Electrolyte Types

Electrolyte types

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

Phenol sulphonic acid (PSA) electrolyte

  • The Ferrostan process, using PSA, was one of the first to be developed and has continued to dominate in most parts of the world. Figure 10 shows a flow diagram of a typical line.
  • The electrolyte consists of a solution of PSA containing stannous ions plus addition agents which ensure good quality smooth deposits over a wide current density range. Over the years a variety of addition agents have been used including such things as gelatine, dihydroxy diphenyl sulphone etc. Since the mid-1960’s, however, only two addition agents have been commonly used: EN SA-6, developed by US Steel and Diphone V, developed by Yorkshire Chemicals.
  • When making up a new electrolyte the tin is added as stannous sulphate; however, no further additions of this material should be required. Concentrations of stannous tin below the recommended range of 25 - 35 g/l may result in high current density defects such as dark deposits on unflowmelted plate and white edges. Free acid concentration is maintained by periodic additions of PSA. Low acid levels reduce the conductivity of the bath and therefore require higher operating voltages to pass the required current. Oxidation of tin (II) to tin (IV) occurs more rapidly at low acid levels increasing the amount of sludge. This is a waste of tin, since once tin (IV) has been formed it is no longer able to be plated out. On a typical well run line, tin lost as tin (IV) should be about 3 - 5 kg per 24 hours operation. If the addition agent concentration falls too low the plating range is reduced, the deposit becomes less bright and in extreme cases adherence will be poor; cathode efficiencies may also be adversely affected. Typical operating conditions are line speeds of 300 - 500 m/min, current densities of 20 - 35 A/ dm2 and temperatures of 40 50°C . A considerable amount of heat is generated by passage of high currents and temperature is controlled by circulating the electrolyte through a cooling system.

Halogen electrolyte

  • The second major electrolyte process in terms of world production is the H alogen process, developed by E. I. du Pont de Nemours, Weirton Steel and Wean Engineering. T he system operates on a different type of line to the Ferrostan process, with horizontal rather than vertical plating tanks. This configuration together with the high current densities used (65 A/dm2), enables lines to be run fast, over 600 m/min being common. The plating tanks are on two decks; each level containing up to 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. T he anodes extend about 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° 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 Sn2+ ), sodium and potassium fluorides, sodium chloride and potassium hydrogen fluoride together with organic additives such as polyalkylene 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 Halogen lines flow melting is usually by induction heating.

Fluoroboric acid electrolyte

  • Fluoroborate systems were first used by Rasselstein in the late 1940’s, but have never gained widespread use mainly due to commercial restrictions. The electrolyte contains tin fluoroborate (30 g/l as Sn2+ ), fluoroboric acid and boric acid to prevent hydrolysis of the fluoroborate ions: again proprietary additives are employed. It is claimed that these lines can operate over a wider current density range than Ferrostan systems 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. Most of the other sections of the line are similar to those described above except that pickling is carried out with hydrochloric acid at room temperature without electric current.

Methane sulphonic acid (MSA) electrolyte

  • Methane sulphonic acid (MSA) based tin and tin alloy processes were first developed in the early 1980’s, and began to gain market acceptance in the electroplating industry shortly thereafter. The advantages of these processes are claimed to include: high conductivity, biodegradability, low incidence of stannic tin generation, wide current density range and adaptability of process attributes to end user requirements through the use of modern grain refiners.
  • By the end of the decade, most tin alloy electrodeposits produced by the electronics industry in Europe and the United States were from MSA based processes, which had replaced the more traditional fluoborate electrolytes.
  • The success of MSA processes in the electronics industry led to further developments for applications in heavy industry. In November, 1989, the first commercial tinplate production line using a patented MSA process (Ronastan TP) developed by LeaRonal (now Shipley Ronal) began plating steel strip at Hoogovens (now C orus), Ijmuiden, The Netherlands.
  • The MSA tinplate process has undergone several improvements in recent years, and has been installed in both vertical and horizontal cell tinplate lines, an attribute shared by no other tinplate electroplating process. Although the chemistry is tailored to meet each individual line’s requirements, the general operating conditions for the process are shown in Table 1.

Table 1 MSA operating conditions

Operating Parameter

Vertical Cell Operating Range

Horizontal Cell    Operating Range

Tin Concentration (g/l)


15 – 22


10 – 18


Free MSA (ml/l)


30 – 60


25 – 35


Primary Additive (ml/l)


40 – 60


40 – 60


Secondary Additive (ml/l)


1.8 - 3.2 


1.8 - 2.8


Antioxidant (ml/l)


15 – 20 


15 – 20


Temperature (°C)


30 – 50


30 – 65


PH


0 – 1


0 – 1


Current Density Range (ASD)


5 – 60


5 – 60


  • The above process does not contain any free phenol, cyanide, chloride, or fluoride compounds, nor any chelates. Simple neutralisation of plating bath effluent will produce tin hydroxides and sodium sulphate, and the filtered liquor can be sent to sewer or river. Neutralised MSA has been reported to have a lower toxicity than table salt.
  • Tin oxidation rates are slightly lower than those given by PSA processes, and are much less than those of the halogen processes. However, tin oxidation rates for the MSA process subjected to oxygen injection, such as is needed for the Nippon Steel Tin Dissolution System, are significantly less than that of the PSA processes, when iron contamination levels in these plating baths exceed 10 g/l. The conductivity of MSA is approximately 30% higher than for PSA at equivalent acid strengths and temperatures. Operating advantages, such as higher production rates and prime yields have been reported by a number of tinplate producers and these advantages have overcome the increased chemical costs of the MSA process.

The alkaline stannate electrolyte

  • Although still widely used for component tin plating this process is now virtually obsolete for plating of steel strip. Brief details are included here for completeness.
  • The main advantages of alkaline stannate processes are that the electrolyte is non-corrosive to steel and that addition agents are not required to give a smooth good quality deposit. However, these benefits are now seen to be outweighed by a number of disadvantages, the main one of which is that twice the theoretical amount of electricity is required to deposit tin from the Sn4+ as from the Sn2+ state; this is obviously very costly. Other disadvantages include the lower operable current density range and the lower efficiencies obtained. In alkaline electrolytes at low current density, tin anodes would dissolve as stannite but if the current density is increased above a critical value, a film forms on the surface and tin then dissolves entirely as stannate. In practice, filming is achieved by raising the anode voltage for a few seconds at the outset.
  • Source: The International Tin Association (formerly ITRI Ltd)
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