When electric current is properly
supplied to a cathodic steel strip immersed in an electrolytic
solution containing metallic ions, the metallic ions lose their
electrical charges by combining with electrons and precipitate
on the cathodic surface as metallic atoms.
A continuous electroplating line is composed of pretreatment
equipment, plating equipment, and post-treatment equipment.
The functions and construction of the pretreatment and post-treatment
equipment are almost identical to those used in a hot-dip line.
To ensure uniform, efficient plating, it is important to supply
the plating solution to the whole strip surface at high speed
and with uniformity. As plating proceeds, metallic ions are
lost from the plating solution. Quick resupply of these lost
metallic ions is essential for uniform, high efficiency plating.
In order to obtain a uniform plating thickness, it is necessary
to ensure that the composition of the plating solution supplied
to the whole strip surface should be uniform. It is also necessary
with electroplating equipment to minimize the electrode distance
between the strip and the anode in order to reduce electric resistance
and hence power consumption. For this purpose, plating cells
of the various types shown in the figure have been developed
and put into practical use. Multiple cells are typically arranged
in series to obtain high productivity.
Examples of electroplating are zinc, tin and chromium plating.
Zinc plating is divided into pure zinc plating and alloy zinc
plating such as zinc-iron and zinc-nickel, which are applied
to produce electrogalvanized sheets for automobiles, home electrical
appliances, and building materials. Tin plate is mainly used
in food and beverage cans. The method of joining the can body
has changed from soldering to cementing and welding. To reduce
the consumption of expensive tin, the production of tin-free
steel has increased. Tin-free steel is a surface-treated steel
strip with a coating comprising an under layer of metallic chromium
and an upper layer of chromium hydrate. |
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