Chris and PJ

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. hmmmm are electric eels a power supply

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

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

Then tell me what is considered to be the most common corrosion cause steel hulls. 1 Is it connection to shore power? 2 Is it something faulty electrical on board? 3 The boat next door? 4 Anodes not working / insufficient? 5 Stray currents or something else I will then tell you how I would test for it in simple steps if it was my boat.

Thank you This is the most helpful reply I have received Trouble is as soon a you say something simple like: `An anode is just a power supply` your faced with a whole lot of derision. I will rethink And most probably forget about it as I have better things to do with my life

Quite honestly this is about the most unfriendly Forum I have ever seen. As such members with something to add are reluctant to speak because of the un-moderated heckling from the too clever.

Absolutely no connection and as Brexit supporter would not promote a French site. Just few sites get it right and I thought this one was easier reading and get it mostly right. Obviously I am wrong to try to help/ inform and understand that taking these kind of measurements will be opposed by those who would become accountable. I have however had Silvion the foremost British manufacturer of Half-Cells make an electrode to my specification so that it will specifically work in canal water. and at a much lower cost because it does not need to function in 500 M sea water. I have no interest in selling anything or does retirement mean something else nowadays.

The whole gavatest.eu site explains it quite well with pictures but their costs of equipment are way too high. There are several UK manufactures of Half Cells I want a narrowboat to buy for retirement. I look at a bunch of hulls (the biggest single reason not to buy a boat is because its heavily corroded) Seems the biggest reason to sell a boat is that the corrosion its got is soon to cost a lot of money Surveyors would prefer not to take measurements in case they became accountable Asking why they corrode I find no-one in this industry(so far) who fully understands why. I understand the why and the remedies. I try to explain the science in writing --- nothing but abuse is sent back I suggest go look at a site that explains it in pretty pictures. More abuse lol Obviously you can all afford the costs of corrosion. Take a chance when you connect to shore-power Take an even bigger chance when you moor in a marina. For sure the marina operators don't want corrosion measurements taken In case they become accountable! From private messages I get. There are a few who cant afford to see their investment devalue. Those who are interested I am happy, (if I can) to help at no cost.

I completely agree with you. The same is right for the majority. I don't know Until he time you inspect your hull and you have a huge corrosion cost! Would you be even more irritated? I just don't believe in luck

Unfortunately in terms of galvanic corrosion there is no naturally occurring dry air on this planet. The only man made metals that will survive to, show that man ever existed. will be on the moon and voyager spacecraft

The only way to find out if your anodes are working, is to measure in the water, the change in the potentials of the materials connected to them. Go have a look at www.galvatest.eu

Complete and utterly pedantic claptrap hair splitting. That if paid attention to, would confuse the reader into thinking that a Sacrificial Anode is something other, than a simple power source. Be assured than Sacrificial Anodes are selected by their power output and duration. For the sole purpose of driving the cathode more negative and serve no other purpose. I inserted a glaring technical nonsense into my posting No. 83. (steel will rust in dry air) That anybody with even the most basic understanding of Galvanic Corrosion would pick up. To see if those, pompously demanding dimensional accuracy over easy understanding, actually know anything at all about the subject. Steel does not rust in dry air, there being no electrolyte! The key principle that I am attempting to impart to this community is that corrosion is controllable, predictable and quantifiable. Sadly corrosion mechanisms are surrounded by myths When I read posts describing expensive hull corrosion that is mostly attributed to shore power connections, with electricians measuring this and that, then speculating on causes. From the perspective of a corrosion engineer it all seems nonsense for the following reason: Any electrical influence on a steel hull in water that changes its electrical potential in relation to the water. Changes its corrosion characteristics. These changes can be simply measured with a Half-Cell. If damaging potentials are detected the source can be simply identified. Boat next door?--- kind of important that you find out For instance dip the Half-Cell and see if the measurement rises or falls as you connect and disconnect your shore power . If the reading changes, so does your rate of corrosion. The same for all your internal electrics if switching on off and something changes the reading its changed you corrosion rate. The Half-Cell offers the only scientifically sound indisputable way of doing this If you find that your corrosion potential change can be attributed to a third party's failure to take reasonable care or apply reasonable expertise ( I am not a lawyer) These readings when confirmed (by qualified corrosion engineer and loss quantified) give you your only route to legal redress. As I have said before. I am happy to help anybody with the loan of equipment and / or expertise. Of course in the strictest confidence.

So what is your point When you connect steel to magnesium in water energy transfers. and you measure the work done in watts The guy is saying that the energy flowing in the case of galvanic corrosion cant be measured in watts and in an attempt not to patronise I did not ask him to ask a 15 year old

Ok you don't understand so I need to explain it in a different way. I do think I understand what you are trying to Say All metals to be usable by man, with a very few exceptions like gold. Only exist on this planet as oxides or salts. (ores) To turn these ores into a usable form Man through various processes. Smelting for iron and steel. Electrolysis for metals like Magnesium force electrons back into the ore . These man made metals are defying the second law of thermodynamics by containing too much energy. Because this law defines what is best described as natures will to turn everything into its lowest energy form. (Entropy) In the case of steel to loose electrons and rust even in dry air The Galvanic series lists these metals in terms of this energy. Volts Magnesium contains more energy than steel so is more active The Pourbaix diagrams describe how these energy levels are effected by the environment (water) and described in Volts Because nature demands corrosion, in water material is returned to its salt state and an electron is released thus in physics it is said that `Work has been done` The fact that Magnesium looses mass, indicates that work has been done and there has been a depletion in the electrons (energy) you cant have one without the other. Therefore a flow of electrons (energy) and work being done can only be described in Watts the unit of power. If you have a better explanation for the existence of a form of energy in the context of galvanic corrosion that explains how anodes work then please describe it. Please also explain your statement (in the context of galvanic corrosion) that. I quote Energy cant be measured in Watts

You show complete lack of understanding of what galvanic corrosion protection is. The bare roots of the subject, when considering the corrosion of metals in water are described in the Pourbaix diagrams for each submerged metal. Let take the Pourbaix diagram for mild steel in water below: The term SHE means Standard Hydrogen Electrode. SHE half-cells are not practical out side of a laboratory so corrosion engineers use Ag/AgCl half-cells instead so there is a slight difference in real world voltages measured against this diagram. Mild Steel when measured in canal water against Ag/AgCl shows a potential (volts) of around -0.4 volts. I know this, because I have measured it. Looking at the diagram at -0.4v the word `red` indicates the formation of Fe III (one of the 3 forms of iron oxide = rust) The diagram indicates that if you can push the potential of the mild steel to be more negative so that it sits in the immunity zone. Industry goes for -0.800 volts. So it will NOT corrode (rust). Can not corrode. Is resistant to corrosion. Too cathodic to corrode. The ONLY way of doing this is by the application of POWER / Energy/ Watts/ whatever you want to call it. The most convenient way of producing this voltage shift is to electrically connect a sacrificial anode A Military spec. Magnesium anode in fresh water produces -1.5 volts in open circuit when measured against Ag/AgCl When this Mag Anode is connected to the mild steel its voltage drops due to the current required to change the potential of the steel, to a level of about -0.800 v When matched to its operating environment the size shape and weight of the anode determines how much current it can supply and for how long. Current times volt = power in Watts. The only way of measuring these potentials is with a Half-Cell. Anything else is pure speculation banded about by those with big egos and little knowledge mixing apples with oranges The above is scientific fact. The added advantage of achieving these potentials in water is that a powerful Ferrous Hydroxide film (coating) is produced that is highly resistant to the dissolved Oxygen in water. Stopping the formation of pits. Microbe attack and metal loss. 0

hey who did you work for. I started in the early 80s with BIX offshore then Stolt Seaway and ended up with Gulmar

be interesting for us all to hear how you get anything other than power from a power supply to correct an anode that is insufficient. As someone who has designed and installed both Sacrificial and Impressed current offshore corrosion protection systems for more than 40 years may I ask what your technical background is? again anode output can simply be described in KW hours

Please explain how an anode works Please explain why external power supplies are necessary in impressed current systems used to directly replace sacrificial anodes to get exactly the same result.

It is an exact science. Anodes of MIL-A-21412A by specification are designed to work in a linear fashion, for a known time to a set loss of original weight and surface area. Oil platform anodes are designed to last for the design life of the platform because they are stupendously expensive to replace If you mean that Narrowboat anodes systems have not being designed to last for the design lifetime of the hull or take into account the varying water types or environmental conditions or no real compliance testing of anodes because they are cheap to replace. You are absolutely correct My example is from an industry that requires the highest standards I entirely agree with you.

Copied from the site that you obviously did not read: Basic Types of Cathodic Protection There are two basic types of CP systems: galvanic (or sacrificial) and impressed current. Galvanic anode systems (also termed sacrificial anodes systems) use a metal that is naturally more negative than the metal being protected and thus when the two metals are connected electrically to each other, current flows from the metal that is more electrically negative to the metal that is more electrically positive. This current flow results in a rapid consumption of the anode, hence the common term “sacrificial anode” is often used to describe these anode systems. Typical galvanic anodes include magnesium, zinc and aluminum as each of these are more electrically negative than carbon steel or other steels. Impressed current anode systems are different than galvanic (sacrificial) systems because they utilize an external DC power supply to create the electrical current flow. The use of an external power supply enables an impressed current system to generate significantly higher current output with fewer, longer lasting anodes than any sacrificial anode system. And from the same site: Impressed current cathodic protection systems have the benefit of using an external power supply to drive current. This makes it possible to protect virtually any structure, regardless of size or current requirements using long life anodes and enough appropriately sized power supplies. PERHAPS THE POWER YOU ARE THINKING ABOUT DOES IS NOT MEASURED IN WATTS (AMPS X VOLTS) PERHAPS YOU THINK CURRENT CAN FLOW IN A CIRCUIT WITHOUT A POTENTIAL DIFFERENCE (VOLTS) ANODES SUPPLY CURRENT. TO SUPPLY CURRENT HAS TO BE A POTENTIAL DIFFERENCE ANODES ARE ELECTRICALLY MORE NEGATIVE THAN THE MORE POSITIVE CATHODE HULL SO THERE IS A POTENTIAL DIFFERENCE. THEY ARE CONECTED TOGETHER SO CURRENT FLOWS CURRENT = AMPS POTENTIAL DIFFERENCE= VOLTS AMPS TIMES VOLTS = WATTS WATTS IS THE MEASUREMENT OF POWER I politely suggest that you do not understand that an anode is electrically more negative of say-1 volt than a cathode of say -0.5volt making a 0.5 volt potential difference or that current flows from the more negative to the more positive If you get a bunch of magnesium anodes put them in a bucket water with a piece of steel and connect a 3v torch bulb between the anodes and the piece of steel it will light. Power to do this comes from the magnesium anode.

Should there be a heath warning regarding possible electrocution if you are earthed and your lips touch the lemon. Perhaps this is why cocktail sticks are non conductive

I strongly advise to go look at: https://www.matcor.com/impressed-current-cathodic-protection/ Anything else I say could be considered patronising

My view on galvanic isolators is that all the circuit diagrams, that I have looked at of how they work tells me that most types rely on the voltage drop of diodes which is fine. My problem with the use of diodes is this application is the fact that %90 of diode failure modes result in a short circuit within the diode. So if they fail, they no longer offer a voltage drop and just conduct like a piece of wire. One thing about diodes is that they don't like high current spikes even of 1000 thousandths of a second. And tend to eventually fail over time if repeated spikes are sent through them. Most ac appliances that contain transformers, especially microwave ovens, upon shut down snub any back EMF`s down to earth through `y` configured capacitors. These high voltage, very high current spikes, of very short duration are normally absorbed by the earth and are unnoticed. Its just a thought and somewhere in this forum a qualified electronic guy will be able to confirm or dismiss this.

The direction of conventional current (the flow of positive charges) in a circuit is opposite to the direction of electron flow, so (negatively charged) electrons flow out the anode into the outside circuit

Anode current rating – all anodes have a current rating based on how long they can be expected to operate at a given current rating. All anodes have some defined expected life based on current output and time – so many Amp-Hours of service life. For example a magnesium anode may have an expected consumption rate of 17 lb/Amp-year (7.8 kg/amp) so if a 17 lb anode is operated at 0.1 amps it would have a life of 10 years.