Backplate corrosion

[Taipeidiver]

I recently made the change from a regular BCD to a BP/W configuration. After the rig's inaugural dive trip, I was disassembling the STA from the plate when I noticed that there was some pinhole corrosion in the plate around the bolt holes.

I have a locally produced plate (manufacturer is called KAG), and a DiveRite STA.

The STA bolts, nuts and spring washers are steel, and the plate and STA washers are aluminium, so basic physics tells us that when the assembled unit is placed in seawater (an electrolyte) they will form a galvanic cell, with the aluminium electrode (the plate) being corroded sacrificially.

The science guff aside, I wanted to know whether any other BP/W users on the Board observed the same phenomena and if it was something that I ought to worry about.

If I should be concerned, is there anything I can do to slow down / stop the process. I can't think of anything other than switching to a stainless steel plate, or trying to get the aluminium one coated.

 

[RICHinNC]

I have seen heavy duty screws (not a good name for them) that are made of plastic. Its the kind that one is a female part and the other is a male part and obviously one screws into the other.

Using those and putting maybe a thin rubber barrier like a piece of rubber tire between the sta and the plate might help.

Just some quick thoughts early in the morning.
rich

 

[FredT]

The steel will drive the aluminum to dissolve. It's simple galvanic action that's inherent in the nature of the universe. Anything that breaks the dissimilar metals contact will work to slow it down.

You can have the plate heavily anodized and sealed, at a significant cost, to reduce the corrosion rate and probably end up simply relocating the corrosion to somewhere else on the plate.

You can add sacrificial magnesium washers to the stack, then change them regularly. At the very least you'll need to tighten the bolts after every other dive. This moves the corrosion from the plate to the washers, and electrically protects both the steel hardware and the plate.

If you made the plate out of 5086 aluminum a good heavy wax coating within a couple inches of the bolt hole, combined with a nylon washer will reduce the local corrosion.

Of course you can do what the rest of us with AL plates that dive saltwater do. Dive the plate, disassemble and rinse it regularly after EVERY dive day, and plan on replacing it every 20 years or so anyway. Or you can dive it, don't worry about rinsing it, and replace it in about 10 years. Most eventually migrate to option 2.

FT