Over zinced?

[walmsleyc]

Attached is a picture on the only bronze through-hull on my boat that shows a "halo". Also some picture of my propeller showing the "scale" that builds up.
Details:

• I'm in fresh water
• I have VC17 bottom paint
• This only bronze thru-hull comes with a zinc (in the picture). It's an Isotherm SP heat exchanger thru-hull where they provide a custom zinc
• It may be electrically connected to the re-frigeration system by the refrigerant piping (I've haven't tested)
• I have a stanless steel propeller on a stainless shaft
• On the propeller shaft I have two magensium anodes and the propeller comes with a nose cone zinc.
• The shaft should be electrically connected to the engine

Observations:

• Halo on the one thru-hull (I saw a lesser version last year, but not sure about previous years as the Isothem thru-hull is relatively new.
• Some mild erosion of the zinc on this thru-hull
• Some mild/moderate erosion of the two magnesium shaft anodes (really not too much)
• virtually no erosion of the propeller zinc (still smooth and dark grey)
• BUT, the propeller and shaft are covered with a matt scale/coating. It's a little blotchy, but that seems due to places where there has been a little bit of marine growth preventing the scale build up.
• This has happened for multiple years (but I've had two magnesiums on the shaft for a number of years too)
• This coating can be removed by mild scraping with a metal paint scraper, or with use of scotch-brite and some elbow grease.

Speculations:

• Could be stray DC current from the Isothem Thru-hull? I haven't tested anything at this point
• But, I do have more than a normal amount of Magnesium/Zinc anodes, and there are lots of reports of over zincing causing such problems
• The coating on the shaft and propeller seem like over zincing could be the culprit.

Just wondering if anyone has observations or seem similar things.

I can certainly clean things up and use only one Magnesium on the shaft. I suppose I could use NO shaft anodes and rely on the propeller zinc (though in fresh water I should be using Magnesiums). I suppose I could have tested for stray current, but I'm on the hard now, so I'm not sure how useful that can be at the moment.

Chris

 

[thinwater]

Yes, that halo is caused by excessive over-voltage, but the cause could be either stray current or over reactive anodes.

 

[dlochner]

The white coating on the shaft and prop is magnesium oxide. Not an issue it is the sacrificial magnesium anode doing its job.

If the anode on the thru hull is indeed zinc, it is worthless. In freshwater zinc forms a hard coating that electrically isolates it from the water.

The anode on the prop looks too good, is it magnesium or zinc? If it is aluminum or magnesium there should be some erosion.

I just saw an article on "Haloing" I think it was by Steve D'Antonio, just can't remember where I saw it. That is what you have around the thru hull. You might try searching for the article.

 

[walmsleyc]

The white coating on the shaft and prop is magnesium oxide. Not an issue it is the sacrificial magnesium anode doing its job.

If the anode on the thru hull is indeed zinc, it is worthless. In freshwater zinc forms a hard coating that electrically isolates it from the water.

The anode on the prop looks too good, is it magnesium or zinc? If it is aluminum or magnesium there should be some erosion.

I just saw an article on "Haloing" I think it was by Steve D'Antonio, just can't remember where I saw it. That is what you have around the thru hull. You might try searching for the article.

• Magnesium Oxide: check - this is what I was assuming
• thru-hull and prop anode: both are zinc (shaft anodes are magnesium), The prop anode has NO degradation, but the thru-hull anode does have some, comensurate to the Magnesiums on the prop shaft
• I'm seeing about getting 3rd party non-zinc anodes for the prop (Aluminum seems to be the only other alternative)
• I've bumped into a couple of articles and seems like both "over zincing" and "stray current" can result in very similar results.

Cheers
Chris

 

[joe28532]

I see this is an older post but I'm going to comment anyways. Zinc does not work in freshwater. It creates an oxide layer and becomes ineffective.