Merccathode questions

[carcraze]

I posted this over in the outdrive engine area but reall got no responses so I thought it might go better here.

Okay I have a new to me Mirage with twins 1988. It is equiped with 2 merccathode units on it and I have no experiance with them other than they are used to stop outdrive corrosion similiar, I assume, to zinks other than a zinc is just a sacraficial metal so the aluminum does not corrode.

I was wondering if someone could give me an explanation of the cathode system as again I assume it stops corrosion through some sort of electrical induced electrolysis or the attempt to stop it hence the term cathode?

The boat was intended to be on a lift but plans have changed and now will need to be in the water for the summer with possibly a pull every 4 weeks for a cleaning. It is fresh water only but the outdrives do have SS props and SS skeg guards and a SS tie bar so I assume that will play a role. Also it has Batt disconnects and I do not see the cathodes tied to the Batts all the time which I assume they would have to be in order to function with the switch off?

Thanks
Mat

 

[wire2]

Re: Merccathode questions

All metals exhibit a specific voltage that they want to be at, compared to other metals.
Any time there's dissimilar metals in an electrolyte it constitutes a battery. A basic dry cell uses carbon and zinc. The zinc is consumed in the process of providing electrons to do a task.

Similarly, stainless and aluminum in water also form a battery. If left alone, current flowing from the aluminum to the ss causes oxygen to form on the aluminum, combining with it, and we have corrosion. At the same time, hydrogen is formed on the surface of the ss, which protects it.

The process is called electrolysis, and can be manipulated for desired results. e.g. zinc is more negative than aluminum, and can be exposed near the outdrive to cause hydrogen to form on the now more positive aluminum. The zinc anode is deliberately sacrificial, can be replaced easily.

A Mercathode forces a voltage on the outdrive for improved but similar results.

FWIW, stainless is an alloy typically including steel, copper, nickel, cadmium, etc. The mix forms billions of microscopic batteries which negates the effects of oxygen in the air (or water), so no oxidization takes place.

 

[carcraze]

Re: Merccathode questions

Thanks for the help.

I have a pretty good background in electronics but needed a little more info. Your explanation is great, a few more questions though.

The zinc trim tabs on the drives are doing their job as they are corroding away, the drives on this thing are immaculate as they were re-done so I am trying to protect them the best I can and it is in fresh water so that shoudl help.

The cathode system since it uses a charge on the drive how does it attempt to negate it? Does it try to neutilize the negative and positive potential so no electrolysis takes place (or reduce it) or does it force electrolysis to take place on a sacrficial unit on the drive? I do not really see a specific probe in the water as I assume to comes through the housing somehwere through a mounting bolt or does it just charge the drive? Are they adjustable and testable as I would assume if they try and reduce potential from pos to neg it could be possible to over do it or tweak it for the best efficency?

Also if it uses a charge then I would assume it needs to be powered all the time in the slip and from what I see if I shut the batt switch off it will also be off so I would have to leave 1 batt on which I guess would mean I do not have to wire a seperate power lead to my auto bilge switch? Do you happen to know the current consumption of these units as to know how long the batt will last powering them? I do not have shore power on the boat for a tender at this point.

Thanks
Mat

 

[wire2]

Re: Merccathode questions

The principal is applied to several areas;

Steel pipelines in the ground often get magnesium anodes connected by an insulated copper wire for the same reason. If an electric service is available, they connect a DC power supply to the pipe and a large corrosion resistant anode buried nearby.

Drill rigs in the ocean use a DC supply, the anodes are a long copper core, coated with titanium, and plated with platinum, hanging from the rig vertically.

Yes, the Mercathode needs a battery connection to impress a positive voltage on the drive, but AFAIK, they use very little current, you should be ok for weeks. I don't have one, so I'm not sure what they use for the anode. Anyone?

Incidentally, there's even a similar unit available to prevent rust in cars.

 

[carcraze]

Re: Merccathode questions

Thanks for the info.

I may have to follow it back to determine where it exits the housing, it looks like all the retainment bolts are painted black though which if one is an anode I doubt it can work that way.

I tried some google searching on the unit as well without much luck i will do some more homework as well see if I can find the anode that should be there.