Chrome re-plating

[linamar]

I have a few small parts whose thin chrome plating peeled of in some areas.
The base metal is brass and I want to redo the coating.
Before re-chroming the old coatoing has to go, but which is the best way to strip it down?
As the base metal is soft, bead blasting would damage the surface.
Any suggestions would be appreciated.
Thanks,
Dan

 

[swall]

There are chemical strippers that would work, as well as reverse current stripping. The plater that will apply the new chrome should be able to perform the stripping of the old.

 

[TVP]

swall is correct-- the two most effective processes will be either chemical stripping (HCl acid works well) or reverse current stripping (immerse in electroplating bath and reverse the current so that the metal is de-plated).

 

[linamar]

Thanks guys.
I have a concern though: would the chemical stripping, or reverse current react with the base material (brass) and incurr any dimensional changes?
This is my major concern; and tipically what is the thickness of decorative/protective chrome plating?
I assume it is not exceeding 10 microns.

 

[kenvlach]

Dan,
Best to let the chromium plater strip the old chromium, but for do-it-yourself, here are the Metal Finishing Guidebook recipes:
Chemical stripping of chromium from brass, copper or nickel.
Hydrochloric acid, 12.5 vol.%, 125[sup]o[/sup]F.

Anodic stripping of chromium from copper, brass, magnesium or steel.
Sodium hydroxide 52.5 g/L + sodium carbonate 67.5 g/L, room T, 6 Volts, steel cathodes.

Some substrate attack is inevitable when stripping a non-uniform, partially peeled, plating.

For polished parts, either the substrate is polished or an electroplating of copper is polished. The copper also fills pitting and rebuilds lost dimensions.

Next, nickel is (nearly) always deposited before chromium plating. The nickel provides corrosion protection, whiteness (hides the copper, etc.), luster (if polished; bright Ni plating baths are also used) and protects the chromium plating solution from Cu & Zn contamination. The nickel thickness is typically 0.0001 inch (2.5 micron), but verify – varies from plater to plater; also, more is used for additional corrosion protection.

The decorative chromium plating is usually only 2-20 micro-inch thick (0.05-0.5 microns). Thicker deposits will typically display visible microcracking, undesirable for decorative work.

 

[linamar]

Thanks Ken,
detailed as always.
I won't do it myself of course, will find a plater that can handle it.
Given the thin layer of Ni and Cr, I believe the surface finish of the base material is preserved. Therefore, I will have to fill any deep scratches and polish the part before the coating is reapplied.
I read somewhere that a satin finish can be achieved at Ni coating that will be preserved after Cr coating, is this right?
I think there is a special process and maybe not every plater can do it.

 

[kenvlach]

Copper plating is useful for filling scratches and is easily polished to give a smooth substrate for the bright nickel. It's also like a sandable primer in that any overpolishing is apparent.

Does satin finish refer to a brushed satin finish?
Warning: every plating chemical supplier (Atotech, Enthone, MacDermid, etc.) offers a satin nickel plating bath which gives a non-brushed, semibright finish.

For brushed satin, you are correct that not every plater does this. I haven't – it seems like deliberate sabotage of corrosion resistance. Maybe just increase the nickel plating thickness, mechanically wire brush or abrade and omit the chromium.