chemical polishing

[linamar]

Hi.
I'm looking for a process to add/restore a mirror finish to a stainless steel surface.
I know chemical polishing is the solution however it is difficult to find any documentation on the processes.
I'm not looking for industrial scale processes for high volume production.
Any help would be appreciated.
Thank you.
Dan

 

[kenvlach]

I am not aware of any commercial chemical polishing process for SS. If there were one, electropolishing wouldn't be such a big business!

Fine polish mechanically (final stages should be wet-polish), clean, and then electropolish.

 

[linamar]

Thanks.
Then can you please share with me a receipe for electropolish? What bath composition, temperature and voltage?
Part at + or - ?
Thanks

 

[kenvlach]

What is the size & quantity of your parts?
Are you looking to spend $7500, $75000 or $750000?
Need acid resistant tanks, pump & heater, chemical storage, wastewater treatment, EPA permits to store, use & treat hazardous chemicals.

[Note: There are proprietary, non-hazardous solutions available -- see

http://www.finishing.com

-- but the license for one is $75000.]

It's probably best to have a commercial electropolisher do your parts, but here are some basics to help decide.

General purpose bath for SS & Ni-base alloys:
Bath: 65 wt% sulfuric acid*, 15% phosphoric acid*, 22% DI water. A little surfactant helps in wetting the surface and reducing bubble size of gas (O2) produced at the part.
*These are absolute concentrations as determined by titration and specific gravity. Commercial sulfuric is 93%, commercial phosphoric is available in 75% or 85% (preferable) concentrations.

You can also use a commercial bath, e.g.,

http://www.ep-systems.com.

At least, read their process info.
Another informative site for "Electropolishing Basics" is

http://www.ableelectropolishing.com/electrop.htm

Temp: 80-175 F (130-160 F preferred).
Voltage: 6-12 Volts as needed to supply a current > 50 Amps/sq. ft (100 Amps/sq. ft is preferable).
Time is 1-15 minutes (best determined empirically for a given anode/cathode geometry, solution & electrical parameters).

Need to preclean parts; electrocleaning is best (but not required).

Parts are anodic. Copper cathodes work best, but I suggest removing (with rinsing) when not in use. Sometimes can use Cu sheet, but need to avoid trapping gas bubbles which cause streaks on parts, dull white areas if trapped. Surfaces to be polished should be nearly vertical but facing upward. Sometimes, need to mask off surfaces, & do parts in 2 steps.
A uniform anode-cathode distance is important, e.g., 1". Sometimes, use polypropylene standoffs [and brush EP tthe covered area afterwards].
You may need heavy copper mesh bolted to a framwork of 1x0.25" bars to conform to a curved part and allow gases to escape. Silicon bronze fasteners last longer than Cu. Make sure the exposed threaded ends face away from the part surface so as not to scratch. Solution agitation, either by mixer or pumping (but not air) is recommended.

Best to spray rinse with DI water as withdrawing the electropolished parts from the bath, then go through a 110 F dragout rinse (captures hazardous metals & acids) & a flowing rinse to remove the sticky EP solution.

Remove any Cu connectors.

Place in a 25-45 vol% nitric acid solution for at least 10 minutes to remove residual sulfates & phosphates & Cu & to passivate the surface.

Rinse, rinse, warm DI rinse & dry.

Handle electropolished parts with white cotton gloves.

Let us know what you decide to do or if need help with specific problems.

Ken

 

[linamar]

Thanks kenvlach.
Well, I'm on a tight budget and the volume of prototypes I have at hand calls for outsourcing.
Thanks for the info, I will call Able to negotiate a price.

 

[linamar]

Ken,
For chemical polishing of SS, what is the composition of the batch and process parameters?
I mean just chemical, not electro-chemical.
Thanks.

P.S. My parts are tiny screws used in micromechanics and the volume is just a few parts at a time.

 

[kenvlach]

I don't believe there's a good chemical polishing solution for stainless steel.

For small amounts, I suggest using SimiChrome polish or else an electropolishing procedure for metallography samples (I think mostly cold perchloric acid combinations; a bit dangerous).

 

[linamar]

Ken,
the slot in the head cannot be polished mechanically then at heat coloring id will stay shiny and base metal colour. I need to polish it chemically so it turns the same colour as the head.
I found information from companies that do electroless chemical polishing (

http://www.hirai.co.jp/content-e/tech/tech/sonota04.html)

but no info on the bath composition.
It is definitely a mix of acids, still have to find which and the proportion.
Thanks.

 

[kenvlach]

If only chemical polishing, why is there a size limit?
"chemical polisher. Processible size is 300x300mm max."

Are you trying to remove "heat coloring" ???
Avoid formation of any heat tint scale by heat treating in vacuum (or dry hydrogen or high-purity inert argon).

 

[linamar]

Ken,
I'm not doing that for money but for pleasure, as a hobby.
I will blue the heads and the slot and if the slot is not polished it won't turn blue and won't look good.
And I'm going to do it at home, not in an industrial environment so my means are limited.

 

[kenvlach]

OK,
The chemical polishers seem to be either electrochemical polishers or mechanical-chemical polishers:

"Electropolishing: Also called chemical polishing or finishing..." ---

http://cr.pennnet.com/Articles/Article_Display.cfm?ARTICLE_ID=244261&p=15&cat=Feat

"High Smoothness Finishing of Stainless Steel Surface by Chemical Polishing" -- uses a barrel tumbler:

http://www.simtech.a-star.edu.sg/publication/ST/Pub-ST2004(c1).pdf

Another site describes Chemical polishing of stainless as a fine etching process:
"The substrate is placed in a chemical solution, which etches the surface to reduce the surface roughness." ---

http://www.mems-exchange.org/catalog/P1684/

For the best results at lowest cost, I still recommend Simichrome. Available worldwide & a 50-gram tube is about $7. If there is much heat treat film within the slot from manufacturing, use an auto bumper chrome polish. The polishes leave a protective film, so you must degrease (hot alkaline detergent or a solvent) before blueing.
Ken

 

[linamar]

Ken,
Thanks for the links, I've already visited the sites; nice but useless, no info.
Probably I have to start mixing acids myself and experiment until I get the right mix.
The screws are so small, thread size 0.5, 0.75mm or even smaller and the slot is even smaller that no mechanical device (brush, felt wheel, thread wheel) will reach inside. I tried with a string and the corners were left untouched.
This is why I'm looking for a chemical solution which apparently is impossible to get. I may try using elecro-chemical with the help of the battery on my car.

 

[kenvlach]

Maybe an extra soft (smallest diameter bristles) (toothbrush will work. Also, you can use the screwdriver tip itself, with Simichrome, for cleaning & polishing.

What exactly needs cleaning from the slot heads?
You can electroclean or electropolish in a beaker. Hang the screws either individually using some soft annealed wire looped around threads or multiply, held in compression between the coils of a small coil spring hung into the solution.

Don't mix up acids on your own.*
The liquid chrome polishes for auto contain acid (if I remember correctly -- my car seems to be plastic).
Descaler for stainless steel welds, available in paste & liquid forms, contains nitric & hydrofluoric acids in correct ratio.
Hydrochloric acid will give stainless a white matte finish that improves coatability. Spray-on aluminum wheel cleaners containing HF will also descale stainless. HF also gives a flat white finish.

*If you have years & $$$ to research this, I can sell you a good selection of metal finishing books, mostly US & UK & translations of German and Russian books, so you will know what not to use.

 

[linamar]

Ken,
The slot is clean, it needs just a very good (mirror like) polish to turn blue when heated.
To give you an idea, the slot width is perhaps 500 microns (0.020") or less, thus hard to polish mechanically.
I will try the polishing compounds first.
I will post a link later for you to get an idea of the size.
Dan

 

[linamar]

Ken,
which is the best way to remove scratches and polish a chromed surface?
I've tried a fine grit compound and Autosol but it's barely cutting through the hard Cr.
Thanks.