View Full Version : Mercathode system

Dr. David Fleming
09-29-2012, 09:34 PM
The Mercathode anti corrosion system used on stern drives. Is anyone familiar with the Magnesium anodes that are optional for use in fresh water? Are there any issues that differ from the Zinc anodes? Magnesium anodes available for the Mercury K-planes?

Doesn't seem like many folks know about the fresh water salt water system. Apparently the Zinc works in both types of water but the Magnesium is better in fresh water and is an option. $100 for the set at the Mercury dealer.

10-03-2012, 03:38 PM
The mercathode system is not the same thing as the anodes. Merathode is an electronic device, there are leads on the drive that it wires up to.

The sacrificial anodes that are also part of the corrosion package work in conjunction with mercathode...

In recent years Merc has moved to a metal blend that is mostly aluminum. It's an in-between metal that is supposed to be good for salt and brackish water. For fresh, magnesium is recommended.

read the pdf attached for a decent techie read.

Dr. David Fleming
10-04-2012, 10:19 PM

Thanks for the reply - seems the Mercathode is not highly discussed - local dealer said the only available sacrificial anodes for the K-Planes were Aluminum and were for brackish water. Had no recommendation for fresh water. Am going ahead with the Magnesium.

My boat is rack stored and sits overnight next to a steel sea wall. I almost have a hole eaten through the skeg from the corrosion. I paint it every year with the Merc aerosol drive paint - corrosion blows right through the paint. Machined edge around the drive cap is starting to peal the paint. All stainless lead wires are attached as is the Mercathode unit and the color code is correct. Do the stainless leads need to be cleaned yearly?

Lake St. Clair water is very low and the boat is near the bottom muck at the marina. Cant figure why is should be corroding so much when it is stored dry most of the summer.

10-05-2012, 12:19 AM
Do you have a battery or dual battery switch, and if so, do you set it to 'off' when away from the boat?

10-05-2012, 12:51 AM
In order for the Mercathode electronic corrosion protecton system to work & perform properly, and the simple sacrificial zinc annodes to propery protect your outdrve & underwater running gear from electrolytic corrosion there must be perfect electrical bonding of all your outdrive's parts.
You should remove all bonding strap connections and brighten the connections up for a good electrical bond.
Use a good multi-meter on the ohm settings to check for electrical issues & possible bad bondng connections which will show up as high electrical resistance which indicates a very poor bonding connection & thus loss of proper corrosion protection..

Any zinc Aluminum, or Magnesium annodes should removed and their electrical bonding connections to the outdrive's metal castings brightened & cleaned-up also.

Morgan's Cloud
10-05-2012, 07:59 AM
It might be worthwhile having someone go over you Mercathode system to make sure it's functioning ok. Because they just sit there , silent , it can be hard to tell if they're actually working.
They also have an inline fuse , normally near the +battery terminal 'cause they should be 'live' all the time , that ought to be checked to make sure it's not burnt out .

10-05-2012, 10:12 AM
It could be hot water too (electric leaks). do your stainless parts get white crap on them overnight?

If you are having this kind of problem I think I would go the Bravo III route. Basically, you can add a second Mercathode unit and more Zincs. I mention Bravo III because they have the most issues so if you search on them you'll find a lot of solutions that should work just fine on Bravo 1 (or Alpha 1)

10-05-2012, 10:53 AM
A few years ago the hardware on my K-Planes and steering ram got eaten up pretty good when we tied up next to a house boat for 4-days. After this I noticed my boat had no bonding out to the K-Planes so I ran two nice big ground wires to both K-Planes. We just spent 3-days tied up on a house boat dock and I didn't get any corrosion on the K-Planes :biggrin.:. I guess I should also bond the steering ram now.

I'm not sure how the Mercathode system works. Is it a ground based system? Is simply bonding/grounding the K-Planes to the motor the extent of it?

Dr. David Fleming
10-12-2012, 11:35 AM

Well Magnesium anodes on the drive and aluminum anodes on the K-Planes, go figure!

You were right, I never thought to check the continuity on the Mercathode power terminal. When the battery switch is OFF the Mercathode is OFF. I have been turning it OFF at the battery switch as a matter of security. Six years it has only been working when the engine has been running!

Might have been nice for Donzi to mention this in their none to helpful owners manual or to have put one of those cute stickers they have all over the boat to tell me how the Mercathode system operates.

The K-Planes I have added they have a set of the stainless ground wires running from their anodes to the frames of the K-Plane. When I installed these K-Planes, I did not see any wiring instructions to ground the them to any electrical systems. The K-Plane pumps are hooked up electrically but not the K-Planes themselves.

Anyone have a factory installed K-Plane set that can verify if they are wired to ground to work with the Mercathode. I'm talking about the K-Planes not the pumps.

Now for some good epoxy over the hole in my skeg and a good Emron paint job!

10-12-2012, 12:07 PM
Very interesting.

10-12-2012, 12:24 PM

My factory K-Planes were not grounded and I've had corrosion problems. I'm on my third set of hyd. hoses, I assume because of this. Therefor, when I did the last hose change I ran grounding wires from the transom bolts to the engine block grounds. I hope this is enough and so far so good with no new corrosion.

This winter I may run a s.s. wire/strap from the grounded transom bolt to the exterior grounding wire between the tab and mtg plate just to be sure.

10-12-2012, 12:44 PM
Dr. David~

It has long been a practice to electrically bond to a good ground any, & all, metal objects that project outside & underwater on any boat hull.
These include~
Outdrives & any drive~ subassembly
Seacock valves
seawater intakes
all metal thru-hull fittings
exhaust outlets

Make sure to use #10-#8 wire, or copper and/or Stainless Steel straps; and link-up undrwater metal parts all to a very good ground on the engine & negative battery terminal.
This ground should not be switched by the large main battery switch.

The Mercathode electronic corrosion protection system ,if you indeed have one,
should also be grounded & bonded and also be kept powerd-up ALL the time.

Morgan's Cloud
10-12-2012, 01:04 PM
You were right, I never thought to check the continuity on the Mercathode power terminal. When the battery switch is OFF the Mercathode is OFF. I have been turning it OFF at the battery switch as a matter of security

That's what I was getting at with my post but I never thought Donzi would be so stupid as to wire the thing so it could actually be turned off.

Like an automatic bilge pump , it should be hard wired to the battery not through the main switch.

If you pull your boat up for any amount of time just make sure you either disconnect the battery or pull the fuse for the system as there's always a small power draw.

If I recall correctly the Merc owner's manual says that the Mercathode really on works properly in still water , so basically , for all those years you were getting zero protection .

10-12-2012, 07:59 PM
Just a thought, but for security purposes, I have left my batteries on but shut off the fuel valve at the tank. It was real easy to do and is hard to find. :shades:


10-12-2012, 09:28 PM
if you keep the boat on a trailer or rack of course not in the water.
you do will not need to worry about ELECTROLYSIS.
make sure your anodes are new and properly grounded.
a boat can not get electrolysis will riding.
this is what i was told.
you can make your test and system.