What Can I do to Prevent Galvanic and Stray Current Corrosion?

Corrosion due to electrical activity can be very destructive and costly. Let’s review the basics so that this silent destroyer doesn’t hurt you!

GALVANIC CORROSION

All hulls which operate in water will be subject to some galvanic corrosion. Place two different metals in sea water, connect them with a wire and current will flow from one to another. The bad news is when the current flows, metal particles from the basic metal will deposit themselves on the noble (more corrosion resistant) metal. Eventually the basic metal will corrode away.

The following items can minimize the problems caused by galvanic corrosion:

  • Use similar metals wherever possible.
  • Make the smaller, more expensive parts (such as propellers, rudders and seacocks) from a more noble metal (graphite, platinum, titanium, stainless steel and copper nickel compounds) than the larger, less expensive items.
  • Insulate dissimilar metals with a gasket or flexible compound to avoid contact (or “electrical conductivity”) between them.
  • Bond similar metals to a “common” ground.
  • Avoid the use of graphite grease. Instead, use a lithium or moly based grease, which are advertised as being “non-conductive.”
  • Provide sacrificial anodes! Since it’s almost impossible to prevent all galvanic action, a sacrificial anode made of zinc is the most common solution. The zinc is placed in strategic locations where it can be monitored. It’s relatively cheap to replace the zinc as it deposits itself on the larger surfaces.

Your engine manufacturer installs zinc rods in the engine’s sea water cooling system. When the new engine goes into service, the zincs should be inspected hourly. As the zinc rods corrode away, a white crust of oxides and salts form on the surface, which will flake off when tapped. Later, as you gain experience, the inspections can be lengthened to daily or weekly until you determine the proper service interval. If the zinc remains clean and like new, it’s not protecting like it should.

STRAY CURRENT CORROSION

Galvanic activity usually progresses slowly, sometimes taking months or years before serious corrosion is apparent. The voltage difference between the two metals may be only millivolts (1/1000ths of a volt). Stray currents, on the other hand, can be thousands of times greater and can destroy expensive components in hours. Here are some things that may cause problems:

  • Poor insulation, especially in damp areas of the boat.
  • Undersized wiring, which causes excessive voltage drops. The electricity then tries to find a better flow path.
  • Cheap appliances which leak electricity.
  • Radio grounds with different voltages than the battery ground.
  • Lack of a common ground point.
  • Tying the AC systems neutral to the boat’s ground system without an isolation transformer.
  • Defective shore power wiring can cause problems between two boats electrically tied together at a marina.

To prevent stray current corrosion, the following practices are recommended:

  • Wire the boat like your house, not your car. Modern homes have three wires to every outlet. One from the electricity source, a return line to the electricity source and a ground wire. (Your car has only one wire that goes from the battery to the appliance. The car’s chassis is used as the return line. There is no path to ground because of the rubber tires.)
  • Use two wire marine appliances, not single wire automotive appliances (e.g., engine alternators and starters, bilge pumps, etc.). Make sure both the electrical supply wire and the return wire are large enough. Devices that work well on land may be unsuitable in a marine environment.
  • Use a common ground for all systems. Use a keel bolt to an external ground plate, not the engine block.
  • Use a Type-B isolation transformer to tie the neutral side of the AC system to the boat’s common ground.
  • Install an isolation switch to disconnect your battery when not in use.
  • Check your boat with a Voltmeter. Look for voltage readings where there should be none.

Finally, if you don’t have a lot of marine experience, call in a pro. Some corrosion problems can be quite subtle and hard to figure out. It pays to hire an expert. It’s much cheaper than replacing expensive components.


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