Fighting Corrosion

One of the important things to protect your boat against is corrosion.

There’s nothing mysterious about corrosion; it’s simply an electrochemical reaction that happens when electrons flow between metals that are connected or grounded through water. The electrical action causes one of the two metals – the weaker one –  to be eaten away. The process is accelerated in salt, brackish or polluted water, or water with a high mineral content.

Under water, metal parts are subjected to two basic kinds of corrosion – galvanic corrosion and stray-current corrosion.

Two Types of corrosion

Galvanic corrosion occurs whenever two or more dissimilar metals (that share a common ground) are submerged in a conductive solution such as polluted water. The chemical reaction and ensuing electrical current flow causes the most chemically active (anodic) metal to erode. If it isn’t controlled, this corrosion will eat away at drive-unit  components exposed to water. (The aluminum parts on the drive are more active than the other metal parts, and therefore more susceptible to corrosion). Another cause of galvanic corrosion is the use of shore-powered hookups that tie the aluminum drive unit to other boats.

The first sign of galvanic corrosion is paint blistering below the waterline, with white corrosion forming on those exposed metal areas. As teh corrosion become more advanced, the exposed metal starts eroding, resulting in pitting of the metal.

On the second kind of corrosion stray-current corrosion.  If an electrical current, flowing along a metal conductor, leaves the metal for a water path, it will cause non-metallic corrosion, or direct removal of the metal.  Stray-current corrosion can casue rapid deterioration of the metal.  It is usually caused by an internal current leaking from an improperly insulated circuit which seeks to ground itself through the path of least resistance – usually the aluminum drive unit.

Common sources of stray current are lead wires in bilge water or with weak or broken insulation. Trolling motors, depth finders, marine radios or electrical accessories that are not properly wired to the boat are also sources of stray current. Check things visually and call your dealer about repairs or proper materials to be used.

There are some things you can do to protect your investment against corrosion. If, however, you don’t feel comfortable with replacing or testing equipment, call you local dealer.

For one thing, you can have a gold drive unit built – some metals are more corrosion-resistant than others, and gold is the most resistant of all. But who can afford a solid gold drive unit? In fact, for maximum performance and durability, a variety of metals and alloys must be used to build an outstanding drive unit.

When  aluminum is used, the pieces will be cleaned with chemicals to ensure proper paint adhesion, sprayed with zinc chromate primer and then painted with a finish coat. In some cases, the paint will be baked on to seal metal  components in a chip resistant, protective coating.

For addional protection from galvanic corrosion, drive units are equipped  with inexpensive and easily replaceable sacrificial zinc anodes, because of their self-sacificing nature, must be inspected periodically and replaced when they have eroded by 50 percent. Check you operator’s manual for the location of each zince anode.

A few tips to remember: Don’t paint zinc anodes; this will prevent them from corroding. Also, when you’re replacing zinc anodes, be sure to scrape the anode mounting surface down to bare metal and tighten the anodes securely. As well , make sure all grounding straps are maintained between underwater stainless steel components, install an anti-corrosion anode kit. If your boat is equipped with shore power, installation of a galvanic isolator is recommended.

A hull potential test by your dealer will determine if additional corrosion protection is needed.

Finally, some food for thought: if we all become more environmentally aware while we enjoy the boating season, would we not be helping the fight against marine corrosion?