Tin plate is a thin steel sheet coated by tin. The thin steel sheet on which tin coating is done is known as black plate. Tin has several properties which make it suitable for use as a coating on steel. It has a good resistance to corrosion in a wide range of environments and in particular retains its appearance and surface properties extremely well in indoor atmosphere. It is easily soldered and the good corrosion resistance ensures effective retention of solderability. It is safe in contact with foods, neither producing risks to health nor impairing flavours. The softness of the metal, although disadvantageous for some uses of coatings, has merit for others, facilitating cold working, giving easy running threads on fasteners and helping to seal joints. Tin coatings are applied either by hot dipping or electrolytically coated. Both processes produce coatings which have characteristics of their own. Hot dip tin coating has advantages in being metallurgically bonded to base steel and of producing a smooth bright surface. Electrolytically coating is able to produce a uniform coating of controlled thickness. With electrolytically coating steel sheet can be differentially coated. In such coating heavy side coating is usually marked. The cross section of tin coating on steel sheet is given in Fig 1.
Fig 1 Cross section of tin coating on steel sheet
Features of tinplate
The tinplate has the following features.
- Appearance – Tinplate is characterized by its beautiful metallic luster. Products with various kinds of surface roughness are produced by selecting the surface finish of the substrate steel sheet.
- Paintability and printability – Tinplates have excellent paintability and printability. Printing is beautifully finished using various lacquers and inks.
- Formability and strength – Tinplates have got very good formability and strength. By selecting a proper temper grade, appropriate formability is obtained for different applications as well as the required strength after forming.
- Corrosion resistance – Tinplate has got good corrosion resistance. By selecting a proper coating weight, appropriate corrosion resistance is obtained against container contents. Coated items should meet 24 hour 5 % salt spray requirement.
- Solderability and weldability – Tinplates can be joined both by soldering or welding. These properties of tinplate are used for making various types of cans.
- Hygienic – Tin coating provides good and non toxic barrier properties to protect food products from impurities, bacteria, moisture, light and odours.
- Safe – Tinplate being low weight and high strength makes food cans easy to ship and transport.
- Eco friendly – Tinplate offers 100 % recyclability.
- Tin is not good for low temperature applications since it changes structure and loses adhesion when exposed to temperatures below – 40 deg C.
The tinplates are specified as per the steel base, extent of tempering, the coating weight, annealing method and the surface finish.
The base steel is continuously cast and aluminum killed. The base steel can be single reduced or double reduced. The base steels are of the following three types.
- Type MR – These base steels are low in residual elements and have good corrosion resistance properties. These steels are widely used in general applications
- Type L – In this type the base steel has extremely low residual elements (Cu, Ni, Co and Mo). These steels have very good corrosion resistance to certain types of food products.
- Type D – In D type aluminum killed base steel is used. These types are used in applications involving deep drawing or other types of severe forming that tend to give rise to Lueder’s lines.
Tinplates are produced in thickness ranges of 0.15 mm to 0.60 mm, width range of 200 mm to 1067 mm and length range 406 mm to 1110 mm. In case of electrolytically coated tinplates, the tinplates are produced in coil form.
The nominal weight of tin coating varies from 1.1 grams/Sq m to 15.2 grams/Sq m. The minimum average coating weight varies from 0.9 grams/Sq m to 14.0 grams/Sq m. The minimum spot value is not less than 80 % of the minimum average coating weight. Tin coating eight is determined in accordance with specific end use applications. Tinplates with heavy coating eight are used for making cans which require a high corrosion resistance. These plates are used as bare without painting and printing. On the other hand tinplates with light coating weights are used for making cans which do not require high corrosion resistance, These are normally used after painting or printing.
The tinplates are produced in the following surface finishes.
- Bright – in this type of finish the surface roughness is aimed at o.25 Ra. Bright finishes are normally for general use.
- Stone – In this type of finish the surface roughness aimed is 0.40 Ra. This type of finish makes printing and can making scratches less conspicuous.
- Super stone – In this type of finish the surface roughness aimed is 0.60 Ra.
- Matte – In this type of finish the surface roughness aimed is 1.00 Ra. This is dull type of finish and mainly used for making crowns.
- Silver – This type of finish is also called satin finish. This is rough dull finish mainly used for making artistic cans
The temper grades of tin plates are as follow. Hardness values are given in HR 30 T. When the tinplates are produced from continuous annealed material then the symbol of CA is added to the temper grade such as T-5 CA
- Single reduced
- T-1 temper grade – It has hardness value of 49 +/- 3. It is used in applications involving deep drawing that require large flexibility.
- T-2 temper grade – It has hardness value of 53 +/- 3. It is used in application that need moderate drawability and some stiffness.
- T-2.5 temper grade – It has a hardness value of 55 +/-3. It is used in application that requires the drawability of T-2 temper grade in combination with the stiffness of t-3 temper grade.
- T-3 temper grade – It has a hardness value of 57 +/-3. It is used in application that requires stiffness to prevent buckling.
- T-4 temper grade – It has a hardness value of 61 +/-3. It is used in application that requires large stiffness such as can ends and crowns etc.
- T-5 temper grade – It has a hardness value of 65 +/-3. It is used in application that requires very good buckling resistance.
- Double reduced – These are thin steel materials with high strength and produced by cold reducing the steel material after the process of cold reducing and annealing.
- DR -8 temper grade – It has a hardness value of 73 +/-3. It is used in application that requires stiffness and strength.
- DR-9 temper grade – It has a hardness value of 76 +/-3. It is also used in application that requires stiffness and strength.
- DR -9 M temper grade – It has a hardness value of 77 +/-3. It has similar application as DR 9.
- DR – 10 temper grade – It has a hardness value of 80 +/-3. It is used in special application that requires strength.
Precautions needed for tinplate usage
While using tinplate the follinfg precautionas are requires to be observed.
- Since tinplate is covered by soft metallic coatings, precautions are necessary to avoid scratches during handling and transportation
- Paintability, printability, solderability and mechanical properties of tinplate tend to deteriorate as time lapses. Hence they should not be stored for a long time and should be used as soon as possible after the receipt.
- Althogh the tinplate has good corrosion resistance, it tend to rust in humid atmosphere. Hence they should be immediately used after unpacking.
- Tin is dissolved by a strongly alkalline solution, hence hen usuing tinplate for making cans for alkaline contents, the internal surface will also need painting.
- Contents which have sulphur content, cause blackening of the tinplate. Hence in this case also painting of internal surface is needed
Tinplates have the following applications
- Cans and Containers – Cans for food and beverages, containers for oils, vanaspatis, grease paints, polishes, chemicals and aerosol etc. Crown caps and screw caps
- Electronics – Electrodes, cable tape and magnetic screen covers etc.
- Engineering – Automotive oil filters, automotive air filters and gaskets etc.
- Construction – Gas meter internal components, heat exchangers, cookware, shelving etc.