924Board.org

[gwsg]

Summer's over so I finally bit the bullet and decided to tackle the leak in my battery box. What a bloody mess! Who in their right mind would design a car that grounds to the body and has its main electrical junction directly under a bucket of acid?

The leak had been 'repaired' by the PO by pop riveting a thin aluminium tray into the bottom, sealed with bituminous body deadener and repainted. But after a heavy rainstorm last spring which resulted in a wet floor and blown fuel pump relay I saw from under the dash that the original bottom was still rusting away and spreading like cancer towards the main ground cluster. He had stopped the leak but not the rust.

When I ripped out the aluminium tray I discovered a rust hole 1-2" wide that ran from the front of the battery bracket right across the bottom to the front outside corner. A bit of poking and the corner itself fell to pieces and I could see that the rust was progressing along the underside edge of the tray into no mans land between the inner mud guard and the fire wall right above the fuse/relay box. Suspicious of where this rust was going I looked under the mudguard above the wheel and noticed a bump in the body deadener which I poked and my finger went right through. This eventually turned into a hole 2" x 3" through the mudguard which actually gave handy access for treating the underside of the battery tray.

I cut all the rust back to solid metal with a 4" angle grinder and dremel but it was pretty hairy with all those sparks flying into the wiring and interior until I stuffed the cavity with damp rags. I"ve treated the metal with rust converter, cold galvanising, and primer. I'm also using thin aluminium [0.5mm] for the patching as its easy to form and gluing it in place with panel adhesive/sealer. I plan to finish with a false bottom of 1.3mm aluminium on to which I will fix ribs or points to keep the battery off the bottom so that it will be able to dry out after rain.

I really hope and think I've killed all the rust because if I haven't its certainly capable of eating a 924. An ugly, ugly disease.
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My 80 924 loves me.

[81turbo]

Well not to be technical but Porsche designed the car as a left hand drive so you kind of brought the bucket of acid above the electricals upon yourselves.

[wdb]

now dont quote me on how this works ,but an acidic or alkaline liquid incontact with metal makes an electrical charge ,thats excellerates the corrosion,just like the aluminum foil disintergrates with barbeque chicken on it .the more concentrated the acid/alkaline ,the higher the charge and faster corrosion . boats use a magnesium anode attached to the hull, so that the magnesium anode corrodes , and saves the important parts of the alumunim outdrive from corroding . hot water heaters also have a magnesium anode in them , for the same reasom ,something about a high electron enviroment ? these magnesium anodes are available and inexpensive,from boating and plumbing supply stores . the kind for boats ,come in a variety of sizes . maybe a handful of small ones under the battery would help stop the corrosion , or beat them into a sheet to lay in the tray . its an idea . after further reading it seems the anode needs to be WIRED to the body to protect it .

ANODE: A positive electrode in an electrolytic system, such as applied in cathodic protection; the electrode at which oxidation or corrosion occurs.

http://www.wastewaterdepot.com/magnesium_anode.html

http://www.radcapproducts.com/prodinfo.html

http://www.radcapproducts.com/testdata.html

http://www.stanford.edu/~erlee/seament/sm_mgac.htm

[gwsg]

The cold galvanising, which is zinc powder suspended in epoxy, is supposed to provide protection through a sacrificial reaction if exposed to water. However I think the vicious action of the acid negates this as the metal was originally galvanised anyway.

I will go to a marine supplies store and ask them about magnesium anodes though and see what they say. Also I remember hearing something about a neutralising tray that you can place under your battery. Any body know anything about these?
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My 80 924 loves me.

[Paul]

Performance Products sells them, I use them in all of my cars.