Where theworld gathers for metal finishing
Q&As since 1989
-----
Electrolytic passivation of tinplate: is there a layer of chromium
2004
Q. Hello,
I'm a student in Paris and I have a project on tinplate. I've read a document about the passivation of tinplate. Tinplate goes through 2 types of passivation: a chemical passivation which permits the layer of chromium oxide and an electrolytic passivation which permits the layer of chromium.
I would like to know if you can explain me how that electrolytic passivation works. You can find the document I read at http://www.sfc.fr/Donnees/mater/febl/texfebl.htm (it's called Brillantage, Passivation) it's in French, sorry for the inconvenience.
Thank you for your help.
Regards,
HO Vivianestudent - Paris, Ile de France, France
----
Ed. note: Sorry, the referenced link is now broken.
A. Passivation of tinplate is usually achieved by either electrolytic or chemical treatment with a dichromate solution. The both processes result in a chromate (hexavalent chromium) coating that seals the surface and reduces the effects of chemical attack. This is also known as a "Conversion coating". A good conversion coating will only be a few nanometers (say about 10) thick. The dip process is purely a chemical reaction and is slower than an electrolytic process. Consequently, to get the same degree of protection, the dip takes much longer than the electrolytic process. The electrolytic process gives a more adherent layer and greater protection. The composition of the conversion coating changes as you move away from the surface. The layer immediately next to the tin will be predominantly CrO3, but as you move out, this will alter to Cr2O3, CrOOH and Cr(OH)3 and perhaps other hydrated and partially hydrated materials. There have been claims over the years that metallic chromium has been detected on the very surface of the tin, but it is discontinuous and, as far as I know, never satisfactorily confirmed. Chromium is difficult enough to electroplate under ideal conditions and using a simple conversion coating electrolyte is, in my opinion, a bit optimistic. However, in theory it is possible. Obviously, you will not get any chromium metal deposited in a dip process and the coating will be more hydrated than an electrolytic coating.
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
2004
Q. Dear sir,
please tell me what is electrolytic passivation?
- Islamabad,Pakistan
April 8, 2013
A. Hi Muhammad. Sorry that you were unable to understand Trevor's posting. Electrolytic passivation simply means the use of DC electricity to assist in the deposition of the chromate conversion coating, for faster operation and better adhesion.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
April 9, 2013
Q. Do you have knowledge of a replacement for the CrVI passivation, since it is now forbidden in Europe?
Francisco Ferro- Lisbon, Portugal
2004
A. Hi Francisco. Trivalent chromates conversion coatings are now widely used on aluminum and on zinc plating. Although they probably need to be tweaked specifically for use on tin plate, I'm confident that they are available from the suppliers to the plating industry. Good luck.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Q, A, or Comment on THIS thread -or- Start a NEW Thread