Nothing is broke!! - Page 2 - PeachParts Mercedes-Benz Forum

vstech · #16 07-16-2007, 10:50 AM

I hear every cloud has a silver lining, but with me, it's duct tape...

barry123400 · #17 07-16-2007, 02:39 PM

Generally speaking you cannot bury rust. It seems since there is no molecular bond between the base metal and rust oxide. Plus whatever you put over it. It still aquires enough moisture probably through condensation and somehow enouigh oxygen to stay active at a lesser rate of course but still active unfortunatly.
I have even wondered if the trapped enclosed moisture breaks down to provide the oxygen by electrolysis. Since there is a minute difference in electrical potential in there as well since you are burying two different compounds. I imagine it is possible. Remember you are burying two different compounds with diferent electrical potentials under whatever youi are using. You then still have a slower acting oxide generator under the stuff.
All the time we spent with a corrosion engineer for several years. Plus the results of various efforts and applications over time have just tended to confirm his viewpoints as well.
The best way to reduce the resultant activity to the minumin seems to be petroleum products. If you still must have the rust oxide. Or total reduction of the oxide itself by converters. That may leave you with reduced electrical potentials but still there.
Better yet is total elimination of the oxide. Then your primary metal is back to it's original state other than a loss of the original amount of it.
I am thinking of longer term time frames here by the way. As keeping a car for ten years or longer with no need to do it again is important to me. The old expression there is never enough time to do it properly yet always enough time to do it over again comes to mind.
For a common sense approach where you cannot eliminate the oxide problem at least cover it with a preliminary covering of fibre fill fibrglass body fill as it is at least moisture proof. The normal body fills absorb and transfer moisture at too high a rate to use them only. Also they probably provide a reserve source to keep the oxide generator going better or longer. Usual results of their application will be bubbling through the paint much, much sooner than the fibrefill or fibreglass if you wish. .
Rust converters would also make more sense if you were not leaving two different compounds burried behind you as well. Anyways the above are just some of the reasons right or wrong that make me get all the oxide out prior to repairing or finishing an area. Another expression that rust oxide is like a form of cancer is not an overstatment in my opinion.
I am also dead gainst the concept or ideal that some moisture setting urathane materials are going to do what is claimed by them long term. I wish it were true but think othewise. If it was so there would be no reason to sandblast steel bridges for example. There is not much argument about the reduction of activity they produce. They do just not totally eliminate it in my opinion.

patbob · #18 07-16-2007, 04:44 PM

Quote:

Originally Posted by barry123400

Since there is a minute difference in electrical potential in there as well since you are burying two different compounds.

Interesting factoid I looked up years ago in HS.. electrochemically speaking, the iron wants to corrode to "protect" the rust. Sort of like zinc will corrode to protect iron. Of course, what it thinks it's protecting it from is beyond me

. Probably explains why rust just keeps going and going and going once started.

I like your idea of electroylzing water to provide the Oxygen, but I don't think the electrical potential is there to really do that.. unless the carbon in the steel plays an electrical potential role here?

babymog · #19 07-16-2007, 05:24 PM

Nothing is being protected. The zinc has a lower voltage potential than iron, thus more available electrons, so it gives up electrons. Electro-galvanic corrosion is all about which is the more noble metal, which has more electrons in a paring, magnesium is worse still. Whenever two metals are togeter it needs a dielectric coupling of some sorts, a rubber gasket etc., the Navy has extensive experience and technical papers on corrosion if the subject interests you further.

In cars, it is usually best to remove any rusted metal and start from there, it is extremely difficult to stop rust. Some treatments such as Naval Jelly will remove substantially all of the rust with an acid etching process, and I have had some luck with "rust converter" paint-on treatments if the rust is all exposed and cleaned first. Sandblasting and cutting is the normal treatment when restoring old automobiles and boats however, complete and permanent.

Hatterasguy · #20 07-16-2007, 10:06 PM

All I know is I painted the rust and filled the holes with an awsome epoxy. I used great rust proofing paint, not POR15 but there are a million brands...yet the rust returns.

Rounds 2:
Cut off wheel, welder, and some new metal!

It won the battle but will lose the war!