Will Sacrificial Anodes Work if Not Submerged?
For anodes to work, they need to be immersed in the same electrolyte as the metals they're protecting.
December 19, 2014
Question: In the photo I sent in you can see my engine shaft stuffing box with a sacrificial anode installed on the shaft. All the gear shown is inside the boat, and none of it ever gets submerged in seawater.
The zinc anode on the propeller shaft inside the boat is doing nothing to protect the metal underwater outside.
A friend of mine and I are arguing over the effectiveness of this anode. I contend that since it’s not actually submerged in seawater (the electrolyte), it’s basically doing nothing. My friend says that as long as it is electrically connected to the metal it serves, it will work just fine. Can you please resolve this argument once and for all? This anode has been on the boat for almost seven years now and it sure doesn’t look to me like it’s sacrificed any metal.
Answer: You win. It’s important for people to remember that all of this sacrificial anode stuff applies to what happens in a galvanic cell, where we have an anode, a cathode, an electrical connection between the anode and cathode, and all of it is sitting in the same electrolyte solution. With everything sized properly, the anode will become sacrificial and protect the cathode from corroding, at least for the expected service life of the anode, which is based on its mass and material. So in your photo, even if the bilge on the boat were flooded, the best that could happen is that the shaft section on the inside of the boat would get some cathodic protection. The propeller and the shaft section on the outside of the hull would not be protected by the anode in the photo.
Whoever installed that anode was definitely misinformed.
