Cathodic protection for a ship?

[Flyback]

Thanks, when ICCP (impressed current cathodic protection) is done, they dont use just a DC power supply to supply the current...they use a train of half sine current pulses....at about 60Hz or so. This is presumably so that fusing is easier? (or is it for human safety?) , since fuse arcs will extinguish easier when the current periodically goes to zero.

So essentially, you would use an AC output amplifier, and just diode rectify its output? Probably a class B amp so that it was more efficient.

Even better, you would use a Class D type switching amplifier, and make it produce a rectified half sinusoidal output current?

How much current and voltage would be needed for a big ship needing cathodic protection?

(I had no idea before today that big metal hull ships are literally dissolving away if they dont have cathodic protection)

 

[Diver300]

Half sine current pulses at 60 Hz would probably come from the ship's mains supply, transformed and rectified but not smoothed.

It sounds like a cheap way of getting DC, rather than being the best waveform for the job.

 

[Flyback]

So its not a bad thing to have flat DC? What about fusing?

 

[Pommie]

The only cathodic protection I'm aware of is a block of magnesium welded to the aluminium hull. However, I'll be following this thread in the hope I'm proved wrong.

Mike.

 

[Buk]

I know nothing about this, but was intrigued to look and found this which you might already have seen:

It came from this page which mentions 8V @ 600A.

 

[atferrari]

The most common are the sacrificial anodes affixed outside on the hull or even the rudder. Most of them are permanently underwater.

They are renewed periodically when the vessel goes to drydock.

Run across of something much more complex in the "wave piercing" ferries (ex Tasmania) where the protection was related to isolation of the hull. Aluminum by the way.

Rather complex to me.

Should see if I could retrieve some info from my files.

 

[Flyback]

Thanks, what is the likely resistance between anode and cathode on these ships?....ie, through the seawater?
Presumbly the electronics is just a pulsed current source?
At what frequency is best?
What pulse current waveshape is best?

 

[atferrari]

Well it seems that all I have actually seen in all vessels I was, were just Zinc.

 

[Pommie]

Well it seems that all I have actually seen in all vessels I was, were just Zinc.

Actually, yes. My mistake saying Magnesium. Wasn't too sure how you'd weld Magnesium anyway.

Mike.

 

[Diver300]

Sacrificial anodes on boats are made of a more reactive metal, so they form a cell with the other metal on the boat and that generates the current that discourages the corrosion on the expensive and hard to replace parts.

The alternative is electrodes that can be made out of anything conductive, and a power supply. It's a trade-off between the cost of running a power supply the whole time and the cost of changing anodes regularly. On small boats that come out of the water every year or more often, sacrificial anodes are used. On ships where the generators are running all the time and they are only very rarely in dry dock, then cheaper electrodes can be used.

Sacrificial anodes aren't welded to the ship. They are bolted on, as they have to be replaced often and they have to be dissimilar metals. On some outboard motors, the trim tab is also the sacrificial anode. A trim tab has to be adjustable, so it is bolted on in a way that allows any angle, so to make it a sacrificial anode, the only changes have to be making it from zinc and not painting it. It is also a device that will be effective even if it's badly corroded.

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