Waste not, want not?
Q
Over the past four years I have religiously replaced the anode on my Vetus 25 bow thruster. Each year I debate whether to let the anode go for another year as the erosion has not been very marked. I have the old anodes from previous years, so this year I weighed them. The one I replaced in March 2013 weighs 27g, March 2014 34g, March 2015 38g and a brand-new one, about to be installed, weighs 42g. These are all Vetus SET0148 anodes. Each year the anode has eroded less. Any ideas as to what is going on? At more than £20 a pop, am I wasting money?
My mooring position at Noss Marina on the Dart (annual usage, etc) has not changed. The boat, a Quicksilver 700 Weekender, only comes out of the water for a two-week period each year. Anodes at the stern of the boat, on the rudder, trim tabs, propeller nut and hull have eroded at much the same rate each year. Most look as if they would do another year, but I am persuaded that they may have become porous, so – safety first – I replace them. The anode in the engine waterway is replaced on about a six-monthly cycle, not because it erodes excessively but as a precaution against it becoming detached from the brass nut and restricting the waterway.
I have read somewhere that the anodes must be in sight of each other to be effective. Is this correct? COLIN BROWN REPLIES: Anodes waste away because they are being sacrificed to protect other metal components that are in the same circuit and are immersed in the same electrolyte. As your anodes are wasting away, they appear to be working. One possible reason for the rate of wasting reducing is that resistance in the protection circuit has been increasing. The voltage ‘pushed’ by an anode is very small, and it only takes a small increase in resistance to reduce effectiveness and reduce the rate of wasting. You could try cleaning all the metal-to-metal contact surfaces between the anodes and the things they protect.
Another possibility could be changes in the water salinity or quality. Full seawater conducts well, and conductivity reduces as salinity reduces. Zinc anodes will develop a white oxide crust in brackish water that can insulate the anode and prevent it from working. The insulation remains when the boat returns to seawater, and the anode can be ineffective. Aluminium anodes are recommended for boats kept in brackish water.
Anodes should be replaced when they have lost approximately half of their volume as beyond that they may not have sufficient surface area to keep going for another season. I’ve never heard of an anode becoming porous, although some partially-worn anodes do look that way.
You are correct in thinking that an anode has to be able to ‘see’ the component it is protecting. This is one of the reasons that I don’t recommend electrical bonding of seacocks: the anode cannot ‘see’ the inside of the seacock and hose tail, and protection is limited. I prefer to see good-quality corrosionresistant components fitted and for them to be left galvanically isolated.