish is most suitable for any given application depends upon the priorities which are relevant to that application, concerning such attributes as corrosion resistance requirement, appearance, wear resistance, coating thickness etc.

Reference 2 and Zinc alloy diecasting – selected corrosion preventative finishes