From Big Medical Encyclopedia

POLUDA — the thin film of tin applied on a surface of metal products (usually a container for storage of foodstuff or ware for cooking) for the purpose of their corrosion protection. Process of drawing P. is called a tinning. To a tinning it has to be applied tin (see) brands 01 and 02 of the superior quality with contents as impurity of arsenic no more than 0,015%, lead no more than 0,25%. For production of a canning tin tin of brand 01 with the content of pure tin not less than 99,915% and impurity is used: arsenic no more than 0,01%, lead no more than 0,025%. The soldering of inside welds of the iron and copper ware used for cooking of ware for milk and other food ware shall be made by the tin-lead solder of the POS-90P brand containing tin not less than 90%, antimony no more than 1,5%, lead of 8,5%, but no more than 10%. Methods of the analysis of the tin used at a tinning and for solder are stated in GOST 15483.0-78; GOST 15483.11 — 78; GOST 1429.0 — 77; GOST 1429.15-77.

The item is put or at immersion of metal products in a bathtub with the melted tin (a hot way of a tinning), or electrodeposition of tin from water solutions of electrolytes (an electrolytic way of a tinning). Other methods of drawing P. are widespread less. The electrolytic way of a tinning has a number of advantages before a hot way of a tinning since allows to reduce considerably thickness of the tin covering a product and it is easy to regulate thickness of the Item.

A tin coating on tin plate at P.'s thickness applied in the industry always poristo. The layer of tin is thinner, the it is more with other things being equal porosity of a covering. Adjoining to foodstuff or the wet atmosphere, every time begins to function as a mikrogalva-nichesky element (tin — iron) that is the main reason for destruction of P. Zhest of an electrolytic tinning, having the increased porosity, is applied in the canning industry after varnishing. At a thickness of tin layer of 0,2 microns the covering has high porosity therefore the minimum thickness of a layer of tin on a tin of an electrolytic tinning shall be 0,32 microns.

The natural oxide films which are formed on the surface of tin have no constant structure and uniform thickness; their protective properties are unsatisfactory. Therefore the tin applied in the electrolytic way is artificially passivated, i.e. create on its surface protective films, structure to-rykh depends on a way of their receiving. Passivation is carried out by a chemical or most widespread electrolytic way. Electrolytic passivation is carried out in the solution containing chromate salts. Films may contain various tin oxides, is lame also metal is lame (see). The most poisonous hexavalent chromium in passivatsionny films is not found. Because a pass-sivatsionnaya the film on a tin may contain chrome and its oxides, at a gigabyte. to assessment of material, from to-rogo cans are made, it is necessary to consider a possibility of migration of chrome on the Wednesdays contacting to it.

Bibliography: Lokshin Ya. Yu. A canning container from new types of a tin and from aluminum, M., 1975; Materials by hygienic assessment of pesticides and polymers, under the editorship of A. P. Shitskova, page 113, M., 1977.

A. P. Shitskova