Irony
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| posted on 18/5/10 at 12:42 PM |
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Electrolysis and Sacrificial Materials
After watching that Richard Hamster show about Sydney Opera House there was a section on Electrolysis.
They said in the days of copper bottom ships a zinc block was used that sacrificed itself (due to it being much more reactive that steel or copper) to
oxidization. Then went the zinc block was knackered they unbolted it and bolted a new one on.
Why was this never used in cars that were prone to rust. Surely zinc is not that costly in such small quantities?
[Edited on 18/5/10 by Irony]
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cd.thomson
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| posted on 18/5/10 at 12:45 PM |
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erm.. it is?!
Craig
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Irony
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| posted on 18/5/10 at 12:48 PM |
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quote:
Originally posted by cd.thomson
erm.. it is?!
I don't know. That is why I am asking.
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snakebelly
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| posted on 18/5/10 at 12:50 PM |
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they are called sacrificial anodes, normally zinc cast around a strip of steel that is then welded to the hull. Although they will work in a limited
way on a car the full benenefit is only realised when both the sacrificial anode and the ships steel hull are submerged in water.
They are also used on Oil rigs, Floating Docks etc etc etc
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phelpsa
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| posted on 18/5/10 at 12:55 PM |
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quote:
Originally posted by Irony
quote:
Originally posted by cd.thomson
erm.. it is?!
I don't know. That is why I am asking.
It's how galvanising works 
EDIT: I may be thinking of zinc plating actually.
[Edited on 18-5-10 by phelpsa]
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cd.thomson
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| posted on 18/5/10 at 01:01 PM |
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i mean you say "why was this never done" but it is done quite alot, zinc plating/galvanising are the big options, but also paintwork
incorporating zinc has been around for a while.
Craig
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mcerd1
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| posted on 18/5/10 at 01:05 PM |
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what snakebelly said ^^
they need to be submerged to work (preferebly in salt water)
it make sence if you can remember your school chemistry......
it works the same way as the hydrogen generators that folk were discussing here a wee while back
you should see the size of the zinc blocks the rigs use (50t or more each) and the power they generate 
I've see the aftemath of not putting the bung back in the bottom of a speedboat before the tide came in - everything connected to the +ve side
of the battery was corroded in less than 8 hours
hot dip galvanizing is the best you can do to get the same kind of protection for steel things out of the water - but its not as good without the
water so any bits that arn't galv'd will corrode, just a bit slower than normal
also it doesn't like thin metal, it needs drain holes everywhere, it can't be easily replaced and paint doesn't stick to that very
well 
zinc plate is fine in theory, but way to thin to last any length of time
so your best be for a car is make it out of good steel (which hasn't been left to go rusty before you make the car!) and coat it in a good
primer (often zinc rich) and good paints, then maybe some kind of anti-chip/ rust proofing stuff - basicaly what most decent cars have as standard
these days
also I should say stainless isn't nessisarly the answer either - if I remeber right - the chloride ions in salt water break down the surface
layer so keeps corroding
[Edited on 18/5/2010 by mcerd1]
[Edited on 18/5/2010 by mcerd1]
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jossey
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| posted on 18/5/10 at 01:30 PM |
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that was a great program.
its more difficult with cars as the water does not pass 1 certain point as it does in tubes etc. so the sacrificial anodes wont work on all the areas.
when water flows through the tubes the zinc blocks are desolved in the salt water.
but with cars the water sits in sills and in wheel arches and under the car in the form of mud so wouldnt work.
good idea though.
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Badger_McLetcher
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| posted on 18/5/10 at 05:59 PM |
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Also this only works for galvanic/electrochemical corrosion which is not such a problem on cars. The bigger problems are gas and liquid phase
corrosions IIRC.
If disfunction is a function, then I must be some kind of genius.
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plentywahalla
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| posted on 18/5/10 at 07:43 PM |
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Electrolytic erosion and oxidisation (corrosion) are two completely different chemical processes.
If two metals of different noble scales are submerged in an electrolyte (possibly seawater) then a potential difference or electrical current is set
up between them.
In an electrical circuit the current flows from positive to negative (cathode to anode) and electrons flow in the opposite direction. Therefore the
anode erodes metal which deposits on the cathode which is the process used for electroplating.
In vessels anodes are fitted made from metal very low down on the noble scale (zinc or aluminum) so that they erode rather than expensive bronze
fittings such as propellers.
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