Electronic Rust preventers, do they work? - Page 2 - PeachParts Mercedes-Benz Forum

layback40 · #16 04-24-2010, 06:00 AM

jeff,
Not quite the theory. Zinc forms an oxide that is near insoluble, thats the dull white powdery appearance that weathered gal has. Problem is that with acid rain the zinc oxide is a little bit soluble in acids so it gets washed off over time. Then more zinc oxidizes & so on until all the zinc is gone. Zinc does preferentially oxidize to iron (steel), hence sacrificial protection. Cathodic considerations come into play when both metals are submerged in the same electrolyte (water).

dguy210 · #17 04-24-2010, 08:04 PM

Supposedly for cars there is a modification to the galvanic corrosion protection system used on boats but with a twist that does away with the need for an electrolyte (i.e. the water) to be present at all times. This works on a capacitive principal. It basically uses a high voltage(~400v) low amperage positive pulsed DC voltage across a dielectric material in contact with the car body. As the positive pulsed DC is applied to the plate a corresponding negative charge builds up within the car body across the dielectric due to capacitive coupling. As the positive pulse ends the extra electrons in the car body (negative plate) bleed off at any sites of corrosion (i.e. bare metal) inhibiting (reducing) the oxidation reaction of the steel. What is cool, is in the presence of a liquid electrolyte solution that makes contact with the bare metal of the car body the paint acts as the dielectric, the electrolyte as the positive plate and the extra electrons from the capacitive coupling in the car body act to reduce the electrolyte instead of using the electrons from the steel.

That is the principal at least, I have no idea if it actually works in practice.

babymog · #18 04-24-2010, 08:11 PM

Quote:

Originally Posted by layback40

<<snip>> Cathodic considerations come into play when both metals are submerged in the same electrolyte (water).

Sorry to simplify. However, I submit that most (if not substantially all) body corrosion on vehicles takes place, ... when it's wet, especially when wet with a very conductive fluid such as salt-water (salted roads, melting snow, etc.).

layback40 · #19 04-24-2010, 09:34 PM

Jeff,
You are correct about the presence of water for corrosion.

For the protection claimed with these devices, both 'electrodes' must be in the electrolyte. We have something that is connected to the electrics of the car & to the body.
There is not a continuum of electrolyte between the 2 electrodes.
In comparison, the thermostat housing on the motor provides corrosion protection to the block. It has the coolant as the electrolyte.
long distance oil & gas pipelines often use cathodic protection. A voltage is applied to the pipe (large earth electrodes are used for the other half) and while the voltage is greater than the electrolysis voltage the pipe has some protection from corrosion.

babymog · #20 04-25-2010, 09:08 AM

Most marine engines have zink electrodes to protect the engine and heat exchangers, inside the coolant system/flow.

I've read about a couple of systems, some time ago, that tried to use emitters much like aircraft use static-wicks, don't think that system was very successful on a car either.

Buy good cars, wash them often has worked for me, and use plenty of water. The car-wash underbelly washes really don't do much, you can still reach into the fenders and find places where dirt is laying (holding moisture and salts), nothing beats a good manual under-car spraying.