Electrodeposition staining

Fig. 12.14 Spread of silver sulphide from discontinuities in gold electrodeposits on silver substrates. The gold was deliberately scratched and the specimen exposed for 24 h to an atmosphere containing 10< o SO2. Immediately after this the sulphide stain extended 0-2 mm. Five years later, the stain extends to about 13 mm, after storage in a normal indoors atmosphere...

Cadmium is more expensive than zinc. It is usually applied by electrodeposition in thicknesses up to about 25 tm and has a superior tarnish and stain resistance to that of zinc. As with zinc, the life of cadmium coatings is proportional to thickness Hudson quotes a life of only 9 months at Sheffield for a 25 m thick coating and approximately 8 years for a 42 m thick coating exposed to a marine environment at Calshot. 

USE In cleansing compds, oil-well drilling, water treatment, cheese emulsification, as general sequestering agent, to remove rust stains, as ingredient of one-fluid ink eradicators, in electrodeposition ol metals. 

The lacquer will electrocoat all uncoated metal immersed in the electrodeposition bath. It should be borne in mind that there will always be at least one area, however, small, of the can which will be uncoated. This is the point or points where electrical contact is made which enables the can to act as an electrode. This is always on the external surface. No uncoated metal must be allowed to come into contact with the contents of the can. In most cases it would be anticipated that the internal lacquer would be applied last, thus the lacquer must not adhere, damage, stain or detract from the appearance of the external decoration. It is feasible that steel cans could have an internal and external lacquer electrodeposited in the first coating operation and use a spray applied internal lacquer as the last operation. In this case, the electiocoated internal lacquer must have the characteristics of the external lacquer, particularly mobility, if a novar system is used. 

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