Most boats experience corrosion of some sort, some a lot more than others. As a kid I well remember working with my dad every spring getting his inboard/outboard-powered wooden boat ready to launch. There was always lots of wire brushing and repainting metal parts and replacing zincs.
Recognizing the need for professional marine techs to better understand what causes corrosion and how to prevent it, boat electrics guru Ed Sherman wrote an article for Marine Electronics Journal focusing on one pesky type of corrosion, galvanic corrosion. Our goal in presenting the information (which is a bit techy in spots) is not to turn you into an expert but to offer some background to help you understand the problem and solution when talking to a pro.
By Ed Sherman
It seems every year around the end of haulout season here in the Northeast where I live, I get at least one and sometimes more than one call asking this basic question: "Hey Ed, how come the zincs on my boat are almost completely used up this year and last year they weren’t even half consumed?” The question is usually followed up by a series of nearly useless explanations about the boats in the adjacent slips and all their presumed faults that could contribute to this phenomenon.
If I’ve learned anything in my nearly 50 years of dealing with marine electrical and corrosion issues, when it comes to corrosion it is almost always someone else’s fault, at least in the eyes of the boat owner. So, let’s make sure that you as technicians can not only explain all of this to your customer, but also have the knowledge to help mitigate some of the problems that do crop up.
Beginning with galvanic corrosion
Let’s start out by making sure you understand what galvanic corrosion actually is. The ABYC Corrosion Certification study guide defines galvanic corrosion this way: "The most prevalent corrosion process in the marine industry. Always involves two or more dissimilar metals submerged in a common electrolyte connected by a metallic (or carbon fiber) path capable of supporting electron flow. "Dissimilar metals” in this case refers to differences in electrical potential (voltage). Galvanic corrosion is one of two types of galvanic processes; the other is simple corrosion.”
So, via this definition we have identified four constituents that fall into what we sometimes refer to as the "corrosion quadrangle”—anode, cathode, common electrolyte and conductive path. Note that rather than refer to metal 1 or 2 we refer to anode and cathode. In our definition we refer to dissimilar metals with different voltage potentials. Understand that metals with a more negative potential are going to be considered as anodic and will be the ones corroding and metals with a more positive potential are going to be considered cathodic. This is all based upon the table of nobility found in ABYC Standard E-2. All of these components need to exist for galvanic corrosion to occur. Elimination of any one of the four constituents will stop galvanic corrosion.
The problem with shore power
One of the problems with shore power systems on the dock is that every point of connection to receive power is interconnected via what we refer to as the grounding system. This is required under electrical codes and should always be the case. Maintaining earth ground continuity throughout a dock wiring system is an important safety concern. The same scenario is also found on a standards-compliant boat.
This whole matter of grounding is worthy of an entirely separate article, but for now just understand that with few exceptions everything electrical, both AC and DC, is ultimately interconnected on board the boat. Additionally, nonelectrical metal items like seacocks and other through-hull hardware may be tied into what we refer to as a bonding system. So, assuming some of these items are located below the boat’s waterline, we have metal in a common electrolyte, electrically tied together via this bonding/grounding system. Furthermore, these same bits of metal are also connected to metal on other boats sharing the same dock and shore power system and we end up with a large galvanic cell that contains all of the components needed to create the corrosion quadrangle. The net result of this can show up as corroded metal or rapid anode consumption on your customer’s boat.
One of the other dirty little secrets we’ve learned over the years is that this grounding/bonding system, normally described as "non-current carrying,” actually does carry low-level current sometimes in the form of AC leakage from certain appliances, in addition to the low-level galvanic current (DC). Any boat that spends much time at the dock and plugged into shore power needs to protect against this. The potential influences on electrical system performance are numerous and go beyond the scope of this article.
The transformer solution
The most obvious is to never plug in at the dock. This is of course not too practical for most modern boaters. Another solution is to install what is known as an isolation transformer
. These transformers are expensive, heavy, take up a fair amount of space and generate quite a bit of heat. It’s a great solution on larger boats and they are used quite frequently. In my opinion they are the ultimate solution because they completely eliminate any hardwire connection between the boat and the dock. Magnetic induction within the unit gets the power from the primary to secondary side of the transformer. It not only provides total isolation galvanically but offers other advantages like stopping electrical noise transfer from the dock system to the boat and the ability to step up or down voltage output. All that said, they are just not practical for use on small- to mid-sized boats due to the size and weight issues.
Next week: the galvanic isolator and a check list
About the author
Ed Sherman is the former longtime vice president/education at the American Boat & Yacht Council (ABYC)
. He is also the author of several technical books, including the Powerboater’s Guide to Electrical Systems
, and writes a regular column for Marine Electronics Journal
called Ed’s Electro-Tech Tips