Fundamental galvanic corrosion questions- zinc/316 stainless

[davidd31415]

Hi:

I believe the galvanic corrosion questions I have are rather fundamental.

First, I have heard that zinc and galvanic corrosion are used to prevent boat hulls from corroding. Is metal above the water line not protected from corrosion by the zinc since there is not a "complete circuit" from the metal to the zinc through the water? If so, this would explain to me why such a method would be useless in preventing corrosion on automobiles.

Second, I am working with a pivot made of zinc and a 316 stainless rod. When exposed to a salt fog (ASTM B 117), a white corrosion builds up on the zinc. Would there be any reason to suspect that part of this build-up is salt?

I have attempted to dissolve it in water to prove that it is indeed not salt and the majority, if not all, of it did not dissolve. My last question is what might this substance be?

I've tried to dissolve it in Sodium Hydroxide to determine if the substance is Zinc Hydroxide but only about 80% of the substance dissolved.

Thanks,

David

 

[strider6]

first question:

To protect a surface, hull, using a galvanic anodes (eg zinc anodes) there are two requirements to be satisfied:
-The anode need to be in electrical contact with the surface to be protected
-it's necessary that there is an electrical path, thorugh water or soil, between the anodes and the surface

So a catohidc protection can't work in air and a galvanic anodes has a range of efficiency, max distance at which the anodes protect the surface and is a function of the resistivity of the medium.
the second question, i think it's a corrosion product of the zinc due to galvanic corrosion between zinc anodic to stainless steel.

http://www.corrosionist.com/Corrosion_Type_Galvanic.htm

S.

http://www.corrosionist.com

 

[EdStainless]

The contact between the Zn and SS in your system will accelerate the corrosion of the Zn. The Zn will corrode some in salt fog, adding a more noble metal only makes it worse.

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Plymouth Tube

 

[CoryPad]

Yes, you should expect salt to be part of the deposits on the parts. The white deposits are corrosion products - zinc oxide(s) and zinc hydroxide(s).

Regards,

Cory

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