ETCHING LIQUIDS 443 



finger tip to remove the surface film formed. Long etching 

 must be carefully avoided. 



Useful with antimony, bismuth, tin and lead alloys, especially 

 babbitts. 



Sodium Hydroxide. — One of the best etching reagents for 

 aluminum-zinc alloys. Start with a very dilute solution and 

 increase the concentration until the proper strength is obtained 

 which yields the best results with the particular alloy being 

 studied. 



Sodium Pier ate. — Prepare a 20 per cent solution of sodium 

 hydroxide, dissolve in it 10 per cent of sodium picrate. The 

 reagent is poured over the polished steel specimen in a small 

 casserole and heated to boiling for about ten minutes. This 

 method was proposed by Le Chatelier and is one of the most 

 valuable for differentiating between cementite and ferrite. 

 Cementite and ferrite both appear white with nitric acid-alcohol 

 etching. With sodium picrate cementite etches black, ferrite 

 remains bright. 



Sulphurous Acid. 1 — Valuable in the study of steels. Cement- 

 ite is not attacked by a solution of 1 part in 25 parts of water. 

 Serves to develop Martensite, Austenite and Troostite, but the 

 appearances obtained are different for these components from 

 those obtained with other reagents. 



1 Zeit. anorg. Chem., 68 (1910), 63. 



