Galvanic corrosion on a thru hull

Galvanic corrosion can happen above the water too.

Galvanic corrosion can happen in many places aboard your vessel.

Many people believe that galvanic corrosion is only a problem below the waterline. We normally use bonding systems to reduce galvanic corrosion on our below water through-hulls, and we install galvanic isolators to prevent galvanic and stray current corrosion on under water metals. But the truth is galvanic corrosion can occur in many other places aboard our vessel and we must always remain vigilant at preventing it. Let us take a closer look at how the process occurs.

Galvanic corrosion requires 3 things to occur:

1 – Two different types of metal (with different inherent voltage potentials)
2 – A conductive path between the two metals
3 – The two metals must be in a common electrolytic solution

We normally think of the electrolytic solution as being seawater, whether it is the seawater around the boat in which your through-hulls and prop shaft reside, or the seawater sloshing around in your bilge. But AIR can also be an electrolytic solution when the humidity is high enough. In the photo below you will see what happened to galvanized steel washers when they are in contact with a stainless-steel bolt and in a damp, humid anchor locker with a wet balsa cored deck above it. As a weak electrical current flows between the more noble stainless steel and the less noble mild steel, the mild steel washers shed ions to maintain the electrical charge. The shed ions result in disappearing washers on the back of the cleat they are meant to secure.

Galvanic corrosion of washers on cleat
Galvanic corrosion of backing washers for a cleat

In this case the proper solution is obvious, stainless-steel washers should have been used with the stainless steel bolt. But what about when you have to use two different metals, such as bolting a stainless steel fitting to an aluminum mast? In this case the key is to electrically isolate the two dissimilar metals by use of a rubber or non-conductive gasket/sealant. Remember, salt spray and salt air are enough of an electrolytic solution to allow galvanic corrosion to progress.

Are you experiencing corrosion problems aboard your yacht? Contact us to conduct a full corrosion survey. Andy Lowe Marine Surveying is an ABYC Certified Master Technician who is ABYC certified in Corrosion.

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