Tinplate and Process of Tinning
Tinplate and Process of Tinning
Tinning or tinplating is the process of thinly coating sheet or strip of steel with tin (Sn), and the resulting product is known as tinplate. Tinplate is light gauge, cold-reduced low-carbon steel sheet or strip, coated on both faces with commercially pure tin. It combines the strength and formability of steel and the corrosion resistance, solderability and good appearance of tin. Within this broad description, there exists today an extremely wide range of tinplate products, tailor-made to meet particular end-use requirements.
Tinplates are widely used for making various types of cans by soldering or welding. They are characterized by the attractive metallic luster. Tinplates with various kinds of surface roughness are produced by selecting the surface finish of the substrate steel sheet. They have excellent paintability and printability. Printing is beautifully finished using various lacquers and inks. Appropriate formability is obtained for various applications as well as the required strength after forming by selecting a proper temper grade. Also, appropriate corrosion resistance is obtained against container contents by selecting a proper coating weight.
Tinplate is used for making all types of containers such as food cans, beverage cans, and artistic cans. Its applications are not limited to containers. Tinplate has also been used for making electrical machinery parts and many other products.
Production of the steel base and its subsequent coating with tin are independent of each other, so that any set of properties in the steel, can in theory be combined with any tin coating. The composition of the steel used for tinplate is closely controlled and according to the grade chosen and its manner of processing, various types with different formabilities (also known as tempers) can be produced. Tinplate is sold in a range of steel thicknesses, generally ranging from around 0.15 mm to 0.6 mm.
The steel sheets can be coated with different thicknesses of tin. Even different thicknesses on the two faces (differential coatings) can also be produced to cater for varying conditions at the internal and external surfaces of a container. Several surface finishes are also produced for diverse applications. Tinplate has a special passivation treatment to stabilize the surface and improve adhesion of lacquers. It also carries a very thin film of an oil to improve its handling and fabrication properties. This oil is, of course, compatible with food products. The resulting wide variety of materials gives the user a great flexibility in choice and the ability to select precisely the right material for a given end use.
Tinplate and packaging of food material
Tin is present in the diet only in small quantities of complex bound Sn (+2) ions. It occurs in most of the food materials. Tin levels are to be as low as practicable because of the possibility of the gastric irritation. Levels are usually less than 1 mg/kg (milligrams per kilogram) in unprocessed food materials. Higher concentrations are found in canned food materials because of the dissolution of the tinplate to form inorganic tin compounds or complexes. Generally a maximum limit of 250 mg/kg for tin in solid foods in cans and a maximum level of 200 mg/kg for liquid foods in cans are specified. Stannous chloride is authorized as a food additive for canned food products upto 25 mg/kg (as tin).
The present major source of tin in the diet is food contact materials, especially the release from the tin cans to acidic food materials. Tin cans are actually steel cans with a thin coating of metallic tin (tinplate). There is often an internal resin-based coating on the tinplate. Tinplate is mainly used in cans, can ends, and closures mainly for glass bottles and jars. However, the use of tin cans is decreasing. Tin is also used to coat kitchen utensils.
Tin is amphoteric, reacting with both strong acids and bases, but is relatively unreactive to nearly neutral solutions. The presence of oxygen greatly accelerates reaction in solution. Tinplate used in food containers is only slowly oxidized. The tin content in food materials depends on (i) whether the tin cans are lacquered, (ii) the presence of any oxidizing agents or corrosion accelerators, (iii) the acidity of the food product in the tin can, (iv) how long, and at what temperature, the tin cans are stored before being opened, and (v) the length of time the product is kept in the tin can after it has been opened.
The oxidation of the tinplate followed by unavoidable migration of the tin ions formed into the food material is the physiochemical mechanism, known as the sacrificial anode effect, which protects the underlying steel from being corroded by the food material. The dissolution of the tin protects the can from possible perforation, and protects the contents from degradation (changes in colour and flavour) during heat sterilization and storage, which is having a typical shelf life of 2 years.
Tin concentrations in food materials in unlacquered cans can exceed 100 mg/kg while food materials stored in lacquered cans have tin levels generally below 25 mg/kg. However, storing food materials in opened unlacquered cans result in substantial increases in the tin concentration in the food materials. Canned vegetables and fruits in unlacquered cans make up only a small percentage by weight of total food intake, while they may contribute 85 % of the total intake of tin. The lacquer coating thickness greatly affects the performance of the lacquered food can.
Tinplate-its corrosion and uses
For hot dipped and electroplated tin, an oxide film forms on the tin in air. The film is fairly stable and provides a barrier to further oxidation. At pH values between 3 and 10 and in the absence of complexing agents, the oxide barrier protects the metal from food. Outside this pH range, however, corrosion of the tin occurs.
Some corrosion can be expected from tin or tin coatings exposed outdoors. In normal indoor exposure, tin is protective on iron, steel, and their alloys. Corrosion can be expected at discontinuities in the coating (such as pores) due to galvanic couples formed between the tin and the underlying steel through the discontinuities, especially in humid atmospheres.
Tinning is an extremely cost-effective process, since tin is readily available and it is much less expensive. It also offers excellent solderability, as well as superior protection against corrosion.
Tinplating can produce a whitish-gray colour which is preferable when a dull or matte appearance is desired. It can also produce a shiny, metallic look when a bit more luster is preferred. Tin offers a decent level of conductivity, making tinning useful in the manufacturing of various electronic components. Tin is also used for food packaging. Because of several advantages, tin is the metal of choice for plating applications in a wide range of industries such as (i) aerospace, (ii) food packaging, (iii) electronics, (iv) telecommunications, and (v) jewelry manufacturing.
Formation of tin whiskers can occur during the tinning process and can negatively impact the final outcome. Tin has the strong tendency to form whiskers. Tin whiskers are small, sharp protrusions which can form on the surface of the pure tinplated sheets long after the conclusion of the plating process. Whiskers have a diameter of 1 mm to 2 mm and can reach a length of about 3 mm. Whiskers can cause significant damage to the finished tinplates. Since the whiskers are electrically conductive, they can cause short circuits in electronic components. Although the exact mechanism of whisker growth is not yet understood in detail, tin whiskers can occur only in electroplated pure tin coatings. As a preventive measure, lead is required to be added to the tin by at least 2 %, or the pure tin plating is to be heated above the melting temperature of tin.
Tinplate is basically a steel product, since it is essentially light gauge steel strip coated with tin on both surfaces. Hence, the production of tinplate falls conveniently into two main stages namely (i) the production of thin low carbon steel strip or sheet having the required dimensions and mechanical properties, and (ii) the tin coating process. Here only the tin coating process is described. The thin low carbon steel strip or sheet on which the tin coating is applied is called ‘black plate’.
Large quantities of relatively strong tinplate are now manufactured by the technique of double reduction. Thinner yet stronger tinplate can be produced by double reduction method, which allows for more efficient material utilization in can making. After an initial cold rolling and annealing, instead of temper rolling, the steel is given a second cold reduction with lubrication, of around 10 % to 50 %. The work hardening effect gives the steel additional strength, whilst the strip retains sufficient ductility for it to be formed into can ends and bodies. Final thickness can be as low as 0.12 mm, the typical range being 0.14 mm to 0.24 mm. A two-stand or three-stand rolling mill can be used for double reduction. In some plants, a dual purpose mill is used which can produce double-reduced material and operate as a conventional temper (skin pass) mill. Double-reduced steel shows very marked directional properties and the grain direction is always to be indicated and taken into account during forming operations with the final tinplate.
Before entering the tinning line the strip is normally edge trimmed and inspected on a coil preparation line. A strip thickness gauge can also be installed so that off-gauge or sub-standard black plate can be cut out. Coils of optimum weight are produced by welding strip lengths together.
There are two processes for the tinning of the black plates namely (i) hot dip tinning process and (ii) electroplating process.
Hot dip tinning process
Hot dip tinning process is the process of immersing the steel black plate into a bath of pure molten tin at a temperature greater than 232 deg C. The coating produced consists of a very thin intermetallic layer which first forms at the interface of the base material and the tin (e.g. when dipping the black plate, an iron/tin alloy is formed) followed by a layer of pure tin.
The steel strip to be tin-coated is first uncoiled and then subjected to a thorough cleaning and, optionally, pickling cycle. Thereafter, its entire surface is wetted with a fluxing agent suitable for the application, usually a standard commercial product. This flux or ‘soldering fluid’ activates the strip surface in preparation of the tinning process. The so-called fluxing bath is followed by the heated tin bath. Typically this is a resistance heated pot, but for high outputs the use of an induction-heated pot can also be considered. Here the molten tin is held at the specified temperature, and the amount of energy removed by the coated strip is substituted. Gas heating system can also be used but it tends to be disadvantageous due to the installation complexity.
Strip speeds reach upto 200 metres per minute (m/min). The tin bath has a temperature of around 250 deg C to 290 deg C (the melting temperature of tin is around 230 deg C). Given the relatively low heat conductivity of tin, bath temperature management needs to be carefully addressed. Downstream of the tin bath, which is to be adequately sized, the core of the system is the design and process integration of the wiping and blow-off unit since it is decisive for the coating thickness and uniformity over the width and length of the strip. Optionally, the air wiper can be coupled with a non-destructive inline coating gauge. This forms a closed control loop ensuring uniform product quality. From the air wiper the newly coated strip enters a non-contacting high-convection cooling zone and then passes through the coating gauge before it is wound up again on the recoiler. The special operating regime of tinning line in stop-and-go-mode provides a dramatic reduction in tin-coated reject material.
The advantages of hot dip tinning process are (i) no waste from production process, (ii) no hazardous substance (such as cyanogen, lead, etc.) is used at all in production process, (iii) plating speed is very high (several times higher than electrolytic plating, (iv) both thick coating and thin coating can be produced at around same speed, (v) thickness of tin layer is set by computer-controlled air knives system, a contact free process which ensures particularly high surface qualities, (vi) tin coating and base metal is strongly bonded as inter-metallic layer is formed during the hot dip process, (vii) risk of whisker growth is very small since hot dip process makes crystal structure of tin uniform and minimizes its inner stress which minimizes risk of whisker growth. The advantages of hot dip tinning when compared to electroplated tin coating include (i) less porous than electroplating, (ii) more ductile than electroplating, (iii) virtually stress-free, (iv) more economical than electroplating, and (v) better corrosion resistance than electroplating. The disadvantages of hot dip tinning is that the thickness of the coating provided by hot dip tinning is not as well controlled when compared to that provided by electroplating methods. Hot dip tinning is not to be used when tight tolerances are needed.
Tinning by electroplating
In electroplating, the item to be coated is placed into a vessel containing a solution of one or more tin salts. The item is connected to an electrical circuit, forming the cathode (negative) of the circuit while an electrode typically of the same metal to be plated forms the anode (positive). When an electric current is passed through the circuit, metal ions in the solution are attracted to the item. For producing a smooth, shiny surface, the electroplated sheet is then briefly heated above the melting point of tin.
Presently, tinplate is virtually produced only by the electroplating of tin on to the steel base by a continuous process (Fig 1). The major reason for electro-tinning of steel strip replacing hot dip tinning process is because it gives a very high degree of thickness control, including differential thicknesses of coating on the two sides of the steel sheet. The electro-tinning process also gives higher outputs of tinplate with superior quality and at lower production cost. Further, with the improvements in the plating technology and steel base chemistry, the thicknesses of steel base and tin coating have been gradually significantly reduced. These days a typical coating thickness is in the range 0.1 to 1.5 microns depending on the end use.
Fig 1 Schematic process flow diagram of a continuous electro-tinning line
There are four basic choices of electrolytic plating processes which can be used to deposit tin. These are (i) alkaline stannate, (ii) acid sulphate, (iii) acid fluoborate, and (iv) acid sulphonate. The stannate process is based on either sodium or potassium stannate. For high-speed plating applications, the potassium stannate is used since it has very high solubility as compared to the sodium salt. For achieving current densities upto 1600 amperes per square metre (A/sqm), a formulation containing 210 grams per litres (g/L) of potassium stannate and 22 g/L of potassium hydroxide is used. The potassium stannate concentration can be doubled in order to reach current density of 4000 A/sqm. Anode efficiencies in the range of 75 % to 95 % and cathode efficiencies in the range of 80 % to 90 % are typical for the alkaline process.
Of all the tin plating processes, the alkaline process has superior throwing power. The process does not require the use of organic addition agents but is to operate at elevated temperatures (70 deg C to 90 deg C). The most important aspect of alkaline tin plating is the critical need for proper control of the anode. If the tin anodes are not properly controlled during the plating process, rough porous deposits result. A yellow-green film is to be present on the anode during the plating operation in order to ensure excellent plating.
Plating solutions based on stannous sulphate (7 g/L to 50 g/L) and sulphuric acid (50 g/L to 150 g/L) can deposit either a bright decorative deposit or a matte finish depending on the type of grain refiner / brightening system used. A semi-bright matte tin finish can be obtained using gelatin and an organic compound, beta-naphthol. A large variety of organic brighteners are commercially available to produce bright, decorative adherent deposits from the stannous sulphate electrolyte. These additives are generally based on aliphatic aldehydes and an aromatic amine. Improved versions of the above consist of wetting agents such as water-soluble polyethylene glycol and a water soluble derivative of ethylene as a primary brightening agent. The bright bath has several advantages over the matte process which include improved corrosion resistance, reduced porosity, resistance to fingerprints, improved solderability as well as its cosmetic appearance.
The acid sulphate process operates between 20 deg C to 30 deg C at essentially 100 % anode and cathode efficiencies. The acid bath does not need the careful anode monitoring of the alkaline stannate bath but does need organic addition agents. However, the throwing power of the acid bath is normally less when compared to the alkaline stannate process.
Another acidic plating process based on tin fluoborate (75 g/L to 115 g/L) and fluoboric acid (50 g/L to 150 g/L) is designed to plate pure matte tin deposits. A major advantage of this process over the tin sulphate is that it can be operated at much higher cathode current densities, up to 10,000 A/sqm (in agitated plating solutions). Gelatin and beta-naphthol are typically used as grain refiners in this process, which is operated in the temperature range of 20 deg C to 30 deg C. Anode and cathode efficiencies are around 100 %.
Recently tin plating formulations based on methane-sulphonic acid (15 % to 25 % by volume) are gaining acceptance because the solutions require simple waste treatment, contain no fluorides or boron, and are less corrosive than the electrolytes based on fluoboric acid. The methane-sulphonic electrolytes, similar to the fluoborate baths, can hold high concentrations of metal in solution (upto 100 g/L tin) permitting plating at high speeds. A major drawback of the methane-sulphonic acid process is its high chemical make-up cost.
All of the acidic tin plating electrolytes mentioned above deposit tin from the divalent state (+2) as compared to the +4 state for the alkaline stannate solutions. The acidic processes thus deposit tin twice as fast as the stannate process and operate at essentially 100 % cathode efficiency. The acid tin processes are easier to control and maintain than the stannate solution. They have the additional advantage of operating at ambient temperatures.
While considering the process flow in the continuous electro-tinning line (Fig 1), black plate coils are fed onto the tinning line, being loaded onto the uncoiler. Two uncoilers are needed for continuous operation. The tail end of the coil being processed is welded to the head end of the next coil to be processed, which 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 quantities of uncoiled black plate (often upto 600 metres). Modern electro-tinning lines incorporate side trimmers after the accumulator to cut the strip to the correct width. Also, many lines now incorporate tension or stretch levellers, which apply controlled tension across the strip to remove distortions.
In the continuous electro-tinning lines, the cleaning time is very short (around 1 second to 2 seconds). Hence, there is need of effective cleaning of the black plate strip. This need is met with the use of electrolysis to aid chemical dissolution of rolling oil residues and other organic contaminants. Heavy current which is passed during the electrolysis produces gases at the strip surface. This results into lifting of the dirt and residue from the strip. The cleaning agent is generally a 1 % to 5 % solution in water of a mixture of phosphates, wetting agents and emulsifiers in a sodium hydroxide / carbonate base. Temperature is generally in the range of 80 deg C to 90 deg C with current density of 1000 A/sqm is normally adequate.
After cleaning, the strip is thoroughly washed, ideally in hot water (70 deg C) using high-pressure sprays. Pickling removes oxide and rust layers and leaves the surface etched for better deposition of tin. During the process the strip is usually made anodic then cathodic with current densities ranging between 500 A/sqm and 3000 A/sqm being employed.
Different types of electrolytes can be used in the tinplating section. The plating cells consist of a series of vertical tanks through which the strip passes in serpentine fashion. The number of plating tank passes in use, the anode length, and the width of the strip determine the effective plating area. This, together with the available plating current, decides the maximum line speed for a particular coating weight. Present day tinning lines achieve speeds of 600 m/min or more with typical strip widths between 1000 mm and 1250 mm. The steel strip is guided through the tanks by sink rolls located at the bottom of the tanks and conductor rollers with rubber covered hold-down rollers at the top. These collect electrolyte from the strip and return it to the plating cell. The conductor rolls need to have good electrical conductivity and low contact resistance between the roll and the wet strip. These rolls are generally made from steel coated with copper and then chromium.
Each plating tank has four anode bus bars and four banks of anodes, one for each face of the down and up passes of the strip. Traditionally anodes are made of 99.9 % pure tin and are 76 mm wide, 50 mm thick and around 1.8 m long. The anode is consumed in the process and is replaced when it is reduced in thickness by around 70 %. A worn anode is removed from one end of the bank and a new one inserted at the other, the others being moved across to make room. In recent years, inert anodes made from titanium coated with platinum or iridium oxide have become more popular. Nippon Steel was the first to use inert anodes in electro-tinning line. In this case stannous ions are produced off line in a generation plant in which high pressure oxygen is bubbled through the electrolyte solution containing pure tin beads, dissolving the tin and making fresh electrolyte.
Inert anodes are positioned parallel to the steel strip in a fixed position. There is no necessity for frequent renewal of these anodes. This results into minimal variations in the tin coating thickness across the strip width. Adjustable edge masks ensure correct anode width to avoid tin build-up on the edges of the strip. Since there is no need to cast and replace tin anodes, use of inert anodes also reduces requirement of manpower.
An alternative system of parallel tin anodes has also been used. In this system the anode bridges are aligned parallel to the strip and are loaded with conventional tin anodes. The anode bank is placed close to the strip reducing the initial voltages required. As the anodes slowly dissolve the voltage is increased to maintain a given current. When the anodes have been reduced to a specified thickness the whole bank is replaced. This system is claimed to give similar control over tin thickness as with inert anodes.
At the end of the plating section there is a drag-out control section which essentially removes residual electrolyte from the strip for subsequent recovery. Tin is deposited as a whitish coating having a slight metallic lustre. Where needed this is flow melted by induction or resistance heating (or a combination) to produce a bright mirror-like finish. In resistance heating, a high alternating current is passed through the strip via conductor rolls. With induction heating the strip passes through a series of internally cooled copper coils through which a high frequency current is passed. The induced eddy current and hysteresis losses heat up the strip and melt the tin coating. This flow melting process enhances the corrosion resistance of the product by formation of an inert tin-iron alloy layer.
Prior to flow melting, the plate is fluxed by treating with dilute electrolyte or proprietary chemicals to prevent surface defects appearing on the plate. Flow melted tin plate has a thin tin oxide film on the surface, which if untreated can grow during storage. In order to improve the tarnish resistance and laquerability a chemical or electrochemical passivation is applied to the strip. The most common form of passivation involves cathodic treatment at temperatures between 50 deg C and 85 deg C in dichromate or chromic acid solution containing 20 g/L dichromate (other treatments which are now seldom used are the use of phosphates or carbonates). This treatment deposits a complex layer of chromium and its hydrated oxides, which inhibits the growth of tin oxides, prevents yellowing, improves paint adhesion and minimizes staining by sulphur compounds. Prior to oiling the tinplate is to be thoroughly dried. Oiling with dioctyl sebacate or acetyl tributyl citrate is carried out in an electrostatic spray process.
Quality inspection is by in-line inspection prior to recoiling and involves checking of the strip thickness, detection of pinholes and tin thickness.
There is another electro-tinning process which has horizontal rather than vertical plating tanks. This configuration together with the high current densities used (6500 A/sqm), enables lines to be run fast, with above 600 m/min speeds being common. The plating tanks are on two decks with each level containing upto 18 plating tanks (1.8 m long by 300 mm deep) with banks of small anodes supported on conducting carbon rests, over which the strip passes. The anodes extend around 130 mm beyond the strip edge and the supports are inclined at an angle across the tank width which ensures constant spacing between strip and anode surfaces for anodes of progressively diminishing thickness. At the entry and exit of each plating level and between adjacent individual plating cells the strip passes between a pair of rolls, the upper conducting roll being termed the cathode roll. Tin is plated on the underside in the first deck. The steel is then turned through 180 deg and enters the second deck where the other side is plated.
The pH of this system (around 3) is high for an acid system, but no free acid is added to the bath. The bath contains tin chloride (around 35 g/L as Sn 2+), sodium and potassium fluorides, sodium chloride and potassium hydrogen fluoride together with organic additives such as poly-alkylene oxides or naphthalene sulphonic acid. The electrolyte continually circulates in the system, overflows the ends of the tanks and is recirculated. In the lower deck the electrolyte is sprayed onto the top of the strip to wet it. After plating the strip passes through rinsing tanks, wringer rolls and a hot air dryer all located in a top third deck. In this process, flow melting is usually by induction heating. The electrolyte contains tin fluoro-borate (30 g/L as Sn 2+), fluoro-boric acid and boric acid to prevent hydrolysis of the fluoro-borate ions. Also, proprietary additives are used. It is claimed that these lines can operate over a wider current density range allowing greater line flexibility. Although the first lines to be built were horizontal, later lines are vertical, containing up to 16 plating tanks and running at line speeds of 640 m/min or higher.
In the production of tinplate, the manufacture of the steel base and the application of the tin coating are independent of each other so that theoretically any tin coating, or combination of coatings, can be applied to any steel base. Thus the range of materials classified as tinplate can run into many thousands, indeed tinplate is available in more qualities than virtually any other light gauge sheet metal product. In practice the range of steel base thickness is from 0.13 mm to 0.60 mm and the tin coating from 0.5 g/sqm to 15.2 g/sqm tin per surface. There are international and national standards which specify the ranges and tolerances for the various characteristics, and methods of verifying them.