BP dissimilar metals - corrosion?

[simbrooks]

Its been a while since i did anything chemistry based at school, but i was swapping over my BP's last night and noticed what appeared to be corrosion on the AL BP where it had been in contact with the SS washers of the STA. Am i correct in thinking that as i used the AL BP in salt water that the two dissimiliar metals formed a battery of sorts and that is why there is a little aluminium oxide directly under the washers? It looks like the kind of corrosion you get on flashlights if you dont open them up and let the battery discharge gas release, white a bit like talc or something. Just wondering if others have found this and what to do about it, if anything at all?

 

[Green_Manelishi]

Its been a while since i did anything chemistry based at school, but i was swapping over my BP's last night and noticed what appeared to be corrosion on the AL BP where it had been in contact with the SS washers of the STA. Am i correct in thinking that as i used the AL BP in salt water that the two dissimiliar metals formed a battery of sorts and that is why there is a little aluminium oxide directly under the washers? It looks like the kind of corrosion you get on flashlights if you dont open them up and let the battery discharge gas release, white a bit like talc or something. Just wondering if others have found this and what to do about it, if anything at all?

I am no metallurgist but I think there will be some corrosion if the Al and Steel remain
in contact. That's one reason I dissassemble the rig for each cleaning then reassemble for the dive. It keeps the contact at a minimum. I've been considering purchasing some aluminum screws and such.

 

[Mako Mark]

yep this sounds like galvanic action dude, generally the softer metal will act as the sacrificial metal, bonding with free hydrogen ions. This is why you put zinc blocks next to things like the propellor shaft on your boat.

I have had stainless and brass clips on an aluminium back pack for years and havent noticed this happening and I dive the salt/fresh water interface in the caves here. Perhaps the tightness of the screw/washer traps water.

have you considered using a poodle jacket??

.

 

[simbrooks]

I have always kept it assembled when washing, will think about that next time i hook up the AL BP with the STA and wing. Now that the waters haved cooled below 80F i am back to the 3mm, so out comes the SS BP for the "winter" to change up the carried weight Yes i can imagine saltwater getting trapped from the dive inbetween the washers and AL BP.

As for poodle jacket, i find all that padding gets between me, the water and my tank, just cant stand it

Are there other washers i can put in place of the SS ones? Plastic, another metal (zinc?) or the like. The thickness of this oxide layer is barely a deep as gentle scratch.

 

[Green_Manelishi]

Are there other washers i can put in place of the SS ones? Plastic, another metal (zinc?) or the like. The thickness of this oxide layer is barely a deep as gentle scratch.

No, no, no.

Use SS or Al. The oxide layer on Al does in fact protect it, unlike iron or steel
which will rust deeper and wider until it's rusted out.

Plastic does and will break; usually at the most inconvenient time.

 

[simbrooks]

Okey dokie - just unsightly, nothing wrong with it, no problem, just curious and dont recall my chemistry/material science all that well

 

[dlndavid]

You can lesson the galvanic corrosion by anodizing the AL or isolating the contact with teflon tape/washers or epoxy paint

 

[Lat. Adjustment]

I've had the same thing happen on my wreck reel, when I assembeled the BP and rebuilt the reel I sprayed the hardware with Boesheild (developed by Boeing for use in aircraft) available at most marine stores, so far so good.