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Grounded or not Grounded

Hi,

We build submarine equipments for long deployment in sea water. We use stainless 316 for some sections expose to sea water. To protect those SS316 sections, we use Zinc anodes. Even so, we end up after a while with corrosion that cause leak to our equipment. Actually our DC ground (Battery negative) is connected to the frame, who's connected to expose to sea water sections. We have no current running on the frame, we only use a single connection point. I know it's important on boat to keep DC ground isolated from the structure, mostly to prevent current to run in the structure. I also check many sea instruments, most are isolated.

So my questions,

Is this an acceptable practice (DC ground to frame) or should we keep the DC ground isolated from the frame? I also wish to know why DC ground to frame can accelerate corrosion if no current run on the frame?

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Jacques St-Pierre
Reply to
Jacques St-Pierre
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re,

DC

Jacques, I'm not sure how to ground a submarine, But sacrificial anodes are used on many (all?) metal hulled water craft. I don't think the ground current has anything to do with it.

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George H.

Reply to
George Herold

s

Good. Not perfect, but good.

DC

In case of leakage, the ground-to-frame MIGHT induce corrosion; if possible, use a set of antiparallel diodes (it's called a 'ground isolator') to keep DC frame current low at modest voltages.

There are always thermal 'noise' currents, and perhaps RF currents, or currents induced by magnetic-field coupling. Your zinc electrodes are sources of current, hopefully in a useful direction, too.

One can 'passivate' stainless steel (in an oxidizing acid, I believe) to help form the corrosion-resistant oxide skin. You can also consider other alloys (titanium is good-to-excellent in seawater tests).

Reply to
whit3rd

You may want to consider using 316L which has better corrosion resistance than 316.

Is any of the SS covered with tightly fitting parts so that seawater cannot circulate over it? This can cause the oxygen content in the covered areas to fall which in turn degrades the corrosion resistance.

John

Reply to
John Walliker

s

re,

DC

One connection from the electronics to the mechanical stuff is ok. More than one can cause trouble by making a voltage difference. Even an AC voltage difference can be trouble. I usually make no connection so that I can check with an Ohmmeter.

Make sure that your screws (if you use any) are of the same type of material as the body.

Grease up any two parts that come together in the sea water.

If you have plastic parts in the system remember that carbon reinforced materials are extremely nasty to the metal near them.

Reply to
MooseFET

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