De zinkification of prop.

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snoozydude

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I discovered on my last haulout I am experiencing a lot of de-zincification of my prop ever since I moved to my latest pen berth. Tips of the prop blades are brittle and the prop is very pitted and blotchy.

The boat has never had an anode that I know of, though it may well have
corroded off before I purchased her as she was in poor condition and
the electrics were in a mess. I turn off all electrics when not sailing. (she is sailed a few hours every weekend).

I never connect to mains power however some of my neighbours are
constantly connected.

Hull type GRP.

Drive train.
IGM10 - Gearbox - Fixed coupling - 20cm bare shaft - PSS seal - 30mm
bare shaft - 3 blade prop.

>check for potential differences (voltages. millivoltages) between
ships components in contact with the water

From the above advice from another post I expect I could do this with a multimeter by checking between shaft and engine block where I should see a significant potential, the cause of my de-zincification (assuming the block is earthed).

How to fix.

Hanging an anode overboard - so how does this work?
Suspend an anode close to the prop?

Does the anode need to be connected via a wire to earth of just hung on a string?

I do not suppose putting an anode on the shaft aft of the gearbox coupling inboard not in contact with the water would do anything would it?

Any other suggestions until I can put on an anode on the small 30mm of bare shaft forward of the prop would be much appreciated.
 
Thats nasty looking .
You'd not expect to see that with a decent bronze prop. Maybe its manganese bronze. That's used for props but is a copper zinc alloy, so is really a brass not a true bronze.

Nevertheless your problem seems to be associated your new mooring. It's connection to shorepower that is the usual first suspect but you are not. Hopefully you have nothing else that is making an electrical connection to the pontoon?

You say you switch off all your electrics when not sailing but I'm a bit worried when you say they were a bit of a mess. I just hope you really are isolating the battery completely. If the negative side is connected to something in contact with the water and there is a stray positive connection to the shaft and prop, perhaps via a piece of equipment, that might account for it but would not link it to your change of mooring.

By all means check with a sensitive voltmeter looking for a positive reading on the shaft and prop compared with some other metal bits in contact with the water. The trouble is that there will naturally be a small positive reading on copper alloy bits so its then a question of what is natural and what is not.

As a temporary solution you could try a suspended anode but it will have to be suspended fairly close to the prop and have a decent cable connecting it to the propshaft immediately inboard of the stern gland with a good strong clip (don't forget to remove it through before starting up) No not on a piece of string, a good low resistance electrical connection is needed. Don't be surprised if the anode wastes away quickly.

You are quite right an anode not in the water will achieve nothing.

There does not seem to be enough space for a shaft anode. You will find a recommended minimum distance between prop and anode on www.mgduff.co.uk I think but it may be necessary to fit it anyway. It will waste away quickly especially if the cause of the present problem is not found. A good sized hull anode as close to the prop as possible could be an alternative but it will have to have a good electrical connection to the shaft inboard of the stern gland. Relying on a low resistance path via the gear box (and a bridging connection on any flexible coupling) may not be adequate. The MGDuff website shows a brush system that can be used.

You really do need to establish the reason for the present situation. Shorepower is eliminated, make sure you have no other connection to the pontoon, make absolutely certain there is nothing silly happening on board with your own electrics.
Nothing funny about the water you are moored in I suppose.

That leaves some totally inexplicable stray currents around your berth as the possible cause /forums/images/graemlins/confused.gif Dodgy mains wiring on the pontoon, dodgy wiring on another boat. Couldn't explain it anyway.

You are going to need a new prop before long I'd guess. Make sure it's real tin bronze. The way scrap metal prices are going maybe someone will pinch the existing one next time you are hauled out then the insurance will buy you a new one!
 
We got to Salcombe before one blade fell off; the rest of it looked like that. But 30 years ago not many people knew about cathodic protection.; fortunately home port was Chichester and we were pointed towards M G Duff and implemented their proposals: one long anode under the quarter, internally wired to seacock and engine. Only later with a fin and skeg design did I add a shaft anode.
That's all it takes, and with respect to the multimeter brigade, you may never quantify the problem or even find the source.
Check annually; if you have to replace every couple of seasons then don't worry, the anode is doing its job.
 
When we got Gladys we had to replace shaft (bent) and prop (de-zinced looking worse than yours). I have just removed the shaft anodes that were put on when she was being repaired October 2006. She was then effectively in the water for 12 months, and the anodes were more than 75% gone. THIS year she's getting two of the LARGE MG Duff shaft anodes...

It's difficult to sea whether there is room behind your prop for a shaft anode, but that would be the easiest way of protecting prop and shaft. You also need to protecrt seacocks etc...
 
[ QUOTE ]
You also need to protect seacocks etc...



[/ QUOTE ] With respect, no you don't. Provided the seacocks are not connected to anything else electrically and they are made of good quality bronze they should be fine without any protection.

However the reason that the prop needs cathodic protection is that it is connected to other metals which are in contact with the water. Hence an electrical circuit is made and electrolytic corrosion occurs. It doesn't need any other connections to batteries etc for this to happen although stray currents can exacerbate the problem dramatically.
 
Re: Dezincification of prop.

[ QUOTE ]
However the reason that the prop needs cathodic protection is that it is connected to other metals which are in contact with the water. Hence an electrical circuit is made and electrolytic corrosion occurs

[/ QUOTE ]

BUT, look at the galvanic series and you will see that copper and its alloys are towards the "noble" end so it is the "other metals" that will suffer from corrosion not the copper alloys.

To be pedantic this is normally called galvanic corrosion, the term electrolytic corrosion being used when it is driven by an externally applied EMF. In Dug's case though it is probably the correct description!

You are right about the seacocks. Due to whatever is causing the prop problem, however, they will be at risk if there is an electrical connection to them and Dug would be well advised to inspect them and remove any electrical connections he finds.
 
Had a look at you picture of the badly corroded prop. Try and remove the prop hang on a bit of wire and gently tap with a small metal object ie old bolt or hammer, should ring like a bell but if you get a dull thud or muffled sound then you should really think about replacing the prop.
If its a case of a new one, hit the old one very hard and see what happens or take a piece off one blade and observe the cut, bet it looks a bit pink or muddy colour, this will show you the extent of your problem and possibly how lucky that you have not lost a blade underway.
 
<<< BUT, look at the galvanic series and you will see that copper and its alloys are towards the "noble" end so it is the "other metals" that will suffer from corrosion not the copper alloys. >>>

But some alloys suffer from galvanic corrosion between constituent phases. The vast majority of propellers are made from 60/40 brass, modified by additions of other elements such as tin, magnesium, aluminium, manganese, etc. The phase structure of the alloy is then (alpha plus beta) brass, the two of which occupy different positions in the galvanic series. As we are aware, they then corrode in seawater, the zinc-rich beta phase preferentially being lost.
DZR (dezincification resistant) brasses have a reduced zinc content (37%) to minimise the beta phase content.

Several copper alloys have similar problems, aluminium bronzes being another example.
 
According to the surveyor who adviced the previous owner of my boat, the prop was dezincified and because there was no room for a shaft annode that he should fit a slipper ring system on the shaft internally, and bond it to the hull annode.
I am intending making this system myself, which unless I am wrong is simply a set of brushes running on a copper sleeve attached to the shaft and connedted to the annode, but I am also wondering if one large brush would do the job as well, as opposed to having two. Any thoughts?
 
Re: Dezincification of prop.

[ QUOTE ]
[ QUOTE ]
However the reason that the prop needs cathodic protection is that it is connected to other metals which are in contact with the water. Hence an electrical circuit is made and electrolytic corrosion occurs

[/ QUOTE ]

BUT, look at the galvanic series and you will see that copper and its alloys are towards the "noble" end so it is the "other metals" that will suffer from corrosion not the copper alloys.

To be pedantic this is normally called galvanic corrosion, the term electrolytic corrosion being used when it is driven by an externally applied EMF. In Dug's case though it is probably the correct description!

You are right about the seacocks. Due to whatever is causing the prop problem, however, they will be at risk if there is an electrical connection to them and Dug would be well advised to inspect them and remove any electrical connections he finds.

[/ QUOTE ]OK Mr Picky. You say tomato I say tomayto.

I haven't heard of the distinction between Galvanic and Electrolytic corrosion being made in this way but you may be right. Its the same process anyway - one's driven by stray currents and the other by naturally occurring currents from different metals on the galvanic scale. The end result is loss of prop' either way...
 
[ QUOTE ]
But some alloys suffer from galvanic corrosion between constituent phases.

[/ QUOTE ] Thanks for confirming that. it's something I always believed was the case but I have never been able to confirm it. I doubt if its the cause of Dugs problem as that seems to be something to do with his new berth.

It should make the alpha brasses at very low risk of dezincification but are they, or is it just that for physical properties reasons the alpha beta brasses are more commonly used.

On the subject of DZR brass what I have found out is that it is a leaded brass which is also heat treated. Do you have any more to add, or is that even wrong? I thought that unless cooled very slowly brasses with less than 39% of zinc were entirely alpha phase although from the phase diagram I deduce that the solubility of zinc in copper is only about 35% at 250C. Changed greatly though I suppose by the other alloying elements. Does the heat treatment of DZR brass involve a fairly rapid cooling to prevent the formation of the beta phase?
 
How big is the gap between the end of the shaft and the rudder? If it is sufficient you could change the propeller nut for one that takes an anode, and uses a stainless tab washer instead of a split pin for security.
 
Yes, I don't disagree with that. Plenty of other posts suggest that the absence of an anode is the fundamental cause of the problem so I didn't bother to repeat it. If you fit a hull anode instead of one on the shaft, make sure that it can 'see' the propeller and place it as close as possible. Your brushes should work fine, it's the method used on high speed machinery to overcome arcing problems. So long as there is electrical continuity between the prop and anode I doubt if the brush design matters a lot.
 
My information is that the phase diagram in this area can be confusing and that the beta can be present down to lower percentages of zinc. Presumably the presence of other elements has an effect. My source names arsenic as the critical element although, like you, I understood that these alloys contain lead. However, I know from other work that lead is often added for completely different reasons, e.g. casting improvements and machinability, so it may be present for one of these rather than corrosion resistance.

I don't really know if either alpha or beta brass has better corrosion resistance but having only one removes the galvanic influence.
 
The phase-diagram indicates that the alloy at appr. 800 degrees C above 34,5 % Zn starts to have beta-phase. Fast-cooling will prevent that the phase-diagram is followed and the condition of the alloy at the temperature of heating will be frozen. Fast cooling does therefore not make much sense, unless one would start cooling from about 450 degrees at which temperature the alpha.phase has the highest solubility of zinc, 39 % as you mentioned.
As far as I know lead is not soluble in copper-alloys. The normal way to make these alloys is to inject the lead in small droplets during the casting of the material as close to the temperature of solidification as possible. The only reaason to add lead is to increase the machinability. The lead is added to alloys in the alpha-beta region as well as to pure alpha-alloys. My textbook writes: "The alloys of the two-phase alpha-beta region are characterized by their good warm-deformability and in combination with lead-additions of up to 3 % for their good machining characteristics". Since the lead is present as pure lead in a finely dispersed form, it could also play a role in galvanic corosion. because it is a two-phase alloy.
 
[ QUOTE ]
....the alloy at appr. 800 degrees C above 34,5 % Zn starts to have beta-phase......

[/ QUOTE ] HEAVEN PRESERVE US! as a simple yottie with a similar prop problem to solve I've been watching this thread in the hope of adding to my meagre knowledge. On the contrary I'm now suffering from dezincification of brain /forums/images/graemlins/confused.gif!!

Barry Edwards, yacht Phoenix, Conwy (Yes Vyv, Barry of Boatropes your rope supplier, thankfully ropes don't corrode galvanically or electrolytically or in any phase at all.... On reflection I'll email you.)
 
[ QUOTE ]
HEAVEN PRESERVE US! as a simple yottie with a similar prop problem to solve ....

[/ QUOTE ]
I see your point! But there are a lot of inquisitive minds who are interested in the science behind it all, and (as is apparent in the above posts) a wealth of experience.
If you want a simple answer, here goes:
If you can afford a bronze prop then buy one. If (like me) you have to make do with a brass one, which most are these days, then it needs the protection of zinc anodes. A shaft anode is good, if you have room to fit one. A hull anode, fixed close to, and in line of sight of, the prop is a good idea. Fix it with a proper anode fixing bolt, and use a grounding wire to connect the anode, shaft and engine block together.
All the above assumes a sound 12v wiring system with no leakage, and no connection to the mains to complicate things. And the MGDuff website is a good source of practical information.
 
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