Thank you all for your response of cleaning, I’ve have still being
experimenting with some chemicals. And also have looked at etching
compounds for stainless steel. these steels contain as main compound
alloys, chromium and nickel. And I have found a way of easy cleaning,
5% hydrogen fluoric acid 30% Nitric acid Just dip for 10 minutes at 20
This cleans the black layer very well, even in the deepest pores. The
hydrogen fluoric acid I’m also using for Titanium.