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Passivate 304 and 316 stainless steel or not?
We are manufacturing SS components from SS 304, 316 . We carry out passivation in a plastic container. Our passivation formulation consists of 20% Nitric acid and 5 % sodium dichromate in water. We immerse our components in this formulation for approx. 45 min, without any heating. When we immerse the components nothing happens to it ( no colour change , pitting etc). We then immersed some samples of the passivated lot in salt water ( using house salt) and kept them for 48 hrs. Nothing again happened to the samples. Does that mean that the components have been passivated correctly. Also surprisingly the non passivated samples when put in salt water also did not show any rusting. Do we require passivation.
A Khan- India
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Hi,
There are few standard test to confirm whether the Passivation is perfect or not. What you have done is Accelerated corrosion testing (when you put the SS material in Salt water the Fe left on the surface will form rust to tell you that there is still some iron left on the surface). Passivation is the process in which the Iron on the surface of SS is selectively removed to enrich the Chromium on the surface. This will provide better resistance against corrosion.
To confirm if all the iron on the surface is removed effectively during passivation there is a test called copper sulfate test.
You need to apply a slightly acidic CuS04 (16 gpl CuS04 and few drops of H2SO4) Solution on the surface of the passivated material. After few mins wash the sample and look for copper deposit on the place where the solution was applied. If you don't find Copper deposit, your passivation is good. This has to be done only on 200 and 300 series SS material. Refer Mil-753 [link is to spec at TechStreet] (replaced by AMS-STD-753 [link is to spec at TechStreet] ) for details on this test.
Good luck ..
Karthik- Singapore
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It is not unusual to have 316 or 304 stainless steel pass the free iron tests (whichever you use- refer to ASTM A967 [link is to spec at TechStreet]) because the base metal is very good even without passivation. Many companies do not passivate these grades for general use. (See A-967 also for the method of passivating austenitic stainless- you do not need the sodium dichromate)Passivation is absolutely necessary if you have imparted free iron to the surface in your machining, processing or handling.
However, if your customer is expecting good corrosion resistance in stressful situations (salt spray, chloride contact, etc.) passivation gives you an extra measure of resistance due to the chromium enrichment of the surface.
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