394 ELEMENTARY CHEMICAL MICROSCOPY 



Useful for all iron-carbon alloys, especially those high in car- 

 bon. Pure iron (ferrite) is not appreciably attacked save after 

 long exposure. 



With low carbon steels a higher concentration than 5 per cent 

 is advisable. 



This reagent not only etches but stains the specimen. Often 

 a surface film, especially with high phosphorus irons and steels, 

 is formed of such a character as to mask the structure. Gentle 

 rubbing with a finger tip in washing will usually clear away the 

 obliterating film. 



Silver Nitrate. Dissolve 5 grams in 100 cubic centimeters 

 of water. After washing, rub the surface very lightly with a 

 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 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. 



Sulphurous Acid. 1 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. 



