Author Topic: Electropolishing experiments  (Read 1113 times)

Offline PK

  • Sr. Member
  • ****
  • Posts: 292
  • Country: au
Electropolishing experiments
« on: August 28, 2017, 06:07:44 AM »
This is another 'we're doing this at work, but everything is relevant to y'all at home' projects.
We've been buffing some TIG welded stainless assemblies for a few years now and it's a bit of a pain:

   1.You get a shiny weld which contrasts the satin finish on the rest of it.
   2.You have to get the wax off.
   3.Then you have to get the stuff you used to get the wax off, off.

We could just use pickling paste, but all the ones that work have HF acid in them and that's nasty even by my lax workplace safety standards.

I decided to look into electro-polishing and a quick search turned up a candidate solution of Citric and Sulfuric acids, neither at nasty concentrations... This looked promising as I'd assumed we would have to use hot phosphoric/sulfuric acid. Out came an old mop bucket, in went some lead flashing for a cathode, and 7 minutes at 10A later out came the part. All it needed was a quick wipe under running water and presto!

I've started a thread on this, because I think I'm going to automate the process and that should be interesting...
PK

Offline Will_D

  • Sr. Member
  • ****
  • Posts: 477
  • Country: ie
    • National Homebrew Club of Ireland
Re: Electropolishing experiments
« Reply #1 on: August 28, 2017, 09:57:35 AM »
If you can't get the part into the bath then you need to buy (or make) a carbon fibre brush and use that as the anode and paint the oxide film away. I used 50% cold phosphoic acid iirc.

For the brush i just pulled carbon fibre cloth into a 6mm copper tube, flattened end to trap fibre, attach wire an the wrap the copper in inulating tape. I used a 8 amp 12 volt power supply.

Idea ripped off a commercial set up!!

like this:

http://www.weldcleaningsolutions.com/weld-postcleen-kits.html
Engineer and Chemist to the NHC.ie
http://www.nationalhomebrewclub.ie/forum/

Offline simon.baldwin69

  • Newbie
  • *
  • Posts: 3
Re: Electropolishing experiments
« Reply #2 on: August 28, 2017, 10:21:58 AM »
Hydrochloric and nitric acid makes a great pickling paste

Sent from my EVA-L09 using Tapatalk


Offline Will_D

  • Sr. Member
  • ****
  • Posts: 477
  • Country: ie
    • National Homebrew Club of Ireland
Re: Electropolishing experiments
« Reply #3 on: August 28, 2017, 11:09:05 AM »
Nitric acid is getting very gard to obtain these days
Engineer and Chemist to the NHC.ie
http://www.nationalhomebrewclub.ie/forum/

Offline PK

  • Sr. Member
  • ****
  • Posts: 292
  • Country: au
Re: Electropolishing experiments
« Reply #4 on: August 28, 2017, 06:29:08 PM »
If you can't get the part into the bath then you need to buy (or make) a carbon fibre brush and use that as the anode and paint the oxide film away. I used 50% cold phosphoic acid iirc.

For the brush i just pulled carbon fibre cloth into a 6mm copper tube, flattened end to trap fibre, attach wire an the wrap the copper in inulating tape. I used a 8 amp 12 volt power supply.

Idea ripped off a commercial set up!!

I did some experiments about a year back with a system like this. I used some carbon from some reinforcing cloth.  It works well for a hobby environment, but each case has just under 1m of weld so you'd be standing there for a while I used rust converter (ie phosphoric acid) which I'm not as happy about as citric acid.
With a tank, you just throw it in and go do something else for 5min..


Offline PK

  • Sr. Member
  • ****
  • Posts: 292
  • Country: au
Re: Electropolishing experiments
« Reply #5 on: August 28, 2017, 06:32:28 PM »
Nitric acid is getting very gard to obtain these days
Ha!  I went into the same battery shop I've been going to for years to buy some more battery acid for this test.
It's not that they wouldn't sell me any, it's their business, they can do as they wish.
It's the look of fear and concern in their eyes as they called the boss.......
I lectured them for about 5 minutes..... man I was pissed off...

PK

Online PekkaNF

  • Hero Member
  • *****
  • Posts: 1814
  • Country: fi
Re: Electropolishing experiments
« Reply #6 on: August 28, 2017, 10:35:51 PM »
....

I decided to look into electro-polishing and a quick search turned up a candidate solution of Citric and Sulfuric acids, neither at nasty concentrations... This looked promising as I'd assumed we would have to use hot phosphoric/sulfuric acid. Out came an old mop bucket, in went some lead flashing for a cathode, and 7 minutes at 10A later out came the part. All it needed was a quick wipe under running water and presto!

....

Interesting. This process is a complete stranger to me.

What sort of concentrations we are talking here? Few %?

Another quesion is about lead cathode....Does it has to be lead?

So, yo need a "tank" that works same priciple than ultrasonic cleaner - drop parts in basket, clse the lid, press "start" and it chime when process is over? Wih pre/post rinse buckets? Not the fullyautomatic conveyor system like in metal plating lines.

Pekka

Offline PK

  • Sr. Member
  • ****
  • Posts: 292
  • Country: au
Re: Electropolishing experiments
« Reply #7 on: August 28, 2017, 11:30:35 PM »

Interesting. This process is a complete stranger to me.

What sort of concentrations we are talking here? Few %?

I just made up 4L of a saturated solution from dry acetic acid (150ish g/l) and added about 1L of battery acid. It sounds a lot worse than it is. I'll be telling the guys to wear gloves, but I didn't. It's certainly no nastier than an anodising tank.

Quote
Another question is about lead cathode....Does it has to be lead?

Nope, anything that lasts will do. You could even try stainless steel. Graphite would work and titanium would be perfect.  Just make sure it has a large area WRT the job.

Quote
So, yo need a "tank" that works same priciple than ultrasonic cleaner - drop parts in basket, clse the lid, press "start" and it chime when process is over? Wih pre/post rinse buckets? Not the fullyautomatic conveyor system like in metal plating lines.
would be nice, but it'll probably be a small rack with Ti fingers that clip into the part. Push the button and the rack lowers into the tank, a few minutes later the power is turned off and the rack rises out of the tank and the unit starts making some irritating noise.   Add in some fume extraction and call it done.  We only do batches of 100 of these parts so there's no need to go too stupid with it...

PK

Online PekkaNF

  • Hero Member
  • *****
  • Posts: 1814
  • Country: fi
Re: Electropolishing experiments
« Reply #8 on: August 29, 2017, 02:28:47 AM »
Thank you. Very nice to know....I'm trying not to gravitate towards stainless steel bending and welding, but it has it's benefits.

Normal grocery store grade vinegar here has about 10% acetic acid and pretty cheap on 5 litre gallons, wonder if that would make the base for that. Mental note: Go and buy battery acid bottle (or two) while it's still freely available.

Pekka

Offline simon.baldwin69

  • Newbie
  • *
  • Posts: 3
Re: Electropolishing experiments
« Reply #9 on: August 31, 2017, 01:54:39 AM »
Well depends on where you are, in the uk you can buy 3% nitric acid without a license and that should do the job, hydrochloric acid is easy to come by but sulfuric acid is difficult to buy at 98% but you can get drain cleaners that are 98%, I refine precious metals!

Sent from my EVA-L09 using Tapatalk


Offline PK

  • Sr. Member
  • ****
  • Posts: 292
  • Country: au
Re: Electropolishing experiments
« Reply #10 on: August 31, 2017, 03:31:56 AM »
but sulfuric acid is difficult to buy at 98% but you can get drain cleaners that are 98%, I refine precious metals!

I get that concentrated acids are dangerous (and useful for many applications.... I feel your pain), but battery acid is, what, about 4mol/L ?
IIRC glacial H2S04 is around 18 or so...

Offline PK

  • Sr. Member
  • ****
  • Posts: 292
  • Country: au
Re: Electropolishing experiments
« Reply #11 on: September 05, 2017, 02:50:43 AM »
 :update: Some progress.
The trolley for the thing arrived.
I welded up the frame that will hold the carrier hooks and get lifted in and out of the tank by an , as yet, unspecified mechanism.
Today we made the foam plug for the fume catching lid...