Silver Half Cell for Anode Check - Am I doing it right?

[duncan_ellison]

OK, here's one for you experts on corrosion prevention, which seems to be a bit of a black art to me.

I bought a Silver/Silver Chloride Half cell electrode on AliExpress a year or so ago and made a 3D printed holder with a weight to put it over the side.

Long story short, it didn't seem to work, so I put it aside for some time and found that the liquid inside had dried up, I believe this is Potassium Chloride and I could see crystals inside. Found there was a port on the side and managed to fill the glass tube with distilled water covering the actual electrode and the crystals dissolved - yay!

I'm getting reading in the 950mV range on the propshaft / Engine / Bonding lines, which suggests an over protection. On the other hand, the boat has just be relaunched with new (zinc) anodes on the propshaft, hull and prop.

As a check, I tested between the cell and a big zinc anode (not connected to the boat) launched over the side and got around 950mV again.

Questions:
1) Is the cell to zinc value about right and is this a valid way to test the cell?

2) Could the readings be high due to the cell having dried out?

3) Should I be at all worried about the high readings?

4) At the risk of igniting a debate, none of the through hulls are connected by design (that seems to be the common way in European boats), but I'm still getting around 250mV from them which suggests that they are erroding. How come when there is no current path?

For reference: Fibreglass boat, bronze prop, stainless shaft, sea water

[duncan_ellison]

136 views and no comments ?? Maybe a little bit of a specialist question?

Anyway, I'm fairly confident my setup is working OK having tested it on a couple of other boats and the results were more or less as expected.

[hlev00]

I believe the value can be as much as -1080 mV and still be valid for protection with your materials. I have a commercial anode on my boat and will take a look at the published values there when I get a chance next time.

I have no idea how to see if what you have built is actually giving you accurate readings though.

[duncan_ellison]

hlev00 - Thanks. On another boat with known depleted anodes, I was seeing around -550mV, so it sounds about right. If I'm correct, a very high (over-protected) vessel, it's more of an issue for a steel hull.