Does chromium oxide reduce conductivity in stainless steel?

[frogintheglen]

Does chromium oxide reduce the effectiveness of a faraday cage constructed from stainless steel mesh? I am using mesh that is like insect screen for windows. It will be in moist environment. I appreciate your help!

 

[.Scott]

I'm not sure how the chromium oxide is involved in this. Is it a coating on the mesh? It would be best if the stainless steel mesh allowed easy conduction of electricity in all directions - including the diagonals. Also, where one mesh intersects another, that entire seam should be conductive with a good electrical connection to the mesh on each side all along the seam. And, of course, the mesh should completely surround the contained volume.

 

[rbelli1]

I'm not sure how the chromium oxide is involved in this. Is it a coating on the mesh?

It is in the passivization layer that forms naturally on stainless steel.

BoB

 

[Tom.G]

Just measured contact resistance with light contact to some SS tableware and to a SS knitting needle my wife allowed me to get near. There is obviously an extremely thin passivated surface layer. Any slight movement of the test probes on the surface varies the contact resistance between about two Ohms and Open Circuit.

Perhaps you could use a welded screen where all the intersections are welded (if such a thing is available) or a punched sheet.
Another possibility is Laser-machined or etched SS sheet if you need very fine holes. That stuff is found in reusable SS coffee filters so it does exist. Don't know how available it is though.

 

[rbelli1]

Does it have to be a mesh? Can you use solid sheets or foil?

BoB

 

[frogintheglen]

Hello, rbelli yes, it needs to be a mesh, unless I can find another material similar to a mesh.

Hello Tom G. "Any slight movement of the test probes on the surface varies the contact resistance between about two Ohms and Open Circuit." I apologize for I am not well educated in electronics and do not understand exactly what this means. A reading of two ohms would not be a lot of blocked electricity, correct? But when you say "open circuit" reading do you mean no connection?
If so, you are telling me that the oxidation layer on SS will possibly degrade the effectiveness of a mesh faraday cage, hence your suggestions of other materials with punched holes rather than overlapping wires.
I think I understand your answer, but please let me know if I misunderstood.

 

[rbelli1]

Yes the wires will have unreliable contact due to the oxide layer.

If you can weld or solder the edges that will help. If you can use a copper or copper alloy (brass, bronze) mesh it might be easier to solder.

BoB

 

[Tom.G]

when you say "open circuit" reading do you mean no connection?

Yes, you understand correctly, and I am sorry for the confusion!

One requirement for a good Faraday Cage is that ALL the pieces it is made of are connected together along all joints. At an edge where two walls come together, the two walls must be connected with solder or welding or very good clamping the whole length of the edge.

Depending on your exact requirements this may be good enough contact for a woven Stainless Steel mesh to work. Because of the surface passivation layer, clamping would not be very effective though. Soldering SS is possible but somewhat difficult, so is welding. Since it is like insect screen then soldering is the way to join them.

(i see @rbelli1 has the same idea)

Because of the good connections needed, first build a frame then use one piece of mesh to wrap around it. If you need a small cage maybe you can cut just one piece of mesh to cover all sides, top, and bottom. That way there are fewer edges you need to solder.

Cheers,
Tom

 

[frogintheglen]

Thank you rbelli1, thank you Tom. This is very helpful.

So the problem is not the connection where the mesh-wires cross over each over, the problem is the seams? I was worrying about intersections within the mesh losing connection due to oxidation, am I to understand this is not a major concern? As for the seams, would it be enough to fasten the edges together with a very thin (maybe 26 gauge) SS wire, by tightly stitching the edges together? (I have much experience in wire-crafting so this was going to be my method of choice.) If soldering is necessary I will do it, just don't have the materials on hand right now.

Once the edges are soldered or otherwise properly connected, how long could this cage remain effective, if left in a moist environment?

Would a brass or bronze mesh be significantly better as a faraday cage material, aside from easier soldering? Aluminum mesh is cheaper but I'd thrown out the idea of aluminum because I thought it would be the material most likely to become ineffective due to oxidation, in a short period of being in a moist environment.

I appreciate all the information.

 

[Tom.G]

So the problem is not the connection where the mesh-wires cross over each over, the problem is the seams?

That depends on the wire spacing, the highest frequency you want to block, and how much attenuation you need, if that is what you are after. Others here have more recent experience with the necessary details. Paging @berkeman

If soldering is necessary I will do it

Necessary.

Once the edges are soldered or otherwise properly connected, how long could this cage remain effective, if left in a moist environment?

Don't know, not my field. Depends on the particular grade of SS. You might try asking a Marine Engineer or metallurgist, or even a Google search..

Would a brass or bronze mesh be significantly better as a faraday cage material, aside from easier soldering?

Electrically, the difference would be little if any at all. Otherwise it depends on the mechanical requirements, flexibility, temperature, etc.

Aluminum mesh is cheaper but I'd thrown out the idea of aluminum because I thought it would be the material most likely to become ineffective due to oxidation

Worse than that, almost all Aluminium dissolves in a salt water atmosphere.

Cheers,
Tom