Marine Corrosion - Part 1

By Andrew Spaulding

Marine corrosion is an insidious force that chews away at our underwater metals. At the start of any corrosion conversation, I think it is important to review terminology. Often the terms used have specific meanings that are muddled in common use, or we use portions of a term leaving the discussion open to interpretation. In an attempt to avoid both of these situations, we’ll start today’s marine corrosion discussion by reviewing some terms.

 Often you will hear the term “electrolysis” used to describe the effects of corrosion, but strictly speaking electrolysis occurs when DC current drives a chemical reaction. So electrolysis could be the cause of your corrosion, but it isn’t the correct term to use for galvanic corrosion or crevice corrosion or AC stray current corrosion - all of which could be the cause of underwater marine corrosion.

Ground is another term that is used improperly. You don’t have a ‘ground’ on your boat. However, you may have up to 5 ‘grounding’ systems onboard. There is a DC negative ground system that acts as a return path for DC current. If you have an AC (alternating current, not air conditioning) panel, you have an AC safety ground system that prevents AC electrocution. You have a bonding system connecting the underwater metals electrically for corrosion protection. You may have a RF/SSB ground plate for counterpoise. And lastly, you may have a lightening ground system for strike protection. All of these systems have a “ground” function, but they are designed to be separate, discreet systems.

Another marine corrosion term that gets misused is “zinc”. Zinc is a type of anode material that is commonly used in sacrificial anodes onboard boats. Anodes can be made of zinc, magnesium, or aluminum. For fiberglass boats in fresh water you should be using magnesium anodes. Zinc anodes don’t have enough potential to protect the underwater metals very well, if at all.

Graphic thanks to Electro-Guard, Inc. (http://www.boatcorrosion.com)

Sacrificial anodes protect under water metals with their galvanic potential. Any two metals that are electrically connected (with a wire or physically in contact) to each other and in an electrolyte generate an electrical potential that is measurable.  If the two metals are lead and copper for example, and the electrolyte is salt water the potential generated is about 100 millivolts.

Properly-sized sacrificial anodes will push the galvanic potential of the boat’s other underwater metals negative by about 200 millivolts. This is enough change in potential to ensure that the anode is sacrificed in place of the other metals. Since fresh water isn’t a great electrolyte, magnesium anodes provide the extra negative potential that is necessary to protect the other underwater metals. Aluminum-hulled boats and certain stern/sail drive applications require the use of aluminum anodes. Wood-hulled boats have different concerns all together – it is critical to not OVER protect the underwater metals in a wood boat or damage to the wood can occur.

The bonding system provides the electrical connection to connect all of the underwater metals into one big galvanic system.  Since we know what the galvanic potentials are for all of the underwater metals, we can measure them with a voltmeter. As we add anode material to the galvanic system we can see changes in the voltmeter. Also, if a piece of equipment onboard is leaking current into its ground system, we will see changes in the voltmeter reading as that equipment is turned on and off. It is through this process that we find what is causing marine corrosion to occur.

Later this week, I am headed to the harbors to conduct a corrosion survey for a boat. I will have the results to report in next week’s newsletter: Marine Corrosion Part 2. Hopefully, I will also have the answer to solve this boat’s annual battle with corrosion.