Electrolytic Tinning Process

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Tinplate is now virtually all produced by the electroplating of tin on to the steel base in a continuous process. The major reasons why electrotinning of steel strip superseded hot-dip tinning, were because it can give a very much higher degree of thickness control, including different thicknesses on the two sides of the steel sheet, and much higher outputs of tinplate, at higher quality and lower manufacturing 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; today a typical coating thickness is in the range 0.1 to 1.5 microns depending on the end use.

The most widely used process is the Ferrostan process (the trade name of US Steel) and this currently accounts for some 70% of world manufacturing output. This general description of electrolytic tinning is based on the Ferrostan process, with differences in line layout for other process types being scussed later.

Blackplate coils weighing between 5 and 15 tonnes 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 that can hold varying amounts of uncoiled plate (often up to 600 m). Most modern lines incorporate side trimmers after the accumulator to cut the strip to the correct width. Many lines now incorporate tension or stretch levellers (Figure 9), which apply controlled tension across the strip to remove distortions.

Because cleaning times are very short i.e. 1 &#; 2 seconds, effective cleaning requires the use of electrolysis to aid chemical dissolution of rolling oil residues and other organic contaminants. Passage of heavy current produces gases at the strip surface which undermine the soil and lift it from the strip. The cleaner is usually a 1-5 % solution in water of a mixture of phosphates, wetting agents and emulsifiers in a sodium hydroxide / carbonate base. Temperatures are normally 80-90°C with current densities of 10 A/dm2 usually adequate. After cleaning the strip is thoroughly washed, ideally in hot water (70°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 normally made anodic then cathodic with current densities between 5 and 30 A/dm2 being employed.

Various electrolytes can be used in the tinplating section and these are covered more fully later. 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 strip width determine the effective plating area. This, together with the available plating current, is mainly responsible for fixing the maximum line speed for any coating weight. Modern tinning lines achieve speeds of 600 m/min with reports of new lines being designed to run even faster; typical strip widths are between 1 and 1.25 m. 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 must have good electrical conductivity and low contact resistance between the roll and the wet strip; they are normally 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. Traditional anodes are made of 99.9% pure tin and are 76 mm wide, 50 mm thick and about 1.8 m long. The anode is consumed in the process and is replaced when reduced in thickness by about 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.

Inert anodes made from titanium coated with platinum or iridium oxide have become more frequently used in recent years. Nippon Steel was the first to use a line totally employing inert anodes. 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 and do not need to be frequently renewed. This means that variations in tin coating thickness across the strip width are minimised; 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 enables manning levels to be lowered.

An alternative system of parallel tin anodes was developed by Rasselstein and has also been used by British Steel (now Tata). 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 required 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 may be fluxed by treating with dilute electrolyte or proprietary chemicals to prevent surface defects such as woodgrain appearing on the plate. Virtually all DWI plate (drawn and wall ironed) is nonflow brightened, and this can be a significant part of the output for many manufacturers.

Flow melted tin plate has a thin tin oxide film on the surface, which if untreated can grow on 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 and 85°C in dichromate or chromic acid solution containing 20 g/l dichromate (other treatments which are now seldom used included use of phosphates or carbonates). This treatment deposits a complex layer of chromium and its hydrated oxides, which inhibits the growth of tin oxides, preventing yellowing, improving paint adhesion and minimising staining by sulphur compounds. Prior to oiling the plate must 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 strip thickness, detection of pinholes and tin thickness.

• Source: The International Tin Association (formerly ITRI Ltd)

Application and performance characteristics of tinplate

1&#; Use of tinplate

Tinplate (commonly known as tinplate) refers to a steel plate with a thin layer of metal tin plated on its surface. Tinplate is a steel plate made of low carbon steel rolled into a thickness of about 2 mm, which is processed by acid pickling, cold rolling, electrolytic cleaning, annealing, leveling, trimming, and then cleaned, plated, soft melted, passivated, and oiled, and then cut into a finished tinplate. The tinplate used for tinplate is high purity tin (Sn>99.8%). The tin layer can also be coated by the hot dip method. The tin layer obtained by this method is thicker and requires a large amount of tin, and purification treatment is not required after tin plating.

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The tinplate consists of five parts, which are steel substrate, tin iron alloy layer, tin layer, oxide film, and oil film from the inside out.

2&#; Performance characteristics of tinplate

Tinplate has good corrosion resistance, certain strength and hardness, good formability, and is easy to weld. The tin layer is non-toxic and odorless, which can prevent iron from dissolving into the packaging, and has a bright surface. Printing pictures can beautify the product. It is mainly used in the food canned industry, followed by packaging materials such as chemical paints, oils, and pharmaceuticals. Tinplate can be divided into hot-dip tinplate and electroplated tinplate according to production process. The statistical output of tinplate must be calculated based on the weight after plating.

3&#; Factors of tinplate

There are many factors that affect the performance of tinplate, such as grain size, precipitates, solid solution elements, plate thickness, and so on. During the production process, the chemical composition of steelmaking, the heating and coiling temperatures of hot rolling, and the process conditions of continuous annealing all have an impact on the properties of tinplate.

4&#; Classification of tinplate

Equal thickness tinplate:

Cold rolled galvanized tin plate with the same amount of tin plated on both sides.

Differential thickness tinplate:

Cold rolled galvanized tin plate with different tin plating amounts on both sides.

Primary tinplate

Electroplated tin plates that have undergone online inspection are suitable for conventional painting and printing on the entire steel plate surface under normal storage conditions, and must not have the following defects: &#; pinholes that penetrate the thickness of the steel plate; &#; The thickness exceeds the deviation specified in the standard; &#; Surface defects such as scars, pits, wrinkles, and rust that may affect the use; &#; Shape defects that affect use.

Secondary tinplate

The surface quality of the tinplate  is lower than that of the first grade tinplate, and it is allowed to have small and obvious surface defects or shape defects such as inclusions, wrinkles, scratches, oil stains, indentations, burrs, and burn points. This does not guarantee that the entire steel plate can undergo conventional painting and printing.