Electrolytic corrosion

Electro-chemical reactions occur when currents are applied to underwater fittings
The first case history describes a severe example of the consequences of stray electric currents on skin fittings
Photographs and text by Jon Markovitz, to whom grateful thanks for his frankness and permission to publish.
I re-engined my boat this winter with a new Beta 25, doing pretty much all the installation work myself, the engineer then checked everything before running up the engine and signing off the warranty. I then connected the seacock and hull-anode bonding wire to the engine block.
Two weeks later, returning from the first weekend cruise of the season, I found I had a dead engine starting-battery (although the day was saved by jump-starting from the domestic bank). As the battery was five years old, I reckoned it was finished and bought a new one at vast expense.
Last weekend, at anchor, we were about to go to bed when I chanced to check the engine battery voltage and found it dead! As we were on a bit of a lee-shore, I jump-started the engine and moved for peace of mind to an empty mooring-buoy for the night. On the following day I had a local electrician on board to check what was going on. What in fact was going on was that I’d secured the seacock bonding wire to the starter-motor positive!! The meter was reading 16amps to the bonding circuit whenever the engine battery was on, whether we were running the engine or not. I’m also reliably informed that I’m not the first to have done this…
Moving to the harbour wall, we were able to half-dry at LW for a visual inspection, where we found considerable damage. After spending the rest of the night on alternate seacock-watches we arranged an emergency lift-out first thing in the morning.
This photographic record shows most of the damage
 New MG Duff large hull anode – lost 1/4″ all round (but still plenty left).


1/2″ seawater inlet to the galley. The skin fitting flange had totally fractured and was held in place by the Sikaflex sealant.

1/2″ seawater inlet to the engine. Appeared to be intact but broke off when touched by hand.



3/4″ galley-sink outlet seacock – badly corroded and weeping. Light visible beneath the flange in the photograph.





3/4″ seacock – threads corroded, penetration on interior of the hull.



The only two seacocks that didn’t seem affected were the heads sink outlet which is on the waterline (it was cleaner than before but hadn’t lost any structural material), and the cockpit drain seacock, which isn’t bonded at all.

Heat exchanger anodes, damaged and new.





Both old Blakes heads seacocks – outer flanges and bolts badly corroded (only replaced these parts)





Heat-exchanger and the rear cap which contains the anode was plastered with deposits from the now non-existent anode (a total of 28 engine hours since new, of which about 24 hours were with the aforementioned wiring cockup). I managed to scrape most of it off, then dissolved the rest in juice from a lemon, before inserting a new anode.

New Featherstream prop anode – lost 1/8″ all round (still plenty left for the season).





Update – more damage to brand new engine parts due to electrolysis appears to have taken place:

* heat-exchanger stack has gone pink, engineer reccomends replacement.
* gearbox (normally bulletproof) is gradually leaking ATF; repair if possible or replacement.


Some lessons and current thinking about this case history

First and foremost the obvious one. Live battery connections should not be connected incorrectly. To anywhere else the violent spark would have made it obvious that something was incorrect. In this case the resistance of the seawater was sufficient that the flow of current was fairly slow, so no spark occurred.

Stray electric currents, involving far less power than in this case, can be a problem for metals underwater. Direct and alternating currents, the latter in marinas with mains power connections, can accelerate corrosion of fittings to a considerable extent. 

All seacocks except one were bonded to the anode, and in this case to the positive terminal of the battery. Modern thinking, partially derived from the well-publicised case of the Random Harvest near-sinking, is that bonding of skin fittings and seacocks is bad practice. All seacocks should be made from DZR brass, bronze, (see Brass and Bronze pages) or another corrosion-resistant material. If this is the case there is nothing to be gained by bonding.


Severe corrosion of rudder pintles
The photographs show the upper and lower rudder pintles on a motorboat that wintered afloat, connected to shore power throughout. When lifted from the water in the spring both parts were severely pitted but other connected parts appeared to be undamaged. No part of the rudder assembly was intended to be connected electrically although it was earthed by the structure. Steering was hydraulic, with rubber hoses carrying the fluid.
Upper pintle with damage. No explanation has been found for the appearance of this damage.
Measurement of the voltage between the rudder tiller arm and the battery negative was found to be 12.7 volts, indicating that a stray current was present in the rudder. Extensive searching eventually found the fault. A hydraulic hose had been resting upon the positive terminal of an electric breaker. Long-term vibrations had worn through the rubber exterior sleeve of the hose, enabling electrical contact between the reinforcing braids of the hose and the positive terminal of the breaker. Current was able to pass into the tiller arm via the crimped steel fittings connecting the hose to the arm.
The exposed terminals of the breaker.

All photos by Jan Lasse Eilertsen


Rapid corrosion of a seacock 
The owner of the boat to which this seacock was attached had disconnected a battery in order to test its resting voltage over a period of some hours. He inadvertently disconnected the live positive cable instead of the negative. The end of the cable contacted a seacock, completing a circuit through the sea to earth potential at the drive shaft. Within a few hours the battery voltage was down to 10.7 volts.
A closer view of the fitting, showing the intensity of the attack. The boat is relatively new and the seacock had been in good condition prior to the event.

Photos: Matt Boney

Unusual chain pitting

This pitting was observed on a 2-year old Maggi Aqua 7 chain. The pitting was almost 1 mm deep in spots but totally confined to the outside faces of the links.  No galvanising was left on affected areas. The first 20 metres was affected, not damaged otherwise.

The pitting is not due to corrosion as it is highly localised on the outside of the links. Any form of corrosion due to mud, sulfurous water or similar would affect the whole surface. Investigation revealed an electrical fault in the windlass, allowing leakage of positive DC into the sea via the chain. The pitting is caused by electrical make-and-break sparking. Loss of galvanising may be coincidental, as this is the active part of the chain.

Photos: Charles Buehler.