One of the questions I get periodically is what the correct numbers should be when checking a boat for appropriate levels of cathodic protection. Or, written in proper lay terms (totally inappropriate technical terms): “So, what should the numbas be if I want to make sure my boat won’t get attacked by electrolysis?” Its a good question and deserves a proper explanation. My lead photo shows some typical numbers for a fiberglass boat with classic stainless steel propeller shaft with bronze propeller along with some bronze through-hull fittings. The range of protection, measured in millivolts, and oh yes these are negative numbers, is -550 to -1100mV. This value will vary depending upon the boat configuration. Wooden boats for example have a protection range of -550 to -600mV. An aluminum boat would have a range of -900 to -1100mV. The imperative thing to remember in all of this is that these numbers come from the ABYC standard E-2 and are based on measurement in SALT WATER at a specific speed and temperature range. Therein lies the potential problem. What if the boat is located in fresh or brackish water in a fast water current area where the water temperature is typically colder? What should the numbers be then?
Well, there is no standard answer to this question. I can share some experiential data, but the reality is this must be handled on a case by case basis. Also, understand that in some areas, this will be a moving target that depends on a variety of environmental factors.
Checking in with one of my ABYC certified technicians located in northern Michigan, where the water is fresh and typically cold, and using a silver chloride reference cell (Ag/AgCL) He says that boats in his area rarely experience corrosion problems and that their anodes last multiple seasons typically. When testing potentials, he tells me he gets readings in the -300 to -400mV range using the silver chloride reference electrode and aluminum anodes. My own test with fresh water gave me a reading of -363mV. Understand that this value can change depending on actual water chemistry and temperature.
As for brackish water, the numbers can cover a much broader range. In my experience, in the -500 to -700mV range. The reason for this is that you will be in a coastal area where there are more contributors to the water chemistry equation. Run-off from the surrounding land is a major factor. Things like petroleum from highway run off, and fertilizer from farmlands abutting the shoreline are contributing to a really interesting blended soup that can change from day to day depending on the weather. Remember that the run-off, primarily some variation of fresh water will stratify at the surface of saltwater.
So, the question becomes what is a technician supposed to do? Well you will need to establish your own very localized benchmarks. I’ve always told people in classes I’ve conducted that a boat must get tested where it is actually going to be berthed. That’s one point. Additionally, keep in mind that in many coastal areas boats actually migrate from their berthing areas to other water chemistries on a daily basis, further complicating these issues. For boats doing that, I typically recommend an impressed current system that actually measures hull potential and adjusts its output in real time. This is the perfect, if not a bit expensive solution. For other less frequent migrators, I recommend protecting for the location where the boat spends most of its time.
So, once a technician has established a baseline for the water they are dealing with, there is one key statement found in the ABYC E-2 standard that applies. “A minimum negative shift of 200mV relative to the corrosion potential of the least noble metal being protected must be maintained.” Keep in mind that Table 1 in the E-2 standard is also based on a saltwater premise. So, typically numbers will be less negative for each of these as well depending upon the actual water chemistry. Keep that in mind when you are trying to figure out the 200mV differential.