By -
Date 08-08-2005 18:04
Hi Tim,
A blue stain is left if there is indications of surface iron contamination (within 15 seconds). It may take up to several minutes for the detection of oxide scale.
Actually this test is not recommended for food-processing applications, beverages, pharmaceuticals or other products for human consumption unless ALL TRACES of the solution is sure to be removed. You have got to be extremely careful when using this test...
Chuck
Tim,
Sorry, I didn't address the "rouging" part of your question. I hit the "post" button before I got to that part.
Rouging, by definition, is the result of the formation of iron oxides, hydroxides, or carbonates either from external surfaces or from the destruction of the passive layer of the stainless steel. Depending on severity, the colors can be from orange to red to black. Either way, it is an idication of surface contamination which, when left, can cause a possibility of formation of intergranular corrosion. Now you've got a real problem. Surface contamination, when detected, can be resolved. Intergranular corrosion, in most cases, goes undetected until a catastrophic faliure occurs. Then it's too late. That is why it is so important to make sure the invisible, impervious layer of chromium is not compromised on the surface of the stainless steel. I mean, in the real world, (refineries, nukes, power houses, paper mills, etc.) stainless steel comes in contact with carbon residue all day long, but it is only detrimental if the passive layer of the SS has been destroyed, then those particles can act aggressively on the unprotected SS surface.
The ferroxyl test is not used to determine the potential for "rouging" or any other contamination potential. It is used only to determine if there are any contaminants on the surface of the SS during the test. Nothing more. With a ferroxyl test, the solution should be removed as quickly as possible as to not compromise the surface of the SS. In my 35 years of dealing in stainless steel, the ferroxyl test is only used when there is an objectionable agreement/disagreement of free iron or iron oxide contamination. If there was any doubt that free iron or iron oxides exist, the surface of the SS should be cleaned without the implementation of a ferroxyl test. Again, that's what I've experienced over the years.
Chuck