Serious corrosion mystery

[pwinkler]

Last month we took our Ranger 33 out for a day sail from Swan Creek on the Chesapeake. We could hardly make any headway. We had the boat pulled for a cleaning and sure enough, the prop was covered with barnacles and growth. The same thing happened to dozens of others this year, we discovered.

However, the shaft ( 1inch bronze) had what appeared to be severe stray current corrosion. It was even pinkish in large areas and had deep pock marks of 2-3 mm. The zinc wasn't touched at all, nor was the prop (Indigo), struct or any other fittings on the bottom. Just the shaft.

The service manager (Doug at Gratitude Marina) suggested it may have been caused by lightening. I used a meter to test all my bonding and everything was correctly bonded. This is an Atomic 4 engine, too. The AC circuitry was checked out and found to be proper as well.

Anyone seen anything like this before? Oh, the hull had large white patches where the bottom paint had come off, too. The bottom and shaft were fine when it was splashed at the end of April.

Very weird.

TIA,
Phil

[Fstbttms]

Don't assume the corrosion you noticed was caused by stray current. There are many causes of metal corrosion in boats. We don't get much lightening out here so I can't speak to the likelihood of that but I have seen any number of boats with issues like yours. Bronze that is "pinked" has been de-zincified, that is, the zinc component of the bronze is depleted, leaving a weaker, brittle material, and may be the cause of the pitting you saw (although "pitting corrosion" is also a possibility.) You make no mention of the condition of the zincs. Were there any left?

Seems like the shaft was significantly compromised. Did you replace it?

[Sapperwhite]

What is the salinity in Swan Creek, zincs don't work in fresh water. I know it's not fresh, but if its brakish or mostly fresh you may have trouble.

[EO32]

Originally Posted by Sapperwhite

zincs don't work in fresh water.

Why would zincs not work in fresh water?

I realize salt water is more conductive, but that doesn't mean fresh water is not conductive.

[tommays]

"Aluminum anodes “self-clean” in freshwater. Zinc anodes can form a coating that stops them working ..."


There is also a lot of problems with buying brand X anodes becasuse of poup quality control of the alloy

[Fstbttms]

Originally Posted by Sapperwhite

What is the salinity in Swan Creek, zincs don't work in fresh water. I know it's not fresh, but if its brakish or mostly fresh you may have trouble.

My understanding is that zinc anodes do not have the electrical potential to overcome the higher resistance found in freshwater. That is why you should use magnesium or aluminum anodes in a fresh water or brackish environment.

[sailingdog]

As pointed out, you really shouldn't be using ZINC zincs in fresh or brackish water. Magnesium is best for fresh water, aluminum can be used in brackish...but it really depends on the salinity.

Originally Posted by EO32

Why would zincs not work in fresh water?

I realize salt water is more conductive, but that doesn't mean fresh water is not conductive.

[SteveInMD]

The salinity of the Chesapeake Bay changes quite a lot over the course of the year. The heavier salt water forms a wedge shape under the fresh water that moves up the bay during the dry summer months and retreats in wet weather. The change in salinity can be fairly dramatic. This year was fairly wet and the salt wedge didn't come very far north so the water was fairly fresh.

Having said that I don't know if I should be using zinc or aluminum in the Annapolis area. I think most people use zinc. What would happen if I used one of each?

[sailingdog]

If there is any doubt, go with aluminum, and check them a bit more often. They're not that expensive and beat having to fix something due to galvanic corrosion damage.