I expect snorts of derision but does............

[john_morris_uk]

Finlou,

I think you will find that with the cathodic protection of boats in seawater, the idea of the anode 'seeing' the metalwork that one is trying to protect is to ensure that other 'nearer metalwork' which may be at a very slightly different point on the galvanic scale is not protected at the expense of the parts that you are trying to protect.

That is why I am happy that the anode that I put on my shaft protects the prop - even though electrically the whole thing is bonded to the keel which is at a different point on the galvanic scale, but further away...

 

[Tisme]

I am beginning to wonder if this shows that an anode <u>can</u> work round corners, but that it is more effective in line of sight?

It seems logical to assume that the electrical resistance will be lower in line of sight, simply because of the old adage that the shortest distance between two points is a straight line.

The question then remains is it more important to be in line of sight, or "electrically" near to the area being protected?

This is relevant on my boat because I have one anode that is difficult to fit in line of sight, but would be very easy to fit "just around the corner".

Any thoughts?

 

[BrendanS]

experience and lots of threads on the subject would suggest line of sight most efficient, but certainly as close as possible to the bits you are trying to protect.


Efficiency dies quite quickly in free flowing water, so if you are based somewhere with tidal flows, then I'd certainly go for line of sight and close

 

[finnlou]

[ QUOTE ]
In the same experiment, how much better is the current if they are in line of sight?

[/ QUOTE ]
You should see very little difference, the key to a galvanic cell is that both electrodes are immersed in the electrolyte not that they can "see" each other

 

[BrendanS]

a lab situation though. In flowing water things are a bit different. To realistically show what happens in real life, you'd need a realistic flow of water over the experiment

 

[finnlou]

[ QUOTE ]
Finlou,

I think you will find that with the cathodic protection of boats in seawater, the idea of the anode 'seeing' the metalwork that one is trying to protect is to ensure that other 'nearer metalwork' which may be at a very slightly different point on the galvanic scale is not protected at the expense of the parts that you are trying to protect.

That is why I am happy that the anode that I put on my shaft protects the prop - even though electrically the whole thing is bonded to the keel which is at a different point on the galvanic scale, but further away...

[/ QUOTE ]
as I said in previous posts, it is different if your hull is metal, however if your hull coating was perfect and the only bare metal was your shaft, then it wouldn't, since the current only has one place to go, the main reason they should be sited as close as possible to the item beng protected is to eliminate the effects of stray currents, if for instance you placed your anode near the bow, and then moored your boat so that it's bow was neer the stern of another boat in front of you, your current will most likely flow to their propellor not yours because it is much closer.

So, to answer the other posters question, yes, closer in an electricall circuit sense is far more important than "line of sight"

 

[finnlou]

The effects of the tide on corrosion rates is more to do with the changes in ph and conductivity of the water as the tide rises and falls and not it's actual flow, it would require a significant turbulent flow to affect the corrosion rate or effectiveness of your anodes.

 

[BrendanS]

that is not what the suppliers say? just look at the sites associated with Mercury active cathodic systems, and previous comments on these forums re tidal flows on anode systems

 

[finnlou]

[ QUOTE ]
that is not what the suppliers say? just look at the sites associated with Mercury active cathodic systems, and previous comments on these forums re tidal flows on anode systems

[/ QUOTE ]
Well, I can't find a single corrosion text book that states that the actual flow of the tide affects it's corrosion rate only that its ph and conductivity change as the tide change, therefore affecting the corrosion rate

 

[finnlou]

quote from NACE
"Special forms of
corrosion are associated with seawater velocity, e.g., (1) erosion-corrosion caused by high-velocity
silt-bearing seawater, (2) impingement attack, where air bubbles are present, and (3) cavitation, where
collapsing vapor bubbles cause mechanical damage and often corrosion damage as well."

Your anode offers NO protection from these flow based corrosions since they are not galvanic

 

[BrendanS]

do a search on river flow then, it may be more productive. I think I've lost the will to live.

 

[Graham_Wright]

Seems to me that dangling anode turned out to be a can of worms. /forums/images/graemlins/tongue.gif