GASES IN METALS 181 



great commercial importance. Tullis has found that the treatment 

 of molten aluminum with gaseous chlorine completely removes the 

 dissolved gases. Subsequent treatment with boron trichloride causes 

 the formation of small grains in the castings. Rosenhain ^^ has 

 reported that both gas removal and grain refinement can be effected 

 in one operation by the use of volatile chlorides such as titanium 

 tetrachloride. Both of these methods are being tested on a commercial 

 scale in England. It is claimed that secondary aluminum, treated in 

 this way, gives strong castings as free from pinholes as does virgin 

 metal. If the cost is sufficiently low and if the industrial hazards 

 attending its use can be controlled satisfactorily, this method of gas 

 treatment should have a wide applicability to refining secondary 

 aluminum. 



The descriptions, given above, of the effect of gases on the properties 

 of metals should indicate how important these effects are and how 

 small a quantity of gas may suffice to produce them. It seems 

 necessary, therefore, to consider gaseous impurities along with others 

 in metallurgical studies. 



Theory 

 The Effect of Temperature on the Soluhility of Gases in Metals 



Metal founders of early times had great difficulty making castings 

 which did not contain blowholes. Knowing that the solubility of 

 gases in aqueous liquids decreased with increasing temperature, they 

 tried to remove the gases from molten metal by heating to a higher 

 temperature before casting. They found, however, that their product 

 was less sound than before. This suggested to later workers a differ- 

 ence between the action of gases in aqueous liquids and in molten 

 metals and several systematic investigations were begun. 



Sieverts, one of the first men to study this problem, found that, in 

 general, the solubility of gases in metals increases with increasing 

 temperature. 1* He found, for instance, that one volume of copper 

 absorbs 0.006 volume of hydrogen at 400° C, and 0.19 volume just 

 below its melting point. As the copper melts, the quantity of gas 

 absorbed increases to 0.54 volume, while at 1550° C. 1.25 volumes are 

 absorbed. The amount of hydrogen absorbed by iron increases from 

 1.05 volumes for the solid to 2.10 volumes for the liquid at the same 

 temperature. 



Some known exceptions to the general rule that absorption of gas 

 increases with increasing temperature are the solubility of hydrogen 



^2 Loc. cit. 



^^ Sieverts published a useful resume of his work, with references to the original 

 articles in Zeit.filr Metallkuiide, 21, 37 (1929). 



