BOATWORKS
By Tom Kennedy
The Mystery of Electrolysis & Galvanic Corrosion
W
hen it comes to electricity, most folks are lost once you go beyond changing a battery or flipping a switch. Electricity is just one of those naturally occurring conditions that we have managed to harness. Most folks accept the principle of how it works as a mystery and know that it can be dangerous and destructive if not harnessed correctly. While electricity is our friend, when electrical circuits are compromised or a condition is in place to promote harmful effects, it can spell problems for the boat owner. Because boats spend a lot of time in water, they are subjected to the harmful effects of electrolysis and galvanic corrosion—especially boats in saltwater. While these two problems are caused by different issues, they both have electricity in common as the main culprit. Metal boats are much more susceptible to both of these issues than wood or fiberglass boats. Nonetheless, knowledge of how electrolysis and galvanic corrosion takes place and adopting some simple and inexpensive preventative measures can save expensive components from damage and huge repair cost. So what is electrolysis and galvanic corrosion? Both electrolysis and galvanic corrosion are caused by electrical current. They differ where the electrical current originates. Electrolysis occurs when an external current is passed through metal components on the boat. Galvanic corrosion takes place when two dissimilar metals come into contact while in an electrolyte. Clear as mud, right? Let’s break these two issues down. Electrolysis The need for auxiliary propulsion, lights and creature comforts has driven boatbuilders to incorporate complex electrical systems using both DC and AC current systems. DC circuits require batteries and charging via engine alternators or chargers connected to shore power. The AC systems are normally operated under shore power. Larger boats also have generators onboard that can develop News & Views for Southern Sailors
AC current. With all these means by which electricity is introduced to the boat, there is a great deal of opportunity for stray voltage to leak from circuits and attack electronics and metal components on the boat. Electrolysis damage is clearly the more aggressive of the two because it is being driven by much higher amps and current. A failure in the ground system or a faulty connection in an accessory on your boat can cause electrolysis to occur. Bilge pumps are common culprits. Many boat owners elect to install multiple electrical bilge pumps in various locations in the bilge. Failing to ground them all to a common ground is a sure set-up for stray voltage that can travel through the water in the bilge and attach metal components like the keel bolts and through hulls. Failure of either could result in catastrophic damage and loss of the vessel. In some cases, another boat can cause electrolysis to occur on your boat. Another boat? Yes, another boat. I see a lot of boats in marinas hooked up to shore power for extensive periods of time. Boats on the same dock at a marina commonly share the same electrical circuit. Any fault in that circuit will be transferred to every boat on that circuit. Additionally, any boat with a fault in its shore power circuit will transfer that fault to all the other boats via the dock circuit. In order to prevent electrolysis, your boat’s wiring system should be routinely checked for corrosion, and incorrect connections. If adding circuits, make sure they are installed using approved components and by approved methods. Do-it-yourself projects like adding inexpensive isolators to the alternator/generator and shore power charging circuits will go a long way to preventing electrolysis. Unless you absolutely need to be hooked up to shore power, it is advisable to disconnect when you are not on the boat at the slip. Galvanic Corrosion Boats spend much of their life in one of nature’s best electrolytes—salt water. Introduce two dissimilar metals to an electrolyte and you now have a
battery capable of generating an electrical current between the two metal components. All metals contain various amounts of charged particles called ions and therefore have differing electrical potential. These metals are categorized by their potential, otherwise known as their galvanic properties. Metals with higher electrical potential act like anodes, and metals with lower potential act as cathodes. The damage occurs when the higher potential metal is dissolved by the ion exchange occurring between the two metals via the electrolyte. The farther apart the two metals are in galvanic properties the higher the propensity for galvanic corrosion to take place. The most common galvanic damage I see occurs to the running gear. Take a bronze propeller, which has high electrical potential, and attach it to a stainless steel drive shaft with low electrical potential and immerse them in salt water and you have a perfect recipe for galvanic corrosion. An electrochemical reaction takes place, and the softer bronze prop becomes the anode while the strong stainless steel shaft acts as the cathode. Over time, the bronze prop will be eaten away. The simple solution is to introduce a softer metal with higher electrical potential than the bronze prop to act as the anode. Installing zincs on the prop shaft serves this purpose. Marine stores offer inexpensive zincs in various sizes and shapes. These zincs can be installed while hauled or by a diver. Some simply attach to a grounding point and hang in the water by a wire. Others are collars that wrap around the shaft. Every time I haul a boat for maintenance, I make sure I check the zincs and replace them if needed. I also check every through-hull and valve. I have seen valves crumble in my hand as a result of galvanic corrosion when a bronze though-hull was mated to a different metal valve. This is a setup for catastrophe when this occurs while the boat is in the water. In summary Electricity is a must for many boats See MYSTERY continued on page 67
SOUTHWINDS
May 2009
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