=======> ASTM A967 STANDARD SPECIFICATION FOR CHEMICAL PASSIVATION TREATMENTS FOR STAINLESS STEEL PARTS
18. Practice E—Potassium Ferricyanide-Nitric Acid Test
18.1 This test is recommended when detection of very small amounts of free iron is required. It is recommended for detection of free iron on austenitic 200 and 300 series stainless steels. This test is not recommended for detection of free iron on ferritic or martensitic 400 series stainless steels because these steels will give a positive indication irrespective of the presence or absence of anodic surface contaminants. This test shall not be applied to parts to be used in food processing.
18.2 The test solution is prepared by adding 10 g of chemically pure potassium ferricyanide to 500 mL of distilled water, adding 30 mL of 70 % nitric acid, agitating until all of the ferricyanide is dissolved, and diluting to 1000 mL with distilled water. The test solution shall be mixed fresh on the day of the test.
18.3 The test solution is swabbed on the surface of the sample representing the lot of passivated parts. The formation of a dark blue color within 30 s denotes the presence of metallic iron.
18.4 The tested sample shall not exhibit the dark blue color indicative of free iron on the surface.
18.5 When the test is negative, the surface shall be thoroughly