462 BURGESS AND MERICA : FUSIBLE TIN BOILER PLUGS 



oxide Sn02, either as a solid mass at the fire end of the plug or as 

 a network throughout the tin filling. It was discovered that this 

 latter form of oxidation in service depended on the presence of 

 zinc in amounts as small as 0.3 per cent. This zinc is insoluble 

 (that is, it does not form a solid solution with tin) in solid tin, 

 and upon long heating at about 180°C. the zinc in such a tin 

 plug coalesces to form a network bounding the tin grains. This 

 zinc is easily corroded by many kinds of boiler water, for in- 

 stance alkaline waters, and the oxide or corrosion products of 

 this zinc and of the tin afterwards attacked, remain and form a 

 solid, continuous oxide mass which in some cases has held the 

 pressure of the boiler even after the tin remaining had melted. 

 Tin plugs containing zinc were made up and heated in an auto- 

 clave with water at from 180° to 195°C. for 500 hours, after 

 which this network structure was seen clearly developed. Plugs 

 of pure tin were unchanged under the same conditions of test. 



Lead and zinc were found to be the principal impurities in tin 

 plug fillings, and since all ''failed" plugs contained these or other 

 impurities the conclusion is reached that if these impurities are 

 eliminated by strict specifications and inspection, which will 

 allow only admittedly superior qualities of tin such as Banca 

 and some others, the dangers of these plugs will no longer exist. 



It was found that a determination of the freezing points of 

 samples of tin by a cooling curve method afTorded a very good 

 criterion of the purity of such tin, as approximately 0.1 per cent 

 of either lead or zinc could be detected. This method was de- 

 veloped and is recommended as a quick and convenient method 

 for the inspection of the purity of tin in tin plugs. 



Various existing specifications for fusible tin boiler plugs are 

 discussed in the complete paper and the conclusion is reached that 

 tin equal to Banca tin 99.9 per cent pure should be used for such 

 plugs where possible; where such tin can not be obtained it is 

 likely that tin of 99.8 per cent purity including all high grade tin 

 qualities would be satisfactory. 



