Ferroxyl solution

The combination of these two reagents in a gelling agent to which sodium chloride is added is known as a ferroxyl solution. Cushman and Gardner made ample use of this medium to reveal local corrosion cells and supported their theories with many pictures in their 1910... 

When the pure metal was treated with ferroxyl (see p. 72), unlike ordinary iron, it remained quite bright for an indefinite time, manifesting no tendency to corrode. On applying pressure locally, however, corrosion set in, a pink colour developing round the pressed portion, and Turnbull s blue appearing round the unpressed parts, indicating solution of the metal. 

Ferroxyl.—An effective method of showing that differences of potential exist between different parts of a piece of iron consists in utilising the ferroxyl indicator devised jointly by Cushman1 and Walker.2 A 1 5 per cent, solution of agar-agar jelly is prepared, a few drops of phenolphthalein added, and the whole rendered perfectly neutral whilst hot by titration either with alkali or acid as occasion requires. A small quantity of potassium ferricyanide solution is now added, and the solution poured into a shallow dish to cool. A clean sample of iron is placed on the solidified jelly and covered with a layer of warm solution, and the whole allowed to cool. After a few hours some very beautiful colour effects will have developed, and may be preserved for several months by keeping the surface of the agar covered with alcohol. 

The ferroxyl test is mentioned in ASTM A 380, Practice for Cleaning and Descaling Stainless Steel Parts, Equipment, and Systems. It has recently been adapted as a rapid and simple corrosion test for sintered austenitic stainless steels [25] and for the detection of contamination of stainless steel powder with iron or low-alloy steel powders [26]. This test uses hexacyanoferrate (n/m) solutions with different chloride contents. Immersion of a sintered stainless steel part into such a solution develops, usuaUy within minutes, a blue precipitate known as TiunbuH s blue in accordance with... 

How to prepare 250 mL of ferroxyl gel containing 3 percent sodium chloride Dissolve 7.5 g sodium chloride in250 mL distilled water. Add 5 g powdered agar agar and boil the mixture until the agar is dispersed. Then add 0.5 g potassium ferricyanide and 5 mL of a 0.1 percent phenolphthalein solution. 

The local action cell on a single metal surface may be revealed by placing an ordinary steel nail in the ferroxyl gel (Fig. 7.43). After a short time, a pink area develops aroimd the shank of the nail, indicating the formation of a cathode. Blue areas develop around the cold worked head and point of the nail where the iron goes into solution, indicating that these areas are the anodes. 

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