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Anode Myths part1.


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39 minutes ago, ditchcrawler said:

It stops mussels attaching themselves as I have witnessed at work when it stopped working all the strainers on the sea water pumps filled wit mussels in a very short time

Interesting :)

 

I wonder if that has any relevance to anaerobic bacteria? (Serious question). 
 

 

 

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On ‎26‎/‎09‎/‎2019 at 17:48, reg said:

Which is very much how I read it, however I thought it necessary to have that pointed out to have absolute clarity on the matter. 

The problem with the op's initial assertion that 'An anode is just a power supply' and his steadfast defence of that assertion left me in a position of wondering how much of the rest of his post was incorrect and therefore whether it had any validity at all.

Now that the op's fundemental error has been identified and cleared up it appears that the original post may have some value after all. Hopefully he sees this whole process as being a submission of a draft document for review. 

Personally I wouldn't mind seeing a redrafted post with ambiguities and clutter removed. 

 

You are mixing apples with oranges

Conventional current flow, as understood by electrical engineers, is that the current flows from the + terminal to the - terminal.

In electrochemistry the current flow is from the negative to the less negative = the electrons travel from a battery negative to the positive. The complete reverse of conventional current flow thinking

Potential in electrochemistry is the amount of negative charge 

REDOX reactions state that electrons are lost when a metal oxidises. simultaneously these electrons are gained by the other connected  metal reducing. (go Google)

In this application of galvanic corrosion in water.

The Anode (magnesium) is oxidising ----- loosing electrons turning the magnesium metal into magnesium ions that dissolve into the water.

The other connected metals, mild steel hull, brass propeller is reducing ----- gaining the electrons from the magnesium a the magnesium oxidizes.

These electrons are taking the path of least resistance through the steel not through the water.

The action of the steel gaining these electrons causes it to become more negative (electrons are negatively charged particles)

The current flow. Flow of electrons. Is from the higher potential Magnesium to the steel.

The Pourbaix diagram for steel in water states that if you increase the negative potential of the steel past around -0.5 v IT CAN NOT RUST

Industry standards require that the negative potential should be -0.800 v for submerged metal. (for additional beneficial electrochemical reasons)

 

When selecting sacrificial anodes to supply the required energy to lower the potential of the steel to the required -0.800 v.

You calculate the surface area of the submerged metal in square meters

You calculate the current density required to lower the potential to -0.800 v per square metre (bare metal in canal water is around -0.400 v)

by multiplying these figures you arrive at the current required in Amps.

Sacrificial anodes are rated in amps / kg and time (they are in almost short circuit.(very low volts) the steel hull has very low resistance)

Of course water resistance (ion density) is a factor, as is anode surface area and geometry but mostly allowed for by using magnesium alloys designed to work in fresh water.

Coatings on the bare metal is also a factor but all paints have porosity. A perfect coating would insulate the whole hull absolutely for ever and no current would flow from the anode.

 

It is clear that a sacrificial anode is supplying amps and volts over time = watt hrs Thinking of it as a power supply added to an existing electrical circuit is valid and makes it easier to understand and calculate its requirements.

 

Change your thinking from, the Magnesium anode prefers to rust so the steel does not.

To:

Because the magnesium anode is rusting its changing the electrical charge of the steel so that it is unable to rust.

 

Further any external factor that interferes with, the unable to rust potential of the steel hull will cause it to rust if the anode cant keep up supply enough current.

Grounding the hull to earth increases the current required. Stray currents can do the same

 

 

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Which takes me back to your previous assertion that

'An anode is just a power supply'. 

No it isn't. I have 4 anodes sitting in a plastic bag under my bed and they are not in and of themselves capable of providing power. 

I understand however how an anode AND a cathode(the Hull) AND both being immersed in a suitable liquid(the canal), can produce a current. In this scenario the anode is only 1 part of the trio i.e ANODE+CATHODE+ELECTROLYTE therefore, and I repeat, the ANODE cannot be in and of itself a power supply. 

I genuinely believe that is not what you really wanted to say or imply but this simple error, as it is so fundamental, has detracted from the rest of your original posting. 

Look at it from my and I suspect others position. If the fundamentals are incorrect then how can I have faith that the more advanced information is correct? 

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2 minutes ago, reg said:

Which takes me back to your previous assertion that

'An anode is just a power supply'. 

No it isn't. I have 4 anodes sitting in a plastic bag under my bed and they are not in and of themselves capable of providing power. 

I understand however how an anode AND a cathode(the Hull) AND both being immersed in a suitable liquid(the canal), can produce a current. In this scenario the anode is only 1 part of the trio i.e ANODE+CATHODE+ELECTROLYTE therefore, and I repeat, the ANODE cannot be in and of itself a power supply. 

I genuinely believe that is not what you really wanted to say or imply but this simple error, as it is so fundamental, has detracted from the rest of your original posting. 

Look at it from my and I suspect others position. If the fundamentals are incorrect then how can I have faith that the more advanced information is correct? 

put your anode in a bucket of water and measure its open circuit voltage with a volt meter. if it is magnesium it will be 1.5v measure its current in the same way

 

Sacrificial anodes are sold by their power output

I am asking you to consider their action as that of a power supply because that is the only way that it is possible to calculate their deployment

 

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1 minute ago, Chris and PJ said:

put your anode in a bucket of water and measure its open circuit voltage with a volt meter. if it is magnesium it will be 1.5v measure its current in the same way

 

Sacrificial anodes are sold by their power output

I am asking you to consider their action as that of a power supply because that is the only way that it is possible to calculate their deployment

 

Exactly

The assertion that 'it is a power supply'

 

is entirely different to

 

'think of it as a power supply when doing your calculations' 

 

I'm sure you can agree on that. 

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6 minutes ago, reg said:

Exactly

The assertion that 'it is a power supply'

 

is entirely different to

 

'think of it as a power supply when doing your calculations' 

 

I'm sure you can agree on that. 

Think of it as a way of trying to put something complex into a straightforward phrase. Seriously, it can be difficult finding the right wording to explain difficult technical stuff in a way that everyone can understand without making it simplistic.

 

Following my attempts to mediate I have had some PM correspondence with @Chris and PJ. All I can say is that you don’t do something professionally at a senior level for a whole working career if you don’t know your stuff. 

 

Seven pages of arguments isn’t helping anyone understand anything of use to them. The issue here isn’t one of technical expertise, it’s about communication.

 

We’d serve the readership better if we got on with discussing the practical aspects of what boat owners can do if they have concerns rather than dragging out this argument.

 

JP

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9 minutes ago, Captain Pegg said:

The issue here isn’t one of technical expertise, it’s about communication

Totaly agree which I believe I have previously said that this original post should be taken as a draft document which has been subjected to review. 

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15 minutes ago, reg said:

Totaly agree which I believe I have previously said that this original post should be taken as a draft document which has been subjected to review. 

It’s a post on a canal boating forum not a technical paper for publication in a journal!

 

If everything on CWDF had to be vetted for accuracy there’d be bugger all left.

 

JP

 

Except for my posts (obvs). ?

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3 minutes ago, Captain Pegg said:

It’s a post on a canal boating forum not a technical paper for publication in a journal!

 

If everything on CWDF had to be vetted for accuracy there’d be bugger all left.

 

JP

 

Except for my posts (obvs). ?

I am surprised that after 160 posts we are still on the same subject

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14 minutes ago, Captain Pegg said:

It’s a post on a canal boating forum not a technical paper for publication in a journal!

 

If everything on CWDF had to be vetted for accuracy there’d be bugger all left.

 

JP

 

Except for my posts (obvs). ?

So after 160 posts of vetting is it now generally accepted here that the potential in volts, of your hull is an accurate indication of its rate of galvanic corrosion regardless of painting or if anodes are fitted or not?

-0.400v or less its corroding and most likely pitting. -0.800v its not corroding and not pitting

 

If there is a technical argument to say that the above is not correct we can move on.

 

15 minutes ago, Captain Pegg said:

I know; it’s amazing. 

 

160 posts on electric eel stuff and nobody has mentioned fish yet.

 

JP

hmmmm are electric eels a power supply

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51 minutes ago, Captain Pegg said:

It’s a post on a canal boating forum not a technical paper for publication in a journal!

But it was written as a technical post for a journal not as a explanation post on a boating forum. 

Anyways up I'm out of here. 

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I've seen many steel framed buildings where the frame is connected to.a small power supply to raise its voltage and thus prevent corrosion.

 

Using anodes on boats is designed to replicate this by using the hull, the water the boat floats in and an anode of zinc, aluminium or magnesium (depending on the type of water the boat is floating in) as a battery.

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I’ve not read all of this, and this might have been said before but, surely, all we need to know is that the anode is made of a metal that gives itself up more easily than the metal of the boat it is attached to. Hence the name, (sacrificial anode).

 

As long as the anode is of the right metal, and is properly connected to the boat, it seems to be reasonable to accept that it’s doing its job. A galvanic isolator is also a good idea for boats on shore power.

 

I suppose we ‘could’ buy a machine that tells us that the anode is, or isn’t, doing its job but, on the whole, things have tended to be mostly fine.

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39 minutes ago, Richard10002 said:

I’ve not read all of this, and this might have been said before but, surely, all we need to know is that the anode is made of a metal that gives itself up more easily than the metal of the boat it is attached to. Hence the name, (sacrificial anode).

 

That may be enough for some people, but others have enquiring minds and like to know how & why it works.

 

That is the core reason of this thread

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