Tar dissolves rust and is the best primer coat on steel

A few years back I reported to the newsgroups that when I put tar
coating on my steel roof and years later peeled away the dry hardened
tar that the steel revealed was crisp and shiny bright with no sign of
any rust.
So last summer I went to further test this observation. I took a old
rusty steel putty knife and immersed it into a can of roof tar and then
set it outside on a drum so that given 6 months of hardening and then
today I peeled the knife off the drum which allowed me to see the
surface of the putty knife. It was mirror shiny metal of steel as if one
polished steel. There was no evidence of any rust.
So again, I believe what has happened is that tar dissolves rust on
steel.
But I need a chemist to write out the chemical reaction taking place.
Is there another compound besides tar that dissolves rust on steel to
use as an analogy?
I think the important message of this discovery would be that all primer
coats of steel that need protection should be a first coat of tar and
then paint over the tar. Especially important for bridges.
Archimedes Plutonium

whole entire Universe is just one big atom where dots
of the electron-dot-cloud are galaxies

"Archimedes Plutonium" skrev i melding news: snipped-for-privacy@iw.net... Tar = Petroleum derivate or extract of wood?
T

An old folk remedy was to mash uncooked potatoes in water. Place your rusted item into it and the rust disappears after a few days.
Al

Are you sure you didn't do some inadvertant polishing when you scraped the tar off?

(snip what I wrote)
I started with a putty knife that was rusty on both sides. Not so much rust as to have pitted the knife steel. And I dipped it into a can of roof-tar or roof asphalt. That black stuff that is sticky and gets hard as it dries. And I left the putty knife on a steel drum barrel for 6 months. And a few days ago I pulled the knife off the drum barrel and so some of the tar stayed stuck on the drum barrel exposing the steel putty knife. The exposed steel of the putty knife was as shiny bright as if it were "new metal".
So I must conclude that tar or asphalt chemistry dissolves rust on steel. I believe it is the sulfur content in asphalt that dissolves the rust.
But I need a expert chemist to tell what the full interactions are.
When I first discovered this feature of asphalt removing rust from steel was when I was working on my sheet metal roof and come to pull some of the old tar from the roof. And everywhere I pulled the old tar off, there exposed was bright shiny galvanized steel.
Archimedes Plutonium whole entire Universe is just one big atom where dots of the electron-dot-cloud are galaxies

I was going to look on the label to see if a chemical ingredient was available and post it. I buy it in 1 gallon or 5 gallon cans called "roof asphalt". It comes in runny type for coating large areas or it comes in thick type for filling gaps or holes. It is very black and sticky and I use "mineral spirits" if I get any on my hands. I suppose it is the same stuff used in blacktop and asphalt.
I would guess it is a petroleum derivative. I will look on the cans tomorrow to see if they give some chemical identity of the ingredients.
Archimedes Plutonium whole entire Universe is just one big atom where dots of the electron-dot-cloud are galaxies

I simply pulled the putty knife off the drum. The same thing happens when I pull old tar off my sheet metal roof in that the exposed steel is "new" with no signs of any rust.
I have some of those old weights of 50 pounds. Those old weights for calibrating scales. Some are badly rusting. So I will apply a thick coat of this asphalt tar next summer and let it sit for 6 months and then pull off some patches. I suspect I will see bright shiny steel that I could even use as a mirror.
Archimedes Plutonium whole entire Universe is just one big atom where dots of the electron-dot-cloud are galaxies

It has been my hypothesis that the process is mechanical removal of rust, as implied here.
It would be interesting to repeat the "putty knife experiment," but instead of pulling off the dried tar, simply allowing the knife to sit in a jar of kerosene until the tar dissolves away. What would the underlying metal look like in that case? Naturally, this experiment should be done with a control.
Steve Turner

It's possible that the tar contains a chemical reducing agent of some type. Over time a reducing agent will convert the rust back to base metal - reversing the process of oxidation that created the rust. The mechanical action of the tar removal may be a factor as has been proposed by another writer.
The use of potatoes to remove rust is interesting. They contain a significant amout of vitamin C - a chemical reducing agent.
RT

I went to look up the ingredients of the asphalt. The brand I use is "BlackJack" roofing coating and roofing cement. Both are black and one needs a putty knife to apply whereas the other can be brushed on.
It gave only asphalt as ingredient.
Looking up for the chemical formula of asphalt talks about asphaltenes and soluble to benzene but not n-pentane. Says that asphalt contains alot of sulphur, nitrogen and organic acids.
Still no formula for this hydrocarbon.
You maybe correct that it reduces because when I look at the peeled off asphalt from the steel I find no trace of "old rust". No trace on the steel and no trace in the asphalt itself.
My next test will be this upcoming spring and summer where I take a heavily rusted 50 pound scale weights and generously coat them with asphalt and then 6 months later when hard and set I will peel off the tar. I expect the weights to be mirror shiny bright steel. And several of these weights have markings of "50" and of "Standard" and other marks.
Thanks for the good news, for I had imagined that the asphalt dissolved the rust much like salt dissolves in water. But with your suggestion of "Reducing" that the asphalt puts the steel in rust back into the metal itself-- well, this is better than my wildest imagination of this situation.
To think that a easily applied chemical compound not only gets rid of rust but puts steel back onto the original steel.
Can someone tell me of a similar chemical reaction with some other metal besides steel?
Archimedes Plutonium whole entire Universe is just one big atom where dots of the electron-dot-cloud are galaxies

Why don't you weigh the weights before and after your experiment?
I think they'd have to be pretty accurate scales.
The weights are marked with their nominal weight. Rust would add to the weight.
Present rusty weight - nominal weight =Amount of oxygen.;
If metal is replaced during experiment,
Weight after experiment = nominal weight
² If metal is taken away,
Nominal weight >weight after experiment.
Have fun,
Joe Cummings.

asphaltenes and soluble There is no likely *chemical formula* of asphalt as its a mixture of hydrocarbons (aromatic as well as paraffinic) along with sulfur nitrogen heterocycles. It may contain some potential carcinogens.
A similiar reaction which restores the original shine of almost black tarnished silver can be easily done by keeping tarnished silver with warm solution aluminum foil containing an electrolyte for 10 minutes. The oxidized silver Ag2S converted back to its reduced form Ag. Note that there no mechanical removal of black layer (and hence no loss of Ag---just like this tar experiment, instead that sulfide coating is forcibly converted back to silver.

Yes, thanks. I suppose polishing of silver instead of electrolyting will remove some silver atoms from the object. I suppose my copper polishing of my RevereWare pots removes some copper atoms every time I polish them.
But it is fascinating that if asphalt is an electrolyte for iron and steel materials, then what is the electrolyte contained within the asphalt?? I do not think asphalt has any aluminum contained within that aluminum in the silver reduction.
And if asphalt is a reducer of iron or steel then how can we measure the extra oxygen released from the FeO2. Would the extra oxygen go into the asphalt compound or released into the atmosphere? If so, perhaps a new way of increasing Earth's atmospheric oxygen.
Archimedes Plutonium whole entire Universe is just one big atom where dots of the electron-dot-cloud are galaxies

ahahahaha.......... Archie, this "FeO2" and its "extra oxygen" are certainly possible in your own Plutonium world. But it ain't happening in your local neighborhood........Go milk a cow now. Suits you better. ahahaha....ahahaha.....ahahanson