Copper Plating on propeller

[macd]

Apart from the mechanical properties of copper as described by Vyv it also loses its antifouling properties when cathodically protected. If there is an anode to protect the prop it will also "protect" the copper plating and completely remove any antifouling effect.

I think you've got that wrong. Anodes protect against galvanic corrosion, which is not the issue. Copper is fairly noble but in salt water will succumb to chemical change (unrelated to galvanic effects). Elemental copper (and later Muntz metal) was after all used for many decades on wooden hulls as combined antifouling and barrier protection against Toredo worm. But, as Vyv writes, copper plating is probably too soft to resist the extreme turbulence of life on a prop, where it will be likely to erode mechanically.

 

[Tradewinds]

Reported to last longer

http://www.ybw.com/forums/showthread.php?337957-Product-report-TK-Line-Clear-Antifoul-spray-for-prop

http://www.ybw.com/forums/showthread.php?364411-My-MBY-report-on-antifoul-update

I hope so. I used it on my prop before I read about the 3 month bit!

 

[Pasarell]

I think you've got that wrong. Anodes protect against galvanic corrosion, which is not the issue. Copper is fairly noble but in salt water will succumb to chemical change (unrelated to galvanic effects). Elemental copper (and later Muntz metal) was after all used for many decades on wooden hulls as combined antifouling and barrier protection against Toredo worm. But, as Vyv writes, copper plating is probably too soft to resist the extreme turbulence of life on a prop, where it will be likely to erode mechanically.

Not wrong I'm afraid. I could bore you with all the reasons and references to studies but I can assure you it is correct. Connect copper to a less noble metal such as an anode and the antifouling properties will be removed.
You are, of course, correct in your comments about sheet copper being used on wooden ships to protect against fouling and borers but it was not connected to any anodes there so retained effectiveness

 

[VicS]

Not wrong I'm afraid. I could bore you with all the reasons and references to studies but I can assure you it is correct. Connect copper to a less noble metal such as an anode and the antifouling properties will be removed.

Well I for one would be interested in learning a little more.

Presumably due to the cathodic protection suppressing a small level of dissolution of copper.

You may well be starting an interesting line of discussion. I am sure very many people fit anodes to protect their props quite unnecessarily but it seems in so doing create a situation in which the prop is more susceptible to fouling??

Good old fashioned fixed blade manganese bronze prop should not need cathodic protection. Fit an anode as a precaution and then suffer from fouling of the prop ???

Different situation once you get involved with folders with a mix of alloys .......

Vyv , any thoughts?

 

[vyv_cox]

Vyv , any thoughts?

Only that if it is actually copper oxide that provides the antifouling properties maybe adding an anode prevents its formation?

 

[VicS]

Only that if it is actually copper oxide that provides the antifouling properties maybe adding an anode prevents its formation?

Yes the conditions at the cathode will be reducing so could well not only prevent the formation of oxide but also tend to reduce any that is already there.

Is the oxide responsible for antifouling properties anyway. Not all afs contains copper oxide, some contain cuprous thiocyanate.

 

[vyv_cox]

No doubt the marine creatures like thiocyanate even less than they do the oxide! However, since copper sheet and Coppercoat work OK I believe it is the oxide doing the job for them.

 

[rogerthebodger]

Some years ago there was a boat built hull built from Cupronickel that it was claimed did not need any anti fouling and stayed free of marine fouling for 16 years.

http://www.bruceroberts.com.au/building/methods/CuNi.htm

 

[vyv_cox]

Some years ago there was a boat built hull built from Cupronickel that it was claimed did not need any anti fouling and stayed free of marine fouling for 16 years.

http://www.bruceroberts.com.au/building/methods/CuNi.htm

Yes, there was a feature on it in one of the mags a couple of years ago. Unpainted either above or below the waterline, I have to say it was not the most attractive appearance I have ever seen. Copper nickel should be a good antifouling material but a rival to Coppercoat using it was spectacularly unsuccessful.

 

[Anders_P42]

I hope so. I used it on my prop before I read about the 3 month bit!

When did you spray on and when is the next lift out?

 

[JumbleDuck]

No doubt the marine creatures like thiocyanate even less than they do the oxide! However, since copper sheet and Coppercoat work OK I believe it is the oxide doing the job for them.

I have wondered for a while why the fairly small amount of copper in copper coat (50%?) seems to work - for some people, at least - while the rather larger proportion of copper in my bronze propeller doesn't. Any bright ideas?

 

[Pasarell]

Well I for one would be interested in learning a little more.

Presumably due to the cathodic protection suppressing a small level of dissolution of copper.

You may well be starting an interesting line of discussion. I am sure very many people fit anodes to protect their props quite unnecessarily but it seems in so doing create a situation in which the prop is more susceptible to fouling??

Good old fashioned fixed blade manganese bronze prop should not need cathodic protection. Fit an anode as a precaution and then suffer from fouling of the prop ???

Different situation once you get involved with folders with a mix of alloys .......

Vyv , any thoughts?

First thing to note is that metallic copper works in a very different way to copper oxide as used in most antifoulings.
In conventional (liquid paint) antifoulings copper oxide is in a soluble form and part of a blend of biocides. They are all contained in a soluble or eroding paint matrix that is designed to allow the active biocides to disperse in the water and kill fouling organisms. For yachts, especially, the paints are designed to erode quite quickly to get enough biocide into the water around the painted surface to prevent fouling from settling at the peak fouling time (June-ish around the UK). The rest of the year they release biocides at pretty much the same rate even though the fouling challenge is much lower.
With metallic copper it is the cuprous oxide surface that prevents fouling due to contact with fouling organisms. Basically the oxide layer is inhospitable to supporting life. The action is far more effective with shell fouling than with weed. When clean copper is first immersed it needs to form the oxide layer as quickly as possible to become effective, and to do that it needs clean (sea) water. If it is immersed in polluted water it will often not form an effective oxide layer at all and fail as an antifouling.
After a period of time cuprous oxide in sea water will convert to form cupric hydroxychloride which is both more fragile than cuprous oxide and less effective as an antifoulant. Weight of fouling will usually make the fragile surface break away to reveal clean copper that quickly oxidises to form cuprous oxide and the cycle starts again. Conversion to cupric hydroxychloride typically occurs every 5 to 7 years.
The difference between conventional antifoulings and metallic copper can be seen when they each stop at an immersed edge. Conventional antifoulings will still have an effect for some way over the unprotected surface as they kill organisms in the water around the surface they coat. With metallic copper the effect stops immediately at the edge of the metal and fouling organisms can live healthily right up to the metal edge.
It is assumed metallic copper stops working when cathodically protected because the surface will not corrode and oxidise although quite a few learned people think this needs more study as there may be other reasons as well.
Pure copper and copper nickel alloys have an almost identical antifouling performance in metallic form but copper nickel is usually preferred. 90:10 where antifouling is most important and 70:30 where mechanical properties take precedence. Pure copper erodes in sea water relatively quickly and at a fairly random rate. Both 90:10 and 70:30 copper nickel erode at a very predictable low rate meaning their working lives are far longer. Although there is no real evidence of marine pollution caused by metallic copper there is increasing scrutiny around the world and the rate of erosion of pure copper is causing some alarm with more cautious authorities.
Several yachts and commercial vessels have been built from copper nickel plate over the years, both 90:10 and 70:30. Provided they have been launched into unpolluted waters they have performed well against fouling and for durability. When Copper Mariner 1 had her hull plates tested for thickness after some 40 years in service there was no measurable reduction although it must be said the measurements were compared with nominal plate thickness during building and the expected erosion would only have been about 25 microns.
Much of the original studies into copper were carried out at La Que Corrosion Labs in N Carolina but they have now closed. Dan Efird was the recognised expert and he still works as a consultant, mainly to the oil industry. Most of his publications are available to purchase. Publications by Powell, Wiley, Anderson and others are also relevant

 

[vyv_cox]

I have wondered for a while why the fairly small amount of copper in copper coat (50%?) seems to work - for some people, at least - while the rather larger proportion of copper in my bronze propeller doesn't. Any bright ideas?

This is a long way from being my area of expertise but here is a thought. Copper, as in Coppercoat, is 100% pure, the other 50% is resin. Brass (assuming your prop to be a manganese bronze one) is not just a mixture of copper and zinc, it is an alloy of two phases, one copper rich, the other zinc rich. When dezincification occurs the pink metal remaining is thus the copper-rich phase, which is a solid solution containing up to 35% of zinc. This alloy does not form the copper oxide film that is needed for antifouling. Perhaps a similar situation to the earlier statement that protecting copper with an anode inhibits its antifouling properties?

I'm not quite sure what the 1% manganese does, probably forms a solid solution with the copper, increasing strength and deoxidising the brass.

 

[Taunto]

I would like to thank everyone who replied to this posting , it has been most interesting . It is not my intention to report weekly on the progress of this little experiment but i thought i would let you all know how the first 10 days have gone . Water temp is hovering around 20 to 21 deg c and the nasty's are copulating at a great rate , the boat has now run on the motor for 1 hour at around 2,500 rpm , the colour is still quite bright copper and no patina has yet to appear also there is no growth of bugs or weed , the plating on the tips have not worn off as someone suggested they might , all looks good at this stage . I will report again in a months time .
Cheers taunto.