Where the
world gathers for metal finishing
Q&As since 1989


-----

Steel etching in caustic chromium strip solutions

Is there an upper limit for Chromium concentration in caustic anodic stripping solutions? We have a stripping tank that we use to strip old chromium off of steel crankshafts. I've noticed that after a while (I don't know about amp-hours, we should keep a log on that but don't) the solution will darken to a brown color from bright yellow and then the cranks will start etching. Obviously we don't want that to happen. I'm guessing that at a certain concentration, the hexavalent chromium starts oxidizing the steel, gets reduced to the trivalent state and that darkens the solution. Anyway, I'm just wondering if that's the case, where should we start being concerned.

Peter H. Millican
- Denver, Colorado, USA
2003

---

2003

You do not mention the voltage you are using. I do not recall off hand the upper voltage we use to use but I would think 6 or less. I also do not recall the concentration of NaOH but do recall it would only hold about 1 opg.

Just a wild guess and a wild stab at an other cure is add some silver nitrate to the water and mix well before adding the caustic. This should grab any chloride in the water and then the caustic.

Hope this helps.

Let me know if not and I'll crack my old books.

Jon Quirt
- Minneapolis, Minnesota