Is there galvanic corrosion in vacuum? 3

[Bronzeado]

Dear Colleagues,

I wonder if somebody could tell me if there is any galvanic corrosion between two metals (say Al and Cu)in vacuum.
As far as I know, there would have to be an electrolyte to form a complete electric circuit in order to permit the electrons flow.

Merry Christmmas!

Herivelto Bronzeado
Brazil

Best Regards,

Herivelto S. Bronzeado
Brasília, Brazil

http://www.linkedin.com/profile/view?id=46319837&trk=tab_pro

 

[NBrink]

If there is no electrolyte, no galvanic corrosion will occur. Not to say that electrolytes can't exist in vacuum mind you, but two dissimilar metals with nothing but vacuum between them won't corrode.

Nathan Brink

 

[Dhurjati Sen]

Bronzeado
Are the two dissimilar metals in contact with each other, even under vacuum? Can't prevent Galvanic Corrosion, then.

 

[NBrink]

Dhurjati Sen,
An electrolyte is required for galvanic corrosion to allow for the ions produced as a result of the corrosion reaction to move. This is why car batteries don't work when they run low on water. Simply having two dissimilar metals in contact with each other is not enough.

Nathan Brink

 

[Bronzeado]

Dear Nathan Brink,

Let's think what Dhurjati Sen said that:

"... two dissimilar metals in contact with each other, even under vacuum, can't prevent Galvanic Corrosion ...."

My feeling is also that the Galvanic Corrosion will take place between the two diferent metal in vacuum (without any electrolyte) as an electric circuit may be formed inside the materials, alowing the transference of electrons between the metal.

This phenomenon seems to be similar to the Galvanic Corrosion that occurs inside the same material under diferent level of pressure.

Thank you for your thoughts!




Best Regards,

Herivelto S. Bronzeado
Brasília, Brazil

http://www.linkedin.com/profile/view?id=46319837&trk=tab_pro

 

[NBrink]

There are countless applications where dissimilar metals are in electrical contact with no ill effects due to the lack of an electrolyte.

If an electrolyte is not necessary, but simply that two dissimilar metals are in contact, how do you suppose chrome plating protects steel?
Or a steel stud into an aluminum engine block? I've got cladding terminations in vacuum distillation towers with no such galvanic corrosion.

https://www.nace.org/Corrosion-Central/Corrosion-101/Galvanic-Corrosion/

NACE's blurb on galvanic corrosion says "in a corrosive electrolyte". I'm sure if when I get back to the office, if I open the ASM handbooks, they'll say the same thing...

Nathan Brink

 

[Bronzeado]

Dear Nathan, thank you for your reply.

My doubt is if two dissimilar metals could criate an electric path inside them taht can cause eletrons to flow and than cause the galvanic corrosion even without any electrolyte arround.

May be this electrons flow will stop if the metals reach somehow an ion equilibrium, which may stop the corrosion in practical terms. Is it correct?

Sorry to insist in this matter. I am not a material engineer and I would like to get some experts thought on that. Thank you!

Best Regards,

Herivelto S. Bronzeado
Brasília, Brazil

http://www.linkedin.com/profile/view?id=46319837&trk=tab_pro

 

[Bronzeado]

Dear Nathan,

"If an electrolyte is not necessary, but simply that two dissimilar metals are in contact, how do you suppose chrome plating protects steel?
Or a steel stud into an aluminum engine block? I've got cladding terminations in vacuum distillation towers with no such galvanic corrosion."

I really do not know about that.
However, perhaps the corrosion is so small that it will be ZERO in practical terms. My question is something "theorical".

Best Regards,

Herivelto S. Bronzeado
Brasília, Brazil

http://www.linkedin.com/profile/view?id=46319837&trk=tab_pro

 

[Dhurjati Sen]

NBrink & Bronzeado

Dear guys, thanks for such a lively discussion.

As NBrink has said, there will not be any Galvanic Corrosion even if the dissimilar metals are in contact (electrically connected).....I agree whole-heartedly, if and only if there is PERFECT VACUUM.

Now, you get perfect vacuum only in text books. In all other vacuum scenarios, there would be some air that would serve as the electrolyte for ionic movements while the electrons move through the metals. Galvanic Corrosion would take place for sure specially if there is an area effect as well as the dissimilar metals are further apart in the Galvanic series!

 

[Compositepro]

Air and other gases are not electrolytes. Oxygen from the air can dissolve in water (an electrolyte) and cause corrosion. Galvanic corrosion requires electron flow (current). Current flows only in loops. Dissimilar metals form a battery with a specific voltage. Place them in an electrolyte, but not touching, and no current will flow. Let them touch and electric current will flow from one metal to the other. But for this to happen there has to be ion flow through the electrolyte to close the loop. Simple contact is not enough because electron flow at contact points between metals can only be in one direction, from negative to positive. It is the formation of metal ions at the interface between metal and electrolyte that causes corrosion.

 

[EdStainless]

There are a few reported cases where metals joined together in inert conditions did have some electron/ion exchange.
It is not galvanic corrosion though as no metal is actually lost. It is caused by the interaction of the oxide layers (semiconductors) on the surface of the metals. This process can result in metals 'sticking' to each other after prolonged vacuum exposure. NASA has put a lot of work into this, along with what grease will survive vacuum exposure.

In theory it might be possible to form solid electrolytes on the surface of metals and drive some galvanic interaction in a perfectly dry environment. But I suspect that those surfaces would not be stable under normal conditions.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[Bronzeado]

Thank you all for this discussion.

So, we should conclude from this discusson that the galvanic corrosion occurs ONLY if an electrolity is present.


Best Regards,

Herivelto S. Bronzeado
Brasília, Brazil

https://www.linkedin.com/in/heriveltobronzeado/

 

[moltenmetal]

Bronzeado- thank you for listening. The misunderstanding of galvanic corrosion is very broad in industry and the question comes up probably once a month here on Eng-Tips.

Lots of things can either reduce galvanic corrosion to an acceptable level or even stop it entirely. The complete elimination of electrolyte is one of those things.