Galvanic Corrosion

[Dhurjati Sen]

Dear experts,

Let us visualise the following:

A zinc block 20 mm thick is casted on a carbon steel plate of 5 mm thickness and left in the open with average humidity 50% to 90% round the year and rainfall moderate with a distinct rainy season spanning 3 months approx.

How would the corrosion pattern look like

a) Over a period of one year?

b) Over a period of five years?

Quite a puzzle, is it not?

With warm regards.



DHURJATI SEN

 

[TugboatEng]

For there to be protective effect from the zinc, both metals will need to be submerged in electrolyte. For you case, it's best to assume both metals will corrode independently.

 

[EdStainless]

If this is in a rural area with clean air and far from the sea then the iron will rust and the Zn will turn whitish-grey.
There will be a very small area where they contact that the iron will have very little rust and the Zn will show some attack.

If this is in an industrial area with polluted air and/or near the coast then the Zn will have a flakey white surface and iron will be very rusty. There will still be a narrow band of iron near the An that is less rusty, and the Zn may show distinct undercutting where it contacts the iron.

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P.E. Metallurgy, consulting work welcomed

 

[BenMacier]

Dear experts,
We have produced slender anodes and the project is on hold until further notice, so we have to store these anodes outdoor because nobody wants to pay for indoor storage.
The project still wants to keep these anodes and everybody is wondering how we can use them in a couple of years.

Include strong wire brushing on the steel lugs before welding them. A bit of wiping on the zinc surface will do.
Just ensure they don't get flooded during the storage period !

* Finding a solution is great * Knowing how to implement it is fantastic * Believing it is the only one and best is naive ?

 

[Dhurjati Sen]

Thank you very much Sir Ed. The corrosion in the zinc would be mostly armospheric, Same holds for the steel. Galvanic corrosion seems to be negligible?

DHURJATI SEN

 

[EdStainless]

Unless this is sitting flat and water can puddle on it the corrosion would nearly all be atmospheric.

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P.E. Metallurgy, consulting work welcomed