THE ALUMNI JOURNAL 115 



PICKLING WITH HYDROFLUORIC ACID. 



Iron wire and iron sheet is almost universally pickled with sul- 

 phuric acid. In such work there is only a smooth even surface from 

 which iron scale is the only material to be removed. Sulphuric acid 

 does this in a very satisfactory manner and at a very low cost, and 

 there is no immediate likelihood that any other chemical will replace 

 it. The sulphuric acid attacks flic sheet and icirc, rather than the scale, 

 by eating- away beneath the scale, so that the scale (or iron oxide) is 

 undermined and falls to the bottom of the tank. The workman judges 

 his pickling by the rapidity with which gas bubbles up through the 

 solution. This gas is hydrogen formed by sulphuric acid acting on 

 iron. Sulphuric acid and iron oxide or iron scale give off no gas when 

 they combine. 



While this method is quite suitable for iron wire, it is far from 

 satisfactory when applied to castings. Here there is an irregular sur- 

 face instead of a smooth one, a considerable amount of sand clinsfs 

 to and is imbedded in the outer surface, and we have more or less 

 scale, all of which must be removed before the casting is machined. 

 Sulphuric acid can only remove the scale and sand by eating away the 

 iron bcneatJi. While this goes on slowly, the acid is rapidly attacking 

 unprotected surfaces, blow holes and flows, to the detriment of the 

 casting. 



On account of these defects many foundrymen now use hvdro- 

 fluoric acid, or a mixture of hydrofluoric and sulphuric acids, on their 

 castings. Hydrofluoric acid attacks the sand and scale directly, and 

 has very little action on iron, so that flows and blow holes are not in- 

 creased in size and surfaces unprotected by scale are left intact. 



The hydrofluoric acid is used cold (keep above freezing point) 

 in the same lead lined tanks that would be used for sulphuric acid. 

 The strength required varies with the kind of sand and amount of 

 scale; usually about a two per cent solution is used (fourteen gallons 

 water to one gallon ordinary thirty per cent acid), but the foreman in 

 charge will probably find by trial that his particular castings require a 

 little stronger or little weaker solution to get the best effects. 



The first castings placed in the solution should be clean in from 

 twenty to thirty minutes ; they are then immediately dipped in hot 

 water (containing a little lime if desired) and will then dry without 

 rusting and without affecting any galvanizing or painting which may 

 subsequently be applied. 



