How some barnacles saved my boat: Galvanic corrosion in the real world

30 Mar

Every now and then trying to fix the boat degenerates into rushing around in circles between the boat and  my workshop and I begin to feel like some sort of frenetic, freshly-decapitated chicken. It does not help when the whole yard loses power just as I’ve finally assembled the correct piles of detritus for the task at hand. That happened twice this week. On the bright side, I (re)found the bolt that could easily have sunk my boat:

I wrote about this in my last post but when noticed the offending fastener in a pile of bits it reminded me of just how dire this situation had been. Here, lets get another angle.

Now let’s cut it in half:
Well, that doesn’t tell us much of anything but it was fun.

Let’s recap. This bolt, with a little caulk, was plugging a 1″ hole in the hull. It was deep in the bilges where it would be exceedingly difficult  to find and plug by the time you noticed the cabin sole awash. By the looks of it it’s likely to have been like this for a long time. A close shave for this pretty little boat. But how did it get so horribly corroded? I decided this was a good chance to learn about galvanic corrosion, something I’ve halfheartedly been trying to wrap my mind around for a while.

Galvanic corrosion (and accelerated forms of it, such as stray-current corrosion) is the process by which when in the presence of an electrolyte two metals of dissimilar composition can create a tiny flow of electrical current. This current flow is the result of an electro-chemical reaction which leads to the accelerated decomposition of the less noble metal. The chemistry is a little beyond me but the process is relatively simple.

 All metals when immersed in a conductive fluid (the electrolyte, on boats this is usually seawater) have a unique electrical potential. If two metals with different potentials also have a direct electrical connection then a circuit is formed (a galvanic cell) whereby current flows through the electrolyte from the metal with a lower electrical potential to the metal with a higher potential. In this circuit the higher potential metal is a cathode and the lower is an anode. Due to something complex that has to do with ions this reaction that allows current to flow through the electrolyte (seawater) also steadily eats away at the anode. The cathode is unharmed. Well, at least in theory.

This reaction is why most modern boats are fitted with one or more sacrificial zincs. Zinc, having a very low electrical potential, is an anode in almost any galvanic circuit and by sacrificing the zinc you are protecting any other metals which are grounded to it. Unfortunately the real world is a lot messier than the theoretical one and just as fitting a sacrificial zinc doesn’t give you perfect protection most galvanic cells don’t operate quite how you might expect them to. There are a lot of other factors at work in the real world which are quite difficult to predict. Nonetheless, we can try. So lets look at our bolt again:

What happened to this thing?! Well before we make any predictions, let’s make some assumptions. If this was bolted into the hull to plug a hole then it must have had another washer so that it could be tightened down instead of just falling through the hole into the boat. So we’ll assume there was a washer on the outside and we’ll assume it was the same metal as the one that remains, which is aluminum. So what about the bolt, what is it? We can reasonably rule out stainless steel because corrosion is more or less even with no signs of crevice corrosion or localized corrosion. So it’s not stainless but it certainly appears to be steel. As crazy as it sounds, I think someone just went to the hardware store and plugged their boat with a plain old mild steel bolt and a couple aluminum washers.

Enter galvanic corrosion. Since the bolts and washers have an electrical connection (through their physical connection) and are immersed in seawater this will have started a galvanic cell. Looking at a table of the galvanic series of metals in seawater (page 199 in the Boatowner’s Mechanical and Electrical Manual) we find aluminum to have an electrical potential of -0.76 to -1.00 while mild steel is -0.60 to -0.71. (For brevity and sanity’s sake I’m skipping over what exactly these values represent.)

This isn’t a great difference but in seawater all it takes is a difference in voltage potential of 0.1 for a galvanic cell to form. So then what? Well since the aluminum would be serving as an anode in this circuit we can predict that the steel bolt would remain completely intact while galvanic corrosion eats the outside aluminum washer up until it is consumed enough that the whole mess fell into the bilges and the boat sinks.

Except this didn’t happen because the real world is far more complex. It does seem that our theory held true at first. It is likely that being closest to the direct physical/electrical connection of the bolt and washer the center of the outside washer was the first to corrode and once our galvanic cell enlarged the hole in this washer enough it simply fell off the bolt. This leaves us with what I found when I hauled the boat – a bolt suspended in the hull with a washer on the inside but none on the outside to hold it in place. Instead it stayed there because of caulk the use of which was the only nod to sanity of whoever installed this damn thing. This caulking throws some interesting wrenches in the gears of our nice theoretical world because it serves as both physical glue and electrical barrier. This is where our play-by-play gets to be mostly guesswork but here’s what I think really happened.

I think that the initial corrosion of the outside washer, being an active galvanic cell, was a relatively quick process and that it fell off early on. Once it fell off it seems the caulking was enough to hold things tight and dry, at the very least slowing the influx of water to a tiny trickle.

At this point the bolt and remaining washer (on the inside of the hull) would have had an electrical/physical connection but without water getting into the boat there wouldn’t have been the electrolyte needed for galvanic corrosion. I suspect at this point the rate of corrosion dropped off drastically.

However, even if the caulking was a perfect seal we have the head of the mild steel bolt immersed in the ocean and doubtlessly beginning to corrode. There’s also a pretty good chance that the bilges were wet at least some of the time which would have allowed another galvanic cell to start on the inside of the boat. This will have eaten away at the remaining washer. Again this shows how complex the real world is. Not only would the corrosion taking place on the inside of the hull be subject to variation from how and when the water in the bilges is stagnant or moving but it is also likely that the electrical connection between the bolt and washer would eventually have been broken as galvanic corrosion turned the conductive aluminum into non-conductive aluminum oxide. This is one reason why in this real world example of galvanic corrosion there is damage to both the aluminum anode and the steel cathode. Fortunately this corrosion seems to have happened at a pretty slow rate and I think things stayed like this with the bolt and inside washer held in place by caulk and quietly eroding for quite some time.

How can I say this with relative assurance? Well, here’s a picture of this part of the hull on the first day I saw my boat:

That spot to the left of the larger mass is where you would see the bolt in question if it weren’t for all the barnacles covering it up. It’s hard to guess exactly how this dreaded marine growth will have changed our ‘perfect world’ galvanic interaction but I suspect it may have saved the boat! These little critters will have been helping by somewhat protecting the bolt from corrosive seawater, slowing or blocking the influx of water into the boat and reducing water pressure on the whole mess. And to think I had completely forgotten about these barnacles until I dug this photo up. How very ungrateful of me.

So there you have it. Galvanic corrosion is easy to start, quite dangerous to a boat, and very complex to predict in the real world. It’s also quite difficult to explain so I hope this post wasn’t completely incoherent!

This article was syndicated from Safe At Harbour But Meant For The Sea: DIY Sailing with Paul Calder


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