Problem with Pickling Steel with 2.5% silicon
Q. I am looking for recommendations on pickling or otherwise cleaning small components made from alloy steel with 2.5% silicon. The surfaces need to be very clean to allow uniform diffusion during carbonitriding heat treatment.
The components are vacuum annealed prior to carbonitriding, but do not take a uniform case. Pickling in hydrochloric acid has helped, but is not completely reliable. I believe the problem is due to a stable surface oxide, and that the silicon makes the surface film more resistant to cleaning than plain carbon steels.
Anyone have experience with cleaning these silicon steels for plating or other processing that requires clean metal surfaces?
A. You may want to check out Patent 8,128,574. They discuss pickling of Si steels in a bath containing hydrochloric acid, ferric chloride and hydrofluoric acid. Ferric chloride will attack the base metal and dislodge the insoluble matter. Hydrofluoric acid will directly dissolve the silicon-bearing oxide. The hydrofluoric acid, however, is extremely difficult to handle due to safety/environmental issues. You might first try just the ferric chloride, either added directly or (as in the patent) produced by chemical reaction with hydrogen peroxide.
- Chiago, Illinois, USA
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