Cathodic Protection

[Bloodhound]

I have an Electroguard 208 cathodic protection and monitoring system which was functioning perfectly when in salt water but now that we're in fresh water the monitor indicates about 25% of the way into the Over-Protected region and the cell current is reading maybe 20 to 30 mA. This is a Westsail 32 with about a 1" thick Fiberglas hull so I'm not concerned about generating caustic and rotting out wood. However, I would appreciate any input to indicate whether there really is a problem or not, whether this is just a manifestation of the fresh water environment and, if there is a problem, what needs to be done to correct it. The sacrificial metal is zinc in good condition. Should it be aluminum in fresh water, or does it really matter that much?

[Vasco]

They say you should use aluminum anodes in fresh water but everyone I know here in Lake Ontario uses zinc anodes. I've done that for over twenty years and my shaft and prop aren't any the worse for it.

[delmarrey]

Are you in a marina?

There could be some stray voltage.

[Bloodhound]

Yes, we're in a very small (13 sailboats on one wharf plus numerous power boats mostly on air lifts) marina. I'm not surprised there is such a small current but the meaning of over-protected escapes me in this situation. I'm beginning to think that sometimes there can be too much "information" or should I say "non-information" that only serves to cause needless concern. I just want to be sure it's really "needless".

[David M]

Being overprotected means you are producing hydrogen...nothing to worry about since it is on the outside of your hull. Aluminum hulls and wood hulls can be damaged by a severe caustic attack if overprotected. With the lower conductivity of fresh water, in your case, all you probably need to do is reduce the amount of sacrificial anodes.

[Bloodhound]

A little further reading indicates that a true sacrificial anode system can never cause overprotection - you need an impressed voltage for that to happen, i.e., you have to apply a potential. So if the Electroguard 208 is truly sacrificial, what the heck does an overprotection region on the meter mean? Another claim is that in fresh water, zinc will form a zinc hydroxide layer effectively isolating the zinc from the water and rendering the system useless. Apparently that's why aluminum is the anode of choice (and can be used in both fresh and salt waters). Perhaps my first experiment should be to don mask and flippers and wire brush the zincs to see what happens? And is the Electroguard 208 sacrificial or impressed voltage? I was under the impression it is sacrificial. The other issue is the use of a silver/silver chloride reference elctrode in fresh water - appropriate for salt water but not for fresh (I seem to remember the correct electrode is based on sulfate, but not for sure). So if the reference elctrode is wrong the voltage reading will be wrong. Is this part of the problem. This is something I really want to understand, and electrochemistry was never my best subject.

[GordMay]

The Electroguard 208 is an Impressed Current system.

See the Troubleshooting guide in the Instructions:
Electro-Guard, corrosion and cathodic protection specialists for boats, yachts and small ships.

[Joli]

Actually in fresh water you should use magnesium.

Boatzincs dot com

Originally Posted by Vasco

They say you should use aluminum anodes in fresh water but everyone I know here in Lake Ontario uses zinc anodes. I've done that for over twenty years and my shaft and prop aren't any the worse for it.

[Bloodhound]

Thanks everyone for your input and suggestions. I read the manual thouroughly some time ago, and again just now but my symptoms don't fit the troubleshooting cases. I have an actual current reading of 20 mA (not zero) and although over protection is indicated, it's not extreme (i.e., it's far from being off the scale). I will disconnect from shore power and see if this has any effect because the troubleshooting guide suggests poor connectivity between shore ground and bonded components as a possible problem. andve dropped a line to the manufacturer and hopefully they will have a solution. I will also clean the zincs. I've dropped an email to the manufacturer (and will followup by phone next week) --- hopefully they might have a solution. Being in fresh water, I'm certainly not concerned about caustic (no sodium source) but hydrogen generation can cause embrittlement of stainless.

[Bloodhound]

According to the manufacturer, the Electroguard system is not an impressed current device. It is a controlled sacrificial anode system that works in salt and brackish water only. It does not work in fresh water and it provides no protection in that environment. The manufacturer stated that no protection for stainless steels and bronzes is necessary in fresh water and that zinc paired sacrificially with stainless or bronze is useless. The claim is that aluminum and magnesium are, too, and that stainless and bronze don't need protecting.

When dealing with fresh (low conductivity) water, it makes sense to me that whether protection is necessary or not depends on specifically what metal is being protected. A steel hull for example would need protection even in fresh water but zinc would be the wrong anode because too low a voltage would be generated when paired with iron. Magnesium would be the right choice.

The bottom line seems to be, (1) forget the meter readings --- they are meaningless in fresh water and (2) no protection of stainless and bronze is needed.