Magnesium sacrificial anodes

I was reading ABYC E-02 about cathodic protection and saw the following note:

2.7.4 Magnesium anodes shall not be directly

coupled to the surface to be protected without dielectric

shielding.

Does anybody have more explanation, details or info about this?
All anodes are magnesium on my boat so the question.

L

I find this company helpful with my Anode questions.
BoatZincs.com (978-841-9978) - FAQs

I'd be interested in the explanation on that if you get one. I used magnesium on our boat currently (one transom plate on mounting studs and one "baseball" anode on each prop shaft). Fresh water, Missouri River. I was surprised how much loss there was after a single season, but I know the loss rate is much faster with magnesium.

https://www.corrosionpedia.com/definition/391/dielectric-shield

I wonder if it is an issue with dissimilar metals?

Standing by for explanation???
All anodes are dissimilar and don't see how they work without being connected electrically to the material to be protected... AFAIK? ???

We are in fresh water and use all alum anodes.

A guess. The passive current in the "system" is driven by the galvanic potential of the metals. Too high of a potential would create a proportionally higher current. The dielectric shield would force the current distribution over a greater area, protecting more of the parent material and making the anode last longer.

I'm making an educated guess based on a few maintenance projects I did with impressed cathodic protection of waterfront facilities at a mill.

Occasionally, the diver would find issues all around the pilings or fixed barge float where the shields fell off, but the metal under the anode was perfect.

Keep in mind with impressed current, you ran electricity through the anodes on purpose, so you could consume them really quickly if they weren't shielded.

Bacchus said:

Standing by for explanation???
All anodes are dissimilar and don't see how they work without being connected electrically to the material to be protected... AFAIK? ???

We are in fresh water and use all alum anodes.

Click to expand...

I had the same thought. In a previous life I worked for my father's commercial diving company. We did a lot of work on traveling water screens at power plants -- essentially giant conveyor belts with heavy wire mesh across steel frames, set across power plant cooling water intake canals. In high corrosion environments we typically welded aluminum anodes to the frames of the steel traveling water screen baskets (the aluminum anode was cast around a mild steel rod that stuck out a few inches on each end for the welded attachment). No dielectric barrier there...

The ones I recall with were dipped in plastic (like plastidip). You cut away the plastic to expose as much of the anode that you wanted to. The plastic remained on the side you were attaching it to. You could cut away more depending on the current reading on the reference anode. Very primative. I'm sure they are much more sophisticated now.

I'm thinking of those anode fish now that some smaller boaters use, those zinc or aluminum things cast to look like fish that you just toss and hang overboard. The other end of the cable usually has a clip on it like a battery jumper cable. What are you supposed to do with that clip? Well here:

https://www.fisheriessupply.com/martyr-grouper-anode-zinc-cmgrouper

Here's the product directions. Hmmm. (Well duh, yeah, don't clip it to a battery terminal, ha!):

Using the cable on a cleat, secure the Grouper's weight below the bottom of the hull before attaching the alligator clip to outboard or outdrive mounting bolt, or to rudder or strut ground strap, etc.
• Clip to grounding strap, where possible, or to a component wired to the boat's common DC ground
• Do NOT connect to battery

Spend two days to seek for information but did not find anything conclusive. Only thing I found that it was recommended to have the anode isolated from the support and connected to the piece to protect in one single point.
Wish to see one of our corrosion expert chime in!

L

Lou_tribal said:

I was reading ABYC E-02 about cathodic protection and saw the following note:

2.7.4 Magnesium anodes shall not be directly

coupled to the surface to be protected without dielectric

shielding.

Does anybody have more explanation, details or info about this?
All anodes are magnesium on my boat so the question.

L

Click to expand...

Suggest you re-read that. They are referring to aluminum and steel hulls only.

boatpoker said:

Suggest you re-read that. They are referring to aluminum and steel hulls only.

Click to expand...

I can believe you but this is not mention in E-02.

Here is the section where I saw this:

2.7.3 Only one current path shall be provided from

each anode to the cathodic bonding system.

NOTES:

1. Sacrificial anodes may be mounted directly on

the metal to be protected.

2. Zinc galvanic anodes may be applied directly to

aluminum, steel and non-metallic hulls without shielding.

3. Better current distribution will be obtained with

a dielectric shield between the anodes and metals to be

protected providing the anode is electrically connected to

the metal being protected.

2.7.4 Magnesium anodes shall not be directly

coupled to the surface to be protected without dielectric

shielding.

L

Lou_tribal said:

I can believe you but this is not mention in E-02.

Here is the section where I saw this:

2.7.3 Only one current path shall be provided from

each anode to the cathodic bonding system.

NOTES:

1. Sacrificial anodes may be mounted directly on

the metal to be protected.

2. Zinc galvanic anodes may be applied directly to

aluminum, steel and non-metallic hulls without shielding.

3. Better current distribution will be obtained with

a dielectric shield between the anodes and metals to be

protected providing the anode is electrically connected to

the metal being protected.

2.7.4 Magnesium anodes shall not be directly

coupled to the surface to be protected without dielectric

shielding.

L

Click to expand...

Don't know where you are getting that from but here is a photo of the appropriate section of the 2013 standard which is the latest edition of E-2.

boatpoker said:

Don't know where you are getting that from but here is a photo of the appropriate section of the 2013 standard which is the latest edition of E-2.

Click to expand...

Oh mine is the 2001 version, need an update
Thank you!

L