zinc anodes

Freddy · Post by **Freddy** » Sat Mar 10, 2012 2:32 pm

I have a 1976 F30 single screw. There are some old anodes on the shaft and rudder, possibly elsewhere. I am wondering if anyone knows where I might find specs or general rule of thumb guidance regarding how to better protect.

jefflaw35 · Post by **jefflaw35** » Sat Mar 10, 2012 4:13 pm

there are a few treads I searched and found alot of info on this Freddy

MadBen · Post by **MadBen** » Sat Mar 10, 2012 5:42 pm

Generally that is all you need. If you have trim tabs or a heat exchanger, there may be some on those. Trim tabs would be similar to your rudder zincs (sacrificial anodes) and a pencil shaped one in your heat exchanger. They need to be replaced periodically.

foofer b · Post by **foofer b** » Sun Mar 11, 2012 8:08 am

When installing the zincs on the rudder. Propellor shaft and trim tabs, be sure to wire brush or sand the surface to make sure the zinc has good contact with the bare metal of your boat for the zinc to work properly,

Big D · Post by **Big D** » Sun Mar 11, 2012 10:55 am

Clean and rough up the surfaces as mentioned. Some shaft anodes come with special instructions (sharp light blow with hammer), these have a little protruding ball embedded on the side that makes contact with the shaft for better continuity. The more sacraficial protection you have, the better. If you don't have any on the trim tabs for example, you may consider adding a 3 inch anode on each tab. You may also want to consider increasing the size of the current rudder anodes. The shaft anodes should be placed within an inch forward of the strut. If you are experiencing extensive corrosion and with no apparent reason for such that you can see based on your vessel, it may be an issue with the marina or your neighbours' boats. In such a case, I would add a transom (Diver 12x6) anode if not in place already. For additional protection, there are galvanic isolators that protect while hooked up to shore power, and impressed current cathodic protection devices such as Merc Marine's Mercathode system that protects while away from the dock as well.

Buzzards Bay? I'm assuming salt water? If so use zinc anodes. If fresh water application, use magnesium anodes.

The Dog House · Post by **The Dog House** » Sun Mar 11, 2012 8:30 pm

Big D wrote: Buzzards Bay? I'm assuming salt water? If so use zinc anodes. If fresh water application, use magnesium anodes.

For the Delaware River (brackish water), we use aluminum anodes.

foofer b · Post by **foofer b** » Sun Mar 11, 2012 8:49 pm

Which of the three is more sacrificial? Corrodes most readily?

Big D · Post by **Big D** » Sun Mar 11, 2012 10:05 pm

There are many factors that affect the type and amount of protection required; water velocity, boat usage, fresh/salt/brachish water environment, condition of equipment protective coatings etc. Metals with higher negative potentials will corrode faster than metals having lower potentials. The list below will give an idea of where some common metals on boats lie in terms of thier suseptability to corrode compared to others. Magnesium being the highest negative potential in this case and therefore corrodes faster in an electrolyte (water) compared to bronze for example. Gold on the other hand has a positive potential thus can be submerged for hundreds of years and still be fine....so all we need do is get gold running gear

Magnesium................ -1.65V
Aluminum aloy............ -1.10V
Zinc......................... -1.05V
Galvanized iron........... -1.05V
Aluminum................... -0.75V
Mild steel................... -0.70V
Cast iron.................... -0.70V
Lead.......................... -0.55V
Manganese bronze........ -0.27V
Copper, brass, bronze.... -0.25V
Monel.......................... -0.20V
Stainless steel............... -0.20V
Nickel........................... -0.15V
Silver............................ -0.00V
Gold.............................. +0.15V

If we look at the list, and dropped a piece of aluminum and a piece of copper into water, the aluminum would corrode faster; in the presence of stray currents, it would sacrifice it's self if you will so the copper is protected against stray currents. The greater the difference in potential between two metals, the better protected the one with the lower negative potential will be.

If you have aluminum running gear such as a stern drive, having a metal close to it that is lower in the scale such as stainless steel would cause the drive to corrode; hence the corrosion issues with Merc Bravo drives or any others with stainless props. This would also hold true on an inboard with a hydraulic swim platform made out of aluminum and the running gear of stainless and bronze; the platform gear would corrode fast as it sacrifices metal in the presence of stainless unless a metal with a higher negative potential is introduced.

The Dog House · Post by **The Dog House** » Mon Mar 12, 2012 6:31 am

The anode websites list magnesium for fresh water, aluminum for brackish water, and zinc for salt water. This would imply that aluminum is between magnesium and zinc. In my personal experience, the aluminum anodes corrode faster than the zinc anodes. Is it possible the anode companies are using a different aluminum alloy?

The Dog House · Post by **The Dog House** » Mon Mar 12, 2012 7:50 am

Just found this reference which places aluminum between magnesium and zinc. Not sure why there's a difference.

Metal Potential, Volts
Calcium +2.20
Magnesium +1.87
Aluminum +1.30
Manganese +1.07
Zinc +0.758
Chromium +0.600
Iron +0.441
Cadmium +0.398
Nickel +0.220

Big D · Post by **Big D** » Mon Mar 12, 2012 7:59 pm

Something's not making sence to me. Did they mention what the refference was for those readings? That would imply that an aluminum stern drive with zinc anodes for example would sacrifice metal to the zinc anode (sacrifice the drive) if we go by that chart. I'll have to confirm those numbers. I agree with the metals you're naming for the water conditions, bare in mind that conditions change things depending on the water type.

Suffice it to say though in spite of differing numbers, that yes;
Zinc or aluminum for salt water
Aluminum for brackish water
Magnesium for fresh water

It should also be noted that for us woodies in fresh water, magnesium should never be use as it will damage wood.

jefflaw35 · Post by **jefflaw35** » Mon Mar 12, 2012 10:13 pm

D I love you to death but you hurt my brain Sometimes. I can't keep up, and that's not possible to me

jefflaw35 · Post by **jefflaw35** » Mon Mar 12, 2012 10:15 pm

Sorry I just had to play

Big D · Post by **Big D** » Mon Mar 12, 2012 10:30 pm

How about I play with your brain some more and throw a wrench into things by giving some reasons why I think you should use aluminum in salt water instead of zinc??

or would that be sacrificial sacrilege by stepping out of tradition

jefflaw35 · Post by **jefflaw35** » Mon Mar 12, 2012 10:35 pm

Ouch I was just teasing

But it wouldn't hurt, I can cut and make aluminum. Just might not look good under water lol. Please keep speaking at me lol. Zinc goes fast!!