Brass and aluminium plates, stainless steel screws = CORROSION?

GarethW

GarethW

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I have designed some equipment that must be exposed to the UK weather for a year or two (typically miserable, cold and damp 99.9% of the time, then ridiculously blasting hot of the other 0.1%). I have concerns about galvanic corrosion between the brass, aluminium and stainless steel screws.

I have sketched the design below. What are people's views about the longevity? I didn't realise the orange part would be brass when I had the alumuminium part made. Having dug around a bit I think it will be reasonably OK, but I'm tempted to have the aluminium parts re-made from brass if it shows signs of corrosion.

Thoughts?
corrosion.png
 
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with galvanic corrosion, you'll want to pick contacting materials that are close together on a galvanic chart like this one http://www.corrosionist.com/Corros1.gif you're likely to see problems in the Aluminum more because of the stainless screws than the brass.
 
MDR said:
with galvanic corrosion, you'll want to pick contacting materials that are close together on a galvanic chart like this one http://www.corrosionist.com/Corros1.gif

you're likely to see problems in the Aluminum more because of the stainless screws than the brass.
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Thanks for the quick reply! So it would seem that mild steel screws with zinc plating might be a better choice than stainless ones to avoid galvanic corrosion with the Al. However they'll just rust presumably?

What would be worse:
a) Mild steel+Zn screws rusting
b) Stainless screws causing galvanic corrosion with the Al.

or, any ideas on a better solution?

Do you think I'll be OK with the brass/aluminium at least? They don't seem too far away in the galvanic series....
 
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You could anodize the aluminum, that will help.
How much stress is the joint under?
You could use plastic or aluminum screws if the joint is not under much stress.
Or stainless with sealing washers under the heads to prevent moisture from getting into the threads and accelerating galvanic corrosion
 
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The stainless screw bolted in Al is not a good idea. Electrolytic reaction has been alrdy discussed, but material hardness is also a parameter you'd have to look especially in the screw thread.

If you can't use inserts in the Al plate, it would be better to drill a hole trough the entire material and use a nut or a weldnut made of stainless steal.

regards,
 
Yep, threaded inserts in the Aluminum is also recommended and can be selected to reduce galvanic action. You will still want to isolate the head of the screw from the Aluminum as well in harsh environments.
 
Thanks for the replies, guys. Here are some of my comments inline below in blue.

MDR said:
You could anodize the aluminum, that will help. Can't do that because it needs to be conductive
How much stress is the joint under? Not a lot
You could use plastic or aluminum screws if the joint is not under much stress. Might be a bit flimsy
Or stainless with sealing washers under the heads to prevent moisture from getting into the threads and accelerating galvanic corrosion That's a good idea. Do you get them as small as M3?

Yep, threaded inserts in the Aluminum is also recommended and can be selected to reduce galvanic action. From what material? You will still want to isolate the head of the screw from the Aluminum as well in harsh environments.
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LOL because of this thread I had a dream last night that the machine I am currently working on had to be placed outside in the wet and sun. I was freaking out!
 
mdr said:
lol because of this thread i had a dream last night that the machine i am currently working on had to be placed outside in the wet and sun. I was freaking out!
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:eek: welcome to my world!
 
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