Originally Posted by Searider
Limescale type deposits on propellers are characteristic of the anodes working well. Known as anodic chalk. Can't quite remember the chemistry but most well protected boats we look at have a film on the propellers (bronze or SS).
Surely it should be known as Cathodic Chalk - since it forms on the cathode?
I'm speculating on the chemistry because I can't see an obvious way to form CaCO3 electolytically. So two reactions can occur at the cathode:
O2 + 4H+ + 4e- --> 2 (H2O)
O2 + 2 H2O + 4e- --> 4 (OH-)
this will result in a local increase in pH which will make CaCO3 less soluble.
doesn't help explain why sometimes it happens and sometime it doesn't though.