Alloys of Lead

The most important alloys of lead are those with tin, which have already been described under the latter metal. Numerous binary and ternary alloys of lead with other metals have, however, been prepared for purposes of metallographic study.

Physiological Action of Lead. - Lead compounds are poisonous, especially when soluble. Lead-poisoning is cumulative, that is to say successive minute quantities, which separately might be innocuous, produce serious or fatal effects after a time, owing to their accumulation in the body. Indeed, a single large dose of a lead compound is less dangerous than repeated small doses, and seldom proves fatal. Chronic lead-poisoning may result from the drinking of water containing lead derived from the pipes in which the water has been conveyed; or from the continued handling of lead compounds, such as white lead. Those engaged in the manufacture of these compounds may be affected, as well as those who use them, especially painters, and pottery workers employing white lead for glazing purposes.

The symptoms of lead-poisoning are general ill-health with loss of appetite, interference with digestion, accompanied by abdominal pains (painters' colic), constipation, nervous prostration, epileptic fits, local paralysis, especially of the wrists, known as " dropped-wrist," followed by general paralysis and death. Signs of such poisoning are seen in the blue line which appears at the edges of the gums, owing to the deposition there of lead sulphide. Occasionally also the teeth turn black, and the skin assumes a jaundiced hue. The lead is distributed in various parts of the body, and is partially excreted by the kidneys. Potassium iodide is said to aid this elimination.

Action of Water on Lead. - On account of the employment of metallic lead for the lining of cisterns and construction of pipes for storing and conveying potable water, the action of such water upon lead is of great importance, and has been fully investigated.

Pure water from which air is excluded is practically without action on lead at ordinary temperatures, since lead was found in such water only to the extent of 0.3 part per million.

The action of water and oxygen on carefully purified lead has been examined by Lambert and Cullis. The distilled metal was submitted to the action of pure water with and without the addition of pure oxygen. The action of pure water on the fresh surface of the metal was inappreciable, but on the addition of pure oxygen very rapid corrosion took place, with the formation first of hydrated plumbous oxide, Pb₂O.2H₂O, and then the ordinary white crystalline lead hydroxide. When the pure, distilled metal has been kept for some months it becomes much less oxidisable, so that it may be exposed to ordinary air for many days without any appreciable loss of its brilliant lustre. This difference in oxidisability between the recently and earlier distilled metal is attributed by Lambert and Cullis to allotropy, a metastable, more active form of the metal being first produced when the vapour condenses.

The allotropy of lead also serves to account for the action of water and oxygen on the metal, which, according to Lambert and Cullis, originates in electrical action due to differences of potential between different parts; for in the purified metal such differences must necessarily be attributed to physical and not chemical heterogeneity. Thus pure water is supposed to initiate action on pure lead, with the separation of hydrogen, and the formation of plumbous ions in solution; but this action is quite inappreciable until oxygen is added to oxidise and remove the hydrogen which offers an enormous resistance to the passage of the current.

Hydrogen peroxide, which is formed during the wet oxidation of lead, is said by Lambert and Cullis to be the product of a subsidiary action, and to have no direct bearing on the process of corrosion. It is responsible, however, for oxidising lead monoxide to the higher oxides.

Water containing only dissolved oxygen acts more readily upon lead than water containing only dissolved carbon dioxide. This confirms the conclusion that the first action on lead of water containing dissolved air is one of oxidation, so that lead hydroxide is formed and passes into solution. Water containing carbon dioxide dissolved under pressure dissolves, however, large quantities of lead. Lead hydroxide in solution subsequently reacts with carbon dioxide derived from the air with the formation and precipitation of a basic carbonate of the composition 2PbCO₃.Pb(OH)₂.

The effect of various salts, present to the extent of 0.2 gram per litre, upon the solvent action of water on lead, was examined by Muir, who placed them in the following order according to their action on the metal: ammonium nitrate, calcium chloride, ammonium sulphate, potassium nitrate, potassium carbonate; he also found that the solvent action was always the greater the more considerable the exposure to air. This difference is due to the different solubilities of basic lead carbonate in dilute solutions of these salts.

Whilst ammonium nitrate in solution appears to exert on lead the greatest solvent action of any known salt, other nitrates show no considerable action, and sulphates, phosphates, carbonates, and silicates actually retard the solvent action of water. This is well shown when clean sheets of lead are immersed in ordinary distilled water and in tap water, contained in stoppered bottles, and allowed to stand side by side for some months. After some time the distilled water will contain basic carbonate in suspension, while the tap water will be clear, because in this case the action has been stopped by the formation of a protective crust of the salt on the surface of the lead.

The inhibitive action of chalk, sand, and old mortar on the corrosion of lead by water has long been recognised; and it appears that the presence of 0.5 grain of dissolved silica per gallon is sufficient to render the water lead-proof. It has been found that the best way to prevent water from attacking lead is to bring it into contact with a mixture of flints and limestone. According to Carnelley and Frew, this is owing to the formation of calcium silicate in solution. Water containing carbon dioxide dissolved under pressure dissolves considerable quantities of lead; peaty waters, containing in solution organic acids, such as ulmic and humic acids, corrode lead; and free lime is particularly active in this respect. Indeed, it is known to plumbers that lead pipes laid in contact with new mortar are liable to corrosion. When water is to be tested for minute quantities of lead it must not be filtered through paper, which adsorbs the lead salts so that only a small proportion of them is found in the filtrate.

From a consideration of the kinds of impurity which cause water to dissolve lead, it will appear that a good potable water containing both temporary and permanent hardness in moderate amount, only a little nitrate, and no appreciable quantity of ammonium salt, has little or no effect upon lead; and, therefore, that such water may be safely conveyed in lead pipes.