Plated 6061-T6 Corrosion Problem 1

[SebagoMan]

I have a electronic unit that mounted to communication & Broadcast towers all over the world. It is made of 6061-T6 AL which is then SILVER PLATED. This plating consists of .0001" -.0002" copper plate class 4 flashing under the silver plating (.0003" - .0005"). Obviously the copper to aluminum are quite far apart on the anodic index, but if the plating is not comprimised, ie cracked, scratch, etc., my experience has been that it will stand up well to salt spray tests & in the field. HOWEVER, I am evaulating a unit that has had the plating comprimised and corrosion has started underneath the plating. The unit was in S. Florida, so it is getting its share of warm salt air.

My question is, will this corrosion stop, or will it continue right through the case? The plating is acting as a "container" holding in moisture. The case is quite thick (.15 min) so I imagine it would take a long time to penetrate...

 

[swall]

These things always get worse, not better! Since corrosion has gotten underneath the plating, I would anticipate that filliform corrsion will set in. This is a "worm track" like corrosion attack the will work to propagate the undermining of the plating. It can be thought of as a meandering,horizontal form of pitting corrosion.

 

[EdStainless]

The case will start looking like vermiculite, all flakey and expanded. Eventually the case will fail.
I know that you can't monitor or inspect the cases. Could you put a corrosion sensor inside? One of the thin wire type. If you could read this remotely then at least you would know when a case had lost seal.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[SebagoMan]

Both of you are confirming some of my fears. Because the plating will not flake off like paint, it traps moisture and exacerbates the corrosion. Even worse the copper is the more cathodic material in this situation. Exactly the opposite of what I'd like to see, as the case (what I want to keep) will continue to corrode and my silver/copper plating will be "fine".

 

[mshimko]

I once saw another type of attack under plating; the "bond" layer completely corroded away, and when we opened the test piece, a fully intact ring (the shape of the plaing) of the outer plating - gold in this case - actually fell out!

BTW, IMHO, one should never assume something will remain perfect.