Tinning (a layer of pure tin or a mixture of tin and)

TINNING, a layer of pure tin or a mixture of tin and lead applied to the surface of metal articles to protect them from oxidation. Tin is little affected by air and water; weak organic acids and fats do not dissolve it. For methods of applying tinning, see Soldering. Solders are metals or their alloys used for soldering one metal surface to another; the solder must produce a strong joint. The most common solders are tin alloys with varying lead content (from 1% to 35–50% and more) with a melting point of 200°C and above. There are solders containing bismuth, antimony, cadmium, zinc, nickel, and other metals. Due to the harmful effect of lead on health, according to the circular of the People's Commissariat of Health of the RSFSR No. 320-32 of December 22, 1927, the following permissible lead content limits in tinning and solders have been established: 1) tinning of kitchen kettles, boiling vessels, samovars, thermoses, and other utensils and devices used for preparing, storing, and consuming food and drinks is permitted with tin containing no more than 1% lead; 2) tinning of tinplate for canned goods must not contain more than 0.04% lead (OST 3319); the stricter requirement is explained here by the longer duration of contact between the contents of the can and the tinning layer (up to several years); 3) in solders: internal solder not more than 1% and external solder 10% lead (by a special decision of the Academic Medical Council of the People's Commissariat of Health of the RSFSR, up to 50% lead is permitted in external solder). In Germany, a lead content of up to 1% is permitted in tinning, and in France up to 0.5% (Law of August 11, 1926). In 1928, the Academic Medical Council of the People's Commissariat of Health of the RSFSR by its decision No. 527 permitted a bismuth content of up to 0.008%, an antimony content of up to 0.01%, and arsenic traces in a tinning sample of 20 g. In practice, it is often found that the lead content in tinning exceeds the legally established norm of 1%. In 1929, Zinoviev, from 39 samples of tinning taken from boiling vessels on steamships, found lead content up to 1% in only 15 samples, while in 24 samples the lead was from 1% to 36.03%. Of 29 samples of tinning taken from kitchen utensils in buffets, lead was less than 1% in only 5 cases. In 1931, Budagyan and Vilkomirskaya (Institute of General Nutrition) established in 43 samples obtained from various places in the Union that in alloys used for tinning, lead was up to 1% in 24 samples, up to 10% in 6 samples, and more than 10% in 12 samples. To avoid lead poisoning, sanitary control over the quality of tinning and solder must be conducted with the utmost care and strictness. When storing products in tinned utensils, lead must not transfer to the contents of the utensil; the transfer of tin to tinplate canned goods is permitted up to 150 mg per 1 kg of product (e.g., OST 2446 for rhubarb puree). - The renewal period of tinning depends on the frequency of use of the utensil and the nature of the food prepared or stored in it, as well as on the method of cleaning and washing; when preparing acidic and fatty food, renewal of tinning will be more frequent; when cleaning with sand, iron plates, or ground brick, tinning wears out quickly. The simplest method for testing tinning for lead (by Budagyan and Vilkomirskaya): tin alloys with a lead content up to 2% when drawn on white paper give a barely noticeable stroke, with a content from 2–8% a faint stroke, and with a content of about 15% a clear (like a pencil) stroke. Chemical methods for determining tin and lead in tinning are described in OST 514.