MOVED: Galvanic corrosion expert required to identify root cause......

[vyv_cox]

Wooden boats have other issues as regards corrosion, ones I know little about.

With GRP the skin fitting will (should?) be made from a material that resists corrosion, so there is no need for protection by an anode. If skin fittings of differing materials are bonded together, e.g. brass, bronze, stainless steel, there is a risk that the more anodic one will corrode trying to protect the more noble. This was judged to be the cause of the Random Harvest sinking.

Bonding is a practice largely used in USA, where its primary reason is lightning protection, not corrosion resistance. Very few (none?) European boats are built with bonded seacocks.

 

[Tranona]

Very few (none?) European boats are built with bonded seacocks.

While I think that is true of sailing boats, maybe not so true of motor boats. Over the years I have had exchanges with jfm on the MOBO forum about the subject he says all his boats (Fairlines I think) have had bonding circuits including all seacocks. Same with Princess. He is adamant that this is correct and that ball valves are "protected" by being bonded to anodes. Such exchanges are amicable and usually end with "agree to disagree".

He is currently having a superb 90 footer built in Italy - see the thread on MOBO. Lots off discussion on the build and in that I have asked if the yard will be bonding seacocks. Promised to check but so far no response (probably much more exciting things going on to occupy his mind!)

 

[Chris_Robb]

None of my boats have ever had bonded fittings or stern gear. None have ever had any problems. I have spent my life in the boat trade & seen more trouble with bonded fittings than not.
Way i have always understood it & what i was taught studying boatbuilding, You put two dissimilar metals in an electrolyte (sea water) you have created a battery & the less noble one gets eaten. This is why we fit anodes. And yet we are encouraged to connect everything together inside the boat with wires, including to the engine with a dirty great alternator on it & batteries, mains power etc. No wonder the bits in the water fizz!
My current yacht still has the same anode on her that she was relaunched with 9 years ago, I keep wirebrushing it every year but its still there.

I was always told that you must never use a steel wire brush on a Zinc anode. It apparently "poisons" the surface. I did this for years on my boat and never had any loss of anode. I eventually replaced them all vowing never to wire brush them - and hey presto they started loosing noticeable amounts especially in a Greek Marina - stray currents? I think it was Vyv Cox that alerted me to this.

 

[vyv_cox]

While I think that is true of sailing boats, maybe not so true of motor boats. Over the years I have had exchanges with jfm on the MOBO forum about the subject he says all his boats (Fairlines I think) have had bonding circuits including all seacocks. Same with Princess. He is adamant that this is correct and that ball valves are "protected" by being bonded to anodes. Such exchanges are amicable and usually end with "agree to disagree".

He is currently having a superb 90 footer built in Italy - see the thread on MOBO. Lots off discussion on the build and in that I have asked if the yard will be bonding seacocks. Promised to check but so far no response (probably much more exciting things going on to occupy his mind!)

My son confirms what you say. His Fairline is fully bonded, shaft brushes, P-brackets etc. He tells me most mobos are bonded.

 

[vyv_cox]

I was always told that you must never use a steel wire brush on a Zinc anode. It apparently "poisons" the surface. I did this for years on my boat and never had any loss of anode. I eventually replaced them all vowing never to wire brush them - and hey presto they started loosing noticeable amounts especially in a Greek Marina - stray currents? I think it was Vyv Cox that alerted me to this.

An explanation. The iron content of anodes to the MIL spec is strictly controlled to very low levels. The reason is that iron inhibits the electrolytic action of the anode. So wire brushing, or recycling old anodes in iron pots, may well produce anodes that last for very long durations because they are doing nothing or very little. A new, reputable anode will waste more quickly because it is protecting whatever it is connected to.

 

[Snowgoose-1]

An explanation. The iron content of anodes to the MIL spec is strictly controlled to very low levels. The reason is that iron inhibits the electrolytic action of the anode. So wire brushing, or recycling old anodes in iron pots, may well produce anodes that last for very long durations because they are doing nothing or very little. A new, reputable anode will waste more quickly because it is protecting whatever it is connected to.

You never stop learning .
Chris's experienced with brushing used anodes is exactly what I have experienced. I always thought that the guts of the anodes was just used up.

 

[jon and michie]

I was always told that you must never use a steel wire brush on a Zinc anode. It apparently "poisons" the surface. I did this for years on my boat and never had any loss of anode. I eventually replaced them all vowing never to wire brush them - and hey presto they started loosing noticeable amounts especially in a Greek Marina - stray currents? I think it was Vyv Cox that alerted me to this.

Could You use a stainless wire brush ?
No Iron oxides
Ive used S/Steel wire brushes to weld Aluminium using a Tig welder - this removed carbon on the surface of the weld area

 

[jon and michie]

My son confirms what you say. His Fairline is fully bonded, shaft brushes, P-brackets etc. He tells me most mobos are bonded.

Ironically its a Fairline - I was going to look at a Targa 43 last year and after a bit of information digging I was informed it had a problem with the bonding circuit - I wonder if this is a common fault with fairlines ?

 

[Tranona]

I am not sure why builders like Fairline carry on with this practice as it is not logical from the point of view of dealing with galvanic action. It seems more logical to deal with each potential problem in isolation. The main problems on inboard shaft drives are the normally stainless shafts and yellow metal props which are in direct contact with each other. Relatively easy to deal with using an anode connected to either the shaft or the prop or in some cases to both such as a cone anode on the prop and a shaft anode. P brackets do not generally need an anode although sometimes they are attached with stainless fastenings. However if the fastenings are bedded in sealant preventing the mating faces from exposure to seawater any galvanic action is unlikely. likewise with seacocks. They are not connected to anything else in a GRP hull.

The misunderstanding of corrosion (dezincification) arises because ordinary brass has zinc in it which will be affected by the copper and gradually leach out leaving structurally weak copper. An anode will not prevent this as the circuit is not between 2 metals with differing potential but internal to the ally. Think of brass screws in damp wood which often end up with just the heads and the threads have disappeared.


Outdrives and saildrives are of course different because the units themselves are constructed of mixed metals, usually stainless and aluminium so these have their own dedicated anodes usually bolted direct to the aluminium.

 

[Keith 66]

Ordinary brass has zinc in it. Manganese bronze commonly used for propellors also has approx 25% of zinc in it, technically its a high strength brass.
Anyone who has ever worked in the boat trade will have come across dezincified brass props, stern gear or skin fittings.
It simply is not to be trusted.

 

[rogerthebodger]

Ordinary brass has zinc in it. Manganese bronze commonly used for propellors also has approx 25% of zinc in it, technically its a high strength brass.
Anyone who has ever worked in the boat trade will have come across dezincified brass props, stern gear or skin fittings.
It simply is not to be trusted.

This is why I replaced all the yellow metal on my steel boat with 316 stainless steel including making a stainless steel 3 bladed propeller

The key is to ensure all the threads sealed complete with sikaflex and prop taper filled with anhydrous lanolin

 

[vyv_cox]

Ordinary brass has zinc in it. Manganese bronze commonly used for propellors also has approx 25% of zinc in it, technically its a high strength brass.
Anyone who has ever worked in the boat trade will have come across dezincified brass props, stern gear or skin fittings.
It simply is not to be trusted.

Manganese bronze contains nearly 40% zinc. Admiralty bronze is the improved 70/30 brass containing about 28% zinc. 60/40 is the casting or hot stamping version, 70/30 is used for sheet, tubing, etc.
Brasses and bronzes

Not sure about 'not to be trusted'. Like anything aboard it has a finite life and needs inspection to determine replacement time. Hallberg Rassey and several other quality builders still fit leaded 60/40 brass skin fittings

 

[jon and michie]

Ordinary brass has zinc in it. Manganese bronze commonly used for propellors also has approx 25% of zinc in it, technically its a high strength brass.
Anyone who has ever worked in the boat trade will have come across dezincified brass props, stern gear or skin fittings.
It simply is not to be trusted.

This is why I replaced all the yellow metal on my steel boat with 316 stainless steel including making a stainless steel 3 bladed propeller

The key is to ensure all the threads sealed complete with sikaflex and prop taper filled with anhydrous lanolin

That said but majority of vessels (ie big ships) are steel hulled and have brass props and anodes a plenty

As the OP @Dorset Dan mentions in post 6 is that the previous owner had then fitted shaft anodes - which are think is standard on a fairline targa 43- if not most shaft boats to which in post 18 he say he hasnt had any problems since - so I think the key here on this thread was missing anodes on the shafts.