Rust or combustion deposits?


What am I likely to find inside the oil furnace flue collector and heat transfer tubes, rust or the products of combustion?
When I took off the metal box that connects my furnace to the flue, because holes had rusted in it, of course it was raggedy. Even on the sides that had no holes, like the inside bottom, there was a thick layer over all of it, maybe a 16th of an inch or more (3 mm?) and the layer came off when I hit or scraped it, or hit the edge of it where an adjoining piece had fallen off.
What do you think this stuff is, rust or the products of combustion that were gradually deposited on this 30 year old furnace? (In Baltimore where the heating season is 4 to 5 months.)
An argument for deposits seems to me to be that underneath it all, the metal is flat and smooth, not smooth like formica, but "textured" with no dips or bumps that are bigger than the others. And it has no layers, even though the stuff that comes off is a separate layer. If it were rust coming off in a layer, I would not expect the part that is left to be the same layer, the same thickness everywhere.
Also the pieces that came off would crumble if I tried to bend them.
The guy who welded it said it was rust, but he was in his 20's.
So, what do you think was flaking off?
Tonight when I was cleaning the heat transfer chambers, they too had stuff that flaked off, a few pieces as big as a quarter, many as big as a penny. They too might have been rust or combustion products. Here too, the inside of the chambers seemed flat and smooth.
So, what do you think was flaking off?
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There is a detectable and allowable small ash content in commercial fuel oil. What you are seeing is the accumulated deposits from 30 years of normal combustion. Nothing uncommon, and not really rust.
Joe
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Thanks. First I thought what you said, but the young guy confused me.
So the metal was pretty thin to begin with, even if the oil company guy claimed I couldn't get one made and had to buy a new furnace.
(I will buy a new furnace before Dec. 2010 to take advantage of hte tax credit and get a more efficient furnace, but I didnt' want to be rushed. I'd rather spend 71 dollars and some of my time to have heat now.)

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mm wrote:

Insulation protective coating to preserve the metal.
You're screwed.
Just kidding.
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wrote:

ROTFLOL
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