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Tinplate

• satyendra
• July 14, 2013
• 21 Comments
• basis box, Black plates, DR tinplate, Electrolytic tinning, finishes, Hot dip tinning, Passivation, tin plate.tempers, tinplate,

Tinplate

Tinplate is a thin steel sheet coated with tin (Sn) metal. The thin steel sheet on which tin coating is done is known as black plate. Low carbon steel with carbon well below 0.1 % is normally used for tinplate production. Tin coating is applied either by hot dipping or electrolytic coating process. 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. Electrolytic coating process is able to produce a uniform coating of controlled thickness.

Tinplate as a food packaging material was first used at the beginning of the 19th century. Metallic materials have played a relevant role ever since. The widespread use of metallic containers in packaging all product types is a consequence of their great versatility and excellent properties, namely mechanical strength and formability, light weight, hermeticity, gas impermeability, opacity, thermal conductivity, relative chemical inertness, easy printing, and recycling. These properties, together with constant development and technological evolution, have extended the application of metallic materials to all food products, although they are best suited for the packaging of preserves, juices, and carbonated beverages.

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.

Until a few years ago, the unit of trade for tinplates was the basis box. The nomenclature basis box is a hangover from earlier times when tinplate was sold in units of 112 sheets, each 14 inch (356 mm) by 20 inch (508 mm). The basis box is an area of sheet of 31,360 square inches (20.2325 square metres). It derives from the time when weighing was the accepted method of measuring both sheet and coating thickness. The surface area of a basis box is 62,720 square inch. The nomenclature survives today as the unit area for the selling of tinplate within some parts of the industry. When tinplate is sold by the basis box, the thickness is known as the ‘substance’ or ‘base weight’. It is defined in terms of pounds avoirdupois per basis box (lb/bb). Material sold in terms of ‘substance’, or in Imperial measurements, is normally available in a specified range of thickness.

Tinplate is today described principally in terms of metric dimensions, e.g. the SITA (System International Tinplate Area) which is 100 square metres. For the conversion to decimal thickness the weight per basis box is to be multiplied by .00011. In the original system, 1 lb basis box meant that 1 lb of tin was applied evenly to both sides of the plate, i.e. each side received 0.5 lb (equivalent to 11.2 grams per square meter or ‘gsm’) of tinplate.

Tinplate as a food packaging medium has survived for over 200 years.  In 1810, Peter Durand received the patent for the idea of preserving food in tin cans from King George III of England. The ‘tin can’ immediately found acceptance and was used for international search for new territories and found its way across continents with Russia, Germany and USA being the initial beneficiaries who then commercialized production and used it to send products to South America and the Far East. Further developments through the 19th century ensured increased productivity in production of cans, sealing of cans, differentiation based on can shapes, pressurized cans. By the time of the World Wars, it was an integral part of packaging.

In 1938, the benefits of preservation of food in tin cans were further established through the chemical analysis of contents of a 100 year old canned product (had been packed for an expedition but was not used), which was found to be in perfect condition. Over the years, packaging improvement has led to more customer friendly designs along with usage of the right combination of forming technology, steel specifications (gauge consistency and clean steel i.e. absence of non-metallic inclusions), infrastructure (particularly recycling infrastructure), and market conditions. The tinplate can has evolved as the consumer demand has required it to, adapting, innovating, and satisfying while preserving the qualities that provide its inherent value which is protection and strength. There is further scope for development with lighter cans. Present development in food processed can making suggests a further potential weight reduction of around 23 %.

The biggest single contribution towards the decrease in the amount of tin on tinplate has been the electrolytic tinning process, as this allows well controlled, but small, amounts of tin to be deposited onto steel sheet. The tinplate is then flash melted, when the surface obtains the characteristic shine and reduced porosity, but at the same time an Inter-metallic alloy is formed. As the quantity of tin has decreased, the relative amount of tin associated with the alloy layer has decreased, making reclamation more difficult.

Tinplate production process

The traditional method for producing tinplate involved dipping or passing the steel sheet through a bath of molten pure tin. It was the first modern technique of manufacturing tinplate, by the hot dipping of single sheets of steel in a bath of molten tin. It has now been discontinued and accounts for only a very small percentage of the global tinplate production.

Tinplate, or tin-coated thin steel strip, is now mostly produced by the electroplating of tin on a steel base in a continuous process. Continuous electrolytic tinning was developed in the thirties and forties in an attempt to reduce the amount of tin needed per unit area, as well as to take advantage of the great lengths of strip (coils) which the steel plants could then supply. The first electrolytic tinning lines began to operate in Germany in 1934 and in the USA in 1937. Tin shortages in the USA during the Second World War stimulated a large-scale rise of electrolytic tinning, which soon became the dominant manufacturing technique of tinplate.

Tinplate is produced in the electrolytic tinning process through the electro-deposition of tin on steel from aqueous solutions of tin salt. The electrolytic plating produces a perfect coating control. To protect the tin coating, the production procedure includes an electrolytic passivation treatment and a final oil coating.

The introduction of the electro-plating process enabled a different thickness of tin to be applied to the two surfaces of the steel. This ‘differential tinplate’ is of economic benefit to the user since it enables the most cost-effective coating to be selected to withstand the different conditions of the interior and exterior of the container during its application.

All tinplates are now virtually produced by the electroplating of tin on to the steel base in a continuous process. The major reasons why electro-tinning of steel strip superseded hot-dip tinning, were because it can give a very much higher degree of thickness control, and much higher outputs of tinplate of higher quality, and lower production cost. As plating technology and steel chemistry have improved, steel base and tin coating thickness have been gradually reduced, significantly lowering the cost of production. Presently a typical coating thickness is being achieved in the range 0.4 micrometers to 1.35 micrometers depending on the end use.

Blackplate coils weighing between 5 tons to 15 tons are fed onto the tinning line, being loaded onto the two uncoilers required to allow continuous operation. The tail end of the coil being processed is welded to the leading end of the next coil to be processed. This necessitates the two coils being stationary during welding. To avoid shut down during welding, lines are fitted with looping towers or accumulators which can hold varying amounts of uncoiled plate (frequently upto 600 metres). Most modern electrolytic tinning lines incorporate side trimmers after the accumulator to cut the strip to the correct width. Many electrolytic tinning lines now incorporate tension or stretch levellers, which apply controlled tension across the strip to remove distortions.

Tin can be deposited either in the state of stannous (Sn2+) or stannic (Sn4+). Virtually all lines now use acidic processes where tin is deposited from the stannous state, the main advantages of this being that it only needs half the electricity compared to deposition from the +4 oxidation state, and since higher current densities are achievable fewer plating tanks are needed.

Plating is preceded by cleaning in a pickling and degreasing unit, followed by thorough washing to prepare the surface. After the plating stage, the coating is flow melted, passivated, and finally lightly oiled. Flow melting consists of heating the strip to a temperature above the melting point of tin (typically 260 deg C to 270 deg C), followed by rapid quenching in water. During this treatment a small quantity of the tin-iron compound FeSn2 is formed. The structure and weight of this alloy layer plays an important role in several forms of corrosion behaviour.

The steel strip is then given a passivation treatment to render its surface more stable and resistant to the atmosphere. This normally involves an electrolytic treatment in a sodium dichromate electrolyte which results in the formation of a film (normally less than 1 micrometer thick) consisting basically of chromium and chromium oxides and tin oxides.

The passivation film which forms on the surface of tinplate through chemical or electro-chemical reaction plays an important role during tinplate storage, transportation, and use. On one hand, the passivation film can block the diffusion and penetration of oxygen and corrosive media (such as chloride ions and sulphur ions) into the interior of the tinplate, thereby enhancing the oxidation resistance and corrosion resistance of the tinplate. On the other hand, the film can play a role in connecting the tinplate and the coatings or paint films when producing cans to improve adhesion. Thus, the performance of the tinplate is directly related to the properties of the passivation film. The fundamental factors which affect the performance of the passivation film on the tinplate depend on the properties of the passivation film itself. Specifically, the performance depends on the composition and structure of the passivation film.

After passivation the plate is given a light oiling to help preserve it from attack and to assist the passage of sheets through container-forming machines without damaging the soft tin layer. Finally the strips are sheared into sheets or coiled, and then packed for shipment to the can manufacturers.

The tin used for the coating of tinplate has to be at least 99.85 % pure. This defines the tin used to make the anodes for electrolytic tinplate production (or the grade of tin used to make up the baths in the hot dipping process). Fig 1 gives flow diagram for the production of tinplate.

Fig 1 Flow diagram for the production of tinplate

Conventional tinplate is a heterogeneous material with a stratified structure, formed by a low carbon steel sheet coated with tin on both sides. Tinplates are produced with plate thickness range of 0.13 mm to 0.50 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 tin coating mass varies from 0.5 gsm to 34 gsm, representing less than 1 % of the steel weight. The tin coating on tinplate is so thin that for practical purposes it can be ignored in considering thickness, so that the specified thickness is essentially that of the steel base.

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 user applications. Tinplates with heavy coating weight 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 need high corrosion resistance. These are normally used after painting or printing.

Double reduced (DR) tinplate are from 0.13 mm to 0.29 mm. Lower gauges down to 0.08 mm are now available for special uses, either in single-reduced or double-reduced base materials. Heavier gauges of conventional tinplate (upto 0.6 mm) are available from several sources. However, in some cases, notably in the USA and Europe, material above 0.5 mm thick is described as ‘tinned sheets’, rather than as ‘tinplate’.

Double reduced tinplate is produced by giving the steel a second cold-reduction, of the order of 15 % to 50 %, following annealing. This operation replaces temper rolling (Fig 1). The resultant DR product is stiff and strong. DR product also has marked directional properties, i.e. its formability is very different in the rolling direction and transversely to it. For this reason, it is especially important to specify the rolling direction and to use the DR tinplate correctly.

Differentially coated tinplate, frequently called ‘differential tinplate’, is electrolytic tinplate, in which one surface carries a heavier coating than the other surface. This material is used mainly for the production of containers which need higher corrosion resistance inside than outside, but can occasionally be used ‘inside out’, for example, in packaging inert materials for shipment to tropical regions, or any other application to optimize the cost of the tinplate products. In order to distinguish material having differential coatings, it is normal to mark one surface. Normally the heavier coated surface is marked, since this normally forms the interior of the can, but the user can arrange with the supplier for the lighter surface to be marked if needed.

The coating of tinplate consists of four components, excluding the steel base which represents the bulk volume of the tinplate. These four components are (i) the iron tin alloy layer of FeSn2, (ii) free tin layer, (iii) oxide/passivation film layer consisting of chromium and chromium oxide, and (iv) the surface layer, which is normally lacquer or oil. In order to get an approximate idea of the relative thicknesses (t), the steel substrate is around 2,540,000 t, the alloy layer is around 1,200 t, free tin layer is around 12,000 t, the passivation film layer is 25 t and the oil layer is around 50 t. Obviously the free tin and iron tin alloy layer thicknesses alter with variation in the amount of tin deposited, but over 99.5 % of the tin present Is in one of these two forms. The oxide layer is stannous oxide (SnO). The passive film on the tinplate is undoubtedly affected by the presence of the oil film. The oil film is applied to the strip to aid de-piling of plates and to aid fabricability in can production. Fig 2 shows structure of tin coating on steel sheet.

Fig 2 Structure of tin coating on steel sheet

Features of tinplate

The tinplate has several features which are described below.

Appearance – Tinplate is characterized by its pleasing 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 skillfully 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 achieved 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 achieved against container contents. Coated items are required to 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.

Safety aspects – Tinplate, being low weight and high strength, makes food cans easy to ship and transport.

Eco friendly – Tinplate offers 100 % recyclability and hence is eco-friendly.

Low temperature application – Tin is not good for low temperature applications since it changes structure and loses adhesion when exposed to temperatures below – 40 deg C.

Specifications

The tinplates are normally specified as per the steel base, extent of tempering, the coating weight, annealing method, and the surface finish. For cans, the quality and aptitude of tinplate are both defined by certain properties, which are normally included in the technical specifications of the material such as tin coating weight, steel substrate, temper, passivation, and surface finish. The tin coating weight is of the most practical interest. At present, the tin coating weights of commercial tinplates are fully standardized. To identify differential tinplates, it is common to draw two parallel lines on the surface having the higher coating weight. Separation between these lines depends on the coating and is defined in the standards. Nowadays, low tin coating tinplates (LTS) are being commercialized for particular use, since they are more economical than standard tinplate.

The base steel is continuously cast and normally aluminum killed. The base steel can be single reduced or double reduced. The base steel is of the following three types.

• Type MR – This base steel has low in residual elements and has good corrosion resistance properties. This steel is widely used in general applications
• Type L – In this type, the base steel has extremely low residual elements (copper, nickel, cobalt, and molybdenum). This steel type has very good corrosion resistance to certain types of food products.
• Type D – In D type, aluminum killed base steel is used. This type is used in applications involving deep drawing or other types of severe forming which tend to give rise to Lueders lines.

Single-reduced tinplates are produced in the following surface finishes. The practice and new standards recognize five basic surface finishes. Surface finish is indicated by ‘Ra’ which is the arithmetic average of the absolute values of the profile height deviations from the mean line, recorded within the evaluation length. Simply put, Ra is the average of a set of individual measurements of a surfaces peaks and valleys. .

Bright finish – It consists of a surface provided by a flow-brightened tin coating on a smooth finish steel base (steel roughness is to be lower than 0.35 micrometers Ra. Bright finishes are normally for general use.

Light stone finish – It consists of a surface provided by a flow-brightened tin coating on a steel base finish characterized by a light directional pattern (steel roughness is to be between 0.25 micrometers Ra and 0.45 micrometers Ra).

Stone finish – It consists of a surface provided by a flow-brightened tin coating on a steel base finish characterized by a directional pattern (steel roughness between 0.35 micrometers Ra and 0.60 micrometers Ra). This type of finish makes the scratches of printing and can making less conspicuous.

Matt finish – It consists of a surface provided by an un-melted coating normally on a shot blast finish steel base (steel roughness over 0.90 micrometers Ra). This is dull type of finish and mainly used for making crowns.

Silver finish – It consists of a matt finish product which has been flow melted. This type of finish is also called satin finish. This is rough dull finish mainly used for making artistic cans.

Double reduced tinplate is customarily supplied with a finish corresponding to stone-finish. It can however, also be available with an un-melted tin coating.

There are normally two grades of tinplates. The first is electrolytic tinplate, standard grade which represents the normal production of lines employing the usual inspection and classification procedures. It permits lacquering and printing over the entire surface. The second is electrolytic tinplate, second grade which is available in some countries. This grade represents the best sheets rejected from the standard grade and can contain sheets showing surface imperfections, tinning defects, and shape and other minor defects. Second grade does not, however, include off-gauge material, or sheets with pinholes.

There is no official third grade, but off-gauge and pin-holed material can be suitable for some non-critical purposes and is sometimes categorized as ‘waste category’. National and international specifications prescribe a sampling scheme for assessing tinplate grades.

Mechanical properties

The mechanical properties depend on a number of factors including steel composition, rolling practice, the annealing cycle, and the degree of skin-pass or temper rolling. There is no single mechanical test which can measure all the factors that contribute to the fabrication characteristics of the material. However, the Rockwell hardness (R30T) test is in general use as a quick test which serves as a guide to the properties of the material.

The term ‘temper’ when applied to tinplate, summarizes a combination of interrelated mechanical properties. Tinplate is available in a wide range of forming grades or ’tempers’. Temper is related to the mechanical resistance of the materials and is a property which measures the quality of the steel substrate. Temper values are measured by the elastic module or by using Rockwell hardness (R30T) testing.

For single-reduced tinplate only the Rockwell superficial hardness (R30T) test is at present specified. However, the determination of tensile strength of the product is a more technically sound and meaningful measure of the mechanical behaviour. This technique is more and more practised. For double-reduced tinplate the determination of tensile properties is already used to determine mechanical properties.

The Rockwell hardness (R30T) test values for tinplate form the basis for classifying tinplate into a system of temper designations, as shown in Tab 1. Tab 1 covers the most normally used temper designations. Particular national standards can have other temper designations. Present practice for single-reduced tinplate is to use the aim Rockwell hardness (R30T) value as the classification and for double-reduced tinplate the tensile value. Formerly a simple numerical grading from 1 to 9 for tempers classification was used.

It is to be noted that, primarily due to the differences in grain size and shape, the mechanical properties of batch annealed and continuously annealed material of the same Rockwell hardness (R30T) value are not identical. A further point is that since the Rockwell (R30T) test does not measure all the factors which contribute to the fabrication characteristics of tinplate, it is customary to specify the Rockwell hardness (R30T) value in terms of an aim value or range rather than an exact value. The principal criterion for acceptance is that the tinplate is to satisfactorily make the required part.

Uses of tinplate

While using tinplate the following precautions are required 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 are not to be stored for a long time and are to be used as soon as possible after the receipt.
• Althogh the tinplate has good corrosion resistance, it tend to rust in humid atmosphere. Hence tinplates are to be immediately used after unpacking.
• Tin is dissolved by a strongly alkalline solution, hence when usuing tinplate for making cans for alkaline contents, the internal surface also needs painting.
• Contents which have sulphur content, cause blackening of the tinplate. Hence in this case also, painting of internal surface is needed.

By far the largest application of tinplate is in packaging industry since it is ideally suited for packaging.  The privileged position held by tinplate in this industry can be explained by its several advantages by virtue of it being non-toxic, light in weight, strong, corrosion resistant and easily formed, and soldered and welded. It also provides a very good printing surface. The tin coating has a low melting point, possesses lubricant qualities and imparts a good appearance. Cans made from tinplate are easy to handle, store and recycle.

The five constituents which serve a precise purpose for the main end user of tinplate are (i) the mechanical properties of tinplate are those of the steel base, i.e. strength allied to lightness and formability, (ii) the tin coating provides excellent corrosion resistance or inhibition towards most interior and exterior can environments, (iii) tinplate provides an excellent base for decorative purposes, or the application of lacquers when additional protection is needed, (iv) tinplate is suitable for a number of assembling processes, e.g. crimping, soldering, welding, and cementing, and (v) tin, being a very soft metal, ensures low wear of the tooling of the can-maker and can even contribute to lubrication in cases of severe forming.

Tinplate is primarily used for packing foodstuffs and beverages, but it is also used in containers for oils, grease, paints, powders, polishes, waxes, chemicals and many other products. Aerosol containers and caps and closures are also made from tinplate.

Other uses of tinplates include electronics (electrodes, cable tape and magnetic screen covers etc.), engineering (automotive oil filters, automotive air filters and gaskets etc.) and construction (gas meter internal components, heat exchangers, cookware, shelving etc.).