black stuff on new steel

What is the chemical composition of the black stuff on new steel? I know paint doesn't adhere to it and needs to be sandblasted.

 

iron oxide (rust)

 

Mill Scale : FeO

 

Or if the steel is imported from overseas - it could be lanolin. That's the stuff that makes sheep waterproof - and is a great anti sea-water preservative for ferous metals. Used on all export vehicle parts.

 

As a college student, I worked in the largest Dutch steel mill that was next to our holidayresort on the beach.
I did hundreds of odd jobs among them the packaging of the steel sheets and coils, some weighing more than 40 tons.
It is as Bergalia says the anti-rust coating that is sprayed on the coils before they are packed. Sometimes they contain also the graphite-based grease deposits from the qualitycontrol and overpacking units. It is definately not rust, that you should have discovered and established right away.

 

not rust. Have used quite a bit of new steel in France. It seems to be an oil or some type. I use white spirit (mineral spirits) and a clean rag to wash/wipe it off with. This works well for preping the weld area as well as degreasing before painting with Ferriprime and other oil based paints.

 

gonzo, if you're asking about the hard black scaley " skin" that's on new steel it is as polarity stated, called " mill scale". i don't know exactly how it forms other than it is from when the white hot steel is shaped under a roller, the very surface is compressed and it scales when it cools. you are correct it must be blasted or ground off . it wrecks havoc with welding, especially if you are using a small 110 v. type. evidently, it must also be non-conductive when thick enough. i am a certified welder and fight with the stuff all the time. i have seen it thick enough to shunt the starting drive of a good welder and not be able to strike and arc at all, until a spot is chipped or ground off where you are going to weld.another spot to clean off is where the ground clamp will be. paint and primers also will not hang on to it. i am around commercial const. everyday and the painters will refuse to paint something untill it is sand blasted, even with tinemic(sp?) paint.

 

Good question, and answer by capt'ron; I use 7/16" o.d. tubing ('dom', drawn over mandral) from an aircraft supplier, and always thought it was a preservation coat, but mill scale seems credible, imho.

 

Capt'n Ron is correct, it is mill scale. I couldn't find the right word for it. Before sandblasting, in earlier days, one left the hull bare outside for some time, so that the mill scale could rust away. There are also deposits from the uncoiling machines, and later on from the cold rolling plant, where most of the mill scale will come off.

 

thanks guys,
gregsport, all of the mech. tubing (both d.o.m. and e.w.s.) i've worked with was relatively clean, except for occasionaly, an oily black coating, a lubrication from being formed. you are correct that it is left on to act as a protective coating, but a good wiping with laquer thinner or similar leaves the tubing shiney clean, ready to weld. mill scale is actually part of the steel, the very surface, super compressed from the high pressure under the forming roller and is sort of bonded to the surface, sort of the same way welding slag is stuck down, some of it will pop off as it cools, the same as welding slag, but allot of it stays put and is a real p.i.t.a. to get off without the right equipment. if left in the weather, rust forms between it and the parent steel and it eventually falls off. if you look at a large welded structure left out in the weather for a long time, there will be a dark outline of the structure on the ground, that's the mill scale that fell off from the structure rusting. the best medium for blasting it off is carborundum granules, but it's pricey stuff.