Hello Heinz,
You can always try. I prefer Diluted Vinigar for think black rust. And brush it away with a brass bursh. It works fine for me. The vinigar is not a strong acid, and les strong iff you dilute it a bit. It wil destroy the stucture of the rust, what you can bush of the next day.
Regards Abdulrahman (formerly Michel)
PS you can also have a look at this product. I realy works!!
Bio deruster link