The River Exe ate my anodes!

[Dull Spark]

We were warned by an old salt when we moved to a mooring on the Exe that the River ate anodes. He was right!

Are all estuaries the same or is the Exe a "special"?

 

[Tranona]

Suggest you check how brackish your water is and what material your anode is made of.

 

[lpdsn]

I was under the impression that Zinc anodes were less effective in brackish water and so weren't eaten as quickly, to the detriment of whatever they were protecting. Are you moored in brackish water or further down where you'd expect it to be as salty as the sea?

Don't remove your batteries and wiring from the list of suspects just yet.

 

[macd]

I was under the impression that Zinc anodes were less effective in brackish water and so weren't eaten as quickly, to the detriment of whatever they were protecting.

That's so. They develop an impervious surface coating of zinc-something-or-other which prevents them working, both in brackish water and even when the boat returns to full salt. The effect obviously depends on the degree of brackishness and seems to vary from boat to boat, perhaps due to varying rates of 'normal' erosion.

 

[[70521]]

Where about are you on the river? I've been on the Exe for years and an anode usually lasts two years.

 

[[3889]]

That's so. They develop an impervious surface coating of zinc-something-or-other which prevents them working, both in brackish water and even when the boat returns to full salt. The effect obviously depends on the degree of brackishness and seems to vary from boat to boat, perhaps due to varying rates of 'normal' erosion.

That is what I was led to believe but personal experience suggests otherwise. I keep my boat in fresh water (from which drinking water is extracted) but sail on the sea. For the first two seasons I used a hanging Mg anode bonded to the shaft, as suggested by MG Duff. The Mg remained untouched but the Zn on the prop and shaft deteriorates steadily so I no longer bother with the Mg. Maybe all low salinity water is not the same?

 

[macd]

That is what I was led to believe but personal experience suggests otherwise. I keep my boat in fresh water (from which drinking water is extracted) but sail on the sea. For the first two seasons I used a hanging Mg anode bonded to the shaft, as suggested by MG Duff. The Mg remained untouched but the Zn on the prop and shaft deteriorates steadily so I no longer bother with the Mg. Maybe all low salinity water is not the same?

There does seems to be a deal of variability. I kept a boat in brackish harbour for several years. When my zincs became ineffective due to an impermeable coating I changed to aluminium anodes which fixed the problem. It was obvious at the club lift-out that some other boats moored nearby had no such issues. I suspect, but have no compelling evidence, that this was because of higher anode erosion rates. Mine eroded quite slowly, so obviously weren't doing much, anyway. The OP seems to be in a very different position.

 

[Dull Spark]

Thanks for your replies. We have a swinging mooring at Starcross. The engine and shafte anodes need to be changed mid-season and I won't get a second year from the MG Duff on the hull. (But perhaps I'm being too cautious.) The old salt was right and for once, I paid attention. Worth knowing for others. Best regards.

 

[[70521]]

Thanks for your replies. We have a swinging mooring at Starcross. The engine and shafte anodes need to be changed mid-season and I won't get a second year from the MG Duff on the hull. (But perhaps I'm being too cautious.) The old salt was right and for once, I paid attention. Worth knowing for others. Best regards.

Are you sure it is not the electrics? That is all salt water not brackish
I've had three moorings in the area and as stated above an anode lasts two years and nobody I've talked to has ever complained of anode issues.

 

[Hydrozoan]

That's so. They develop an impervious surface coating of zinc-something-or-other which prevents them working, both in brackish water and even when the boat returns to full salt. The effect obviously depends on the degree of brackishness and seems to vary from boat to boat, perhaps due to varying rates of 'normal' erosion.

The passivation is usually attributed to zinc oxide, though I suspect that zinc hydroxide and a zinc hydroxy-carbonate may also form in fresh waters.

A recent study on zinc-coated steel in seawater has reported a zinc hydroxy-chloride, a zinc hydroxy-carbonate and the oxide as solid corrosion products on the zinc coating (abstract and initial pages at https://link.springer.com/article/10.1007/s00343-016-5269-9). I would not expect a hydroxy-chloride in truly fresh water, such as might be used as a drinking water source.

Interestingly, that study refers to the products in seawater being ‘loose and porous’, so perhaps those in fresh water tend to be less so – possibly because, echoing your observations about rates in the above quote, and on different boats at #7, they form more slowly? (Perhaps, too, a relatively high rate of erosion may lie behind Quiddle’s experience at #6 – either because of frequent/long excursions into salt water, or the innate rate on his particular boat, or both.)

Whilst in principle particular dissolved components – such as organic substances – might conceivably affect the corrosion chemistry of zinc, I doubt in practice that they would make a single river exceptional, and I would expect the corrosion products everywhere to be dominated by zinc oxide/hydroxide, hydroxy-carbonate and hydroxy-chloride, depending on salinity.

The advice to the OP to check his electrics seems sensible.

 

[Corribee Boy]

I've been on a swinging mooring on the Exe for several years and haven't had to change the main hull anode: I wondered if it had become passivated and abraded the outside each year, but still no erosion. This last year the bonded hull pear anode eroded by about 50% and the small, unbonded anode on the skeg went completely. The prop anode didn't erode much at all,( nor did the prop!)

I did ask Richard Bentley, the mooring contractor, if he knew of anything that might have caused it: he hadn't heard anything.

 

[[70521]]

It will be all these Seahorses that the Seahorse Trust have recently found.

 

[Hydrozoan]

It will be all these Seahorses that the Seahorse Trust have recently found.

More likely 'The oysters ate my anode!', I think.

 

[alahol2]

One for the Exe-files.

 

[fisherman]

We left some hot dipped galvanised pots in the Helford once for about three months, they deteriorated very badly compared with those at sea. Mystery to us, we thought maybe something in the mud or runoff from the land.

 

[Hydrozoan]

We left some hot dipped galvanised pots in the Helford once for about three months, they deteriorated very badly compared with those at sea. Mystery to us, we thought maybe something in the mud or runoff from the land.

Were they located where they were likely exposed to very variable salinity?

 

[Capt Popeye]

One for the Exe-files.

:encouragement:

 

[fisherman]

Were they located where they were likely exposed to very variable salinity?

Some were off Frenchman's, most below t.he gear mark so almost at sea. Very little fresh water coming in at Gweek and Polwheveral plus a few smaller ones.

 

[Hydrozoan]

Some were off Frenchman's, most below t.he gear mark so almost at sea. Very little fresh water coming in at Gweek and Polwheveral plus a few smaller ones.

Thanks - no obvious reason then, unless perhaps sulphide from the mud.

 

[[70521]]

More likely 'The oysters ate my anode!', I think.

There is an active mussel farm http://www.exmouthmussels.com/ on river and they do like to attach themselves to boats. In my experience don't taste as good as the ones we have in France, but that might be the chef.