Water when absolutely pure has no action on lead, but in the presence of air the lead is quickly attacked, with formation of the hydrate, Pb(OH)2, which is appreciably soluble in water forming an alkaline liquid. When carbonic acid is present the dissolved oxide is soon precipitated as basic carbonate, so that the corrosion of the lead becomes continuous. Since all soluble lead compounds are strong cumulative poisons, danger is involved in using lead cisterns or pipes in the distribution of pure waters. The word “pure” is emphasized because experience shows that the presence in a water of even small proportions of calcium bicarbonate or sulphate prevents its action on lead. All impurities do not act in a similar way. Ammonium nitrate and nitrite, for instance, intensify the action of a water on lead. Even pure waters, however, such as that of Loch Katrine (which forms the Glasgow supply), act so slowly, at least on such lead pipes as have already been in use for some time, that there is no danger in using short lead service pipes even for them, if the taps are being constantly used. Lead cisterns must be unhesitatingly condemned.
The presence of carbonic acid in a water does not affect its action on lead. Aqueous non-oxidizing acids generally have little or no action on lead in the absence of air. Dilute sulphuric acid (say an acid of 20% H2SO4 or less) has no action on lead even when air is present, nor on boiling. Strong acid does act, the more so the greater its concentration and the higher its temperature. Pure lead is far more readily corroded than a metal contaminated with 1% or even less of antimony or copper. Boiling concentrated sulphuric acid converts lead into sulphate, with evolution of sulphur dioxide. Dilute nitric acid readily dissolves the metal, with formation of nitrate Pb(NO3)2.
Lead Alloys.—Lead, unites readily with almost all other metals; hence, and on account of its being used for the extraction of (for instance) silver, its alchemistic name of saturnus. Of the alloys the following may be named:—
With Antimony.—Lead contaminated with small proportions of antimony is more highly proof against sulphuric acid than the pure metal. An alloy of 83 parts of lead and 17 of antimony is used as type metal; other proportions are used, however, and other metals added besides antimony (e.g. tin, bismuth) to give the alloy certain properties.
Arsenic renders lead harder. An alloy made by addition of about 1⁄56th of arsenic has been used for making shot.
Bismuth and Antimony.—An alloy consisting of 9 parts of lead, 2 of antimony and 2 of bismuth is used for stereotype plates.
Bismuth and Tin.—These triple alloys are noted for their low fusing points. An alloy of 5 of lead, 8 of bismuth and 3 of tin fuses at 94.4°C, i.e. below the boiling-point of water (Rose’s metal). An alloy of 15 parts of bismuth, 8 of lead, 4 of tin and 3 of cadmium (Wood’s alloy) melts below 70°C.
Tin unites with lead in any proportion with slight expansion, the alloy fusing at a lower temperature than either component. It is used largely for soldering.
“Pewter” (q.v.) may be said to be substantially an alloy of the same two metals, but small quantities of copper, antimony and zinc are frequently added.
Compounds of Lead.