When the anode is 70% eroded it needs to be replaced.

Martyr ( Canada Metal (Pacific) Limited ) only uses mil-spec mil-a-l800l alloy in all of their premium marine anodes. This alloy contains mostly zinc but also contains parts of cadium (0.1%) and aluminum (0.25%) the cadium helps the anode erode inward giving the anode a sandblasted appearance.

Canada Metal uses a special steel combination fastener that makes it possible to use their Balldrive, which is a hex driver with a universal ball tip.

What causes corrosion? Seawater is a good conductor and freshwater a bad conductor, so corrosion is worse in seawater. Generally, corrosion rates increase in proportion to the amount of oxygen in the water. However, cracks and crevices, which are areas starved of oxygen, become anodic and corrode also. Higher temperature increases corrosion rates - doubling for every 30 degrees C (55 degrees F). There are various types of microorganisms that can contribute to corrosion, either by removing protection or causing a corrosive environment.

Why do I need anodes? You need anodes on your engine because when two different metals are in contact, electrons will flow from the more negatively charged metal (anode) to the more positive metal (cathode). If you want to protect both types of metal from corrosion, you must add a third metal such as zinc, although magnesium and aluminum are also used. This active metal becomes the anode for both metals. The zinc or aluminum sacrifices itself to protect the other two metals, hence the term "sacrificial anode".