Galvanic Corrosion Voltage Readings

RWS · Post by **RWS** » Mon May 04, 2009 9:37 pm

Iâ€™m concerned that I may have a galvanic corrosion voltage problem.

The boat is on the lift behind the house, connected to shore power for the fridges, air circulation fan and 3 bank battery charger. Otherwise she is in the water.

When on the lift during extreme high tide, the propellers touch the water, the shaft zincs and hull zinc do not.

I ran a 14 gauge wire from the bonding system to a volt/ohm meter. From the VOM I ran 14 gauge wire to a 2â€ x Â¾â€ worn zinc in the water (salt/brackish)

Here are the voltage readings I took both in the water and on the lift.

AC VOLTS

In water shore power breaker OFF .018 AC volts

In water shore power breaker ON .016 AC volts

Out of water shore power breaker ON .312 AC volts

Out of water shore power breaker OFF .327 - .355 AC volts

No noticeable differences with the 12v breaker panels ON

OHMS RESISTANCE m OHMS 20 scale

In water old shaft & hull zincs (bad shape)8.63 - 8.68 no difference when shore power is on or off.

In water new shaft & hull zincs 4.29 - 4.66 no difference when shore power is on or off.

Out of water 11.3 - 12.9 no difference when shore power is on or off.

Am I O.K. or is there a galvanic issue going on here?

THANKS FOR THE VALUABLE REPLIES!

RWS

prowlersfish · Post by **prowlersfish** » Tue May 05, 2009 6:59 am

why not hang one of these off the side lower then the props ?

http://www.boatzincs.com/grouper.html

or just a zinc on a wire

you may find this helpful

http://www.boatzincs.com/faqs.html

Stripermann2 · Post by **Stripermann2** » Tue May 05, 2009 7:52 am

The only reason I mention this link is that, Marinettes are aluminum and keeping galvanic corrosion to a minimum is critical. Reading over some of their info may help.

http://www.marinetteboat.com/forum/yaf_ ... osion.aspx

gjrylands · Post by **gjrylands** » Tue May 05, 2009 9:15 am

First I have to say I don't know much about galvanic corrosion in salt or brackish water since all of my boating experiences are in fresh water. From what I have read, you should be taking DC voltage readings not AC voltage readings. To protect a fiberglass boat with inboard engines you want to have readings of -750 to -1000 mV. This is a DC voltage reading since they want negative voltage readings.

When your boat is on the lift, since the props are still contacting the water, you should use the submerged zinc and bonded it to the boat to protect it. You should use a large enough zinc to get readings between the recommended range. You should make your check with the shore power connected since that is the way the boat is normally stored. There shouldnâ€™t be different readings whether the boat is connected to shore power or not. If you get much different readings you should look for the sources of the voltage leaks; i.e. frayed wire, wire connections in bilge water, bad bilge pumps, bad float switch, or any other source of voltage leaks.