Revelations about zinc electrodes.

That wont work very well Tom.
It will protect a very small radius within a conductive film of electrolyte. You need ion exchange for cathodic protection, there's no other way for the zinc to give up its electrons to the car body.
The protection is limited to the area of ion exchange. The problem is that the wet film is so thin that you'll only get protection just below and maybe if you're lucky, for an inch around the periphery of the zinc block.

 

I wonder then why the zincs are corroding and my vehicle isn't.

The theory says the least noble metal will be the one that oxidizes making it sacrificial and preserving anything bonded to it.

They use it, effectively, on pipe-lines all the time.

Are you getting confused with galvanizing?

-Tom

 

Remember you also need reduction not just oxidation.

The ions have to migrate through an electrolyte. A thin film such as a wet under-surface of a vehicle won't allow them to migrate very far at all.

Pipelines are protected with Zincs when they are buried or immersed. They cannot be effectively protected that way in the atmosphere. The zinc will just oxidise it's surface and passivate.


If you completely isolated your car zinc block and subjected it to the salt road spray it will self corrode. Also your car is painted with a good underseal, try the same exercise with areas of that underseal scraped off in various locations .

 

John and Mike,

Now let see whether I have it right.

If I paint that boat below the line, I have to use 1600 Kg. However if I have a bare non painted bottom part, I also have to use 1600 Kg of Zinc.

Could you please elaborate on this? Or am I really ready for the Metal institution?.
Bert

 

Bert

No, the example, shown here:

..this vessel has a surface area of 4500m2.

So if your boat has this amount of surface area (ie a super tanker!!) then yes. But some how i think your boat is a bit smaller..yes??

The underwater surface area is the key input to calculating how much you need.

So, for example, if your boat is say 20 tonne displacement this equates to roughly 7.5m2 surface area, under the water. If, given all things being the same, this is pro rata: 7.5/4500 x 1600 = 2.7kg of zinc

 

Hi John,

No, we have a misunderstanding.

I mean, I always thought that there is a difference between a 100% properly painted below the waterline, without scratches and contact with the electrolite (seawater) and if the full non painted hull surface area is exposed to the seawater.

I am under the impression that in the 1st instance, the zinc will be less "eaten up" , while in the second instance, the zinc will be eaten up as per your calculation.

By the way, I have only approx 10m^2 surface area and indeed your zinc weight is spot on, should I make a metal boat. But it will be a wooden boat with only approx 5m^2 powder coated metal surface below the waterline. Keels only. I need thus very little zinc. I am buying a special round clip-on zinc for my 30 mm shaft.

But for the powder coated keels I am listening very careful to You, CDK and Mike's arguments. At present the stand is that CDK is ahead, I am inclined to follow his advice at present.
Bert

 

I guess you meant 75 m2 surface area (and 27 kg), since my 3 tonne sailboat already has 16 m2.

But there must be some kind of assumption about the paint condition. Perfectly epoxy painted steel will not need that much, if any, current.

 

Tom, I am inclined to agree with you. When it rains and at the coast, it has salt in the air, the automobile car body will be getting moisture all over the bodywork. Thus in that case the electrolite is present, and thus your bodyworks is protected.
As soon the sun is there and the body is dry and warmed up and no moisture is around, indeed the zinc will only protect a small area, only there where maybe some salt/sand is kicked up from the road. Should you be living inland, in that case only the moisture from the sand/salt thrown from under your car by the wheels will be the electrolite. Thus why should your complete car not be protected?. This proves your point.
Bert

 

Joakim, AdHoc has my boat weight and dimensions. He meant, 2 Tonnes instead of 20 tonnes and 7,5 m2., thus 2,7 kg is correct.
Bert

 

Ad Hocs formula sounds like what is commonly used. Ive just re anoded an 20 year old aluminum hull under guidance of MgDuff...100sq meters underwater surface. Sea water. 100\4500 x 1600 = 35.5 kg The epoxy barrier coated , international Micron antifouled hull carries six, 7kg 220mm zinc pie plate anodes , a prop anode, plus a 4 strap anodes on the thrusters and two strap anodes inside the sea chest. There is no exposed propeller shaft. The anode stock on this vessel lasts 4 years, with the forward high water velocity pie plate anodes and the thruster anodes eroding first , sea chest strip anodes and prop anode are replaced yearly

http://www.mgduff.co.uk/leisure-craft/fitting-instructions/aluminium.html

 

Lets get this branch off CDK's thread.

I'll start a new one.

-Tom

 

With no bare metal exposed, there will run no current between the electrodes and the hull, so the zinc will not be active and it is irrelevant what amount of zinc is installed.

I think (?) the guidelines specifying current and zinc mass are based on experience and statistical data on paint flaws, scratches etc.
But these guidelines are pretty old and do not differentiate between paint systems. They may even assume protection by tar instead of modern paints.
Of course there will always be conductive patches on a hull as large as a supertanker, but why should you apply the same criteria to yacht?

Also, they are based on conventional electrode mounting where -as pointed out earlier- the zinc protects its own iron parts and depletes faster than necessary.

 

That makes common sense.

Can I assume correctly. We are going to store a boat out of the sea, somewhere in the backyard, but close to the sea. Should it rain and there is sufficient salt in the air, the metal will still be rusting, if not protected by zinc. Assuming, it has no cover over the boat. We refer to the previous info, about the automobile in Canada.
Bert

 

The automobile case as I explained earlier is a nonsense. it cannot protect anything other than the immediate adjacent surface, (a few mm in you are lucky) if the film is a reasonable electrolyte (not fresh water).

Exactly the same goes for the boat ashore. In fact in rain water the zinc surface will usually passivate and become more noble than the adjacent steel not that that makes any difference to this case.

There is apparently some serious misunderstanding as to how cathodic protection actually works.

 

Who'd ever think the zinc topic would be still ongoing. It's a very complicated subject, and i have to take my hats off for the effort you guys are putting into it. I have a related question. The subject so far relates to corrosion protection using the passive action of zincs(passive meaning corrosion control without an actual electrical/electronic device doing the actual corrosion control) Some 100 years back i helped a friend of mine install an active device (control box) wired to some sort of metal blocks that were epoxied to the steel hull of his big power yacht.I do recall him saying they use similar set ups on big ships and that it was expensive. I've often wondered especially when i had an aluminium hull does that setup prove to be effective and if it is still available and used today. Geo.

A yacht is not defined by the vessel but by the care and love of her owner