Zinc, could somebody explain why and how they work?

[themaniam1]

I have been around boats for a while and have always replaces the zincs when they get too corroded. I have noticed that in some marinas they corrode faster than others even on the same lake and diffenenty corrode at different pace depending on the lake.

Here is my questions. How do they work and why? I know you need them do your outdrive does not corrode but I always thought aluminum did not corrode in fresh water. I guess I just want more understanding on the use of them.

 

[smokeonthewater]

Re: Zinc, could somebody explain why and how they work?

google galvanic corrosion..... you'll find more than you could ever get in a forum

 

[Mischief Managed]

Re: Zinc, could somebody explain why and how they work?

When you put two or more dissimilar metals that are electrically connected to each other in water that does not have a neutral ph, the metals react with the water and produce a small amount of volatge. To produce the voltage, the less noble metals slowly dissolve.

The "zincs" you choose need to be the least noble of all the metals that are in the water on your boat, by a good margin, so that they dissolve, rather than drive parts, seacocks, propellers, rudders, struts etc. Different water types require different "zincs". Many are not really zinc, they are magnesium or special aluminum alloys.

If you add an external voltage source to the mix, you can either accelerate corrosion or nuetralize it (that's what a mercruiser mercathode does). Boats connected to shore power (which connects all the boats together electrically) in marinas often have accelerated galvanic corrosion due to stray voltage from one or more poorly maintained boats. Sometimes this voltage is high enough to actually make the water between boats a lethal electrocution risk. You can reduce the corrosion on your own boat by using a galvanic isolator, disconnecting shore power, or using an isoltaion transformer.

 

[Philster]

Re: Zinc, could somebody explain why and how they work?

Zinc and other metals are simply sacrificial.

For a variety of reasons you are free to read about, metal corrodes. It corrodes in air and water.

If you can put two metals together in a corrosive environment, the weaker metal (less noble chemically speaking) will suffer the corrosion (Zinc gives itself up so that the better metals suffer less corrosion.) As long as the Zinc is there attached to the other metal, the corrosion works against it.

It can work against you, too. In the case of Zinc, you are protecting an out-drive by using Zinc intentionally. I've seen a number of custom car exhausts corrode completely in a year or two, because the metal brackets on the exhaust were attached straight to a car/truck frame. In this case, the reaction corroded the exhaust in no time. It became the sacrificial metal... unintentionally, but it did.

 

[themaniam1]

Re: Zinc, could somebody explain why and how they work?

Good info. I will google it for more info and look into getting a shore power isolator since I use shore power. Thank you for the input.

 

[Chris1956]

Re: Zinc, could somebody explain why and how they work?

One of the reasons that the anodes corrode faster in some marinas is due to voltage leakage from shore power systems. The only protection you can get is to have an iolator on your boat and some distance from those boats that do not have an isolator, and have voltage leaks.

 

[Silvertip]

Re: Zinc, could somebody explain why and how they work?

To illustrate this, take a penny and a dime and spit on them. Place them together and using a digital voltmeter you may or may not be surprised to learn you just built a small battery. Amazing what dissimilar metals will do under certain circumstances.