Zinc anode theory and practice

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alexhibbert

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Hi all,

I know anodes can be quite a pandora's box, but I had a couple of specific questions for those who are very experienced with using anodes on seawater boats.

1. Is there a rule of thumb regarding proximity? I know the action is via electrical potential difference not a 'barrier' defence - but for instance would a hull anode, mounted forward on the hull, but then sent through the hull and internally connected via a cable to the stern drive, still protect the stern drive? Or does the physical anode need to be within a few inches of what it's protecting?

2. If it's necessary to have interior fixtures and fittings that contain unlike metals, so there's a risk of galvanic corrosion, what's the best way to protect using anodes? Can you simply clamp on small anodes near the at risk areas? Or if a large structure, connect a larger anode and expect this to act to protect all the various interfaces between, for example, galv steel and 304 A2 stainless?
(I've seen the debates on this about how the theory that any mixing of metals quickly ending in rusty tears is often untrue in practice, so long as there's no standing water or a high ratio of one metal to the other)

Thanks.
 
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A hull anode will not protect a stern drive housing no matter where it is placed. Galvanic action on sterndrives (and saildrives) comes about because of the mixed metals (stainless and aluminium) use in their construction. Anodes need to be attached to the aluminium housing itself as is standard. Even if you were able to bond an external anode to the housing close to it, for example using a hanging anode, the built in anodes would erode first because they are closest so your external anode would only come into play when the standard anodes have gone.
 
alexhibbert said:
Hi all,

I know anodes can be quite a pandora's box, but I had a couple of specific questions for those who are very experienced with using anodes on seawater boats.

1. Is there a rule of thumb regarding proximity? I know the action is via electrical potential difference not a 'barrier' defence - but for instance would a hull anode, mounted forward on the hull, but then sent through the hull and internally connected via a cable to the stern drive, still protect the stern drive? Or does the physical anode need to be within a few inches of what it's protecting?

2. If it's necessary to have interior fixtures and fittings that contain unlike metals, so there's a risk of galvanic corrosion, what's the best way to protect using anodes? Can you simply clamp on small anodes near the at risk areas? Or if a large structure, connect a larger anode and expect this to act to protect all the various interfaces between, for example, galv steel and 304 A2 stainless?
(I've seen the debates on this about how the theory that any mixing of metals quickly ending in rusty tears is often untrue in practice, so long as there's no standing water or a high ratio of one metal to the other)

Thanks.
Click to expand...
1. An anode should be s close as possible to the item it is fitted to protect. have a good electrical connection to it and "be able see it"
An anode mounted on the forward part of the hull will give no effective protection to an item at the stern
Note what Tranona says about anodes on various parts of a sterndrive

2. I dont understand what you are getting at with your second question. Interior fittings are not usually immersed in seawater.
 
You can check the protection level of your underwater fittings with a reference electrode and a multimeter, Ag/AgCl electrodes are available commercially (at a price) otherwise you may use the medical electrodes, those with a sticky pad come at a few euro for a dozen, their coating is thin so you can do a few measurements with one then you have to scrap it and use another, no big deal.
Connect one probe of the multimeter to a piece of insulated cable with the electrode and put it in the water next to the sterndrive, then with the other meter probe touch some bare metal of the stern drive.
If you read between 0.9 and 1.1v then (light alloy) is correctly protected, anything below that (should be around 0.7v for totally unprotected alu) then you need to add more anodes.
I put some photos of a few tests made at home with some salt water and pieces of different metals found in the kitchen :)
Controllore protezione galvanica fai da te
 
VicS said:
1. An anode should be s close as possible to the item it is fitted to protect. have a good electrical connection to it and "be able see it"
An anode mounted on the forward part of the hull will give no effective protection to an item at the stern
Note what Tranona says about anodes on various parts of a sterndrive

2. I dont understand what you are getting at with your second question. Interior fittings are not usually immersed in seawater.
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Thanks for the varied advice all.

1. Ok, that's understood. Although, this video appears to show sterndrive protection via a proximate anode, connected only via a cable to the internal part of the sterndrive.

2. This is the question really - is there a great deal to be worried about with unlike metals mechanically fixed together inside a boat, with no water immersion, but inevitably exposed to sea air? Some elsewhere appear to think galvanic corrosion will be an issue.
 
alexhibbert said:
Thanks for the varied advice all.

1. Ok, that's understood. Although, this video appears to show sterndrive protection via a proximate anode, connected only via a cable to the internal part of the sterndrive.

2. This is the question really - is there a great deal to be worried about with unlike metals mechanically fixed together inside a boat, with no water immersion, but inevitably exposed to sea air? Some elsewhere appear to think galvanic corrosion will be an issue.
Click to expand...
1. The video in the link did not appear to mention stern drives, but maybe I missed it. However a hull anode may give some protection to a stern drive or sail drive provided it is bonded internally to that part or parts of the drive that are , externally, immersed in the water. It should only be seen as back up protection in case the anode(s) mounted on the drive become completely lost. In the case of a sail drive a hull anode can be bonded internally to the sail drive assembly to provide emergency protection if the ring anode is lost. In the case of a stern rive there are numerous external parts usually each with their own anodes. Unless they are all boded together a single hull anode will not protect them all .
Maintain the protective coating and make sure you identify all the anodes on a stern drive and change them when necessary. You should be able to get a full set of anodes for a stern dive as a complete kit.

2 . Galvanic corrosion requires the two dissimilar metals to be electrically connected and to be immersed in ( the same pool of ) seawater. An anode to protect them must be bonded to the least noble and immersed in the same pool of seawater. This is not a situation which should exist internally. An anode not immersed in the same pool of seawater will do nothing.

Some corrosion may occur in damp conditions but its not something you are likely to be able to prevent with an anode. Avoidance of dissimilar metal contact, Isolation of dissimilar metals or protective coatings will be the way forward
 
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Some good information in the above, I'll try to add some other advice from my experience:
  • Electricity is "lazy" - that is to say, it will take the path of least resistance. The further away an anode is from it's cathode, the less likely it will be to work. I have seen cases where a vessel moored one way round will eat a hull anode in one season, moored the other way round it would last two. This is against a steel piled quay.
  • The connection between the anode and it's cathode need to be really good, otherwise the circuit won't be effective and it won't offer protection. The most common issue I have seen with anodes is insufficient bonding (too small cable size, corrosion on connections, broken terminals, ring terminals on paint or sealant, etc). Most times the connection on the sterndrive/transom bracket end is poor.
  • The anodes fitted to sterndrives are sized only to protect the drive & transom bracket. They will not provide protection for a badly wired pontoon, the rusting steel ship hull in the water nearby, or a plethora of through hulls all bonded to it with no additional protection - I have seen all these cases in real life....
  • If fitting an additional hull anode to protect a sterndrive, install it on the transom - between the drives if a pair, next to it if a single. It won't interfere with any water flow over the hull and is close enough to provide some additional protection (see notes about proximity & proper connection above). As already mentioned, the drive will consume it's own anodes first, but a properly bonded hull anode will slow this down.
  • "If you bond one, bond them all" If adding or changing your cathodic protection, pay attention to what the boatbuilder has already done regarding through hull penetration's (skin fittings, drives, thrusters). The rule of thumb is either to bond none, and let the anodes on each component take care of those individually, or bond them all, and ensure the total cathodic protection for the vessel is appropriately sized (this will usually require an additional hull anode). It is not recommended to bond some fittings but not all, as if you get a stray current leak this could be disastrous (I have seen a sterndrive literally fall off due to stray current corrosion).
 
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