Barnacles on Anode

[makingplansfor]

On lifting out at the weekend, I was surprised to see that the main pear shaped anode had not eroded much and was covered in barnacles. The shaft anode had eroded as expected and was barnacle free. I have searched the forum for similar problems without success so looking for opinion. I suspect that the answer is about connectivity of the anode, however, from what to what etc. I guess that gives an indication of my knowledge on the subject!! Advice therefore appreciated.

 

[VicS]

To be effective the anode must be bonded with a good low resistance electrical connection to whatever it is to protect as well as being reasonably close to it.

If the hull anode is to protect the prop and shaft then it must be bonded to the engine or gearbox and any flexible coupling also bridged to complete the circuit. Or connection may be made directly to the shaft inboard of the stern gland using brushes or MGDuff's "Electroeliminator" or similar

 

[VO5]

On lifting out at the weekend, I was surprised to see that the main pear shaped anode had not eroded much and was covered in barnacles. The shaft anode had eroded as expected and was barnacle free. I have searched the forum for similar problems without success so looking for opinion. I suspect that the answer is about connectivity of the anode, however, from what to what etc. I guess that gives an indication of my knowledge on the subject!! Advice therefore appreciated.

Try this:~

First replace the anode and tighten the nuts.
Then at the inboard end loosen the nuts on the inboard studs which connect the cables to the anode.
Take a circuit tester.
Check if current flows from one inboard stud to the other via the anode.
If it does, you have connectivity and the problenm lies elsewhere (either the connections in the cables or the cables themselves).
If there is no current flow the problem is in the conact beteweeen the studs and the anode.
It may be that the studs are mucky. If so, wirebrush thoroughly and start again.
I hope this helps.

 

[makingplansfor]

Thanks for the help, I guess it stands to reason that the anode is close to what it is supposed to be protecting - I think I get it........... Will take it apart and test as suggested.

 

[makingplansfor]

I removed the anode today and was a little surprised to see a rubber backing piece which means that continuity was through the grubby nuts at the front. I am guessing that a good clean will sort it, however, I was wondering what purpose the rubber performs and if it is needed?

 

[coreng]

Barnacles proliferation

Barnacles proliferation and anodes not worn are generally a strong sign of over-protection. Next step might be bubbles on paint. To confirm, a potential measurement of concerned item is necessary (see thread galvanic corrosion, electrolysis and anodes on motor boat forum).

 

[VicS]

According to MG Duff a backing pad must be fitted between the anode and wooden or GRP hulls and renewed with the anode
(The fitting studs should also be sleeved or painted to insulate them from wooden hulls)

 

[makingplansfor]

Thanks VicS the MG Duff website provides some good info so I am happy why I need the backing layer.

Coreng - I have looked at the link and still dont really understand! Does "over protecting" simply mean that I have too much zinc in the water and if so is it safer to remove one of the anodes?

 

[coreng]

Surely not, it's more complicated ! Potential of items should not exceed - 1100 mV, otherwise it attracts hugely barnacles!

Have a look to this doc http://www.galvatest.com/WP_Cathodic_Protection.pdf

 

[john_morris_uk]

Surely not, it's more complicated ! Potential of items should not exceed - 1100 mV, otherwise it attracts hugely barnacles!

Have a look to this doc http://www.galvatest.com/WP_Cathodic_Protection.pdf

I was interested to read you article - which you call a 'white paper'.

There are one or two statements in it which don't appear to make sense.

Below the lower limit of this range (towards - values),
there will be “over-protection“ with damage that can be
significant for coatings or even for the materials
themselves (hydrogen evolution). Furthermore, overprotection
promotes the proliferation of limpets
(barnacles)…

I am interested in this statement; in all the time I have been involved with boats I have never heard of this phenomona, Perhaps others might like to comment?

Measurement of the hull potential is made using a
correctly calibrated control electrode.

I feel that we are drifting into the land of crystals and tree hugging here. I agree that everything has a potential - but the voltage that you read off your non-conductive GRP hull with some multimeter of unknown impedence is irrelevant. With respect, you might sell silver 'reference' rods, but how do you make the leap of logic that the answer has anything to do with the cathodic protection of underwater fittings. And where on earth do you get the statements about 'for a given hull they should be within these ranges...?' I note that you later admit that their use is disputed.

• Cable fault in the ground cable of the electric
propulsion motor. A resistance of 0.01 ohms will
cause a voltage drop of 200 mV in a 20 A alternator
circuit, by simple application of Ohm’s law (V=IR).
This voltage drop will become a leakage current
flowing along a propeller shaft or Z-Drive5 with
consequences one can only imagine…

Whilst the maths is correct - there is no automatic connection (pardon the pun) between electrical resistance in charging circuits and stray currents through prop-shafts. This is just scaremongering? For example, my engine is ENTIRELY electrically isolated from my propshaft and propellor and for my boat your statement cannot be true. In fact it would take exceptional circumstances and some strange wiring anomolies for it to be true in most boats.

I note that the article appears to be a translation - perhaps from French? and therefore the phrasing and wording is not always conventional English. I have tried to read through these quirks - and allow for them in my analsyis. You often use phrases that are not normally used in this technical area.

I am also sorry that despite that there appear to be real anomolies in the text and theory because much of the work is accurate and the advice is good.

Finally, I am always sceptical when a website tries to sell me something and there is no address or telephone number or e-mail in the contacts page.

 

[coreng]

Hello,

No crystal here but underwater corrosion science, just science.

And though corrosion knowledge is not always quite clear, it's a job practiced daily by thousands of engineers, but, obviously, not in pleasure crafts industry... According to the price of such boats, guess why?

1) Of course there is no potential in a GRP hull. Measurements refers to immersed metals : interconnexion or shaft, mainly.

2) Where on earth? Just search a bit about "underwater corrosion" or "galvanic scale" on Google, even on Wikipedia... Every corrosion specialist on earth knows these values and every single ship at sea is today protected as is with impressed currents... Values are not disputed, opportunity of interconnecting items is.

3) This is just an example, and of course, like every example, it only applies in conditions of example.

PS For more info search for NPL Cathodic protection documentation and ABYC E-02 recommendation on the web. Or ask for it by PM...

 

[john_morris_uk]

Hello,

No crystal here but underwater corrosion science, just science.

And though corrosion knowledge is not always quite clear, it's a job practiced daily by thousands of engineers, but, obviously, not in pleasure crafts industry... According to the price of such boats, guess why?

1) Of course there is no potential in a GRP hull. Measurements refers to immersed metals : interconnexion or shaft, mainly.

2) Where on earth? Just search a bit about "underwater corrosion" or "galvanic scale" on Google, even on Wikipedia... Every corrosion specialist on earth knows these values and every single ship at sea is today protected as is with impressed currents... Values are not disputed, opportunity of interconnecting items is.

3) This is just an example, and of course, like every example, it only applies in conditions of example.

PS For more info search for NPL Cathodic protection documentation and ABYC E-02 recommendation on the web. Or ask for it by PM...

No need for PM's: I've got nothing to hide and I think that several of my questions still haven't been answered. If you have more information which you believe would be educational and helpul, then this is one of the places you can publish it.

I am very happy to discuss science and engineering - having been an electronics engineer in the past. The diagram in your article does not make it clear that you want to use your 'silver rod reference' against the underwater metal - it shows the meter against the hull which is clearly nonsense.

There are lots of people on here who know lots about galvanic corrosion and the pitfalls and cures.

I also strongly dispute that the leisure industry ignores galvanic action of underwater metals and their protection. I don't know what your comment about cost means or implies? There may be ignorance amongst some owners, but I have yet to be persuaded that engine manufacturers and boat builders don't take necessary precautions against underwater corrosion.

I'll ask some more questions:

"If I have zinc anodes connected to a stainless shaft and a bronze propeller, what is the potential difference between the two items?"

"If the electrical connection failed, what would be the PD?"

"Can this PD ever change?(assuming the salinity of the seawater remains the same)"

You say that there are no crystals involved, but I am very allergic to snake oil as well - with respect.

I have zinc anodes providing galavanic protection on my boat. I make sure that the electrical connection between the anode and the metal I want to protect is good. I understand about the galvanic scale and the various degrees of nobility of the metals involved. What I utterly fail to understand is why I should buy a silver stick as a standard against which to measure the PD of the items involved. This is not rocket science.

One last mistake in your paper: You say that stainless steel loses its passivity underwater. This is NOT TRUE. It loses its passivity in deoxygenated water. In ordinary water or seawater its fine. Its claims like these in your article that make me anxious.

I say again - much of what you have written is accurate and fair - but there are one or two lines that don't make sense or are plain untrue and that reflects badly on the rest.

 

[coreng]

1) I don't hide. Forum or not forum, documents are copyrighted, so I won't make them public. Ask owners for them...
2) OK. The whole naval industry is making nonsense then. Possible, though doubtful... Because it is actually specialized surveyor's best practice to do so... And any Ag/AgCl calibrated electrode should do the job... Why an Ag/AgCl electrode? Read above mentionned documents. After that, you will just have to find a reliable one, which is not so simple at affordable price...
3) Dispute. I maintain...

Your questions, assuming I understood them :

a) active inox -550 mV vs Ag/AgCl and bronze -280 mV against Ag/AgCl. Difference is -270 mV. Thus, in contact and immersed in an electrolyte bronze will oxyde inox (see galvanic scale of metals)...
b) natural potentials won't move but current cannot flow...
c) of course, this is what anodes and impressed currents are intended for...

For the rest, and before telling i'm wrong, may I suggest you search for other sources (there are thousands of reports on the subject on the net and a very simple one is wikipedia : marine grade stainless). Just wonder why inox shafts pits...

 

[john_morris_uk]

1) I don't hide. Forum or not forum, documents are copyrighted, so I won't make them public. Ask owners for them...
2) OK. The whole naval industry is making nonsense then. Possible, though doubtful... Because it is actually specialized surveyor's best practice to do so... And any Ag/AgCl calibrated electrode should do the job... Why an Ag/AgCl electrode? Read above mentionned documents. After that, you will just have to find a reliable one, which is not so simple at affordable price...
3) Dispute. I maintain...

Your questions, assuming I understood them :

a) active inox -550 mV vs Ag/AgCl and bronze -280 mV against Ag/AgCl. Difference is -270 mV. Thus, in contact and immersed in an electrolyte bronze will oxyde inox (see galvanic scale of metals)... (Yes I know all about the galvanic scale - but actually the PD is close to zero as they are connected together.)
b) natural potentials won't move but current cannot flow...
c) of course, this is what anodes and impressed currents are intended for...

For the rest, and before telling i'm wrong, may I suggest you search for other sources (there are thousands of reports on the subject on the net and a very simple one is wikipedia : marine grade stainless). Just wonder why inox shafts pits...

There is much stuff on Wiki that is part of my specialist field that is complete nonsense, so we will have to agree to differ on sources. I don't see it as an authorititive source.

I am not disputing much of what you say - but you haven't answered some of my questions. For example - the claim about 'over protection producing barnacles and damaging coatings on the GRP. If you can't see that or understand that we don't seem to be communicating and I don't want to have a slanging match about it. I will allow others to make up their mind about your 'product'.

As I have said in every post, I am happy to acknowledge that much of what you say is true but a few statements you make are NOT true.

Scientific and technical literature is generally in the public domain inasmuch as you can quote things so long as you acknowledge the source - as you should know. Why not quote some of your 'sources' to support some of your more esoteric claims?

For those who are really worried about their galvanic protection I suggest reading the M G Duff website guides and/or asking a marine surveyor.

As we don't seem to be communicating - I will allow others to make up their mind about your statements and product....

 

[GrahamM376]

That was an interesting paper and answered one question which had puzzled me. With my previous engine, I relied on the shaft being bonded to the engine & anode via a bridging wire across the flexible coupling. On one haul out, when I checked the bonding with a meter it showed high resistance so, I added a shaft anode. Next haul out, the shaft and prop were covered with barnacles - far more than seen before. I suspected that the altered electrical "field" had attracted them but couldn't get confirmation, until now.

On the new setup, the Featherstream prop has its own anode which only seems to last a season and I was wondering whether to add a shaft anode again to increase its life. Not so sure now.

Another niggling thought - as antifoul contains a lot of copper, is it wise to antifoul (as most of us do) marine bronze through hulls or, is this possibly creating a problem?

 

[coreng]

Sorry John,

1) Did not see the claim on GRP and I seem to have answered your other questions. So, why this offended attitude? When overprotection occurs, barnacles are on shaft and propeller of course. GRP is protected by not conductive antifouling.
2) MgDuff is a company who has something to sell. As a matter of fact, technical information is usually detained by technical companies selling products regarding their job. Our job is effectively corrosion control on boats.
3) I gave you evident and easy sources of information and left it up to you to find them on the net (it's quite easy) or ask through PM if you don't agree with wikipedia, which I understand.


Graham,

If you know perfectly what you are measuring it's fine. I guess you deduced the cause of resistance and solved it. Shaft anode is a better idea because protection is then dedicated to the shaft. As paint won't go everywhere and usually won't stand on bronze, you need an anode on prop, too. You hardly need anything else on a GRP boat. But the ONLY way to know if all this stuff works well is measuring equipment potential with an Ag/AgCl reference electrode of any kind and of any maker. If every boater did this, shaft and props would'nt need replacement any more, just like on bigger ships... You can do this yourself, or, as says John, ask a surveyor if you know one who practices cathodic protection testing. The problem is it should be done, say, every 3 months. It's continuous (real time) on big ships.

 

[coreng]

Apologies

I apologies John. One remark on white paper was wrongly placed... You were right on this point...

 

[john_morris_uk]

Sorry John,

1) Did not see the claim on GRP My comment about GRP was because your diagram shows the meter connected between the hull and the reference b ut you have answered that question - thank you. (Perhaps you might consider changing the pictorial diagram in your advert?) and I seem to have answered your other questions. So, why this offended attitude? I am not offended and am very happy to have an exchange of questions. When overprotection occurs, barnacles are on shaft and propeller of course. GRP is protected by not conductive antifouling. I am sorry to ask more questions, but this makes no sense at all to me. It sort of doesn't make sense in English. Why should GRP be protected? Is that what you really mean - (I assume you don't mean that...)
2) MgDuff is a company who has something to sell. As a matter of fact, technical information is usually detained by technical companies selling products regarding their job. Our job is effectively corrosion control on boats.
3) I gave you evident and easy sources of information and left it up to you to find them on the net (it's quite easy) or ask through PM if you don't agree with wikipedia, which I understand. I might suggest I have a fair grasp of cathodic protection in shipping and in small craft. With respect I think its up to you to justify some of your points. If you want to quote scientific papers to support your claims, just make sure you reference them properly and quote them publically and there is no problem. This is not a subject that is contained in 'technical data from companies that need to protect their interests.' Its well documented in scientific studies - and its not rocket science.

It appears that you are suggesting that all small craft should 'check the cathodic potential of their underwater fittings' using either one of your references - or a reference from another reputable manufacturer. I suspect that there is more than one person on these forums who are as sceptical of this suddenly proposed 'necessity' for small craft. You quote big ship common practice which I happen to have some familiarity with. Steel ships have good reason to monitor their cathodic protection - they are large steel structures that have the possible additional problems of residual currents from lots of electrical systems, let alone the various metals in contact with the water and the possible problems that entails.

Small craft have very different problems. They are nearly all GRP and have a limited number of dissimilar metals underwater. Good electrical connection between appropriate anodes mounted near to the underwater metal you need to protect is all that is necessary in most cases. Monitor the wastage and you won't go far wrong. Some metals don't need protecting at all. Are you aware that the latest recommendation of many surveyors is that on many wooden boats, anodes do more harm than good. If good quality bronze fittings are used underwater they often need no protection at all.

My conclusion is that I remain UTTERLY sceptical of the suggestion that all small boat owners ought to be checking the PD of their underwater fittings every three months.

 

[coreng]

Debate

Of course GRP hulls don't need cathodic protection and then, won't suffer over-protection. But antifouling around protected items will, while wooden hulls might delignify.

Anodes usually harm wooden hulls because they are not fitted properly by yards... And this concerns some metal and GRP one, too (no names)... So, I maintain corrosion protection is widely misunderstood in pleasure crafts industry...

There ares some few very simple reasons why GRP crafts should monitor (and they all concern money saving) :

1) it tells you if your anodes are worn without pulling boat out of water...
2) costs of repairs or replacing shaft and propeller are huge vs costs of auto-control. When pitting is noticeable, it's too late...
3) aluminium sterndrive are particulary concerned and can go off within weeks (see pics on other forum)...
4) it will help identify stray currents from board or shore, and this is a very reasonable motivation...

 

[VicS]

It must be pointed out that at several places in the so called "white paper" the metals and alloys, at the top of Table A, with the most negative electrode potentials, namely magnesium, aluminium and zinc, are refererd to as being "electronegative".

This is wrong!

These metals are electropositive that is to say they have a tendency to form positive ions ( cations) in solution with the release of electrons which give rise to the negative potentials.

The Group 1 elements, the alkali metals lithium, sodium potassium etc are the most electropositive elements and have the most negative standard electrode potentials.

The Group 7 elements, the halogens fluorine, chlorine, bromine etc are the most electronegative, tending to form negative ions ( anions) in solution.


Its an easy mistake to make, and I would not guarantee that its not a mistake I have made myself but its is not a mistake that should appear in a document being promoted as a "white paper".

Mistakes in the basic terminology like this discredit the whole document.