steel hull corossion & zincs

[Pasarell]

Sorry - been travelling for a few days and no PC access. As Vic has already said it is surface area that counts, not mass. It's all a bit of a black art I'm afraid as there are so many variables and anodes are only one part of the equation. Water temperature, salinity and coating thickness all make a big difference. Basically the more anodes and the higher the temperature the greater coating thickness you need to avoid cathodic disbondment. After that different coatings types give different levels of protection etc etc! Obviously surface preparation and application play a part too

 

[fuss]

Re: steel hull corossion & zincs

Hi Peter,
How long do your anodes last? I have an aluminium boat and after sorting the stray current problems have not replaced any anodes in the last 2 years. They are eaten but no longer very fast.
I also checked my systems with a silver cell.
John

 

[Rick Simpson]

Try the M G Duff website.

I have 14 anodes on my hull (and none on the rudder). She was built for the French Navy. When I bought her I used a specialist steel vessel surveyor (who had worked for M G Duff). He thought she "might" be overprotected.

Apart from a mains earth problem I had when moored alongside a small ship (which stripped alot of the paint off) :0( I have had no problems. I replace 3 or 4 anodes a season, and swap them round to even up the consumption. Some do develop a white crust (same location every time). It is obviously much better that they are consumed - you know that they are working; however, I wouldn't be able to judge what excessive consumption was, but overall I'm OK with 4/14=30% per annum.

It's good to have a voltage measurement process and baseline figures though (thanks), at least I can then monitor changes if any.

One bizarre thing I was told though, was that anodes should be able to 'see' each other ie any anode should have line of sight to one other anode. I can't see the logic of this.

Hi there 'R'. To answer your comment on anodes, anodes operate on a line-of-sight principal, hence their 'throw' operates in a straight line on the same plane, and it cannot go around sharp corners or 90 degree bends. Another anode would need to be fitted on a different plane of the hull.
As mentioned above, too many anodes on a surface can create a situation of "over-protection" and this results in the formation of hydrogen at the cathodic end of the reaction cycle, so hydrogen bubbles will form underneath the coating. The cathodic reaction also causes the formation of alkalinity on the surface of the steel. Alkalinity is the enemy of paint, destroying the binder and causing loss of adhesion. This is why the paint in affected areas will form blisters, and when they are popped open the steel underneath is still silver due to the cathodic reactions.
Once the blisters are opened, the reaction becomes anodic and red rust will begin to form on the steel.
The same reactions are caused when using an impressed-current system on a boat and the voltage is set too high.
On aluminium-hulled boats this can result in holes forming in the hull within a month or less.
So blisters due to the paint problems are extremely rare indeed, and 99% of them are due to cathodic reactions on the hull.
On wooden hulls the reactions between anodes and the bonded system on board that cause alkalinity, normally joined via a though-hull fitting, will cause the wood around the fitting to start rotting, as alkalinity is also the enemy of lignin, the resin that holds wooden the fibres together.