Sciencemadness Discussion Board
Not logged in [Login - Register]
Go To Bottom

Printable Version  
 Pages:  1    3
Author: Subject: Passivation of MMO anode?
woelen
Administrator
********




Posts: 6085
Registered: 20-8-2005
Location: Netherlands
Member Is Offline

Mood: interested

sad.gif posted on 9-1-2011 at 01:15
Passivation of MMO anode?


A friend of mine just purchased an MMO anode with titanium substrate off eBay and used it to make KClO3. He used diet salt (a mix of 1/3 part NaCl and 2/3 part KCl by weight). He had a run from a 5V power supply and let this cell run until current had dropped strongly and as a consequence the cell cooled down and solid KClO3 crystallized. He used no additives in the solution and used a stainless steel cathode. After a few days of electrolysis he had a lot of solid KClO3 which easily could be separated.

He disassembled the cell, cleaned the electrodes and worked up the KClO3. Now he has purchased pure KCl and wants another run, but to our horror, the anode does not work anymore! When a concentrated solution of KCl is used, then only 0.2 A is running through the cell. Different cathodes were used, but always the same result, just around 0.2 A of current. He tested the power suppy and the cathode and these are just fine. It really has to to with the MMO anode.

It seems as if the anode has passivated. The anode was thoroughly cleaned and kept in clean water for some time, hoping that it would remove some invisible layer, but this also does not work.

What is causing this problem and what can be done about it? I do not have real knowledge about this subject and could not find good info on internet. Does anyone else have a similar experience or know of this?




The art of wondering makes life worth living...
Want to wonder? Look at http://www.oelen.net/science
View user's profile Visit user's homepage View All Posts By User
Jurre10
Harmless
*




Posts: 17
Registered: 9-1-2011
Location: The Netherlands
Member Is Offline

Mood: Experimenting

[*] posted on 9-1-2011 at 01:37


Hi,

I'm the one Woelen is talking about.
He exactly writed how it went.

So we think there is an (invisable) layer on the anode,
but we both don't know how to remove it. :(

Hopefully, somebody who know this problem can give us a good answer.

Jurian

[Edited on 9-1-2011 by Jurre10]
View user's profile View All Posts By User
hissingnoise
International Hazard
*****




Posts: 3663
Registered: 26-12-2002
Member Is Offline

Mood: Pulverulescent!

[*] posted on 9-1-2011 at 03:24


I haven't used MMO anodes so I'm just guessing . . .
I assume you've checked and double checked the connections.
Does the Losalt possibly contain additives that could be deposited onto the anode as a polymeric insulating material which could be removed by a solvent other than water?
A good microscope might reveal something . . .

View user's profile View All Posts By User
Jurre10
Harmless
*




Posts: 17
Registered: 9-1-2011
Location: The Netherlands
Member Is Offline

Mood: Experimenting

[*] posted on 9-1-2011 at 03:52


Hi,

Thanks for your tips, but LoSalt is just 66%KCl and 33%NaCl
So i think it can't be something from, the LoSalt.

Maybe it's a bit of the glue i used to glue the anode/cathode in place?
I don't think so because i used Epoxy glue that easely holds at 100°C.

The pont is that i don't have a microscope. :(
There is a little bit of corrosion on the lug, but i think that won't make the connection so bad that there is just 0.23A.


Jurian




If you try your best, but you don't succeed.
Experience is what you need.
View user's profile View All Posts By User
hissingnoise
International Hazard
*****




Posts: 3663
Registered: 26-12-2002
Member Is Offline

Mood: Pulverulescent!

[*] posted on 9-1-2011 at 04:51


Hmmm . . .
LoSalt - from wiki; "An iodised version is also available in the Nederlands and Middle Eastern markets".

View user's profile View All Posts By User
Jurre10
Harmless
*




Posts: 17
Registered: 9-1-2011
Location: The Netherlands
Member Is Offline

Mood: Experimenting

[*] posted on 9-1-2011 at 05:13


hi,

I know, but thats a different color package and there's a little text on it hat says : iodised
So i don't have the iodised version, just naturel.

I have threats of this on 3 different forums but nobody knows whats happening?
So i'm afraid i have tobuy a new one :(

Jurian




If you try your best, but you don't succeed.
Experience is what you need.
View user's profile View All Posts By User
hissingnoise
International Hazard
*****




Posts: 3663
Registered: 26-12-2002
Member Is Offline

Mood: Pulverulescent!

[*] posted on 9-1-2011 at 05:29


Quote:
So i'm afraid i have tobuy a new one

Or save some pennies for Pt foil . . .
But don't discard your old one - keep it for further tests.
Solving the puzzle might be advantageous at some point!


View user's profile View All Posts By User
Jurre10
Harmless
*




Posts: 17
Registered: 9-1-2011
Location: The Netherlands
Member Is Offline

Mood: Experimenting

[*] posted on 9-1-2011 at 05:44


Quote: Originally posted by hissingnoise  
Quote:
So i'm afraid i have tobuy a new one

Or save some pennies for Pt foil . . .
But don't discard your old one - keep it for further tests.
Solving the puzzle might be advantageous at some point!




hi,

I know, hopefully i will find it out.
but when i buy Pt, im also able to make KClO4

i just hope my mmo anode will come back to life :)
Because it waas not that cheap ;)

Jurian




If you try your best, but you don't succeed.
Experience is what you need.
View user's profile View All Posts By User
woelen
Administrator
********




Posts: 6085
Registered: 20-8-2005
Location: Netherlands
Member Is Offline

Mood: interested

[*] posted on 9-1-2011 at 06:13


Testing the connection can be done easily. Just put a clamp on the titanium strip of the anode and put another clamp at an other position of the strip and connect these through a decent 5 V lamp (e.g. lamp from a bicycle, which takes at least 500 mA). But I do not believe this is the issue, I really think this is due to formation of some ultrathin layer, which passivates the anode. And no, having an optical microscope at hand will not help here. Usually such layers are just nm sized and they can only be observed using extremely powerful (and expensive) microscopes, such as electron microscopes.

Quote:
I have threats of this on 3 different forums but nobody knows whats happening?
So i'm afraid i have tobuy a new one :(

Just wait for more input from other members, this thread is just a few hours old.




The art of wondering makes life worth living...
Want to wonder? Look at http://www.oelen.net/science
View user's profile Visit user's homepage View All Posts By User
hissingnoise
International Hazard
*****




Posts: 3663
Registered: 26-12-2002
Member Is Offline

Mood: Pulverulescent!

[*] posted on 9-1-2011 at 06:49


Yeah, people like dann2, or Swede, although I haven't seen posts from them for a while.
A bit of experiment seems in order though - like reversing polarity for a few minutes, or increasing voltage, current density, or both.
Dipping the anode in acid or basic solutions (H2SO4/HNO3/NH4OH) for short periods.
Carefully buffing a small area of the anode with a soft wheel and then checking the treated area for increased gas evolution.
And if all else fails, high quality graphite rods give very acceptable chlorate production rates with NaCl . . .

View user's profile View All Posts By User
metalresearcher
National Hazard
****




Posts: 417
Registered: 7-9-2010
Member Is Offline

Mood: Reactive

[*] posted on 9-1-2011 at 06:57


Recently I tested a Ti anode which passivated shortly. By scouring with fine sandpaper I removed the passivation layer.
But I don't use it anymore for my chlorate cell. Last week I also ordered an MMO anode via ebay (probably the same as @woelen) as graphite anodes corrode and I purchased probably the same LoSalt as @woelen from a large supermarket chain called Albert Heijn..
I don't know whether the scouring of the MMO anode is a good option as it might destroy the MMO layer.
I know that LoSalt contains very small amounts of MgCO3 and KI (iodated) but that should not disturb the cell.
@woelen: did you try a carbon rod just for test whether now it really draws 5 A ?
Now I am warned what to do when I receive my MMO anode as I have the cell ready.


losalt.jpg - 17kB
View user's profile View All Posts By User
Jurre10
Harmless
*




Posts: 17
Registered: 9-1-2011
Location: The Netherlands
Member Is Offline

Mood: Experimenting

[*] posted on 9-1-2011 at 07:25


Hi,

@hissingnoise : @Woelen told me Swede is verry ill :o

@metalresearcher so you just removed the layer with sandpaper :) sounds good!
But isn't it dangerous? i think you may damage de MMO layer?

I don't have graphite rods right now so i can't try them.


Jurian

b.t.w The LoSalt on your photo is the iodised version....




If you try your best, but you don't succeed.
Experience is what you need.
View user's profile View All Posts By User
hissingnoise
International Hazard
*****




Posts: 3663
Registered: 26-12-2002
Member Is Offline

Mood: Pulverulescent!

[*] posted on 9-1-2011 at 07:33


I'm sorry to hear about Swede, woelen; I hope it's not too serious and that he recovers fully soon.
I had missed his very informative posts for a while now . . .

View user's profile View All Posts By User
Jurre10
Harmless
*




Posts: 17
Registered: 9-1-2011
Location: The Netherlands
Member Is Offline

Mood: Experimenting

[*] posted on 9-1-2011 at 07:57


I'm sorry to tell you
I just heard it from Woelen.

I gonna ty things like higher voltage and reversing polarity for yust few secs.


Jurian




If you try your best, but you don't succeed.
Experience is what you need.
View user's profile View All Posts By User
hissingnoise
International Hazard
*****




Posts: 3663
Registered: 26-12-2002
Member Is Offline

Mood: Pulverulescent!

[*] posted on 9-1-2011 at 09:19


Quote: Originally posted by metalresearcher  

Now I am warned what to do when I receive my MMO anode as I have the cell ready.

I'd use NaCl rather than KCl for chlorate cells even with MMOs - NaClO3's high solubility make it less convenient but it seems somewhat less corrosive than KClO3 (possibly because of Na's smaller atomic radius).

- Jurian, I'm wondering if having two chlorides with widely differing solubilities in the cell could have had some kind of effect on the surface oxides.
But, perhaps dann2 will chime in with a different perspective . . .


View user's profile View All Posts By User
woelen
Administrator
********




Posts: 6085
Registered: 20-8-2005
Location: Netherlands
Member Is Offline

Mood: interested

[*] posted on 9-1-2011 at 09:38


Yes, Swede seems to have health issues (at least that is what I know from my last contact several weeks ago). He also is very busy with his work. Whatever is the situation at the moment, let's hope he will be back soon in good health with his invaluable input and knowledge.

Treating the anode with sandpaper is really a last resort action only to be applied when everything else fails. The MMO layer is very thin and very little abrasive action is needed to remove that layer completely. If this is done, then a very gentle abrasive action should be used (e.g. ultrafine CeO2 or SnO2 may be used, applied by gently rubbing with a wet piece of fabric which has some of the powder on it).

The other suggestions can be used, such as increasing the voltage. But only do that for a brief amount of time. High voltage may actually increase passivation and more and more voltage may be needed for operating the cell.




The art of wondering makes life worth living...
Want to wonder? Look at http://www.oelen.net/science
View user's profile Visit user's homepage View All Posts By User
metalresearcher
National Hazard
****




Posts: 417
Registered: 7-9-2010
Member Is Offline

Mood: Reactive

[*] posted on 9-1-2011 at 09:43


What about using nonpolar solvents like white spirit or acetone ?
View user's profile View All Posts By User
Jurre10
Harmless
*




Posts: 17
Registered: 9-1-2011
Location: The Netherlands
Member Is Offline

Mood: Experimenting

[*] posted on 9-1-2011 at 09:55


oke guys,

Something really strange happened!
I first applied 5v and there was a little 0.15A trough the cell.

I had two things i wanted to test :
- 12V instead of 5V
- Reverse Polairity

I wanted to start, by applying 12V for 5 sec, so i did but what happened?!
The stainless steel cathode begon bubbling verry hard! Also the anode wass bubbling, but the bubbles where much smaller.
I looked to my multimeter en gues what! : 7,53A!!!

Then i stopped the cell picked up some 5V wires... just 0.13A.
So is it the anode?I don't think so!


Jurian

b.t.w i did test the power supply, it gives the right voltage,but not amperage?!

[Edited on 9-1-2011 by Jurre10]




If you try your best, but you don't succeed.
Experience is what you need.
View user's profile View All Posts By User
woelen
Administrator
********




Posts: 6085
Registered: 20-8-2005
Location: Netherlands
Member Is Offline

Mood: interested

[*] posted on 9-1-2011 at 10:16


It is the anode, but its overpotential has increased. Your power supply is perfectly fine, also its 5 V part.

If you look at the operation of an electrolysis cell, then you can understand what happens. The redox reactions occurring in a cell needs some voltage (around 2 volts total for both the anode and cathode reactions). The product of current and this redox voltage is the amount of energy, which is converted to chemical energy (new more energetic compounds).

So, in theory, there would be a reaction if the applied voltage is just above the redox potential. In a real cell, however, there is more than that. Each of the electrodes needs takes some more voltage. This voltage is the so-called overpotential needed for the reaction to occur. It depends on the material of the electrodes, it depends on how the bubbles of gas are formed and on the concentration of the electrolyte. Finally, there is someohmic resistance of the cell. This mainly depends on the size of the ions in solution and on the geometry of the cell.

Now let's have the following simple example:

2 V for redox potential
1 V for overpotential at anode
1 V for overpotential at cathode
ohmic resistance is 0.5 ohms

When 5 volts is applied to such a cell, then 5 - 2 - 1 - 1 = 1 volt remains for the ohmic resistance. With 0.5 ohms, a current of 2 A will flow through the cell.

Now assume that due to some layer on the anode, the anode overpotential rises to 1.9 volts. In that case, only 5 - 2 - 1.9 - 1 = 0.1 volts remain. With a 0.5 ohm resistance only 200 mA can run through the cell. If now you apply 12 volts, then 12 - 2 - 1.9 - 1 = 7.1 volts remain. With that current you have 14 A through the cell.

So, what I assume here is that indeed the overpotential on the anode has increased.




The art of wondering makes life worth living...
Want to wonder? Look at http://www.oelen.net/science
View user's profile Visit user's homepage View All Posts By User
Jurre10
Harmless
*




Posts: 17
Registered: 9-1-2011
Location: The Netherlands
Member Is Offline

Mood: Experimenting

[*] posted on 9-1-2011 at 10:48


Hi,

So we are back to where we begun there's a layer on the anode.
But i dont know what kind of layer and don't know how to remove it

I gonna try to reverse polarity for short times and else..:


Quote:

Dipping the anode in acid or basic solutions (H2SO4/HNO3/NH4OH) for short periods.



Jurian



[Edited on 9-1-2011 by Jurre10]




If you try your best, but you don't succeed.
Experience is what you need.
View user's profile View All Posts By User
metalresearcher
National Hazard
****




Posts: 417
Registered: 7-9-2010
Member Is Offline

Mood: Reactive

[*] posted on 9-1-2011 at 11:03


This layer is MMO, deliberately deposited on the anode before you purchased it I think.
I think @woelen is right. Applying more voltage will help. Moreover to get more chlorate ions ClO3- a higher bath temperature is better. So the 'waste' energy as heat is not actually wasted, but increases ClO3- formation.

And running the first hour on 5 volts changes the composition of the electrolyte (e.g. forming ClO3-) so continuing would require a highte voltage.

These are just my thoughts, I do not (yet) have esxperience with MMO anodes only with carbon rods in achlorate cell.

And did you add some K2Cr2O7 to improve efficiency ? That might also have some (adverse?) influence on the current through the cell.
View user's profile View All Posts By User
Jurre10
Harmless
*




Posts: 17
Registered: 9-1-2011
Location: The Netherlands
Member Is Offline

Mood: Experimenting

[*] posted on 9-1-2011 at 11:18


Hi,

So i can run my cell on 12V? or not?
I do have some Potassium Dichromate, that Woelen give me.

But i didn't use it yet.

Jurian




If you try your best, but you don't succeed.
Experience is what you need.
View user's profile View All Posts By User
woelen
Administrator
********




Posts: 6085
Registered: 20-8-2005
Location: Netherlands
Member Is Offline

Mood: interested

[*] posted on 9-1-2011 at 11:33


No, don't run it at 12 V. You will have really excessive production of heat and strong wear of the anode. First try reversing polarity for a brief amount of time and then use the cell in normal operation at 5 V. That might reverse the effect of formation of a thin layer. If that does not give satisfactory results, then you can use a higher voltage, say 6 to 7 volts. This can be achieved by using a series resistor between the 12 V power supply and the cell. A good value to start with might be 2 Ohm's (use a parallel connection of five 10 Ohm resistors, each resistor being a 10 W model). With that value you'll have 3 A through your cell.

Applying 12 V directly to the anode simply puts too much wear on the anode and your cell will become very very hot.




The art of wondering makes life worth living...
Want to wonder? Look at http://www.oelen.net/science
View user's profile Visit user's homepage View All Posts By User
Jurre10
Harmless
*




Posts: 17
Registered: 9-1-2011
Location: The Netherlands
Member Is Offline

Mood: Experimenting

[*] posted on 9-1-2011 at 11:41


Hi,

Ok, then i gonna try to reverse polarity this evening.
Hopefully, the layer will remove ...

Jurian




If you try your best, but you don't succeed.
Experience is what you need.
View user's profile View All Posts By User
Jurre10
Harmless
*




Posts: 17
Registered: 9-1-2011
Location: The Netherlands
Member Is Offline

Mood: Experimenting

[*] posted on 9-1-2011 at 12:40


hi,

Started with normal polarity : 0.15A
Reversed polarity : 4.54A

again : normal :0.20A?!
Reversed : 4.73A

Again normal 0.30A
Reversed:5A

Every time of reversed was 10 seconds.
So it looks like after each time its going better!

Stopped after the 3 times.
Can i go further with this or wil i damage it?

Jurian




If you try your best, but you don't succeed.
Experience is what you need.
View user's profile View All Posts By User
 Pages:  1    3

  Go To Top