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Subject: Zinc's and Thru Hull Grounding Author: JD [ Next Thread | Previous Thread | Next Message | Previous Message ] Date Posted: 7:35pm Galvanic corrosion (what your trying to prevent) is a bass ackwards process. I.E. what common sense says is happening is not what's really going on. The Truth is every bit of metal on your boat is constantly loosing electrons to the acidity of seawater. This isn't like oxidation where the molecules are changing with oxygen thru bonding but instead a process where electrons leave causing the bonds in the metal to litterly fall apart. Electron starved metal may look the same but lacks the glue to hold itself together. Nobal metals like bronze loose electrons slowly, anodic metals like zinc fast. Some metals are weird....Oxidised Stainless (nobel) slow. Reduced Stainless (oxigen starved) (anodic) fast, so stainless is good in the open air but not always that good in a wet bilge. Anyway to keep this short here's what you need to really know. Anodic metals don't just loose electrons to seawater but also to any metals they touch, at least metals that are more nobel then they are. That's why every peice of metal on your boat that touches seawater needs an electrical conection (wire) to your zincs. That includes thru hulls shafts props etc. So back to your zincs. Sure most likely they are loosing some electrons directly to the sea but most of the elctrons they loose are actually traveling through their connection wire to other crucial things that they are protectecting. Hard to believe but your connected zincs are giving up their electrons to replace those lost by your expensive two inch stainless shafts and Bronze propellors. That's why they call them sacrificial zincs. As your prop and shaft loose electrons to the sea the zinc supplies electrons to fill the holes (lost electrons) As time goes on the zinc just falls apart litterly dissolving into seawater because it no longer has the electrons to keep it's molecules together. Zincs not connected to other metals still Galvanicly Corrode they just don't protect anything. They still look like they are working but actually they are just loosing electrons to the water, not to other metal on the boat. It's a common myth that corrosian singles out the weakest metal and attacks it ignoring other metal. It's not true that hanging a zinc over the side will divert the proverbial Galvanic wolves aiming for your prop. If your prop and zinc aren't electricly linked (wired together) Galvanic Corrosian accures in both just faster in the zinc because it's anodic. SO what I'm saying here is that your boat should have a system ground where every piece of metal that contacts seawater is hard wired to your zinc or zincs. It's really pretty simple. The zinc needs to be in the water, a wire needs to be connected to the zinc, and everything you want protected needs to be connected by this wire to the zinc. That includes engine props shaft and bronze thru hulls. As long as your zincs are stuffing the other metals with elctrons your fine. The electrons change the charge potential of the protected metals. Electrons are negatively charged, the metal gets more negative (polarised) Polarisation can be measured with a voltameter. If your worried about it, test it.... Get a portable voltameter that reads to 1500 millivolts, a silver (silver cloride) electrode and about ten feet of wire. Hook the electrode to the wire and the wire to one of the voltameter probes ( I'm not sure which one) At any rate drop the electrode over the side of your boat. With the free end start checking your metal fittings in the bilge. They should read somewhere around -400 mv -0.40 volts For instance that fancy stainless prop shaft. Rub the probe on it till you get a reading. -600 mv, outstanding, -150 mv it's oxygen starved not connected to your zincs and freely corroding away. Hope all this helps Tight lines Jim P.S. You want a real Can-O-Worms ask if you should connect this Anti Galvanic ground system to your battery electrical ground system. They almost always are if for no other reason then the engine block is usually grounded to both. In a perfect world I'd say seperate them. This is because stray current leaks can do weird things to your galvanic ground system. I once saw an instance where a stary short in a sailboats charger was cooking metal on very boat on the dock, some seriously. It's funny now but it took a lot of trouble to convince the old salt that his trusty rusty charger was bad news. I gather you slip. You might want to look into a isolator if you leave shore power hooked up 24-7 [ Next Thread | Previous Thread | Next Message | Previous Message ]