Will this design cause corrosion from a ground loop?

Question

^{simulate this circuit – Schematic created using CircuitLab}

The design is an undersea light.

• The connector provides the customer ground-line-neutral.
• The line and neutral are connected to an LED driver board.
• The green ground wire is connected to the ground plane of the driver board.
• The ground plane of the driver board is screwed to the first housing component with good contact
• A 3 inch ground wire is run to a second housing component which is isolated from the first by hard a nodize.
• Both housing components are wetted by seawater.

This was tested in fresh water for 3 months with no issues. But a prototype version with soft anodize started to corrode pretty quickly at high temperature.

1. Will the cause galvanic corrosion as-described?
2. Will this cause galvanic corrosion when (not if!) the anodize is damaged?
3. If the housing were not connected to the earth ground, what would be the purpose of the earth ground?

Answer 1

A double-insulated construction may be the most promising here. The outer housing can be aluminum, if that is a stipulation; but keeping it isolated from the ground connection is the only way to ensure its survival under conditions of uncertain DC bias. Perhaps if a sacrificial anode (Mg) or polarizer (or both!) can be arranged, but that seems expensive and inefficient (polarizer, inert anode, DC supply) or expensive to maintain (scheduled replacement).

By "double insulated", I mean that the electrical circuitry would be surrounded by insulation -- at least air gap, but given it's underwater, potting is likely recommended -- and the safety ground can still be used on the inner section, but depending on applicable standards/code, may not be required at this point. Thus the outer shell can be isolated, or absent entirely.

AFAIK, most marine equipment is thoroughly potted (to exclude moisture spaces that will inevitably fill with water -- all plastics are permeable to water, it's just a question of rate), and usually housed in plastic rather than metal. Avoiding metal, and ground loop, probably helps with shark attack too, if that's a concern. But it may be worth consulting with a marine design expert for this device, or inspecting comparable commercial offerings for clues on how they did it.

Answer 2

Connecting dissimilar metals in an electrolyte creates a direct current (DC) "battery" and the scientific explanation as to why galvanic corrosion occurs has to do with things like migrating electrons and electrical potential. As a practical matter, a boat owner need only be aware that dissimilar metals, especially dissimilar metals left unprotected below the waterline, can cause horrendous corrosion problems to the least noble or "anodic" metal. As a general rule, the more dissimilar the metals (see chart), the more current will be created and the more likely galvanic corrosion will occur.

Saltwater is a more effective electrolyte than freshwater, which means that galvanic corrosion takes place more quickly in saltwater. Galvanic corrosion can even occur when dissimilar metals are joined above the waterline, since spray and moist salt air will act as an electrolyte.

https://www.boatus.com/expert-advice/expert-advice-archive/2020/january/the-current-scoop-on-galvanic-corrosion

So it really depends on what other metals are incompatible with the aluminum around the 'device', if you have dissimilar metals on the chart then you could have galvanic corrosion.

https://www.monarchmetal.com/blog/galvanic-corrosion-common-questions-answered/

It also looks like the corrosion occurs over time so it is something that you could probably inspect periodically.