Hello Mat, scary expression -Anodes are wearing away nicely. When I pull my boat out of the water I like to see minimal oxidation of the anodes. This tells me that I don't have a big electrolysis problem. Now I can only guess as to your situation but here goes. If you are using self eroding antifoul the reason for the growth around the anode may be that as you sail/motor along, the water passing over the section of the hull with the anode is disturbed and lifts away from the hull. It's like the anode is dragging its own bit of water along preventing the self eroding antifoul from breaking away from the hull.
Second possibility - When an anode is working it gives off oxygen which may encourage the growth of marine life above and closely around its location. This probably wouldn't be noticeable in a healthy water environment but some locations where boats are kept, like dead end marina with minimal tide movement, have a reduced amount of oxygen in the water. So your anodes may be providing a better location for growth.
If your boat is in a location that lacks oxygen, beware of crevice corrosion in any stainless steel fittings that are below the waterline. Stainless steel (being an alloy) also needs oxygen to oxidise to protect itself, just like aluminium.

I think the correct term for what happens underwater with electrolysis is electrical potential.

The greater the difference between two metals, ie anode and cathode (on the atomic scale) submerged in the electrolyte (sea water), the greater will be the electron flow between the two metals and therefore the erosion of one of them.

If you have a metal hulled yacht, anodes are critical.

Normally one could expect anodes to last for a year before replacement is needed.

If at a haul out the anodes are half eroded, they should be replaced. They are cheap enough in comparison to the risk of not replacing them. The second half I believe will erode quicker than the first half.

If they need replacing after only 6 months, it would be a good idea to investigate why. There are plenty of experienced people around who could probably spot it in a moment.

Hello Matt
Sorry for the late reply. Yes you are correct, the anodes are sacrificial. But it is the rate that they dissolve (for want of a better word) that indicates how hard they are working. Yours could be wearing way correctly and nothing needs to be done. There is a way of measuring the potential difference with a Silver Silver Chloride Half Cell

Theoretically to make a metal last forever, simply raise its relative voltage 225-250 mV by placing sacrificial anodes near it. When you test with the Silver Silver Chloride Half Cell you would need to put the cell in the water near the fitting and the other side of the meter on the fitting (inside the boat). My boat has a steel hull with S/S prop shaft and some more S/S in the rudder but I would do the test in different locations around it due to the impedance (resistance) of the steel.

Gundy

Bit of reading to help you sleep


http://www.marinemechanic.com/site/page37.html

www.marinedirect.com.au/catalogue/c2000/p107159

www.boatzincs.com/corrosion-reference-electrode-specs.html?gclid=COvwxpqtv68CFWlLpgod1yrEwA

MatM14 and others,

Sorry this is a late reply. I have only just started reading the forums. The contributors to this thread are correct about the sacrifical nature of "anodes". The rate of sacrifice will however be increased if you spend a lot of time in a marina and plugged into the shore power. As marinas normally have a common earth is very common for an earth leakage from a boat or number of boats to find its way into your boat via the earth of your power cable and then travel out into the marina via your boat. The most likely place for this stray power to leave your boat is via your anodes. This causes the anodes to be eroded at a higher rate than they should. The fix is realively simple -either don't plug into marina shore power (often not possible) or have an electrician check your shore power for leakage into your boat and if leakage inwards is found fit a "electrolysis blocker" to your earth lead. When we had ours checked we found we had 4 amps coming in via our earth lead.This device stops stray current coming into your boat while still allowing leakage to go to earth from your boat if a fault occurs. Your sparkie shouild be able to source the "blocker" for about $250.