This is a video of an outboard engine connected to a corrosion meter. You can see the potential drop as the outboard goes into the water. To summarize simply, this is because because of the anodes attached to the lower unit. As the lower unit enters the water, the cathode (other metals on the outboard engine) are flooded with negative electrons from the anodes which drives the electrical potential around the outboard engine down. This is how your anodes on your boat and outboard engines significantly slows that galvanic corrosion process down and wears at the anodes instead of the more noble metals on your boat or outboard engines. br br Remember as the anodes wear away, the negative electrons also decrease and your more noble metals began to wear faster. It is best to change your anodes when they are more than 50 worn. I have seen significant galvanic corrosion damage with 15 of the anodes still remaining around the protected metal. br br More about galvanic corrosion can be found in ABYC E-2. br br Capt.