Tinning (a method of mechanically applying pure tin or)

TINNING, a method of mechanically applying pure tin or tin containing lead to the surface of metal articles (iron, copper, brass) to give them a more beautiful appearance or to protect them from corrosion. The process of tinning proceeds differently depending on the nature and type of the article being processed. The main stages of work in tinning consist of the following moments. Articles intended for tinning are first subjected to thorough cleaning and degreasing with hydrochloric or sulfuric acid and then the rust layer is removed by scraping; after this, the articles are dipped in melted fat and then immersed in molten tin or a mixture of tin and lead. To remove excess tin and zinc chloride, which is also often used for preliminary treatment of articles, the articles taken out of the molten metal are subjected to manual cleaning with brushes. - In contrast to the ordinary course of the tinning process just described, many articles are tinned using the so-called tertnek-alloy, which contains about 2/3 lead. In this method, the article is preheated, either with a soldering lamp or more often over a fire on braziers or furnaces, and then the tinsmith, holding a block of tertnek in his hand, passes it over the surface of the article. In this way, various types of tanks and kettles (for water, gasoline, acids, etc.) are usually tinned. - Iron sheets are tinned in a special way; they are passed through special machines equipped with troughs and rollers, where they successively pass through different layers—molten tin, zinc chloride, oil; after the tinning is finished, the sheets are polished with bran or flour. The use of acids during tinning causes the formation of acid vapors during work with them, which, entering the air of the room, exert their harmful influence on the workers, expressed as strong irritation of the mucous membranes of the eyes and respiratory tract, as well as damage to the lungs (inflammations) and teeth. More serious consequences with sharply expressed symptoms of occupational poisoning can occur in the case of the use of acids containing arsenic (sulfuric acid obtained by the chamber process). Here, from the action of acids on metals, the formation of arsenous hydrogen is possible and workers may exhibit symptoms characteristic of poisoning by this toxic gas. Preliminary cleaning of articles from rust, in which sand is often used, as well as the removal of excess metal from the surface of the article after tinning, contribute to the formation of silicate and metallic dust, especially iron oxide. Individual cases (Metke 1) of deposition of these substances in the lungs as a result of the production of the indicated works have been described. The most serious and important occupational hazard characteristic of the profession of tinsmiths is the use of lead during tinning. With the exception of tableware intended for the preparation and storage of food and drinking water, the use of lead in which is limited by law [according to Soviet (Bulletin of NKZdr, No. 24, 1927) and German laws its content in tin should not exceed 1%], the use of lead in tinning all other articles has become very widespread. The content of lead in the metallic mixture used for tinning varies from 14% (babbitt) to 65% (tertnek), and during lead coating, which is a variety of the tinning process, articles are immersed in pure lead. Tinning belongs to hot work, in which, due to the use of lead, lead vapors can form, which turn into very fine dust particles (size less than 0.5 µm) upon cooling, possessing the ability to penetrate into the deepest parts of the lungs, where they are absorbed and very quickly enter the bloodstream. This serves as the direct cause of the appearance of signs of lead poisoning in workers. Recently, Froboese established that in processes analogous to tinning (soldering, lead coating, etc.), lead is released into the air not only in the form of dust or vapor noticeable to the eye, but also in the form of a special colloidal state, which represents lead oxide. The content of lead in the air of the working room in the form of a colloid must represent a particularly serious danger for persons inhaling this air, since under these conditions all the lead distributed in it, being in a state of very high dispersion, will be completely absorbed by the respiratory organs. This apparently constitutes the reason for the relatively frequent cases of lead poisoning observed among tinsmiths, despite the fact that the concentration of lead dust during these works was insignificant—0.07–0.09 mg/m³. - In addition to inhaling lead, tinsmiths have to directly come into contact with lead during work, which is why a fairly significant contamination of their hands with lead is noted, varying depending on different working conditions and giving in some cases more than 20 mg of lead in hand washings. Such contamination of the hands of tinsmiths can present a certain danger in that lead, mixing with fat, with which the hands of a tinsmith using oil during work are abundantly lubricated, can form a readily soluble compound with fatty acids, possessing the ability to be rapidly absorbed through the skin (Flury), which can increase the danger of lead poisoning in tinsmiths. Measures to combat the hazards of work during tinning must in the main consist of the following. It is necessary everywhere, where possible, to completely exclude lead from the metallic mixture used for tinning, or at least reduce its content to minimal sizes. Only acids completely free of arsenic should be allowed for use in these works. To extract dust during processes accompanied by the formation and release of it into the air, it is necessary to install local dust-removing ventilation devices. Heating of articles, their treatment with acid, and the tinning process itself should be allowed exclusively in rooms specially adapted for these works and equipped with appropriate ventilation installations. In addition, general rules of personal hygiene recommended for other works associated with the danger of lead poisoning (see Lead) must be observed by the workers here as well. Lit.--see lit. to the article Lead.