I understood that it is not as apt to happen in fresh water.
But stray current in a marina can trigger a problem in fresh water.... Is this correct?
What should we watch for? Just inspect? By the time one finds a problem damage has probably occurred, right?
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Mooched Beachcats in the past
Time to try ownership with Nacra 5.7
Port Clinton, Lake Erie Islands, Ohio
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bent striker rod
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- Rank: Lubber
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Posted: Jun 10, 2013 - 09:06 PM
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Posted: Jun 10, 2013 - 09:17 PM
It can happen in fresh water too...may take longer
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John Schwartz
Ventura, CA
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Posted: Jun 10, 2013 - 11:32 PM
You have several things in your favour Scott, freshwater, & Ohio,(the season is much shorter, & the off season is cold).
The corrosion in the photos is a result of ion migration between metals with different electrical potentials. In very simple terms, if two dissimilar metals, in close proximity to one another, are connected by an electrolyte, ions migrate from the less noble metal. The "loser" is the anode, the more noble, or "winner" is the cathode.
In our case, aluminum is the anode (less noble). In fact it is one of the least noble metals, the only commons ones that are worse are zinc & magnesium, & it should dissolve quicker than a tin can. (I once witnessed the resurrection of a Japanese Zero, the wing spars were magnesium, & could be flaked away with your fingernail).
Interestingly, aluminum "rusts"(oxidizes)very, very quickly, so, why don't our masts & beams dissolve quicker than a common nail? The saving grace is that the result,(aluminum oxide) is a very good barrier.
Another major consideration, is the relative sizes of each piece of metal, also known as the ratio of Cathode to Anode (C/A). Many will remember the chemistry lab, where you covered a steel tray containing common salty foods with aluminum foil. The foil was pushed down onto the food with a pencil point, resulting in a few very small contact areas. This served to concentrate the electrical potential in a very small area. The foil was quickly eaten with "moth holes"at these points.
Consider the following two situations. Case A, an aluminum plate,(anode) containing an SS rivet. The rivet is more noble,(cathode), BUT it is very small in comparison to the aluminum plate, making for a small C/A. Corrosion will occur from the aluminum plate, but it will be slow, & this is what we use on our Cats, large aluminum extrusions, with small SS rivets. Incidentally, monel rivets possess the same potential as passive 304 SS.
Case B, an SS plate,(cathode) containing an aluminum rivet,(anode). In this case the C/A ratio is high, & corrosion of the the aluminum rivet will be severe, even though the electrical potential,(difference), was the same in both cases.
What lessons can we glean from all this?
Best, keep the parts out of an electrolytic substance, not possible in our case.
Better, eliminate areas where the C/A is high,(small aluminum in proximity to large SS).
Good, prevent the current from flowing between the dissimilar pieces, achieved with a barrier, ESPECIALLY WHERE THE CONTACT POINTS ARE CONCENTRATED.
This works well for the portion inside the compression tube, not so well where the rod exits the beam. The grease is quickly squished out of the way, & the contact point is concentrated against the thin wall of the beam. The problem is exacerbated by the use of the large SS washers directly against the aluminum beam. Some grease on the inside of the washers would help.
Take a close look at Philips photo, the worst corrosion is where the rod touches the beam walls. A cure could be had by enclosing the rod in a very thin plastic tube, but such a tube would not last long, the beams edges would soon cut through it.
Ultimately, what we are depending on is Case A, above. A large anode(beam) vs a relatively small cathode (rod).
In your case, the fresh water makes a much weaker electrolyte, hence less current, resulting in little or very slow corrosion. There are differences, depending on the pH value & hardness of the water, but it is much less than sea water. I personally have not greased my 5.7 in 4 years,(freshwater), I doubt if the PO ever did, & it shows zero effect,as can be seen from the album photos.
For those operating in corrosive environments, it would be an interesting experiment to utilize sacrificial anodic protection. A small zinc strip,(even better would be some magnesium wire) attached to both the rod & beam should work. It must be connected to both, then the zinc, being less noble than aluminum should become the anode. Someone should try it & report back.
Edited by Edchris177 on Jun 10, 2013 - 11:42 PM.
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