nmanno

Carbon fiber hull - corrosion

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Hi guys,

I've got a Carbon hull race/cruiser and live in warm Asia. 

I have an aluminium sail drive and aluminium rudder stock and they are getting eating like soft cheese by what I assume is galvanic corrosion.

The sail drive has the usual anode and I've even put an anode connected to the rudder stock when the boat is on her mooring which still doesn't seem to help.

The antifouling used is VC Offshore which is a copper based hard racing antifouling but the yard here paints 1m2 around the sail drive with a non-copper based antifouling. 

In my marina, there were heavy tunnel works over the last couple of years and I wonder if those works were a reason for the strong corrosion (i.e. potentially some strong electrical currents in the water).

Anyhow, what should I do as it's not viable to change the entire saildrive and rudder every 4-5 years! How best to test for stray currents etc...

Any views appreciated.

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From what you're describing its going to be very hard to stop.  Carbon fibre + Aluminium + salt water = battery you need to make sure the aluminium components are isolated from the carbon structure.  If the boat has carbon engine beds this won't be easy or cheap.  Some light reading here to help you diagnose the issue.  Or call this bloke and offer him a trip to Hong kong.  He spends his life in planes anyway.

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You can remove the aluminum bits and powder coat them, which will help a good deal. There are other ways to isolate the carbon from the aluminum, one trick is to apply a layer of fiberglass to the carbon in the area of contact but what may get you are any fasteners still provide an electrical path. They can be painted which will help, or at least the liberal use of TefGel will do wonders.

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If the Carbon is coated properly it shouldn’t give any galvanic problems. As long as the sea water does not touch the carbon (the barrier coat should take care of this) there won’t be a galvanic chain. I suspect the copper will be a bigger problem, lower on the galvanic chain than carbon but a big area and a small area of Aluminium (the bigger the ratio the bigger the corrosion)

 Big steel structures could be a problem depending on the shore power connection you are using, earthing, galvanic transformer, etc...

the picture below is for copper and stainless, but you can change those to carbon and Aluminium, or copper and Aluminium. The picture gives an indication of the different ways you can try to avoid galvanic corrosion.

ways-to-prevent-galvanic-corrosion.png

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I don't know if CF hulls need to be treated differently but I would think not.

The basic test for a 'hot' marina involves measuring the current in the main ground wire. 

You could ask fellow boaters if their zincs are requiring frequent replacement. Also if there are any ali bottomed RIBS ask the owners if they are having issues with corrosion. I have seen 2 where the bottom was eaten away in a year. A hot marina was the suspect. 

If you are plugged into shore power you want to look into using a galvanic isolator.

 

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19 minutes ago, voodoochile said:

Are you sure you're using the correct anode?  And enough of the correct anode?

and not too much of the right anode..

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Just call Brian Gatt, linked in my first post,  a return flight from Sydney to Hong Kong is nothing compared the the boat bucks you're already spending.

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I am not sure what your complete solution is. But if you are at RHKYC that place is hot as aych eee double chopsticks. Crimeny, you've got 220v power supply to a couple hundred mooring balls that are like 30+ years old. 

Hopefully that may help someone t/s the problem. 

 That marina eats metal for fun I think. Also very warm and very polluted.

 

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Some sail drives are meant to be electrically isolated from the engine and the rest of the grounding system. Often in installation or subsequently this isolation Is breached. One thing I'd do is check with an ohmmeter and make sure they are still isolated.

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On 12/4/2018 at 4:40 AM, SCARECROW said:

Just call Brian Gatt, linked in my first post,  a return flight from Sydney to Hong Kong is nothing compared the the boat bucks you're already spending.

Absolutely, I will go down that route if needed, just want to try a couple of ideas first.

 

On 12/4/2018 at 6:58 AM, DDW said:

Some sail drives are meant to be electrically isolated from the engine and the rest of the grounding system. Often in installation or subsequently this isolation Is breached. One thing I'd do is check with an ohmmeter and make sure they are still isolated.

You see, that's an interest angle here, as I went around the boat with the ohmmeter, I got a reading between the bare carbon hull and the drive, the engine, the rudder stock etc... it seems like my entire boat is a huge battery with both the sail drive and rudder stock being the 2 electrodes!

 

On 12/4/2018 at 4:50 AM, Baldur said:

I am not sure what your complete solution is. But if you are at RHKYC that place is hot as aych eee double chopsticks. Crimeny, you've got 220v power supply to a couple hundred mooring balls that are like 30+ years old. 

Hopefully that may help someone t/s the problem. 

 That marina eats metal for fun I think. Also very warm and very polluted.

 

Yes, RHKYC marina in causeway bay, indeed, very hot, polluted and with so much works and power running in the water, I wonder if the entire place is not a power plant! 

And, yes I use the right anode and change it regularly but I don't see the anode being "eaten away" faster than the sail drive.

On 12/4/2018 at 12:25 AM, TQA said:

I don't know if CF hulls need to be treated differently but I would think not.

The basic test for a 'hot' marina involves measuring the current in the main ground wire. 

You could ask fellow boaters if their zincs are requiring frequent replacement. Also if there are any ali bottomed RIBS ask the owners if they are having issues with corrosion. I have seen 2 where the bottom was eaten away in a year. A hot marina was the suspect. 

If you are plugged into shore power you want to look into using a galvanic isolator.

 

I am not connected to shore power, just a mooring in the middle of a marina.

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I had a similar problem with my carbon fibre race boat with my Yanmar SD20 sail drive. There are four preventitive actions I did to remove the problem which I believe eliminated my sail drive being eaton up :

1. I found that the carbon fibre plate installed around my sail drive was making contact with the sail drive and causing electrolysis through to the sail drive as this is the area being eaton up and disappearing. I removed the carbon plate and replaced with fibre glass plate to provide an insulator to the carbon fibre haul to the sail drive

2. Installed a 0VDC isolator from all my batteries to avoid any stray currents from the batteries to the carbon haul.

3. Installed anodes on each side of the boat connected to the sail drive via a cable to one of the bolts. This was only when moored.

4. Use probspeed on the prob which is a 3 blade Gori racing version

the problem has all gone away as a result of these changes with minimal loss of anodes on my sail drive. The carbon fibre boat next to me had a similar issue and the owner only installed 0VDC isolators on his batteries and I believe his issues also went away.

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When CF and Aluminium meet, does the CF get eaten away? All the literature I find says it corrodes the Al; close up pics of eaten aluminium. What happens to the CF?

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According to the galvanic table, carbon eats everything - even platinum

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20 minutes ago, SloopJonB said:

According to the galvanic table, carbon eats everything - even platinum

Carbon just edges out Ti

A galvanic series has been drawn up for metals and alloys in seawater, which shows their relative nobility. The series is based on corrosion potential measurements in seawater. The relative position of the materials can change in other environments. The further apart the materials are in this series, the higher the risk of galvanic corrosion. (reference)

Most cathodic, noble, or resistant to corrosion

Platinum

Gold

Graphite

Titanium

Silver

/ Chlorimet 3

\ Hastelloy C

/ 18-8 Mo stainless steel (passive)

| 18-8 stainless steel (passive)

\ Chromium steel >11 % Cr (passive)

/ Inconel (passive)

\ Nickel (passive)

/ Silver solder

| Monel

| Bronzes

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Gotta start making those solid gold saildrives now.

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The carbon just sits. It will dissolve aluminum. Stainless steel + carbon isn't very safe either. Titanium is good.

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1 hour ago, Zonker said:

The carbon just sits. It will dissolve aluminum. Stainless steel + carbon isn't very safe either. Titanium is good.

Carbon Southern Spars mast w/Aluminum butt, nested on Stainless hinge... check out those biscuits.

 

thumbnail_IMG_2320.jpg

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Nice. Have to save that photo to show naysayers

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2 hours ago, Irrational 14 said:

Carbon Southern Spars mast w/Aluminum butt, nested on Stainless hinge... check out those biscuits.

 

thumbnail_IMG_2320.jpg

That doesn’t seem like a good plan. Whose idea was that?

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That was the plan of a spar designer who:

  1. Expected the boat to be built with proper e-glass isolation in the step
  2. Expected the mast to be removed regularly for tear-down and inspection

 

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16 hours ago, Zonker said:

The carbon just sits. It will dissolve aluminum. Stainless steel + carbon isn't very safe either. Titanium is good.

Thanks for confirming! 

I'm intrigued when you say "Stainless steel + carbon isn't very safe either." -- that's what all small carbon cats use for mast fittings. I've had my cat for ~3 years now in the FL saltwater, and the stainless fittings in the mast have very minor rust stains. Doesn't seem to be an issue in practice -- or am I blind to internal damage?

How reactive is it?

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11 hours ago, Moonduster said:

That was the plan of a spar designer who:

  1. Expected the boat to be built with proper e-glass isolation in the step
  2. Expected the mast to be removed regularly for tear-down and inspection

 

Agree. Anything can go to hell if there is no one to look after it.

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8 hours ago, martin.langhoff said:

Thanks for confirming! 

I'm intrigued when you say "Stainless steel + carbon isn't very safe either." -- that's what all small carbon cats use for mast fittings. I've had my cat for ~3 years now in the FL saltwater, and the stainless fittings in the mast have very minor rust stains. Doesn't seem to be an issue in practice -- or am I blind to internal damage?

How reactive is it?

The stainless parts on a carbon spar will actually rust. The same stainless part on an aluminum mast will merely discolor.

How reactive? You want numbers? It depends. Stainless has two electrical conditions: active, and passive. It is good when it is passive (self-made protective oxide film). Active happens when the galvanic interaction is too strong for the oxide to remain stable. The relative volume of the anode and the cathode has an influence.
But on a UFO you aren't permanently in the water so it will take a while.

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+1.

If the parts were electropolished they are probably in a passive state. Or if pickled.

If they are worked/machined and the raw edge is merely sanded smooth that part might be "active" and more prone to corrosion.

And if you're only seeing light surface rust great - but inspect carefully and look with a 4-8x jeweler's loupe. Sometimes minor rust stains hide nice cracks and crevice corrosion.

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On 12/8/2018 at 9:11 PM, Chucky said:

I had a similar problem with my carbon fibre race boat with my Yanmar SD20 sail drive. There are four preventitive actions I did to remove the problem which I believe eliminated my sail drive being eaton up :

1. I found that the carbon fibre plate installed around my sail drive was making contact with the sail drive and causing electrolysis through to the sail drive as this is the area being eaton up and disappearing. I removed the carbon plate and replaced with fibre glass plate to provide an insulator to the carbon fibre haul to the sail drive

2. Installed a 0VDC isolator from all my batteries to avoid any stray currents from the batteries to the carbon haul.

3. Installed anodes on each side of the boat connected to the sail drive via a cable to one of the bolts. This was only when moored.

4. Use probspeed on the prob which is a 3 blade Gori racing version

the problem has all gone away as a result of these changes with minimal loss of anodes on my sail drive. The carbon fibre boat next to me had a similar issue and the owner only installed 0VDC isolators on his batteries and I believe his issues also went away.

What is a 0V DC isolator?

some euro boats have a battery switch in their negative as well as positive battery leads, is this the same thing?

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