Rusty anode stud

lumphammer

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While preparing for new anodes I noticed that one of the mounting studs for the main anode is very rusty inside the boat.
Does this mean that there is a bad connection on this stud as it should surely be protected by the anode?
 

VicS

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Most anode studs are mild steel. The anode won't protect against rust either inside or outside.

The outside part will be protected.
They are normally zinc plated to protect them from rusting when out of the water.
Stainless steel ones are available if one wants
 

alahol2

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My anode studs were definitely protected from rust, until they went into the water. End of the season they were covered in rust. It's only surface rust and the nuts come off easily and can be re-used for several years.
 

lumphammer

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So forgive my ignorance, but I assumed that because the anodes are bonded to internal metalwork i.e. the engine, they are supposed to reduce corrosion of the parts they were connected to, which I supposed also included the stud.

Is it only metal in contact with sea water that is protected?
 

alahol2

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As I understand it the anode is supposed to protect the external metallic bits of a boat that are in contact with the sea. So that means the through hull fittings and the prop/shaft. It is normally recommended that the anode be connected to the items to be protected, close to the items to be protected and within 'line of sight'. The engine itself is unlikely to receive much protection from an external anode, it normally has an internal anode for that purpose. The anode is usually connected to the engine only because it creates the circuit to the prop (as long as any flexible coupling is bridged). I think the anode studs do receive some protection from severe corrosion but they will still get some rust.
Having said all that, I have had boats with no anode, anode with no bonding and anode fully bonded. In all cases I have never seen any noticeable corrosion so is it all snake oil?
 

coreng

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Which corrosion?

May I help?

For galvanic or electrolytic corrosion to occur you need 2 dissimilar metals in contact immersed in same elecrolyte (sea water). Metals immersed are protected, metal non immersed are not and are subject, then, to atmospheric corrosion (except if kept wet, though...). Anodes fight galvanic corrosion only...
 
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VicS

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This article from the National Physical Laboratory makes useful reading into the theory of what we usually call "galvanic" corrosion and the use of sacrificial anodes. http://www.npl.co.uk/upload/pdf/cathodic_protection.pdf

( The term "electrolytic" corrosion is usually reserved for corrosion driven by an external emf as opposed to that produced by dissimilar metals)
 
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