Anode Myths part1.

[Chris and PJ]

The sole purpose of a sacrificial anode is to change the in water corrosion potential of an electrically connected metal to level that makes corrosion scientifically impossible. It does this by sacrificing metal to produce a negative voltage into anything electrically connected to it of a low resistance.

A mild steel hull submerged in canal water will have a natural corrosion potential of approximately (minus) -0.400 Volts when measured against a silver/silver chloride half-cell (Ag/AgCl). At corrosion potential voltages more positive that -0.700 Volts (Ag/AgCl). mild steel will naturally corrode in water in all the ways you see on an incorrectly protected hull, Pits Microbes Heavy rusting etc.

A mild steel hull, submerged in water that has corrosion potential more negative than -0.700 Volts when measured against a silver/silver chloride half-cell (Ag/AgCl). Has become too Cathodic to be able corrode. If the Anode(s) inducing this negative potential, can supply enough current (Amps) they will also protect against stray currents interfering with the -0.700 Volts minimum protective potential.

An Anode if magnesium and compliant to MIL-A-21412A will produce -1.500 Volts in water, measured open circuit (not connected to a hull) against an Ag/AgCl half-cell. Most importantly it will continue to supply this voltage over its design lifetime until %90 is sacrificed.

Corrosion industry standards use the figure of -0.800 Volts as the ideal corrosion potential voltage for submerged mild steel when measured in water against an Ag/AgCl half-cell. Further it is a scientific fact that these more cathodic voltages, when maintained, produce a Ferrous Hydroxide self-healing film upon the water metal interface that seals the surface from the water contained Oxygen additionally effectively stopping the formation of pits MIC etc

I have checked the in water corrosion potential of several narrow boats and have yet to find one correctly protected. I have also seen several boats out of water displaying rampant corrosion consistent with little or no attention to if the anodes are actually sufficient / working or even of the characteristics necessary.

I am happy to help anyone with the loan of equipment expertise etc. and particularly welcome technical criticism of the above, perhaps something else is occurring that does not happen in the offshore oil industry salt or fresh water where corrosion protection is taken very seriously.

[pete.i]

To be honest it would have been far better to explain WHY most narrow boats are not correctly protected instead of all the, above, gobble de gook that 99.999999%  of boaters, including me, will not understand.

[ditchcrawler]

Just now, pete.i said:

To be honest it would have been far better to explain WHY most narrow boats are not correctly protected instead of all the, above, gobble de gook that 99.999999%  of boaters, including me, will not understand.

Or read

[Chris and PJ]

6 minutes ago, pete.i said:

To be honest it would have been far better to explain WHY most narrow boats are not correctly protected instead of all the, above, gobble de gook that 99.999999%  of boaters, including me, will not understand.

I guess the WHY is the Boat builders, Surveyors, Marine engineers, Anode suppliers all being happy to take your money. Without offering any guarantee or responsibility that any work performed or cathodic protection equipment specified / supplied / installed is actually fit for purpose and or functioning correctly 

Only when the customers start to take their own corrosion potential measurements or insist that the surveyors gain the necessary qualifications to take and understand the measurements, will it change.

 

[Guest]

Thanks for posting this.
Read loads about this over the years, and seen many recommendations including hanging anodes into water off handrails when on mooring etc etc 
Ii seems reasonable to me that measurements must be taken (every boat is slightly different) rather than bunging a couple on each end and hoping for the best.
I am still convinced that shore power does  a great deal of damage. The guy who slipped our hull for two packing at 10 years old (previous bitumen) looked at the hull and said "no shore power then?" (or words to that effect.) I said "no" and he just nodded. 
I would welcome a "dummies guide to measuring corrosion potential" using basic equipment if possible (seems to work inside engine for antifreeze.)

Edited September 17, 2019 by Guest

[Arthur Marshall]

Well, the guy who blacks my boat has been living on his for the last twenty years and has never bothered with anodes.  Mine happily sat unreplaced and unchanged on the boat for twenty years.  Maybe some boats are more prone to whatever it is than others. I admit that I did have to have the boat replated, but it was about fifty years old at the time, so I'm not sure the anodes had much to do with it.

 

Everybody knows that boat hulls get thinner because fish keep bumping into it and ducks eat it.

 

[Guest]

3 minutes ago, Arthur Marshall said:

Well, the guy who blacks my boat has been living on his for the last twenty years and has never bothered with anodes.  Mine happily sat unreplaced and unchanged on the boat for twenty years.  Maybe some boats are more prone to whatever it is than others. I admit that I did have to have the boat replated, but it was about fifty years old at the time, so I'm not sure the anodes had much to do with it.

 

Everybody knows that boat hulls get thinner because fish keep bumping into it and ducks eat it.

 

Out of curiosity, did your boat spend many years on shore power, and specifically amongst other boats on shore power?  Cheers
ps the guy who blacked our previous boat was happy to take the money for welding anodes on, but said that he might as well weld them to my garden gates for what good they would do!

Edited September 17, 2019 by Guest

[blackrose]

10 minutes ago, catweasel said:

Thanks for posting this.
Read loads about this over the years, and seen many recommendations including hanging anodes into water off handrails when on mooring etc etc 
Ii seems reasonable to me that measurements must be taken (every boat is slightly different) rather than bunging a couple on each end and hoping for the best.
I am still convinced that shore power does  a great deal of damage. The guy who slipped our hull for two packing at 10 years old (previous bitumen) looked at the hull and said "no shore power then?" (or words to that effect.) I said "no" and he just nodded. 
I would welcome a "dummies guide to measuring corrosion potential" using basic equipment if possible (seems to work inside engine for antifreeze.)

 

Shore power has had no ill effects on my boat after 15 years, 99% of the time connected to a land line. How would that guy explain that? Shore power may damage steel hulls of course, but only if it's not correctly isolated.

Edited September 17, 2019 by blackrose

[Chris and PJ]

3 minutes ago, catweasel said:

Thanks for posting this.
Read loads about this over the years, and seen many recommendations including hanging anodes into water off handrails when on mooring etc etc 
Ii seems reasonable to me that measurements must be taken (every boat is slightly different) rather than bunging a couple on each end and hoping for the best.
I am still convinced that shore power does  a great deal of damage. The guy who slipped our hull for two packing at 10 years old (previous bitumen) looked at the hull and said "no shore power then?" (or words to that effect.) I said "no" and he just nodded. 
I would welcome a "dummies guide to measuring corrosion potential" using basic equipment if possible (seems to work inside engine for antifreeze.)

You are right

It also makes sense to hang anodes if the corrosion potential measurement taken call for it in those circumstances

Go look at www.galvatest.eu

 

I use Silvion half cells and have for 40 years www.silvion.co.uk they make one suitable for canal water for about £70 type CB10

 

Guess I need to write a guide on how to do these measurements.

 

 

[Guest]

8 minutes ago, blackrose said:

 

Shore power has had no ill effects on my boat after 15 years, 99% of the time connected to a land line. How would that guy explain that? Shore power may damage steel hulls of course, but only if it's not correctly isolated.

That is the nub. I have read that some isolators work better than others. A bit sketchy but I seem to recall that some leaked when phone chargers were used? I have never used shore power, but should the day come I have a serious bit of kit removed from a school lab. on standby. It has the "Gibbo" seal of approval. Basically an isolating transformer to stop kids from getting electrocuted in science lessons.
 

Edited September 17, 2019 by Guest

[Chris and PJ]

1 minute ago, catweasel said:

That is the nub. I have read that some isolators work better than others. I never use shore power, but should the day come I have a serious bit of kit removed from a school lab. on standby. It has the "Gibbo" seal of approval. Basically an isolating transformer to stop kids from getting electrocuted in science lessons.

Again any electrically changing of the corrosion potential of your hull can simply be measured with an ag/agcl half cell. Non functioning galvanic isolators being just one cause. Of course the best isolation is achieved by the air gap within an isolating transformer.

[Steve_C]

31 minutes ago, Arthur Marshall said:

Well, the guy who blacks my boat has been living on his for the last twenty years and has never bothered with anodes.  Mine happily sat unreplaced and unchanged on the boat for twenty years.  Maybe some boats are more prone to whatever it is than others. I admit that I did have to have the boat replated, but it was about fifty years old at the time, so I'm not sure the anodes had much to do with it.

 

Everybody knows that boat hulls get thinner because fish keep bumping into it and ducks eat it.

 

Tony by any chance (just going off your location)?

[Guest]

23 minutes ago, Chris and PJ said:

Again any electrically changing of the corrosion potential of your hull can simply be measured with an ag/agcl half cell. Non functioning galvanic isolators being just one cause. Of course the best isolation is achieved by the air gap within an isolating transformer.

That has always seemed the logical solution to me. Isolate from the mains supply and earth, then create your own air isolated supply and earth on the craft, which (hopefully) cant connect to the earth on other craft. Pretty much like having an inverter on board.

Edited September 17, 2019 by Guest

[Chris and PJ]

4 minutes ago, catweasel said:

That has always seemed the logical solution to me. Isolate from the mains earth, then create your own earth on the craft, which (hopefully) cant connect to the earth on other craft. Pretty much like having an inverter on board.

That of course begs the question of if you should bond your new isolated neutral to your new isolated earth within the vessel. fortunately being a corrosion engineer  you would need to ask an expert lol I have always found it interesting that generators of the building site B&Q variety have their earth by regulation, centre tapped at 120v in relation to both live and neutral and yet people use them as shore power to narrow boats?

[Guest]

7 minutes ago, Chris and PJ said:

That of course begs the question of if you should bond your new isolated neutral to your new isolated earth within the vessel. fortunately being a corrosion engineer  you would need to ask an expert lol I have always found it interesting that generators of the building site B&Q variety have their earth by regulation, centre tapped at 120v in relation to both live and neutral and yet people use them as shore power to narrow boats?

Can cause problems with RCD devices? 
Our inverter when new had no N-E link and as such the RCD device didn't function when tested. After checking with manufacturer I put a N-E link in and RCD then functioned. Don't know if this affected the hull of course! Certainly tripped when I dropped a running angle grinder in the drink.
I think an RCD will still function with a centre tap  if wired correctly (40 years since I studied this.)?

Edited September 17, 2019 by Guest

[Chris and PJ]

2 minutes ago, catweasel said:

Can cause problems with RCD devices? 
Our inverter when new had no N-E link and as such the RCD device didn't function when tested. After checking with manufacturer I put a N-E link in and RCD then functioned. Don't know if this affected the hull of course! Certainly tripped when I dropped a running angle grinder in the drink.
I think an RCD will still function with a centre tap  if wired correctly (40 years since I studied this.)?

RCD` being current operated I don't think will mind.

Certainly from a corrosion point of view any induced voltages will be AC and having no path will not effect the corrosion potentials. As with all marine earth bonding to the hull should be performed at only one location.

[Mac of Cygnet]

1 hour ago, Chris and PJ said:

I guess the WHY is the Boat builders, Surveyors, Marine engineers, Anode suppliers all being happy to take your money. Without offering any guarantee or responsibility that any work performed or cathodic protection equipment specified / supplied / installed is actually fit for purpose and or functioning correctly 

Only when the customers start to take their own corrosion potential measurements or insist that the surveyors gain the necessary qualifications to take and understand the measurements, will it change.

 

 

I guess Ditchcrawler meant HOW not WHY.  In which case I entirely agree with him.   99% of boatowners (or more!)  have no wish to become involved with technicalities like this, but are quite happy to let 'experts' decree best practice. (At the risk of inviting the classic definition of an expert..)

Edited because I see the thread title says 'Part 1'.  You mean there's more?

1 hour ago, Chris and PJ said:

 

 

 

Edited September 17, 2019 by Mac of Cygnet

[WotEver]

27 minutes ago, catweasel said:

I think an RCD will still function with a centre tap 

Yes it will; no problem at all. Some inverters are CT earth. 

[Guest]

1 minute ago, WotEver said:

Yes it will; no problem at all. Some inverters are CT earth. 

Was 99% sure  
When I said 40 years since studying, it is actually 50, so my maths haven't improved any!

[Arthur Marshall]

1 hour ago, catweasel said:

Out of curiosity, did your boat spend many years on shore power, and specifically amongst other boats on shore power?  Cheers
ps the guy who blacked our previous boat was happy to take the money for welding anodes on, but said that he might as well weld them to my garden gates for what good they would do!

No, never. 12 volt only, and not a lot of that! 

53 minutes ago, Steve_C said:

Tony by any chance (just going off your location)?

Yup. 

[Chris and PJ]

11 minutes ago, Mac of Cygnet said:

 

I guess Ditchcrawler meant HOW not WHY.  In which case I entirely agree with him.   99% of boatowners (or more!)  have no wish to become involved with technicalities like this, but are quite happy to let 'experts' decree best practice. (At the risk of inviting the classic definition of an expert..)

Edited because I see the thread title says 'Part 1'.  You mean there's more?

 

Yes certainly there I more and part 2 will examine how `best practice` relates to the myths

[Guest]

3 minutes ago, Arthur Marshall said:

No, never. 12 volt only, and not a lot of that! 

Yup. 

Cheers, interesting.

[blackrose]

1 hour ago, catweasel said:

That is the nub. I have read that some isolators work better than others. A bit sketchy but I seem to recall that some leaked when phone chargers were used? 

 

Switch mode mains appliances can induce interference currents, but it takes a lot of RFI to force a decent spec GI into conduction.

 

About 12 years ago I did a test with ChrisW (remember him?) using his ossiloscope. We had three mobile phones charging, two laptops, two battery chargers and several other appliances running at the same time but we still couldn't get my GI to go into conduction. But if it's a concern then as you say, get an isolation transformer. Then there's absolutely no rub at all.

Edited September 17, 2019 by blackrose

[Guest]

4 minutes ago, Chris and PJ said:

Yes certainly there I more and part 2 will examine how `best practice` relates to the myths

A bit of an aside, but what are your thoughts on measuring the voltage within an engine to  "test" the corrosion control of the coolant? Lots of stuff about it on you tube but no real science expalined. BS?

[Guest]

3 minutes ago, blackrose said:

 

Switch mode mains appliances can induce interference currents, but it takes a lot of RFI to force a decent spec GI into conduction.

 

About 12 years ago I did a test with ChrisW (remember him?) using his ossiloscope. We had three mobile phones charging, two laptops, two battery chargers and several other appliances running at the same time but we still couldn't get my GI to go into conduction. But if it's a concern then as you say, get an isolation transformer. Then there's absolutely no rub at all.

Yes I remember Chris well. I suspect like most things there are good ones and poor ones. Sounds like yours is up to the job, proved by testing and the fact that your boat still has a hull
I already have a transformer, but nowhere to plug it in! Not that bothered TBH as we keep moving home, but would be good in the darkest days of winter.