Case Study: "My Anodes Are Yellow..."

"My anodes are yellow..."

Without much more information than that, we're capable of making a few logical assumptions.

1) The anodes in question are magnesium.

2) The anodes are being used in freshwater.

3) The anodes are working perfectly well.

Invariably, those are all sure to be the case.  The yellow is the result of iron carbonate building up on the magnesium anode.  The magnesium alloy is acting as a sacrificial anode to, in this case, the metals of the saildrive.  But the same reaction is at play with the dissolved ions in the immediate environment, as well.  Not enough of one to prevent the anode from doing its job (and, in the case of magnesium in freshwater - a very good job at at that)  But enough that something of a fingerprint of that electrochemical reaction is left behind.  The magnesium acts as a sacrificial anode to the iron ions in the water, which break down into iron carbonate and, partially, serve to plate the magnesium with a distinctive yellow tint.  All it means is that there's enough iron in the water to produce this effect.  It's kind of a sign of a healthy anode, actually.

Not every magnesium anode is going to have this exact reaction play out.  But it's common enough that we do hear about it every few months or so.