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Old 16-05-2015, 07:56   #1
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Galvanic Corrosion Question

Back in Sandinia, I've just began my two week, pre-season preparation work. I was going to fit three new water pump intake bronze screens today, but when I examined two through hull fittings that were installed last season, I noticed that all the bronze screws had corroded.

Last season, the screens were part of the through hull (a one piece design) pictured. This year I had planned on just installing the bronze screens over the existing mushroom though hulls using an adhesive bonding material along with bronze screws. Last year, I did the same, but as you can see by the pictures, the screws are now missing. The two that were fitted last season are part of the boats electrical bonding system. The two to be installed won't be a part of the system.

Question: Should I go ahead as planned? Will the new bronze fittings end up totally corroded over a one or two year period. Or.... Should I instead use a marilon screen along with bonding and stainless screws?

Thanks in advance for your replies.

Ken
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Old 16-05-2015, 08:01   #2
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Re: Galvanic Corrosion Question

Here's a picture of the results after one year (three months in the water).
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Old 16-05-2015, 08:05   #3
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Re: Galvanic Corrosion Question

Here's a picture of what I intended to install this season using bronze screws and adhesive bedding material.
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Old 16-05-2015, 09:38   #4
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Re: Galvanic Corrosion Question

The green pantina on the bronze strainer is totally fine and not a problem. The screws however are the problem. I've found that bronze screws from the hardware stores are not really bronze, but brass with too much zinc or more likely aluminum in the mix to be used under water. They corrode just like the ones in your photo.

Bronze use to be a mix of copper, tin, zinc and lead. But the lead is expensive and not used that much any more. Manufacturers have substituted aluminum at 10-15% with a dash of tin and copper for the rest.

Even the best bronze nowadays is 85%copper, 5% tin, 5% zinc and 5% aluminum. The problem is the aluminum replaced the lead and it will leach out of the bronze, right after the zinc. So at best, bronze nowadays will loose 10% of it's material to galvanic action, where it was only 5% in olden days.
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Old 16-05-2015, 10:08   #5
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Re: Galvanic Corrosion Question

If you use 5200 on it with the screws, you won't have to worry if the screws are still there or not, and to remove the screen next time all you will need is a propane torch to heat the screen up slightly to remove it. During my refit, I have been going back and forth on the whole bonding thing and while I'm planning on bonding, I am not really sure it's all that it is cracked up to be.
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Old 16-05-2015, 12:05   #6
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Re: Galvanic Corrosion Question

The bronze screen scoops are made of naval bronze, so I'm thinking you are correct about the screws being made of a lesser grade of bronze even though they were purchased from West Marine. Going to go ahead and mount what I have with 5200 plus the screws.

Thanks to both of you for your input.
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