Tinplate is a low carbon steel sheet which is the
base metal that is coated by tin on both the sides by electrolytic
process. Tinplate is got by electroplating tin on the cold rolled steel
sheet surface.
The surface of the Tinplate sheet has a metallic luster. Tinplate is
widely used as a major raw material for the packaging industry. Tinplate
is corrosion resistant, weldable and solderable. Tinplate easily takes
the desired shape and are widely used for making containers and cans.
It is also malleable and easily recycled. Tinplate was previously used
for cheap pots and as holloware but galvanized vessels[vessels coated
with zinc] are now used, this is because iron is protected by zinc
electrolytically since zinc dissolves in preference to iron, whereas tin
will serve as a protective layer till the tin surface is not broken.
This is the main reason as to why one should not food from a rusty can.
Characteristics of Tinplate Steel Sheet
The Tinplate sheet is used in a wide range of products. The utility of
the tinplate sheet lies in it's end usage. The tinplate products have
certain characteristic features like:
- Good paintability and printability
The painting and printing with the usage of different lacquers and
inks has remarkable results on the Tin plate sheet.
- Corrosion Resistant
Proper corrosion resistance against container contents is obtained
by way of selecting a proper coating weight.
- Good Formability & Strength
Appropriate formability required for various applications is
attained by selecting a proper temper grade.
- Good Solderability & Weldability
Selection of a proper temper grade leads to appropriate formability
which is useful for various applications and also for strength which
is required after forming.
Manufacturing process of Tinplate steel sheet
- Electrolytic tinning
This is the process by way of which Tinplate is majorly produced.
In this process the steel base is electroplated by tin in a
continuous process. The previous technique of manufacturing tinplate
by dipping steel sheets in bath of molten tin is now outdated and is
very rarely used. This procedure accounts for a very minor share of
the world Tinplate production.
- Acid sulphate process
This procedure was evolved and developed by the United States Steel
Corporation. This procedure means Tinplate manufactured on the line.
At the beginning of the line the strip is ncoiled and welded and at
the end it is accumulated in a looping tower. The next step is that
the strip enters into a series of vertical tanks that help in
electrolytic cleaning and pickling. The alkaline cleaning solutions
are made up of combinations of alkaline phosphates and silicates,
sodium hydroxide and surface active agents. After this step the
strips are transferred into a scrubbing and rinsing unit wherein the
traces of the detergent are removed before pickling. This section
has rotary scrubber brushes and high pressure jets for the process
of scrubbing. Making the strip anodic in sulphuric acid solution at
25-40"C is known as pickling. 5 and 30 A/dm2 is the current
density that is employed and the acid content is not made to
increase 25 g/l. After this step the strip is put into the
electroplating section after final rinsing and scrubbing.
- The halogen process
The second major electrolytic tinplate process contributing to the
world Tinplate production is the Halogen process. Stannous chloride
and alkaline fluorides are the components of aqueous electrolyte
solution. The me molar ratio of soluble fluoride and stannous
chloride is an important operating factor as related to bath pH.
There are addition agents like naphtholsulphonic acids or
polyalkylene oxides which are used to get coherent and smooth
deposits. Gelatin or peptone are the other agents that may be
present.
- The Fluoborate process
This process is exclusively used for the commercial production of
Tinplates. The first lines were horizontal but for better speed the
vertical lines were introduced. The tinning speed in the vertical
lines of 150m long goes upto 640m/min. The coil is then
electrolytically cleaned [cathodically/anodically] and is then
pickled without passing electric current in the hydrochloric acid at
room temperature. The sheet is then finally rinsed and brushed.
- The alkaline stannate process
The major advantage of alkaline electrolytes are that they are
non-corrosive to steel and hence requires no additional agents for
coherent and smooth deposits. The major disadvantage is that they
must be so heated that operating cathode current density is lesser
than the anode current density. In this process the electrical
energy required to plate a sheet is twice than that plated with tin
from the acidic electrolytes. This process is very less used today,
only in some older plants. The process has both advantages and
disadvantages. The electrolytes that are based on alkaline stannate
solutions are enabled to produce high quality deposits and are easy
to operate.
- Hot dipped tinplate
This is the oldest procedure for producing tinplate. Now this
procedure has taken a back seat but even then certain plants still
involve this procedure. The production range is much less in this
case as compared to the electrolytic plant, like a modern hot
tinning line produces 1000 tonnes per month but an electrolytic
plant produces more than this. The coating of a hot dipped tinplate
is thicker and less uniform than electrolytic tinplate. This process
was a dominant process till the 1940s. But of recent the
electrolytic process has gained huge importance because of it's
advantages.
Application
Tin plate is obtained by the application of Tin on the surface of steel
sheet. Tin plate steel sheet is extensively used in making containers.
The major ones being beverage cans, food cans, artistic cans and 18
litre cans. Tin plate sheets are also used in making protective cover
for optical fiber cables and is also used in making photographic film.
In the recent times Tinplate sheet is used for making electrical
machinery parts and varied other products.
Precautionary measures:
There are certain measures that are to be taken care of while using tin
plate products. This is because the tinplate has a soft metallic tin
coating. The certain specifications are listed below:
- Improper and rough handling or vibrations while transportation
of these sheets causes scratches, hence such sheets should be
handled with care.
- The specific characteristics like- printability, paintability
and solderability tend to go down with the passage of time hence
tinplate sheets should be used soon after the production.
- The corrosion resistant feature of the tinplate sheet tends to
deteriorate in humid conditions, hence it is advisable to use the
tinplate sheet soon after unpacking.
- The sulphur has a tendency to blacken the tinplate and hence the
inner surface of the can made out of tinplate sheet should be
painted.
- The tinplate cans made for containing alkaline solution should
be painted on the internal side since the alkaline content dissolves
tin.
Buyer's consideration
There are certain factors that should be kept in mind while placing a
bulk order for Tinplate steel sheets, the points are:
- Thickness
A buyer should be very particular while purchasing Tinplate.
Actually the thickness of the Tin coating is so thin that it can be
easily ignored, the only thickness is that of the steel base. The
usual international standard for thickness is 0.15 mm to 0.49 mm.
There are number of producers who produce heavier gauges measuring
up to 0.6 mm, in USA, Europe material above 0.5mm thickness is known
as 'tinned sheet' than tinplate.
- Surface Finish
This is another factor that one should be aware of while placing
order for tinplate sheets.
There are different surface finishes to choose from, like:
- Stone finish
This finish is characterized as a steel base finish by a
directional pattern. This kind of a surface is provided by
flow-brightened tin coating.
- Bright finish
This kind of finish is got by flow- brightened tin coating on a
smooth steel base.
- Silver finish
A highly roughened shot blast finish steel base serves as a
surface over which there is a flow-brightened tin coating.
- Matt Finish
Such a surface finish is achieved when an unmelted coating is
put on a steel base which is shot blast finish.
