Sea Worthy: A rusty kind of business

Electrolysis — I hear the word constantly at work, at marinas, in chat rooms and watering holes wherever boaters are found. Seldom is the word applied correctly in the conversation though.

The word electrolysis has grown into a universal term to describe disappearing or rusting metal on boats.

I must admit that a universal term is handy to describe three different things, but it lacks a certain amount of accuracy that may prove useful when one is attempting to eradicate a symptom.

There are actually three primary causes of metal corrosion on boats.

Boats of all construction and propulsion when immersed in seawater (electrolyte) are subject to corrosion in one or all of these three flavors: galvanic, stray current and crevasse corrosion.

Galvanic corrosion is the simple chemical reaction that takes place when almost any metal is immersed in an electrolyte (as in seawater).

The metal creates a specific “potential” (think voltage). Because different metals make different voltages, the lesser of the two will plate off or sacrifice itself to the greater.

One trick to controlling this corrosion is to make all metals the same voltage by wiring them together to even their potential, and at the same time offering a sacrificial weak metal.

The green bonding wires and zinc anodes perform this function. We will cover shore cord born galvanic corrosion at a later date.

Stray current corrosion is what happens when DC current is introduced into the seawater or through the metals touching the seawater (including bilge water).

This type of corrosion can be identified by the speed in which it destroys expensive parts.

Some of my readers will attest to losing tens of thousands of dollars in running gear to a bad alternator in less than a week.

Others have lost outboard and stern drive legs in under a month to a loose wire or bad bilge pump.

There are three primary causes of this dilemma that I encounter on a regular basis.

The first is abandoned wires or connections in the bilge water, another is faulty bilge pumps and poor connections, and the third is engine alternators going bad and shunting raw current through the case ground into the running gear.

Crevasse corrosion is basically what happens when metal is constantly wet, but is starved of oxygen.

Think of a screw in wet wood. Most metals used in boat rigging and construction rely in some way on oxygen to form a corrosion resistant barrier.

An orange stain bleeding from the fitting is a good clue. Once the fastener begins to bleed it must be replaced and re-sealed.

Good rigging practice can avoid these failures when assembling or servicing a boat. The basic idea of prevention is to keep moisture from entering and/or stagnating around the metal.

A few tips: When bolting, over drill the hole by a size, then countersink. This allows the sealer to coat the entire bolt shank and form a ring of sealer under the bolt head. When screwing again, counter sink the pilot hole.

When installing inboard propellers, take the extra time to lap the keys and prop tapers.

Corrosion problems tend to be expensive to repair and difficult to accurately diagnose. If you suspect your zinc anodes are experiencing accelerated wear or you are suffering drive line failures, engine cooler failures, rampant orange stains from mounted hardware or even shocks when you touch the steering wheel, you may want to consider retaining a marine electrician or qualified surveyor to help you track down the culprit.

By the way, if readers have specific questions for me, I’ll be happy to address them in upcoming columns.


Capt. John Campbell is an accredited marine surveyor, who is associated with the Society of Accredited Marine Surveyors, the American Boat and Yacht Council, and the Collier County Marine Trades Association. His expertise includes boat and yacht surveys, damage claims work and marine-related consultation. In this introductory article, Campbell makes his suggestions on how best to secure boats when severe storms threaten.

© 2008 All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

  • Discuss
  • Print

Comments » 0

Be the first to post a comment!

Share your thoughts

Comments are the sole responsibility of the person posting them. You agree not to post comments that are off topic, defamatory, obscene, abusive, threatening or an invasion of privacy. Violators may be banned. Click here for our full user agreement.

Comments can be shared on Facebook and Yahoo!. Add both options by connecting your profiles.