Electrical schem for cathodic protection system

[cupoftea]

Hi
Do you know a schematic for a cathodic protection system for a ship?
I googled for it , etc but found nothing

 

[igeorge]

Can you be more explicite ?
“Cathodic protection system for a ship” ???????????
What ship are you talking about ?
What protection ?
How is your cathodic protection works ?
If you want an answer, then develop your question .

 

[KlausST]

H,

corrosion protection with a sacrificial anode?

Klaus

 

[igeorge]

Thank you, Klaus.
Unfortunately I do not know.

 

[betwixt]

Sacrificial anodes and cathodes do not normally have any connection other than to the base metal it is protecting.

Brian.

 

[FvM]

Sacrifical anode doesn't involve an electrical circuit. Thus I guess the question is about impressed current (ICCP) system https://en.wikipedia.org/wiki/Cathodic_protection#Impressed_current_systems

You can easily find some electrical parameters, e.g. https://dieselship.co.uk/2020/07/07/impressed-current-cathodic-protection-iccp-on-ships/

 

[cupoftea]

Thanks, when ICCP 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 , 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?

 

[KlausST]

Hi

How do you think this should work?
When ship is still? Or in the ocean?

Klaus

 

[dick_freebird]

I would go real simple. A marker light to the zinc from a battery that gets topped up by engine but blocked by diode so your zinc doesn't run down the starting battery as it throws "plating current" 24/7. Lamp provides indication of function.

Current will go with exposed metal surface area, some experimentation or research to determine effectiveness vs current (more than needed just eats zinc for nothing and runs down aux battery).

 

[c_mitra]

How do you think this should work?
When ship is still? Or in the ocean?

It is very often a simple metal bar (aluminum or magnesium) welded nicely to the hull. It must touch the water level.

It works like a battery whose terminals have been shorted. The sea water is the electrolyte and the metal is dissolved away in preference to the iron (because of electrochemical potential). It does not mater whether the ship is in the harbor or in the ocean. It stars working as soon as the sea water makes contact with it.

In its absence, different parts of the same body (iron) may develop small potentials (for various reasons) and cause corrosion.

--- Updated Nov 10, 2021 ---

Thanks, when ICCP 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 , since fuse arcs will extinguish easier when the current periodically goes to zero.

What is the idea? what are the voltage and current levels involved? can you please be more explicit?

 

[KlausST]

Hi,

@c_mitra:
I know how such a protection system works.

But for my understanding the anode will be destroyed. Ions will be dissolved in water and have to move to the "defective" place and will become a protective metal surface (being recovered).
This works great in enamelled water boilers. .... because the metal ions are enclosed in the boiler volume and have enough time to move from anode to the defective place.

But do the ions in the open sea alo have enough time .. without being washed away. The sea is an almost infinite volume....

...yes, and I know that the system automatically stops when the defective surface is repaired.

Klaus

 

[c_mitra]

But for my understanding the anode will be destroyed. Ions will be dissolved in water and have to move to the "defective" place and will become a protective metal surface (being recovered).

My understanding is different.

The ions getting dissolved from the bar do not move to another place on the hull; they just disappear in the ocean.

For example, consider a Leclanche cell: two elecrodes are Zn and Cu. When current passes, Zn dissolves as ZN++, SO-- and Cu++ deposits on the electrode surface. What would happen if the Cu electrode were indeed placed in a salt water bath containing very little copper ions?

In the ideal case, each electrode is in contact with its corresponding ion. When the copper ion is missing in solution, another cation with a lower deposition potential will be deposited. If the copper electrode is immersed in a salt water, hydrogen gas will evolve from the cu electrode.

It does not provide any protective surface; it provides a reducing potential to the hull so that the sacrificial electrode will preferentially get dissolved.

Whatever may be the case, the net result is the dissolution of metal into the great ocean.

 

[KlausST]

Hi,

My understanding (referring to corrodic protection of boilers)

there is the galvanic series of materials.
two different metals, electrically connected in (not deionized water) will cause a voltage to build up
(according galvanic series. used in batteries)
Current flows.
This makes the least nobal metal to be destroyed and the higher nobal electrode will be plated with the less nobal material. As soon as it is plated both electrodes have the same voltage and the current gets reduced.

So in boilers there are magnesium anodes and the boiler is made of steel.

Further my understanding:
In an ideal case this takes a rather short time and all is in optimal condition. corrosion is stopped.

But the problem is that the water in a boiler is exchanged (fresh water in, warm water out) thus new corrosive mertial comes in (oxygen, acid, ...) that may destroy the thin metal coating.
So it comes to a somehow continous process.

... and even worse with a ship in salty sea wather.

But as said. It just how I understand this. There´s a good chance that this is not (completely) correct.

Klaus

added:
your Zn and Cu test.
If you do this in a glas of water, then I´m rather sure that the Cu gets plated with Zn. (Cu electrode becomes grey)
When you apply external voltage you will speed up this process.
As a child I did a lot of "galvanic experiments" ... with applied small DC voltage.
(and almost any piece of metal I found laying around)

 

[c_mitra]

This makes the least nobal metal to be destroyed and the higher nobal electrode will be plated with the less nobal material. As soon as it is plated both electrodes have the same voltage and the current gets reduced.

So in boilers there are magnesium anodes and the boiler is made of steel.

Your understanding is correct.

However, (i) for both electrodes to have the same potential they must be same material and as one electrode is pure magnesium, the other electrode must be completely and fully coated magnesium (unbroken) layer. (ii) magnesium deposition is not possible in aqueous solution (magnesium is electrolyzed from molten salt) and instead hydrogen will be produced.

However, in my example with Leclance cell, with Cu and Zn electrodes, it is theoretically possible for Zn to be plated on the Cu electrode if the Zn ion concentration becomes very high.

In case of boiler (water tube type), it is cheaper to use deionised water because magnesium will first consume all the dissolved oxygen (like they used to be used like getters in vacuum tube devices).

 

[KlausST]

Hi,

In case of boiler (water tube type), it is cheaper to use deionised water because magnesium will first consume all the dissolved oxygen (like they used to be used like getters in vacuum tube devices).

In my house boiler for heating drinking water
* there can´t be deionized water (it´s not very healthy ;-) )
* ..if I´m not mistaken there is a magnesium sacrificial anode

I´m not that good in chemistry to say which way Mg and/or Zn work...

Klaus

 

[dick_freebird]

For uses like protecting your outboard motor, you
can just screw on a zinc from the boat store. The
sacrificial anode is intimately connected to the
"body" and it's pretty compact electrically and
mechanically.

To protect a large structure with lots of pinholes in
the paint, you may need to throw some current at it
or distribute multiple anodes about the structure.

If the anode is "satisfied" by closer-in cathode-points
then remote ones may go unprotected. Surface
current density at the anode and "accessible" cathode
points, solution ion concentration are influences in
this "plating bath" turned upside down. A gross
imbalance in surface areas may argue for some forced
current to "push" the anode past its natural throw-rate
(but not more, than helps anything).

 

[FvM]

I was under the impression that passive anodic protection with sacrificial anodes is commonly known and thus the OP is specifically asking for an active ICCP schematic.