Passivation and Oiling
What is passivation?
- A chemical or electrochemical passivation treatment is normally applied to the surface of electrolytic tinplate. The principal purpose of the treatment is to stabilise the surface and thus to inhibit the growth of tin oxide on the surface. Excessive oxide can produce discoloration in prolonged storage and especially during stoving operations associated with lacquering or printing.
- The passivation treatment is applied to the strip in the electrolytic tinplate line after flow brightening and before oiling. The treatment consists of passing the strip through an aqueous chemical solution, with or without applied current.
- The most commonly applied treatment uses a sodium dichromate solution with applied current, the strip being negative. An alternative treatment comprises a simple dip, without current, in a similar solution or in a chromic acid solution. A cathodic treatment in sodium carbonate solution is occasionally used when milk products are packed and there are other treatments which may be used from time to time, but are non standard and not available from every supplier.
- Normally, unless otherwise agreed by the purchaser, the first of the above treatments, viz. cathodic sodium dichromate, is applied. Many tinplate producers adopt a numerical code to define passivation treatments. A typical system is one based on a three digit code. Where the first digit is used to indicate the solution used (1 - chromic acid, 2 – chromate phosphate, 3 – sodium dichromate, 4 – sodium carbonate.), the second digit refers to the polarity of the tinplate in the solution (0 – non-electrolytic, 1 – cathodic, 2 – cathodic/anodic) and the third digit refers loosely to the coulomb level employed.
- Culomb is The basic unit of electric charge, equal to the quantity of charge transferred in one second by a steady current of one ampere, and equivalent to 6.2415 × 1018 elementary charges, where one elementary charge is the charge of a proton or the negative of the charge of an electron.
Oiling
- Electrolytic tinplate normally carries a very thin coating of oil, in order to facilitate the separation of sheets or wraps of a coil. The oil must be of a type that is acceptable for use in food packaging. The materials in most common use are dioctyl sebacate (DOS), butylstearate (BSO) or acetyl tributyl citrate (ATBC). Whilst not normally specified when ordering, the oil film is usually in the range 0.0025 to 0.0099 g/m2 of surface.
- Source: The International Tin Association (formerly ITRI Ltd)