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Hello all, Currently, I only have the ground bolt from the inverter connected to the hull of our aluminium sailboat. I would like to know if there are any experts on galvanic corrosion within the marine environment that could offer some input or a website with images on the best practices for earthing/grounding dc battery pack, inverter, ac power and our electric ev motor. There seems to be a lot of differing opinions on this topic. Thanks Dennis
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This is an issue all yacht owners fear confronting. Two mechanisms are at play in underwater corrosion of expensive metal bits.
1. Galvanic corrosion. This is a spontaneous process (no battery required) caused by two metals being immersed in an electrolyte (salt water) and having an electrical connection between them. The galvanic potential difference between the two metals causes a current to flow between the two metals. The different potential between the two metals can be seen on a galvanic potential scale.
This form off corrosion is limited to voltage potential developed on the Galvanic scale. It is always less than about 1 volt so is not fast acting.
2. Electrolysis. (Stray current corrosion) This is when some external voltage is superimposed on the underwater metal and causes a current to flow.
This form of corrosion can be from the 12 or 24 volt system and can be brutally fast at destroying underwater metal parts.
In your case Galvanic corrosion can be a problem but the real fast acting killer is Electrolysis which is stray current corrosion.
You rightly mention some advice varies on how to avoid corrosion. That is true and the difference is a trade off between various scenarios where one technique may be superior in one instance but not in another. For instance advice may be given that is valid in a marina but less valid on the swing mooring.
Some good reference is found in Boatowner’s Mechanical and Electrical Manual by Nigel Calder. It is good reference but not the full story. The Marine Electrical and Electronics Bible by John Payne is also ok but for an aluminium yacht, no mistakes can be made. I know it may be difficult to find an expert in this discipline, but with an aluminium hull I recommend you do find an expert and seek their advice.
I hope this helps.
Regarding your point #2 I know in a marina or port tied up beside other boats is a risk that has to be taken since its tough to address stray current from other boats or the marina itself. For the past 2 months since there has bene sun, we have not been plugged into any shore power so leakage through the ac ground from shore is not an issue.
I have read that ground ac should be put to the hull in a single point.
I have read that dc negative should be put to the hull in a single point.
I have read that dc should be tied to the saildrive for grounding to water.
I have read that some aluminium boat owners don't let dc positive or negative touch the boat and keep it floating.
I am still undecided/uneducated on the correct approach for grounding ac and dc. I will keep searching for a definitive answer as I am sure the other thing that needs to be considered is lightning!
Hello @Piraya . Many other corrosion mechanisms can take place that destroy expensive metal bits like dissimilar metal corrosion and stress corrosion but the two mentioned are the principal cause of underwater components being destroyed. It is a predominantly DC current phenomena. AC Earth leakage is an unbalancing of the active and neutral currents because for some small current value the return path is the earth wire. That current is contained to the earth wire. It is contained in the earth wire because the impedance of the earth wire is orders of magnitude lower than the path to ground through seawater.
DC current cause problems because an anode and a cathode are established, the anode being eaten away. AC currents continuously reverse the anode and the cathode so it appears the removal of ions is immediately followed by the replacement of those ions. That seems to me to be the reason AC currents are not referred to when dealing with corrosive currents.
My impression from Victron documentation was that AC grid connection without isolation transformer could be a major cause of corrosion problems on a boat, which is why an isolation transformer is recommended. Does this problem sort under your definition of electrolysis corrosion? Does your DC statement mean that the earth leakage acts like a DC current?
An isolation transformer is simply to separate the neutral earth bond. A neutral earth bond allows a DC path ( even though it is an AC supply ) between boats in a marina. A DC path allows the various boats in a marina to be joined by the AC earth connection when they all plug into the mains outlet on the dock. This DC path allows Galvanic corrosion because various metals are joined below the water line by the AC earth connection.
The maximum voltage that can developed is the voltages shown on the Galvanic scale, let’s say about 1 volt. It can be improved by a Galvanic isolator which present a 2 x diode forward diode voltage block. For silicon diodes that is about 2 x 0.6 volt or 1.2 volts. Enough to stop Galvanic currents or even better to use an isolation transformer where the primary and secondary are isolated meaning the neutral of the boat is no longer connected to earth at the marina distribution board. The isolation transformer prevents Galvanic Corrosion between boats in a marina ( but not galvanic corrosion on the boat itself ) but does not prevent electrolysis ( stray current corrosion within the boat).
AC earth leakage ( the type that trips an RCD) does not act like a DC current. It is an AC current and is quite different from DC leakage current that causes electrolysis.
Ok so the simple answer to galvanic corrosion between boats, but not on the boat itself, is an isolation transformer, f.ex. the Victron. Very clear spoken, thanks. Then a couple if more questions is interesting:
1. What then is the simple(st) answer to prevent galvanic corrosion on the boat itself? Equal wiring to commom earth (f.ex. a Glomex copper plate under the hull) from all metal cocks and parts exposed to salt water?
2. What is the simple(st) answer to prevent eletrolysis? Make sure that none of these parts are exposed to battery plus? (battery minus is connected to sea water through engine anyway)
3. Would a central zinc anode in addition to those on the propellers/drives do any good? F.ex. Volvo Penta has one for sale.
(I am not talking of a metal hull like the original question.)
In common recreational vessels protection can take two forms.
1. All underwater metal bonded together electrically and connected to an underwater sacrificial anode.
2. Underwater bits remain isolated apart from shaft, prop and P bracket.
For instance Jeanneau yachts do not bond all of their underwater bits and attach those to an anode. Jeanneau only bond prop shaft and P bracket and on shaft drive yachts rely on one anode on the stainless shaft and one on a fixed prop to protect both prop, shaft and P bracket..
Many boats do bond all underwater bits and connect them to an anode.
Advantages of total bonding.
1. In the event of any stray current, the return path is very low resistance and will blow the fuse feeding that path. Leakage current is then brought to the attention of owner prior to damage being caused.
2. All underwater metal is at the same potential and the sacrificial anode is destroyed protecting the underwater metal.
Disadvantage of total bonding.
1. In the event that the yacht is located near an electrically leaky boat, the potential gradient in the water causes current to flow in the bonding system causing electrolysis corrosion on bonded metal.
Advantages of isolated system.
1. Potential gradient in water caused by being located near an electrically leaky boat does not cause any current to flow, hence no impact on underwater metal.
Disadvantage of isolated system.
1. If any voltage accidentally appears on an underwater metal part, it will be destroyed by the electrolysis current.
Electrolysis comes about when some leakage from the positive side of the system finds its way to the negative side of the system through an underwater metal piece. For instance if a brass thru hull somehow has a low resistance path from a positive potential and that path is completed through the salt water back to the prop shaft. Soon the brass thru hull disintegrates. Wiring needs to be carefully done so as not to allow this leakage current to develop.
Care should be taken in adding more anodes than the manufacturer intended. Over protection can cause problems in timber boats, aluminium boats and steel boats. GRP doesn’t get damaged but antifoul can blast off the area of overprotected.
