The blue dot test is a method to test the quality of pickling passivation on the surface of stainless steel. Its principle is to detect whether there is iron ion contamination on the surface of stainless steel. Iron ions form galvanic cells on the surface of stainless steel, which will cause electrochemical corrosion of stainless steel.
The specific method for blue dot inspection is: 1g potassium ferricyanide K3[Fe(CN6)] plus 3ml (65%~85%) nitric acid HNO3 and 100ml water to make a solution (preferably used now). Then, after immersing the solution with filter paper, attach it to the surface to be tested or directly apply and drop the solution on the surface to be tested. Observe the appearance of blue dots within 30 seconds. Blue dots are unqualified, and no blue dots are qualified. The test needs to be carried out after the pickled passivation surface is basically dry.
If the passive film on the surface of stainless steel is not perfect or is polluted by iron ions, there will be free iron ions, then the following reaction can occur: 2Fe+K[Fe(CN6)]=KFe[Fe(CN6)]↓dark blue+ 2K
The blue dot inspection method is generally used in the manufacture of stainless steel chemical containers. This is a very strict inspection method. In order to pass the inspection, the process regulations must be strictly followed. We recommend:
1. There must be a dedicated place for stainless steel, wood or rubber must be laid, and it is strictly forbidden to mix it with carbon steel;
2. All kinds of processing equipment, such as plate rolling machine, plate shearing machine, etc. should be cleaned, and the rollers and pressure angles should be painted with varnish to ensure that the stainless steel does not directly contact the carbon steel;
3. The fixtures are made of stainless steel or gasket stainless steel;
4. The steel wire brush for pickling should be stainless steel wire brush;
This method refers to the standard HG 20584-1998.