Anode assesment

[KenMcCulloch]

Our prop is protected by a chunky pear shaped zinc anode which was almost new when we firts had the boat 2 years ago. Last winter we decided not to do anything with it as it was only very slightly eroded. This year we have lost the MG Duff raised lettering and the surface is eroded all over. There's plenty of mass left, maybe 75% of original but I assume that as the anode erodes and the surface area decreases the rate of loss will increase? It looks to me as though it might last another season but I might be wrong about that.

Is there any useful rule of thumb or general advice about the frequency of replacements?

 

[Skipper_Christoph]

If it's got 75% left, I wouldn't replace it.

 

[awol]

The easiest replacement decision is when the anode erosion rate is high - it indicates it is doing its job. Low erosion, and I would classify 2+ years with your question as "low", leads to the question of - is it doing its job? Presumably there is no indication of de-zincification on the prop - usually a more pinkish colouration, maybe with pitting? Do check the electrical connection between the anode and the prop - it should show no resistance. Last, has the anode been cleaned with wire wool or a wire brush? It has been alleged that doing so can passivate the zinc and reduce the anode's effectiveness.

If all that is fine then another year could also be fine but as you are asking the question and only have to pay a half or a third of the cost (how's the hunt going?), why not just do it? I can recommend http://www.zincsmart.com/ as a source and usually cheaper than your local corner chandlery.

 

[xeitosaphil]

" has the anode been cleaned with wire wool or a wire brush? It has been alleged that doing so can passivate the zinc and reduce the anode's effectiveness."

I was under the impression that it should be cleaned with a wire brush to remove the salt/calcification or whatever it’s called to expose fresh Zinc.

Have I been making its effectiveness worse then?

 

[vyv_cox]

I clean mine with a wire brush, or a file if the pitting is deep. I cannot think of a way in which the effectiveness of a zinc anode (or any other metal) would be reduced by wire brushing. An iron keel is a different matter

I get two or three seasons out of my shaft anode. A big anode will protect the relatively small galvanic voltage between a brass prop and a stainless steel shaft for an appreciable time.

I think the only criterion for decision making regarding whether the anode should be replaced or not is whether it will last another season. In the OP's case there seems little doubt that it will.

 

[Stork_III]

" has the anode been cleaned with wire wool or a wire brush? It has been alleged that doing so can passivate the zinc and reduce the anode's effectiveness."

I was under the impression that it should be cleaned with a wire brush to remove the salt/calcification or whatever it’s called to expose fresh Zinc.

Have I been making its effectiveness worse then?

Cleaned yes, with wire brush NO. Use a carborundum wheel or similar.

 

[awol]

From the Volvo website - thus the use of "alleged".

If an anode for example looks yellow or is covered in white crust it has been pacified and needs to be brushed or changed to provide protection. This can be done by brushing the anodes using sandpaper or an emery cloth. Note, never use a wire brush with steel bristles. You have to use an emery cloth without iron oxide otherwise the anode might be pacified. - See more at: http://www.volvopenta.com/volvopent...ges/FAQ_corrosion_1.aspx#sthash.WvuIcnvu.dpuf

 

[xeitosaphil]

Cleaned yes, with wire brush NO. Use a carborundum wheel or similar.

Ok will try that instead then when next ashore, it will be interesting to see how much of the new anode fitted this year has gone in comparison to previous years.

 

[coreng]

The job of an anode is to lower the potential of the protected metals by delivering a very small quantity of current (about 20 mA/m²) during a scheduled period of time. So far, amps and volts are linked by Ohm's law. Current recommended naval practice is then to measure potential of immersed items (vs an Ag/AgCl electrode) to ensure anodes are - still - connected, working properly and that there are no stray currents from boat or shore.

 

[vyv_cox]

I have been trying to find the source of the information that anodes should not be wire brushed. Although this text is not the original, it does explain the findings. http://www.boatzincs.com/use_milspec_a18001k.html

Very small amounts of iron (0.0014%) in pure zinc cause it to become passivated. Alloying the zinc with small amounts of aluminium and cadmium can increase this figure to 0.005%.

Whether wire brushing or filing the anode can increase its iron content is another question! What does seem certain is that any such action could only affect a shallow surface layer of of the anode.

 

[Baddox]

" has the anode been cleaned with wire wool or a wire brush? It has been alleged that doing so can passivate the zinc and reduce the anode's effectiveness."

Despite having seen the claims of passivation, I wire-brush my anodes to remove any insoluble salts that have built up and could form a barrier between the anode and water. It’s very unlikely that any iron from the brush will passivate the zinc anode; the only way I could think of that happening is if a surface coating of iron was deposited over the entire surface of the zinc almost like a waterproof shell. Any gaps in the coverage would leave points of attack for the anode to dissolve and allow water in behind the coating.

 

[KenMcCulloch]

Thanks for the comments and suggestions. The prop is fine, there was electrical continuity between prop and anode when checked in March. When the boat came out last year there was quite a bit of hard white deposit over the surface of the anode but that was scraped off and there is much more erosion evident from this season. We will think about it some more.

 

[daviddb]

I can recommend http://www.zincsmart.com/ as a source and usually cheaper than your local corner chandlery.

Thanks for the link Mr.Awol

 

[VicMallows]

Thanks for the comments and suggestions.

As already indicated, don't fall into the trap of thinking that the anode SHOULD wear away. I have only changed my pear anode once in 30 years - and then it was more because of the state of the studs on the inside of the boat. Prop and shaft are original and in perfect condition.

 

[sunquest]

" has the anode been cleaned with wire wool or a wire brush? It has been alleged that doing so can passivate the zinc and reduce the anode's effectiveness."

I was under the impression that it should be cleaned with a wire brush to remove the salt/calcification or whatever it’s called to expose fresh Zinc.

Perhaps I am lucky, when the boat is lifted and jet washed they jet wash the anodes which blasts off all the built

 

[joha]

Rule of thumb is to replace when it loses 50% of its mass I believe. So you have 25% to lose before replacement.

J