Black radiator water

S

Safari bob

Member
While working on a friend's car with a freshly rebuilt engine, I flushed the radiator after running it for 30 minutes - I had added 1 gallon of vinegar. I drained and flused it (didn't look at the water) and refilled with plain water. It again spit water out and left rust spots. I decided to drain. The drained water was black. You could not see rust, just very black water.

I drained it all out but did not flush. Filled the radiator with pure vinegar and ran the engine for a while. I just drained some out and the water is still black. What could be causing this? Thanks.
 
The reaction between red rust in the block and vinegar is seen at.

http://www.thenakedscientists.com/forum/index.php?topic=15708.0

Vinegar contains acetic acid CH3COOH which reacts with rust FeOOH:
3CH3COOH + FeOOH --> Fe(CH3COO)3 + 2H2O
and iron (III) acetate Fe(CH3COO)3 is water soluble.
Click to expand...
Similarly electrolytic decomposition of rust converts red rust (Fe203) to black rust (Fe304) which is yet another iron oxide state. Black rust is not stable, however, and left out of electrolysis will pick up oxygen from the air to revert back to red rust.

Black rust is a good thing for restorers as it can be painted over and paint will stick to it. Like red rust though, black rust has no mechanical strength and for body panels you should remove any loose black rust before painting.

I think the acetal reaction outlined above is similar. Likely left exposed to the air, the black will eventually turn to red and the acetate will either be released (as acetone vapor?) or combined in solution.

Give it a few days. Drained will probably make the reverse reaction faster.

Best for preservation sake would be to bring the radiator/block to a pH of about 10 and bring nearly to a boil to drive off oxygen - which is what is done commonly on steam boilers as being found most preserving of virgin metal, and promoting a Fe3O4 black rust coating.

One can tell on a newly opened boiler how concientious the operators were on chemical water treatment as the boiler will be uniformly black on the inside. One has to be careful when opening the man-door as the interior black surfaces will IMMEDIATELY pick up oxygen from the air within the boiler and begin reversion back to red oxide - and create a situation where one might enter an "oxygen deprived" atmosphere.

This was one of the incentives for creating Confined Space Entry Inspections for OSHA. After a few days in the air, the boiler interior turns a pleasant brown or red color and the threat passes.

Joe K
 
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assuming that the rebuilt engine block was hot tanked and free of rust, why not just have the radiator cleaned out when it was removed or replaced with a new one. a common mistake is running an old clogged radiator on a new engine or dealing with the consequences afterwards which happens alot because of radiator cost. i would flush it with a pipe connected to the upper hose while adding fresh water into the radiator. once you clean out all the black water and junk you have been experiencing remove the radiator and do it right.
 
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Flush it out and run it. Does it overheat? If not, have fun. If so, look into getting the radiator looked at by a professional radiator guy.
 
You should, also, leave the vinegar in longer than a little while. A couple of weeks is better. Give it a chance to work. It is only a very mild acid. It is fit for human consumption.
MIKE :) (mikeburch)
 
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