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?