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electrolytic derusting

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duncan webster07/05/2022 22:49:43
3990 forum posts
65 photos

I've just tried this for the first time with excellent results. No surprise there, it has been widely reported. What did surprise me was the effect on the anode, which was a bit of scabby old plate, some covered in paint. All the paint very quickly fell off. Anyone know what's going on? The only thing I can postulate is that the electrolysis turns the washing soda into caustic soda, but I never was all that good at chemistry. I once answered a question in an organic chemistry exam with 'under the right conditions of temperature and pressure, anything can happen'

AJW07/05/2022 23:39:41
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377 forum posts
137 photos

I use the process for my stationary engine parts and, like you, have been most impressed even used it for the insides of petrol tanks.

I'm no chemist either!

Alan

Graham Stoppani08/05/2022 06:28:32
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124 forum posts
26 photos

In this process the electrolyte doesn't change. The paint loss is either due to the metal underneath it being eroded away on the sacrificial anode or possibly by the oxygen bubbles being formed.

simple explanation Link

long winded explanation Link

Rod Ashton08/05/2022 07:20:41
340 forum posts
12 photos

Thanks you Graham and thanks to Andrew Westcott in the simple link above. Excellent explanation. Both useful and timely!!!

SillyOldDuffer08/05/2022 09:59:54
Moderator
8699 forum posts
1967 photos
Posted by Graham Stoppani on 08/05/2022 06:28:32:

In this process the electrolyte doesn't change. The paint loss is either due to the metal underneath it being eroded away on the sacrificial anode or possibly by the oxygen bubbles being formed.

...

Partly , but Washing Soda (Sodium Carbonate) is a junior paint-stripper in its own right. Washing Soda and Caustic Soda are both alkalis, which attack grease and oil.

As many paints are held together by a drying oil, they can be stripped by an alkali - it destroys the binding agent and the paint falls apart. Caustic Soda works faster because it's strongly alkaline, but Washing Soda would loosen paint even if the power wasn't turned on as well!

Might even be essential. I think what's happening is the paint surface is first penetrated by the Soda, allowing electrolytic effects to develop behind the paint and at edges, thus accelerating it's separation by removing the supporting metal and levering flakes off with microbubbles.

Probably best to start by removing paint first because it weakens the mix, slows down the main attack and is a bit messy, but I don't suppose it matters much!

Dave

duncan webster08/05/2022 10:44:30
3990 forum posts
65 photos

The bubbles form on the cathode, the paint was on the anode

KWIL08/05/2022 10:53:25
3554 forum posts
70 photos
Posted by duncan webster on 08/05/2022 10:44:30:

The bubbles form on the cathode, the paint was on the anode

Both electrodes will bubble, one with oxygen and the other with hydrogen.

Tim Stevens10/05/2022 20:50:01
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1598 forum posts

And, to follow Kwil's comments - the hydrogen will turn red rust to black magnetite, and the oxygen will, er, oxidise anything - such as aluminium to oxide (= anodising). It can be handy to remember this, as the container can be corroded if you get it wrong.

Cheers, Tim

Bill Davies 211/05/2022 10:29:38
284 forum posts
11 photos

I found Graham's long-winnded link to rust removal by electrolysis very interesting. It is a collection of multiple articles in one document, so it is a bit repetitive. Anyone skipping it should note to avoid using stainless steel anodes, as (for example) comments on using stainless steel on page 11. Stainless steel produce toxic chromates and must be avoided. Now, where's my rusty angle plate I made during my apprenticeship over 50 years ago?

Bill

Graham Stoppani11/05/2022 14:28:30
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124 forum posts
26 photos
Posted by SillyOldDuffer on 08/05/2022 09:59:54:
Posted by Graham Stoppani on 08/05/2022 06:28:32:

In this process the electrolyte doesn't change. The paint loss is either due to the metal underneath it being eroded away on the sacrificial anode or possibly by the oxygen bubbles being formed.

...

Partly , but Washing Soda (Sodium Carbonate) is a junior paint-stripper in its own right. Washing Soda and Caustic Soda are both alkalis, which attack grease and oil.

As many paints are held together by a drying oil, they can be stripped by an alkali - it destroys the binding agent and the paint falls apart. Caustic Soda works faster because it's strongly alkaline, but Washing Soda would loosen paint even if the power wasn't turned on as well!

Might even be essential. I think what's happening is the paint surface is first penetrated by the Soda, allowing electrolytic effects to develop behind the paint and at edges, thus accelerating it's separation by removing the supporting metal and levering flakes off with microbubbles.

Probably best to start by removing paint first because it weakens the mix, slows down the main attack and is a bit messy, but I don't suppose it matters much!

Dave

Did I mention that I used to be Managing Director of Turco Products Ltd? Manufacturer and supplier of paint strippers to the aerospace industry? wink Not disagreeing with what you say, just keeping it simple for the OP.

Mark Rand11/05/2022 17:11:14
1275 forum posts
28 photos
Posted by Bill Davies 2 on 11/05/2022 10:29:38:

Anyone skipping it should note to avoid using stainless steel anodes, as (for example) comments on using stainless steel on page 11. Stainless steel produce toxic chromates and must be avoided.

 

I've only ever seen green (chromium III, Chromites ets.) colouration when using a stainless anode, not red/orange (chromium VI, the dangerous one, including Chromates).

Along those lines:- If you use lead anodes with washing soda, you will get lead carbonate (white lead) deposited in the tank.

 

Edited By Mark Rand on 11/05/2022 17:16:21

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