When is it time to replace your zinc (magnesium)

[DannyS]

In the past, I've always replaced the magnesium anode every time we hauled the boat (northern climate, we haul every fall). But I got to wondering, do I need to do this? I've attached a photo to show the state of my anode after one season, I bought a new one but want to know if this one would still do the job. The obvious answer is to replace it for max protection, but can I go two years between replacement? Is this one shot or will it continue to dissolve down to nothing?

 

[Steve Dion]

Danny: Most of the boats in our marina are in the water 365 days a year and are probably hauled every 3-5 years or less. There is very little wear on the zinc/mag in fresh water.

 

[DannyS]

In the past, I've seen very little pitting on my anodes. The anode in the photo was only on the boat in the water for about 6 months. Would this be considered a normal amount of degradation for a magnesium anode? The reason I ask is that in the past, they come out of the water looking like they were new, but this time, I'm getting more pitting. Is this an indicator of something else going on in or around out boat/ marina?

 

[sailingdog]

If you're getting more pitting on your anodes, it may be due to the alloy composition changing. If it was an electrical issue in your marina, the magnesium anodes would likely be GONE.

On fresh water, the anodes do last a fairly long time. Consider yourself lucky...

 

[Warren Milberg]

I think I'd be more concerned about ambient electricity in the water in around your slip, coming from your boat, nearby boats, and the shore power connections. While you may be able to control the first type, you cannot with the other two. As such, I'd replace that zinc at least annually. A 1-inch zinc typically cost less than $10 and is worth its weight in gold in what it prevents from happening to your underwater metals.

 

[Newbie]

So what is normal wear for salt water? How often do you have to change it?

 

[Don S/V ILLusion]

So what is normal wear for salt water? How often do you have to change it?

There are so many variables that it is difficult to generalize. Conditions such as nearby boats, faulty grounds, too many grounds, size prop and even the size, type and number of anodes you have can effect the lifetime of a zinc.

The best thing you can do,especially with a new boat or one with which you have no prior experience, is to check it to see what is happening.

Having said that, a few months to a few years can be expected.

 

[DannyS]

In Lake Superior, we use magnesium. They're at least double the cost of zinc... around $25, cheap insurance, yes, but what I'd really like to know is is the one in the photo still usable or should that one be changed in order to maintain good protection? In looking at the magnesium in the photo, if it was on your boat, would you replace it?

 

[Blue Nose]

Some of the boats in our marina will change them out annually then take the old zink and hang it is the stern on a string to give more protection. At least that way you feel like you are getting your moneies worth out of it. Can't hurt.

 

[Ted]

When is it time to replace your zinc

Another option is to keep your old zinc on the shaft and add a new zinc this season. This way if the old zinc fails you are still protected by the new one. You might be able to add a new zinc every second or third year depending on what you see when you haul in the fall.

 

[fifedog]

Re: When is it time to replace your zinc

I was trained to use 2 zincs in series on the prop shaft. The thinking is that the weaker one will "sacrifice", leaving the "stronger" zinc in tact. That way, you replace one every other season. Remember to sand down to bare metal where attaching the zincs, and don't paint over them.

 

[DannyS]

I like the idea of using two, if for no other reason than experimentation.

 

[zeehag]

zincs are not magnesium. there is a difference in metals and magnesium is not the anode used in marine environment. if in salt water, they are zinc. usually magnesium is for wheels on sporty cars. i havent ever heard of that element used as a sacrificial anode in marine usage, but i donot do fresh water boating...
replace zincs when they are no longer there or close to not there. do not paint them. do not cover them.

 

[sailingdog]

Zeehag—

There are three types of metal usually used for sacrificial anodes:

For freshwater, Magnesium is usually used

For brackish water, Aluminum or magnesium are used, depending on the salinity—the higher the salinity, the more likely that aluminum will be used.

For salt water, the anodes are usually ZINC.

If you were to use zinc anodes in fresh water, there would be almost no protection, due to the lower reactivity of the zinc, as it is more noble than aluminum or magnesium.

I'd point out that the likelihood of someone in the upper mid-western United States being in a saltwater environment is pretty small. The poster is talking about a boat that is in or near Bayfield, WI.

I'd also point out that it is generally wise to replace zincs when they're at about 40% of original size. Waiting until they are almost gone or gone generally means that you'll have some galvanic corrosion, and galvanic corrosion damage is CUMULATIVE.

zincs are not magnesium. there is a difference in metals and magnesium is not the anode used in marine environment. if in salt water, they are zinc. usually magnesium is for wheels on sporty cars. i havent ever heard of that element used as a sacrificial anode in marine usage, but i donot do fresh water boating...
replace zincs when they are no longer there or close to not there. do not paint them. do not cover them.

 

[DannyS]

Yup, magnesium in the Great Lakes. Funny enough, my local West Marine only carries zincs. They didn't even have magnesium in the store, they had to order it for me. "That's what they sent us" is the answer I was given when I questioned them about it. "You'd be surprised, quite a few of our boaters have boats in salt water." I said "Why not make them have to order the things since the zincs work here." No answer.
I'm sure many of the boaters here are using zincs and don't realize they should be using magnesium.

 

[sailingdog]

Yup, magnesium in the Great Lakes. Funny enough, my local West Marine only carries zincs. They didn't even have magnesium in the store, they had to order it for me. "That's what they sent us" is the answer I was given when I questioned them about it. "You'd be surprised, quite a few of our boaters have boats in salt water." I said "Why not make them have to order the things since the zincs work here." No answer.
I'm sure many of the boaters here are using zincs and don't realize they should be using magnesium.

and damaging their boats by doing so....

 

[Gary_H]

I replace in the fall and then check again in the spring. During the summer I keep a steady check on it's condition because I'm often under the boat. I hate cold water so it doesn't get checked much in the winter.