Revelations about zinc electrodes.

I have found it very interesting, and humbling.

 

Yes impressed current protection works very well (when its working).

 

Hi Mike,

I am never too old to learn. If we do misunderstand the cathodic protection actual work, could you explain why in many instances, it is working on an object, if you say it is nonsense.

Can you explain why the one object is getting rusted and the other who has no protection get rusted on the scratches during dew and low hanging clouds and rain, which seem to have some salt content close to the coast.

I believe that the very thin layer of dew is sufficient to make the cathodic protection actual work. Dew get everywhere.

I do know the capacitance principle, whereby two plates are being mounted on an object and by alternating the potential, the object becomes the dialectricum. I am not too sure how well that one is working, but cathodic protection could work on a boat stored at home, close to the coast. Do you really believe that the electrolyte layer is too thin? I should take an Ohm meter and measure whether we actual have a conductive circuit.

Will let you know, when we have dew.
Bert

 

As Mike has noted, these impressed current systems do work, sometimes! They are mostly used on steel hulls, the main problem though, is the electrical supply. If the boat has shore supply, that's great, simply because of the amount of current required day in day out.

Take a typical 10m boat and assume the current density of around 50mA/m2, this would nominally consume around 1.5amps, ir 250amp hours per week! Hence very large batteries and a charging system would be need...heavy and expensive..is it worth it? So these systems are generally used to protect small items such as an out-drive.

 

Hi Geo, I agree with your statement.

That black box could be the alternating ( approx every 1 second) of the potential of the two electrodes. You alluminium yacht will be come the dialectric . By alternating, the electrons are being accumulated at the one side, and in another second at the otherside. Hereby preventing the object (Your yacht, which is the dialectric) from corroding. I sometimes had the suspicion that the big metal blocks were actual zinc, but never bothered to check.
P.S. The plates has to be reasonable large, but I am not willing to put my head on the block that it actual is working. I have one unit here at home. I will one day disolve the epoxy and see what is inside.

 

You don't need to and perhaps just look outside your terms of reference for a minute. There are lots of rusting galvanised items.

Look at a galvanised sheet of corrugated steel roofing with a patch where the zinc is worn off. It's almost completely covered in zinc, so it has a massive surface area of Zinc protecting a small bare patch of exposed steel. But you'll observe that small patch of steel rusting happily away when the sheet of roofing is wet.
In a coastal environment it rusts even faster. Dip it in the ocean and it stops rusting, in fact it gets deposited material on the bare steel surface and the zinc erodes.

That's the most obvious illustration I can suggest.

If you really want to understand cathodic protection you need to understand both properties of materials and enough chemistry to fully understand redox reactions. It's never a case of simply believing something, it needs to be a sensible hypothesis for starters. That hypothesis should then also be clearly observable.

It would be really nice if it worked, but it is nonsense sorry.

 

John, is there a reason why you use such a high value of 50 mA/m2 I thought that 5 - 10 mA/m2 is more than enough. Except if the boat is a neglected boat with lots of scatches and paint pealed off. Is there a reason?
Bert

 

Thanks Mike,
Your point is taken. Good example. Bert

 

Would there be commercial interest in an autonomous cathodic protection system where maintenance is limited to replacing just a single "zinc electrode" while still protecting the hull from bow to stern?

For a large vessel the electrode would in fact be an array in a convenient location near the waterline.

 

Hi John,
I am not criticising your finger slip on the keyboard (1,5 Amps should read 0.5 Amp it also happens sometimes to me), but I would say: 10 m2 x 10mA/m2 = 0.1amps x 7 days x 24 hours is only 16.8 Amperhour. A solar panel could deliver that per week. But the normal zinc protection is more practical and the energy from the solarpanel could be used for something more useful. Except if it is proven that 10 mA/m2 is sufficient and one like to waste energy.
Bert

 

Actually the energy demand is much less Bert.
Supplying John's 1.5 Amps to a metal hull can be done with a potential of no more than 0.5 V.
The demand in a week would be 1.5 x 0.5 x 24 x 7 = 126 Watt/hours. Taken from a 12V battery that would be slightly more than 10 AH.

Your example would use 0.1 x 0.5 x 24 x 7 = 8.4 Watt/hour or 0.7 AH from a 12V battery and a very simple voltage converter.

I omit the 10% converter loss here because it is not significant for the calculation.

 

You are totally right. I was not thinking in line of voltage regulator down to 0.5 Volt, but more thinking in line with protection of oil pipelines we did with solar cells. Certainly an efficient voltage regulator would be a good solution.

CDK, when should one go for forced protection and when should one go for natural protection. I assume if a yacht is an harbour and there are lots of stray currents around, one would opt for forced protection.
Bert

 

If you buy a fiberglass boat like a Searay with stern drives nowadays, it comes standard with a Mercathode system (funny name huh, not Meranode), so manufacturers recognize its usefulness.

If I were responsible for the exploitation of a large ship I would go for an active system because having divers replace zinc electrodes every two years seems like an expensive operation to me. For a steel yacht taken out every few years for an anti-fouling coat it would depend on the installation cost of such a system.

In principle an active system needs not be more expensive than zinc, but you know how the boating world is organized: if it works better and/or has a display, it costs a fortune.

 

I would like to thank the entire community for a very pleasant read. It would never have occurred to me that I would read an eleven page "discussion" on cathodic/anodic corrosion. My ear is tuned to CDK's voice, and I (as always) appreciate the methodical clarity brought to subjects he is interested in.

Thank you all. (Maybe you're not done

 

The Impressed current systems need reference electrodes, they may vary their voltage and current to correctly polarize the protected metal relative to seawater.

The voltage between the hull and the surrounding seawater is maintained at around 1 volt (I think the recommended minimum is 0.95V now and no longer 0.85V). Note that is the seawater hull minimum potential difference over the protected area.

The common Platinum electrode material is usually deposited on titanium ( from memory) and these electrodes are run at up to 10 volts to achieve active protection ( that's the limit for a the bonded titanium_Platinum electrode). Other Electrodes are run at higher voltages. The amount of current will depend on the area protected and the paint film condition. Recommendations are for active systems that can accommodate 10% paint film damage.

This is one reason why zincs are so good, they will protect any amount of bare steel in return for a reduced life. An active protection system will just run out of it's ability to supply energy and the hull will corrode.

The resistance of Seawater with which these systems can work can vary up to 6 Thousand Ohms at 3m radius. An electric field is present in the seawater around the active electrodes encompassing the hull, and with concentrations at the paint film discontinuities. Film formation alters the surface properties both inorganic and bio-organic films can passivate a surface or reduce it’s relative potential significantly, this occurs an both the hull and the active electode, so much larger voltages are required than you would initially expect.

This is not a trivial simple application and it is not the same game as a passive Anode. There have been some notable and expensive failures with impressed current systems and they have their limitations.