GALVANISM VS. RUST

Q:

Hi Doc. Why not just use stainless-steel bolts to eliminate galvanic corrosion? William Gunter Hernando, Florida

A:

Hi Glen. Stainless steel is formulated to resist oxidation, aka rust. Stainless is not free from galvanic corrosion, though it happens to be more resistant to that process than many metals. But that resistance does nothing to protect other metals that will corrode more readily in the presence of stainless if both are immersed in an electrolyte such as seawater.

So, the question isn’t, “Why not stainless (or any metal)?” It’s, “Why not (insert metal) in this application using these protections?”

For example, aluminum corrodes more readily when immersed in seawater with stainless. Yet there are many outboards with stainless props and aluminum housings. This works not just for one reason, but due to a variety of factors: 1) The two are not always submerged. 2) The engine is fitted with sacrificial anodes that corrode more readily. 3) There is a series of wires—bonding—that maintains consistent electrical potential. 4) Not all outboards are used in an electrolyte. (Salt or polluted fresh water are electrolytes; clean fresh water is not.) So, in the application of outboard engines, it works. (Though, were you to check a warranty’s fine print, outboard-makers do not warranty against corrosion damage.)