Anodes will they work for a 22 Ton Beast?

Thanks Switchman,

The boat rests in fresh water (Lake Ontario) from May to October. The water is very shallow at the marina and by mid summer we're sitting in a couple of feet of sediment. The same was true at a former marina we stayed at in Georgian Bay and the hull suffered no damage.

I will take your advice and have the spot looked at by a professional, I can say that at present the penetration is not critical, however if it were to be attacked again with equal agression there could be a problem. I will also check for current.

As I said in my first thread the remainder of the hull and the prop are fine, but I'm going to invest in some zinc. I'm going to grind prime paint and seal with Coal Tar epoxy and monitor the spot throughout the summer.

Frank

Frank, after reading your reply I asked a fellow about this and he pointed me to a paper he had previously run into. Since you are in fact in fresh water (which does usually act as a good electrolyte) you'll want to use magnesium, rather than zinc.

Read this paper on this link and take a good look at what he is saying on page 6 about the oxygen concentration cell. Was this area of corrosion an area that would be sitting in the sediment you mentioned? Possible that could be a factor...some sort of contamination or something.

http://www.roscoemoss.com/tech_manuals/fmcf/fmcf.pdf

Good luck with it.

This is just about all anyone needs to know on this subject in good simple English. You beat me to it!!

Galvanic cell is considered a mode of attack. For such reason, when damaged happened to the coal tar epoxy coating, such corrosion is started.

In addition, the combination of dissimilar metals lead to galvanic corrosion. The base metal itself (white metal) combined with rust as mill scale especially when such broken happened to mill scale (form other dissimilar metal) which act as a cathode, and base metal acts as anode, and water as electrolyte. Hence a galvanic cell is formed and such a current flows between cathode and anode through water which leads to corrosion of anode (base metal).

The best solution is to repair the corroded areas, followed by shot blast & paint and coal tar epoxy coating. And in this case no need to increase or to add a sacrificial anodes.

Note. There is a schematic drawing showing how the breaks in mill scale (Fe₃ O₄) can lead to galvanic corrosion of steel, but, unfortunately I can't attach it.

I would like to thank everyone who has contributed to this discussion. Your insight has been invaluable. I have read the related material and taken into account all of your suggestions on and off line to formulate a plan to stop the carnage. I will test for stray current, I'll have the bottom sampled in our berth, and prior to launching I will inspect the electrical components and wiring and will repair the damaged area.

Thank again to all of you