Cathodic protection

The Trident 24 is a bit of a metallic cocktail beneath the waterline: cast iron ballast keel; mild steel bilge keels and rudder; stainless steel prop shaft; bronze propeller and (usually) bronze P-bracket.

The bilge keels and rudder were galvanised 40 years ago. But little of that zinc remains. To try to combat corrosion Lottie‘s  bilge keels and rudder are equipped with zinc anodes.  A prop shaft anode is used to protect the bronze prop.  The P-bracket (electrically isolated from the shaft anode by the cutless bearing) is not protected.  Lottie’s original bronze P-bracket seems to have been replaced with a stainless steel one which shows no sign of galvanic corrosion.

Anode performance

When we bought our previous Trident 24 in the 1979 it showed evidence of  galvanic  corrosion on the rudder shaft (shiny bright steel at the waterline where the shaft emerged from the bottom bearing) and on the bronze prop.  These were countered with a  zinc anode bolted to the mild steel rudder blade and a prop shaft anode.  The rudder anode seemed to prevent any further corrosion on the shaft.  The zinc  showed very little activity and never required replacement for the following seven years.  The prop shaft anodes usually lasted two seasons between replacements.  [Click on images to enlarge]

The galvanising on Lottie‘s rudder and keels is more than 20 years older and seems to offer less protection to the bilge keels.  The rudder anode showed comparatively little activity until 2010.  But since 2007 there have been signs of deep pitting corrosion on the outsides of the bilge plates and enough activity on the anodes to require them to be replaced after a season or two.  Until 2010, a single 1kg doughnut anode was through-bolted to the outside of each bilge plate.

2010:

For the 2010 season, the bilge plate anodes were replaced and doubled-up by placing 1kg zincs on the inside of each bilge keel as well as the outside (using the same bolt).  Almost as soon as Lottie was launched in 2010, the paint around the anodes seemed to bubble off.  Was the additional zinc responsible for this?

The fact that barnacles grew in places on the keel and rudder where they had never done so before confirms that the initial paint loss happened soon after launching when the free swimming barnacle larvae are still in the water.  By the end of the season the paint adhesion on the rudder seems also to have failed quite extensively – though whether these two phenomena are connected is unclear.

One theory is that there was too much cathodic protection in 2010 resulting in gassing on the metal surfaces near the anodes.  This increased activity may be the result of one or more of the following:

1. 

   Bilge keel treated with Fertan rust converter

   Fitting all-new anodes in 2010

2. Increasing the number of anodes from 4 to 6
3. Cleaning the anode contact point back to bare metal to improve conductivity

2011

For 2011, all the underwater steelwork was  cleaned back to the metal and treated with Fertan rust converter followed by  5 coats of International Primocon.

All zinc anodes were removed other than the propeller shaft anode. 

Lottie launched in April 2011 and by October it was clear that the paint was surviving on the steelwork better than it did the previous year when anodes were present.  After lift-out in November 2011 there were just one or two eruption spots of rust evident on the bilge keels and none on the rudder, though both keels and rudder had rust where the paint had been worn off when grounding.

Scrubbing the prop in June had also prevented any of the build-up of barnacles by the Autumn experienced previously.

The rust spots and grazes were scoured with a stiff wire brush in an angle grinder (while wearing safety goggles) and spot primed with five further coats of International Metallic Primocon.

 

The anode on the propellor shaft was largely wasted away after just 7 months afloat.