Especially in salt water, anti-seize compounds can cause corrosion if they come into contact with water.

Question: I’ve been using anti-seize grease on the threads of the bolts that hold the lower part of the drive leg in place on my Mercruiser inboard/outboard drive. I’ve been doing this so that the bolts won’t corrode into place and to make it easier to remove the lower case when servicing my water pump. A friend of mine says this may be the worst thing I could do. Who’s right?

Answer: I’m afraid your friend is correct, especially if you’re operating in salt water. Using anti-seize grease on any fasteners that are going to end up underwater is high risk! Here’s why: anti-seize paste or grease is made up of microscopic bits of metal such as aluminum and copper, among other things. Any time you mix dissimilar metals with an electrolyte (the water your drive is sitting in), you run the risk of inducing what is known as galvanic corrosion. In fact, using anti-seize in this application could induce corrosion much more rapidly than using a waterproof grease.

The issue is of course most important in salt water, but not all fresh water is pure so it is still a potential problem.

For years I’ve been using a product actually sold by Johnson / Evinrude dealers known as “Triple Guard”. Mercury has their own version. Regardless of brand, the best product to prevent bolts from seizing in any underwater application (or wherever water may accumulate on your boat, like the bilge) is a waterproof grease; NOT anti-seize.

For more information, read a previous post about Protecting Against Marine Corrosion.