Papillon":2e0wu15o said:
In some waters you do need antifouling paint on everything below the waterline. SW Florida is one such place. Do not use the same bottom paint you have on the hull on any metal items , the coppers in the paint will turn those items into a battery. Not Good.
They make special paints for lower units and trim tabs so just keep looking and do it right the first time.....It's always cheaper in the long run.
This is absolutely correct!
My Bennet Trim tabs are painted with Pettit Trinidad anti-fouling paint which is 75% copper. However, tabs are stainless, and the paint and copper don't seem to be a problem on them.
I'll illustrate the problem of anti-fouling paint on aluminum alloy by telling you that the shop that painted my Sea Ray's bottom also painted up to the edge of the outdrives, resulting in a lot of corrosion. Here's what happened:
The Mercruiser outdrives have a galvanic protection system on them called the Mercathode System. It electronically senses the galvanic potential between the outdrives and the water, then applies enough voltage to the outdrives to neutralize the corrosion potential. Pretty neat in principle and normal operation!
But when you paint the outdrive (aluminum) with paint containing metal, or even have your metal containing bottom paint touching the outrives, a battery like galvanic cell is created, charging both the outdrives and the metallic painted hull electrically.
This charge makes it impossible for the Mercathode unit to sense the corrosion potential accurately, and the galvanic cell (battery) created between the hull and the outdrive causes the metal on each of them to become corroded.
Painting the outdrive or leg of an outboard creates a similar galvanic cell with the associated corroson with or without the help of a corrosion control system.
This is just a specific example of the general principle that any time two different metals are bonded together, a galvanic (battery) cell is created and the potential for corrosion exists. Putting this newly created battery cell into water, especially salt water (an electrolyte), enables the two metals to freely go ahead and corrode at full maximum speed.
Metals corrode because they conduct electricity and are chemically active (except for gold, platinum, etc.)
Each metal has a different level of, or potential for, chemical activity.
When two are touching each other, they both are trying to chemically react, and one will be stronger than the other.
Add either water, oxygen, salt, acids, or bases, and they can freely react with each other (and whatever else is around) to chemically change, which we call corrosion. . Electrons are exchanged in the process, and if the metals are connected with wires, a current will often flow, making a battery.
Some alloys, like stainless steel, have mixtures of metals in them so that the atoms tie each other up, and the potential cor corrosion is greatly reduced.
Whenever you're planing a project, think about the potential for corrosion between different metals, especially if it will be exposed to water, and salt water in particular.
Joe.
