Galvanic series (but non boaty!)

[MapisM]

Afaik, stainless steel is nobler than Aluminum, so it's aluminum which suffers galvanic corrosion.
In seawater, at least. In fact, I seem to remember that the galvanic series order can change depending on the environment.
Now, I recently noticed that some stainless steel screws used outdoor (in a non-marine environment, but continuously exposed to sun and rain), in contact with alu plates, are getting badly rusty, whilst alu parts are fine.
Is it possible that the order in galvanic series gets inverted in a non-salty environment, or should I think that those screws aren't actually stainless steel as they were supposed to be?

 

[RobWales]

I think it all depends on the grade of stainless as not all stainless is strictly stainless........

 

[epervier]

if you are really into the properties of stainless steel, this document is for you:-

http://www.aalco.co.uk/technical/datasheets/Aalco_Datasheet_St_St_316.pdf

enjoy /forums/images/graemlins/grin.gif

 

[VicS]

The galvanic series we are normally considering applies to metals immersed in sea water. It would not apply to metal combinations exposed just to sun and rain. However I would not expect stainless steel screws to rust in that situation, in contact with aluminium or not.

HERE IS a pretty good galvanic series. Note some alloys have a different potential in stagnant sea water.

 

[MapisM]

[ QUOTE ]
However I would not expect stainless steel screws to rust in that situation, in contact with aluminium or not.

[/ QUOTE ]Good point, but the alu plates are there to cover an aluminum fan, driven by an electric motor. And all these components are in contact each other.
That's why I suspect galvanic corrosion to have something to see with the rusty external screws. Or am I missing something?

 

[RobWales]

Yes you are!
As I said previous not all stainless is "STAIN-LESS"
its all down to the grade as others have indicated. /forums/images/graemlins/grin.gif

 

[MapisM]

Well, I did consider that, if you look at the last sentence in my OP, actually.
But I also know what the spots of surface rust in s/s parts typically look like.
And those screws are much worse! Hence I suspect that there might be more than the s/s grade behind it.
Shame that I couldn't take any pic, unfortunately those parts are on the roof of my house, and going up there isn't trivial...