Has anyone tried this for cars? Seems that a small block of zinc strapped somewhere to the frame, with a tiny current supplied by the battery, would work quite well in preventing corrosion in those areas where a lot of salt gets dumped on the roads each winter.
The effect is very localized to where the zinc (or AL for that matter) is attached to the steel. This is why coating the sheet steel with zinc (galvinizing) is the best option.
Won't work. Each non-insulated FE crystal MUST make a complete circuit with the block of ZN.
Why? That's certainly not the case in ships, offshore platforms, and other environments where cathodic protection is used. So long as the steel bits are all in good electric contact with each other, the cathode should sacrificially corrode in preference to the anode.
Well, what's the problem? The bits of steel in direct contact with the cathode are making a complete circuit. The bits of steel in direct contact with the bits of steel in direct contact with the cathode are making a complete circuit.
I know what's needed in cars. Being submerged in salt water probably changes things.
The salt water is just an electrolyte, a bridge between the anode and cathode. Road salt and icemelt should be a pretty reasonable facsimile.
Hmm..., you might want to refresh your chemistry knowledge concerning galvanic reaction vs. crystal orientation. Two crystals made of the same material but having their crystal axises oriented differently will have different electromotive potential between each other. That is why FE rusts in the first place. You would have to attach a peice of ZN to each crystal or a wire from each crystal leading to a large ZN anode. The other way it could be done would be to immerse your car in either salt water or wet dirt and attach a wire from it to a large ZN anode nearby. If your vehicle were one big crystal, then galvanic protection using 2 different materials could be done as you suggest.
maybe if you sprayed the underbody with some sort of conductive oil, but then the oil would probably do more to stop rust than the sacrificial metal. underbody oil spraying is an effective rust inhibitor.
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The concept has been tried on cars for decades and nothing has proved out. If it were that easy the car companies would love to save all the money they pour into corrosion protection in favor of a few bucks on zinc. Or is this like the 200 MPG carburetor that the oil companies refused to allow?
Zinc rich undercoats have been used very successfully, BUT as a previous poster suggested, every part to be protected must be in virtual contact.
Whether the cathodic current comes from a zinc or aluminum couple (galvanic) or from a driven electrode doesnt make much difference.
The EXTERNAL corrosion current path is important. In salt water environments like offshore platforms, the anode will provide protection in the near vicinity of its attachment because the current 'throws' (see electroplating theory), or distributes, somewhat.
These shitteaux cathodic protection devices for cars which you see advertised in the likes of J.C. Whitney are snakeoil. They play on the almost scientific jargon that is associated with them, but are next to worthless.
If you want to attach a magnesium, zinc, or aluminum anode to your car, then do so. It will protect within a very short radius when it is wet.
Wish I could draw a picture using a browser. Makes a lot more sense than trying to explain it.
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