Electrolytic and Galvanic Corrosion

Electrolytic and Galvanic Corrosion

Thursday 28 Dec 2023

Well if you have read the title … you have been warned! This blog entry may be a little technical and dry to some of you (i.e. incredibly boring). However, you might take some comfort in knowing that I won’t be discussing our battery capacity, as cruisers (men only) are apt to do at great length! Seriously, if you aren’t interested in marine electrical issues you might just want to take a quick look at the photos and move on.

As some of you know, for a couple of years now, we have been dealing with a problem of the anti-fouling paint “burning off” around the metal fitting on the outside of the boat. We were concerned that this indicated electrolytic corrosion, which can be a serious problem. It first became evident in Key West, Florida in early 2021 when a diver told us we had a “haloing” issue. We had the boat checked out by a marine electrician and he couldn’t find any issues and therefore, as we had never seen it before, we assumed it was due to stray currents from a faulty dock somewhere along our travels. However, the following year the same thing appeared to be happening so it was obviously our boat. We worked hard to identify the cause and then spent thousands of dollars at Safe Harbor New England Boat Works trying to address the problem. I’ll just say that we weren’t happy with their service and leave it at that. However, I thought I had pinned the problem down to a -ve battery cable that was not attached to the engine (as it should be) and, for the remainder of that season, it seemed like the issue was resolved. Prior to departing Morehead City in early November, I had a diver clean the hull and replace the zincs and I was distraught to hear that we once again had a “haloing” problem. In desperation, I wrote an email to BoatZincs, a company that specializes in making sacrificial anodes (zincs) and marine corrosion issues. I got a quick response from one of their experts, Bob Olsen, who said that rather than cover things in an email that we should chat on the phone. It was a fascinating and insightful conversation. He said that he had reviewed the data I had generated using a reference silver electrode (purchased from BoatZincs) and concluded that we do NOT have an electrolytic corrosion issue. It is simply a reaction between the exterior metal components and the copper in the anti-fouling paint because there is inadequate primer between the metal components and and the antifouling. He says that he handles half a dozen enquiries like this every year and that two coats of primer over the metal components (where possible) will solve the problem. The start of this issue does coincide with my re-finishing the underside of the boat and it’s quite possible I didn’t pay enough attention to this aspect, focussing mostly on getting epoxy barrier coat on the fiberglass areas. There’s nothing that we can do now except scrape barnacles off the areas where there is no longer any anti-fouling and address the problem when we haul out in the summer. 

I do want to give a shout-out to Bob Olsen at BoatZincs, he was amazingly responsive and helpful.

Bob commented that the values I reported relative to the silver reference electrode (-0.80 volts) indicated that the boat components are protected from galvanic corrosion (an unavoidable problem from immersion in salt water), but he also said that the main zinc was working really hard and suggested we use a larger sacrificial anode (zinc). We had a diver here in Jolly harbor look at the zincs and he replaced both the main zinc and the one on the end of the propeller. The latter was not a surprise as it is quite small and had been on there since June. However, the main zinc was 50-60% gone and had only been on the boat since the end of October. I don’t want to hire a diver every two months to replace it and so I need to find a way to extend the life. I couldn’t find a bigger zinc that worked with the current set up and I can’t change or add an additional through-hull fitting for a second zinc until we are up land, so I designed and built a system where I hang a second zinc off the side of the boat whilst at anchor and have it wired into the grounding system. The new zinc is big! I bolted it to an old plastic chopping board that we no longer use and attached a line and an electrical cable. I also ran a connecting cable up through the opening in the foredeck where the spare anchor chain emerges. Now I can just tie off the line, lower the cutting board/zinc over the side and plug together the wires and we have a second sacrificial anode in place. The boat reference values shifted from - 0.8 volts to - 0.9 volts, suggesting it does work, and I really think it should help preserve the life of our main zinc!

How fun is that? The most exciting blog post I've ever written!

Mike

This is an old photo (2022) showing the haloing around the grounding plate. You can't paint over this fitting (it is designed to ground the boat electrics and prevent lightening strikes). However, it does look like I painted right up to, and over, the edges of this bronze plate.

This was the new pristine main zinc on the underside of the hull, showing the impeccable anti-fouling paint ... prior to launch earlier this year (It's about 6 inches long, 3 inches wide)

The propeller and main zincs as they are now after being replaced by a diver (~6 months and ~2 months respectively in the water)

Here is the new extra zinc for deployment whilst at anchor. (~12 inches long, 2 inches wide and weighs maybe 5 lb). You can see why we no longer use that particular chopping board!

The new sacrificial anode (zinc) system hanging off the bow of Cotinga

This is what it looks like on deck, with the grounding wire emerging from the secondary anchor chain opening