Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Depositing predominantly alloy coating
Patent
1993-10-12
1995-01-03
Niebling, John
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Depositing predominantly alloy coating
205252, 205253, C25D 356
Patent
active
053783468
DESCRIPTION:
BRIEF SUMMARY
This invention is concerned with improvements in and relating to electroplating baths and to electroplating processes using the baths. In particular the invention is concerned with electroplating baths for the deposition of tin-zinc alloys.
Tin-zinc alloy deposits are recognised as a potential alternative to the toxic cadmium as corrosion resistant coatings. Tin-zinc coatings have proved to be useful in the radio industry and for the plating of components in the automobile and aircraft industries where their special properties, e.g. their good resistance to corrosion attack by hydraulic fluids, have been of great value. Other examples of the use of tin-zinc coatings include the protection of components for the electrical industry; the protection of hydraulic pit props; and as coatings for steel panels used in the construction industry.
At the present time, tin-zinc alloys are industrially plated from alkaline sodium or potassium stannate/cyanide baths as developed in the 1940's.
Since the mid-1960's the use of tin-zinc coatings has declined considerably. This may be partly because of the unpopularity of cyanide solutions and also probably owing to the fact that the tin-zinc cyanide plating baths are difficult to operate and require constant monitoring and control. For example, after plating for a few hours, the percentage of zinc deposited with tin starts to drop and this necessitates constant maintenance of the bath. Another possible reason for the decline in tin-zinc plating may be the fact that the matt tin-zinc finish is considered less attractive than a number of the bright finishes which are now becoming available.
It is also difficult to plate a wide range of alloy compositions from the same cyanide plating systems.
It is an object of the present invention to provide an improved electroplating bath for the electrodeposition of tin-zinc alloys.
Basically, a plating bath in accordance with the invention comprises an alkaline aqueous solution of three basic components, namely an alkali metal (sodium or potassium) zincate, an alkali metal (sodium or potassium) stannate and an alkali metal (sodium and/or potassium) tartrate. The invention also provides an electroplating process for the deposition of a tin/zinc alloy using the bath defined above.
The bath of the invention may be used for the electrodeposition of a tin-zinc alloy of any relative alloy composition (e.g. 0.05 to 99.95 wt. % of Zn) on any suitable conducting substrate, especially ferrous or copper alloys. The bath is suitable for use in rack, barrel and brush plating processes.
The desired proportions of tin and zinc in the deposited alloy are determined by the bath composition and the operating conditions during plating.
Tin-zinc alloys may be plated from the baths of the invention at current densities lower than the cyanide plating systems, with better cathode efficiency and with good covering and microthrowing powers. The deposits are ductile and have corrosion resistance properties which are superior to pure tin or pure zinc coatings and indeed, tin-zinc alloy deposits of equivalent composition obtained from the cyanide baths. The corrosion protection of the deposit is comparable to a cadmium deposit from a cyanide bath. The plating process gives a compact and fine grained deposit.
The baths of the invention are essentially solutions containing zinc and tin sources. The tin ions are introduced into the bath as sodium and/or potassium stannate. The stannate is the reservoir for the tin deposited at the cathode. Its concentration is not critical, but at low concentrations cathode efficiency is depressed and at high concentrations drag out and other losses will give higher operating costs.
The alkaline zinc source is preferably formed from a zinc oxide or from a suitable zinc salt or zinc metal and a strong base such as sodium or potassium hydroxide. The predominant source of the zinc ions in the bath is a zincate complex obtained from the reaction between the zinc oxide, salt or metal and the sodium or potassium hydroxide and is prepared as
Ashiru Oluwatoyin A.
Blunden Stephen J.
Niebling John
Wong Edna
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