I'm working with 316L stainless steel and trying to improve its roughness with the use of "piranha" solution. No changes can be noticed on SEM images of the surface, even though I use concentrated "piranha" (96% sulphuric acid + 30% hydrogen peroxide, 2:1 ratio) at 80 degrees.
The situation is the same when the surface is initially ground with abrasive paste or when it undergoes initial electropolishing (in the mixture containing phosphoric acid and sulphuric acid) and is dipped in "piranha" afterwards.
Earlier I read the follow recipe - treatment of steel surface in an aqueous solution containing 10-20 wt% HNO3 and up to 2 wt% HF (or NaF); temperature 50-70 Celsius degrees; time - up to 15 min.
OR 20 wt% HNO3 and 5-10 wt% HF at room temperature (time should be determined experimentally). Pure HNO3 and other acid compositions seems to be non-effective.
I find that 10-30% aqua regia for up to 30 minutes works very well. The depth of etching can be varied by time and the surface texture can be varied depending on the concentration.
Try using H2S04 and HNO3 with varying conc (10 to 30%). HNO3 is known oxiding solution however you may need to clean the surface with H2SO4 as you are doing rightly. I am not sure why H2O2 doesnot work, maybe the surface isnot enough "clean".
Bear in mind that stainless steel is "stainless" because of a tenacious oxide layer that protects the bulk metal. Putting in H2O2 (or other oxidant) just ensures that that layer is reformed rapidly whenever your acid/whatever gets through it.
Dear all,
many thanks for your ideas.
As for HF, I am aware that people apply this on stainless steel, but I am trying to avoid it since it's hazardous to use.
What I am trying to do is to increase roughness of the SS surface, so it's not neccessary to get rid of oxides layer.
I will try to use aidic mixtures as you suggested.
For austenitic stainless steels and nickel superalloys we get very good results using fresh Aqua regia. Try with that solution, but as with everything else it takes some practice before you perfect out procedure in detail.
Look up metallographic etching solutions used for stainless steel. Metallurgists do a variety of etching solutions to bring out different surface properties of metal surfaces.
Thank you again for your help. It's good to know that aqua regia works well on stainless steel and I may use it in the future. However, I don't need to apply it now. I am working on stainless steel surface modification method and it appeared that I have improved the adherence of my coating layer without deep etching of the steel surface.
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