Bronze prop with no anodes – what will happen?

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BabaYaga

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This scenario: Feathering propeller made from nickel-aluminium bronze (AB2, nibral) on stainless steel shaft, with stainless steel prop nut in GRP boat. No anode on prop or shaft, also no hull anode. Shaft electrically insulated from gearbox. No bonding of other underwater metal objects.
What would be the most likely outcome, in terms of corrosion or other damage? In other words, what is the anode/anodes protecting against?
 
Typically the prop will suffer in favour of the stainless steel shaft, although NAB is fairly resistant; what does the prop manufacturer say about anode protection?
 
My Flexofold 2 blade has no anodes and is fine. The reason why many feathering and folding props have anodes is to protect the "bronze" from the stainless steel pivot pins and in some cases gears, rather than the shaft.
 
The manufacturer supplies anodes (£ 37 + shipping), regular replacement is a condition for warranty, though mine is well out at 16 years.
No moving or bearing parts from stainless steel in the prop, only the six bolts holding it together.
My intention is to have the blades copper-plated and skip the anode.
Will probably have to take it apart for inspection on an annual basis the next few years, to pick up any signs of corrosion early.
 
My variprop had some noticeable pitting after a couple of years in the water with no prop anode fitted, though there was another anode nearby. I put one on, and also added a galvanic isolator
 
vyv_cox said:
Nickel aluminium bronze is one of the most highly corrosion resistant alloys in seawater. Most ships props are made from it as it also resists cavitation well. AFAIK they do not use anodes in most cases.
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Ship certainly do use anodes. In addition to fixed, Impressed Current Cathodic Protection systems . You will usually find that sacrificial anodes are fitted in the rudder area.
 
Alex_Blackwood said:
Ship certainly do use anodes. In addition to fixed, Impressed Current Cathodic Protection systems . You will usually find that sacrificial anodes are fitted in the rudder area.
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They do not necessarily protect the prop(s). Anodes are located and bonded ro the objects they are expected to protect and in general hull anodes and those attached to rudders are nothing to do with propellers.
 
Alex_Blackwood said:
Ship certainly do use anodes. In addition to fixed, Impressed Current Cathodic Protection systems . You will usually find that sacrificial anodes are fitted in the rudder area.
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Of course ships use anodes in conjunction with impressed current, as do offshore platforms, semi-subs, etc. But mostly not for protection of Nibral props.
 
BabaYaga said:
The manufacturer supplies anodes (£ 37 + shipping), regular replacement is a condition for warranty, though mine is well out at 16 years.
No moving or bearing parts from stainless steel in the prop, only the six bolts holding it together.
My intention is to have the blades copper-plated and skip the anode.
Will probably have to take it apart for inspection on an annual basis the next few years, to pick up any signs of corrosion early.
Click to expand...
Assume you have been fitting anodes up to now. How long do they last? They are only doing something if they get depleted. I expect the maker recommends anodes to cover their backs. For a long time Flexofold did not fit anodes, but changed partly because customers expect them. The anodes are very small and in many cases last for several years.

Copper plating will reduce the volume of the bronze exposed to seawater, as will painting with Velox which I have done, but its value is more in reducing fouling than corrosion.
 
Tranona said:
Assume you have been fitting anodes up to now. How long do they last? They are only doing something if they get depleted. I expect the maker recommends anodes to cover their backs. For a long time Flexofold did not fit anodes, but changed partly because customers expect them. The anodes are very small and in many cases last for several years.

Copper plating will reduce the volume of the bronze exposed to seawater, as will painting with Velox which I have done, but its value is more in reducing fouling than corrosion.
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Original anodes from the manufacturer last a season or two, as in practice they lose material around the bolt holes, despite isolating the bolts (not a very good design). Lately I have used a beefier anode from an other manufacturer (same bolt distance), out of which I get three seasons.
The main purpose of the experiment though is not to avoid the cost of anodes, but to reduce (eliminate hopefully) fouling by barnacles. The hub/gear housing will be painted.
 
burgundyben said:
Do you know the pros and cons of NAB vs Manganese Bronze?
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There is a world of difference between them. NAB is a genuine bronze given additional properties by the nickel. As said earlier, its corrosion and cavitation resistances are excellent, making it the top choice for propellers and piped seawater systems.
Manganese bronze is not a genuine bronze, it is a 60/40 brass with small additions of other metals. Hence its susceptibility to dezincification.
Lots about these on my website Brasses and bronzes
 
BabaYaga said:
Original anodes from the manufacturer last a season or two, as in practice they lose material around the bolt holes, despite isolating the bolts (not a very good design). Lately I have used a beefier anode from an other manufacturer (same bolt distance), out of which I get three seasons.
The main purpose of the experiment though is not to avoid the cost of anodes, but to reduce (eliminate hopefully) fouling by barnacles. The hub/gear housing will be painted.
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I you can find it, Velox is the thing to use to deter barnacles . That is Trilux on the hub as get very few on there and does not affect propulsion. Been on 3 years and so far nothing more than a bit of slime similar to what you get on Coppercoat.
IMG_20200623_150111.jpg
 
Tranona said:
I you can find it, Velox is the thing to use to deter barnacles . That is Trilux on the hub as get very few on there and does not affect propulsion. Been on 3 years and so far nothing more than a bit of slime similar to what you get on Coppercoat.
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About 70 my of copper on the blades is said to last at least 5 years – we shall see.
Probably Trilux on the hub.
 
Tape over the surfaces not to be plated before they go in the tank; a decent plater will either do it for you or tell you what type of tape or coating to use in order to not contaminate the plating solution. The plating is only needed on the outside anyway so the hub could feasibly be plated while assembled and the orifices plugged with cork or rubber. Presumably the blades have gear teeth and bearing surfaces anyway?
 
I suppose that is possible, but would also be somewhat complicated. There is already taping to be done on the internal parts of the blades. Maybe next time around, if this experiment turns out well.
 
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