Galvanic Corrosion Between Stainless Steel and Aluminium 2

[NalaCap0ne]

Hi All,

I've run into a bit of an odd issue (for me anyways) on site the other day and was hoping to get some helpful input. For info, this site is in London, UK.

There is a 10mm THK stainless steel bracket (shown in green in the sketch, grade unknown) which is fixed to an aluminium profile that sits inside a brise-soleil panel. The fixing is not protected by the panel as is shown in the drawing, the connection is open to the elements.

As you can see in the photographs I have provided (one of the bracket, one of the aluminium profile), when the brise-soleil panels were removed, a later of aluminium oxide (I am assuming) has formed on the stainless steel, in the area where the two metals were in contact. What I think has happened based on the information I have gathered over the last few days is that the two dissimilar metals in contact and exposed to rainwater has created a galvanic reaction, causing the aluminium to corrode. The galvanic series chart which I am referencing shows stainless steel twice...once in the passive state and once in the active state. Again, what I am assumign has happened is that the stainless and aluminium oxide layers were damged during installation, and since no oxygen can get to the surfaces of these materials, corrosion began to occur. This coul dbe completely wrong...its just my thinking at the moment.

Can anyone provide me with any input on why this is occuring, when a stainless steel is passive or active, and what sort of long term effects can be expected?

Thanks in advance.

 

[SwinnyGG]

This looks to me like it's pretty clearly corrosion due to the galvanic couple. It's going to continue to happen indefinitely until:

1) someone installs this hardware with a better detail that isolates the two metal parts from each other

2) someone installs this hardware with a better detail that prevents all moisture exposure at the joint

3) the components corrode to the point where they fail

 

[TugboatEng]

Without doing any engineering here, aluminum alloys such as 7050 and 7075 have similar strength properties to stainless steel. A bracket fabricated from these materials "could" be a direct replacement and would minimize the galvanic potential.

 

[NalaCap0ne]

Thanks for the replies, I apologise I am just seeing them now.

The remedial strategy at the moment is to remove the panel, install some sort of material seperator (rubber pad or tape likely), then reinstall the panel. It would be nice to have replaced the stainless bracket with an aluminium replacement.......unfortunately they embedded the bracket in silicone when they installed it! So thats not coming out haha.

Can either of you clarify/explain the active vs passive stainless steel? I cant quite wrap my head around it....

 

[TugboatEng]

Stainless steel is passive when the surface is clean and a a protective layer of chromium oxide has formed it's surface. It's active when conditions prevent or dissolve the layer of chromium oxide and exposing the bare metal. In your case, with crevice formed between the aluminum and stainless overlap, both metals are likely active.

 

[EdStainless]

and these two materials have a significant difference in potential so it doesn't take much of an electrolyte for galvanic corrosion, water that is at all conductive will do it. In this case the Al will be active and that should keep the SS passive, and greatly accelerate the corrosion of the Al.
You know to isolate them the bolts have to be fully isolated. There can be no continuity between the Al and SS. This involves and insulating sleeve in the hole and insulating washers.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[NalaCap0ne]

I didn’t think about having to isolate the bolts as well, but that makes complete sense. Any contact between the dissimilar metals will complete the circuit wouldn’t it. Luckily this hasn’t been implemented yet, and is still in the design phase.

Thanks all for the help. The issue makes a lot more sense to me now.