Free Iron Test
The Free Iron Test detects unpassivated iron particles on stainless steels after processing by using potassium hexacyanoferrate(III). Immediate color reactions occur with unpassivated iron.
Physical-Chemical Mechanism of Action
Potassium hexacyanoferrate(III) reacts with free iron (Fe²⁺ and Fe³⁺) in the weld. This leads to an immediate color reaction, which shows up as a red or brown precipitate.
Metallurgical Relevance & Welding Risks
Unpassivated iron residues can lead to corrosion and cracking because they are susceptible to chemical attack. The Free Iron Test helps detect these risks early and avoid them.
Workshop Practice & Action Guidelines
1. Prepare a solution of potassium hexacyanoferrate(III).
2. Spray the solution onto the weld to be tested.
3. Observe the color reactions: a red or brown precipitate indicates unpassivated iron residues.
4. If positive reaction occurs, correct the processing procedures and re-test.