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. — 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 Picrate. — 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.^ — Valuable in the study of steels. Cement¬ 
ite is not attacked by a solution of i 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. 
