The Cr nanolayer (formed by the PVD method) is resistant to strong acidic solutions (i.e., H2SO4, HCl, ...), however, the bulk Cr metal dissolves slowly. I think maybe this is because of the passive layer on the nanolayer. If it is true, is there any chemical procedure to dissolve the passive layers?
Massive chrome is always passivated by a nanometer oxide film, so this is not the issue. It may be that the chrome applied by the PVD method is not chrome, but chromium carbide, which is easily formed when there are traces of hydrocarbons in the chamber. Unlike chrome and its oxide, chromium carbide does not dissolve in sulfuric and hydrochloric acid.
To etch chromium films a special etchant is used, often called chrome etch, which contains ceric ammonium nitrate and nitric acid. You can find it commercially
Maybe the coating is chromium carbide, especially because of its dark color. But if it is true, why does the chromium layer dissolve forthwith and completely when I scratch the coating under the sulfuric acid solution?
Vadim Verlotski
Chromium carbide is light, so it is possible that your coating is indeed chromium, but with a fairly thick oxide film on top that gives it a dark color. The thicker oxide film may explain why the reaction with hydrochloric acid does not start immediately.
Chromium is a very active metal that reacts quickly with sulfuric and hydrochloric acid when the oxide film is destroyed. It is not even necessary to destroy the oxide film mechanically to initiate the reaction of chromium with acids. In hydrochloric acid, this film dissolves slowly at normal temperature, and in sulfuric acid - when heated to at least 60°C.
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