Do I need an anode?

[alec]

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Fully agree. There is more male cow manure talked on here about anodes than anything else, for which the previous post by c*pns*ns*bl* is a perfect example.

Anodes are fitted in order to preferentially corrode, thus protecting something higher in the galvanic series that would otherwise corrode. So if nothing is corroding then nothing needs to be protected.

My GRP boat has an anode on the propeller and another on the shaft. Nothing else. I take the earlier point about the P-bracket but it is a difficult shape to protect and my preference is for epoxy coating it as an alternative. My skin fittings, almost certainly brass with a small tin content, have been in service for 22 years with no evident dezincification. What else is corroding?

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Yes. My boat is 34 yrs old (GRP) and still has the same bronze seacocks. Neither has it ever had an anode other than the one for the prop.

I would mention though that the prop does lose material without one. A few year's ago it failed the 'ring' test and went a bit pink. The anode has a tendency to fall off.

I am now finding a 'threaded' anode a bit more permanent.

 

[CaptainBob]

Reading this thread with interest... is there a good online guide to the science behind this?

Also - how do you tell if a brass skin fitting has de-zincified or not? And/or how do you tell if it's actually made of bronze? (bronze is immune to this problem right?)

 

[VicS]

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Reading this thread with interest... is there a good online guide to the science behind this?

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This article on the National Physical Laboratory website http://www.npl.co.uk/upload/pdf/cathodic_protection.pdf covers the basic theory. You'll note reference to impressed current systems as well as sacrificial anodes. Unfortunately much of what you will find to read refers to those systems

A link to a second article on the NPL website unfortunately does not seem to work.

The MGDuff website has this on the subject. IMO it is worth remembering that they are in business to sell anodes. They are good at telling you why you should fit them but may not be so good at explaining why you may not need them.

Brass skin fittings will dezincify unless they are made of dezincification resistant (DZR) brass. That is a leaded brass which has also been heat treated. If a brass fitting takes on a pinkish colour that is a sign that dezincification is occurring. The zinc is lost, leaving just the copper behind with little or no strength.

The "hammer test" ie whack it with a hammer, is well known. If it breaks it is dezincified. The down sides of this are fairly obvious!

Difficult to tell bronze and brass in service apart. If filed off to a clean surface the experienced eye may spot a difference in colour. Chemical or spectroscopic analysis is the only certain way I think you will find.

 

[capnsensible]

Cheers and understand. My only real point is that the year we spent alongside a steel yacht really took chunks from our keel and took time and worry to repair. I can only base my views on what I have seen. There is far more corrosion happening, in my observations, than a text book will give credit to.

 

[MarkGrubb]

Thanks for your comment. I had little idea how lead would react, which was why I was careful to say that it may need protecting...I wouldn't want to mislead people.