Removing Mill Scale

[rbattelle]

I've got about 20 ft of 2x4x11ga hot rolled A36 that I'm working with. In the past, I've removed mill scale using a sanding disk on my angle grinder.



But this 2x4 stuff has some extreme scale on it. Rather than being black and grimy like the other stuff I've dealt with, this stuff is a medium gray color and is absolutely impervious to everything except a grinding wheel (I've tried a wire wheel, knotted cup, various sanding disks, a flap disk, and acetone).



I'd rather not use a grinding wheel because it introduces surface imperfections I'd prefer to avoid and it will take me a lifetime to remove all the scale from that much steel.



I've heard of 2 commonly available chemical methods: muriatic acid and vinegar. I'd rather avoid the muriatic acid because of the danger and the large volumes I'd need for this job. But before I go invest in several gallons of vinegar, I thought I'd do a little research.



This page suggests mill scale should not be removed, because it forms a protective coating. But most paint manufacturers say it must be removed prior to painting (I will be painting the steel after it's assembled).



I am welding this steel, and it seems to me that the scale doesn't conduct as well as bare metal does.



So what, exactly, is the harm in leaving mill scale in place? It seems to me that it seals the steel and prevents rust (which is good). Is paint adhesion the only reason to remove it? If so, then it's a non-issue for me to leave it on.



Ryan

 

[cojhl2]

Ryan, Would you please post a pic so we can tell what you are dealing with?



If mill scale is what we normally think it is,, it is usually not removed before painting.



Depends on what the finished product is I guess.



If you want a super finish you should be working with cold rolled.



anyway,, I would like to see a pic.

 

[Flinty]

Yes, without a pic it's hard to say. But we use lacquer thinner and the piece looks like chrome when we are done. Slick as a whistle.

 

[rbattelle]

Here's a picture. You can see a few spots I've hit with a grinder to make welding the seam easier.



Ryan

 

[Nate]

I'm not sure what that coating is exactly, but what I normally do is grind where I'm welding, down to bare metal and that's the only thing.



Before I paint, I clean it off with brake cleaner to get the oil, etc off it.

 

[WilsonJ]

I don't think it's mill scale. Some steel companies put a lacquer type coating to keep the steel from rusting when sitting in the yards. I'd just clean the weld area back at least 1/2" or so from where you'll weld and it should be OK. Paint ought to lay pretty good on it with a little sanding of the coating.

 

[barbwire]

Ryan, I sometimes get foriegn material off metal, by lighting the torch, and adjusting the flame to where you would if you were going to cut. Then by moveing the torch at a somewhat faster speed, over the metal to be cleaned, and hitting the cutting lever quite often ( in short bursts). But, do not let the metal get preheated to the degree that it takes to actually cut. Try it on a scrap piece of metal, and see if it works for you. This method works quite well for me, when I want to remove mill scale or paint. Good luck!

 

[cojhl2]

Hi Ryan,



Please excuse my ignorance,, but that looks like welded tubing. Is that what A36 designates. I'm not acquainted with that terminology.



anyway are you talking about the uper right corner in the pic and down the right side which to me looks like rust, or are you talking about what the inside of the tube looks like?



If you are talking about the same as the inside of the tube then like someone else indicated just clean it to make sure no oily residue and shoot it.



JHL

 

[WilsonJ]

A36 is the code used to ID the material. A36 is plain carbon steel. If you clean the scale off with a grinder and want a smoother surface, go over it with a sanding disk on a grinder. Ought to take any contaminants out the grinding rock might have deposited.

 

[rbattelle]

that looks like welded tubing. Is that what A36 designates.

A36 is an ASTM designation. In theory, it means the yield stress of the steel is at least 36000 psi. Modern A36 usually has a higher yield. For example, this steel is actually stamped "A38737", which implies (to me) a yield stress of 38737 psi.



I didn't think this was welded tubing, I thought it was extruded. I never really considered what the seam is (I guess I assumed it was a "seam" left by the extrusion mold). Geez, I never really thought about it before. Someone more knowledgeable should be able to answer. [Why can't I find ASTM grade A36 in Machinery's Handbook? )



Interesting read. Interesting read.



I think I'll just leave the coating (whatever it is) in place and grind around the areas to be welded.



The upper right side in the picture and down the right side isn't rust - that's where I've ground it for welding. The remainder (inside and out) is in its as-delivered state.



Sorry about the crappy picture; the colors just don't look right.



Ryan

 

[cojhl2]

Thank you for the info Ryan. The last time I had to know these kind of things was in 1967 so I forgot so much I guess.



As I remember we designated steel by a 4 dig number two digits had to do with tensile strength and the remaining two were percent of carbon. I just never knew the ASTM designations I guess.



However, there were some addl designations that i thought were trade names of Ryerson like Corten, T3 etc.



Thanks Ryan,, I allways like to learn something new.



JHL

 

[terrywerm]

That looks more like cold formed tubing with a welded seam. No mill scale, but it may have a laquer coating on it. I would just grind the areas to be welded. When the project is finished, sand the whole thing and use laquer thinner to remove any traces of oil.

 

[sdstriper]

Mill Scale Metal Treatment & More

I've got about 20 ft of 2x4x11ga hot rolled A36 that I'm working with. In the past, I've removed mill scale using a sanding disk on my angle grinder.



But this 2x4 stuff has some extreme scale on it. Rather than being black and grimy like the other stuff I've dealt with, this stuff is a medium gray color and is absolutely impervious to everything except a grinding wheel (I've tried a wire wheel, knotted cup, various sanding disks, a flap disk, and acetone).



I'd rather not use a grinding wheel because it introduces surface imperfections I'd prefer to avoid and it will take me a lifetime to remove all the scale from that much steel.



I've heard of 2 commonly available chemical methods: muriatic acid and vinegar. I'd rather avoid the muriatic acid because of the danger and the large volumes I'd need for this job. But before I go invest in several gallons of vinegar, I thought I'd do a little research.



This page suggests mill scale should not be removed, because it forms a protective coating. But most paint manufacturers say it must be removed prior to painting (I will be painting the steel after it's assembled).



I am welding this steel, and it seems to me that the scale doesn't conduct as well as bare metal does.



So what, exactly, is the harm in leaving mill scale in place? It seems to me that it seals the steel and prevents rust (which is good). Is paint adhesion the only reason to remove it? If so, then it's a non-issue for me to leave it on.



Ryan

I have used this on stripped down body panels, works as they claim. I even had a fender I tested it on, stripped to bare metal place on a damp blanket on one of those fold up stands, it sat there all summer (6 months +) and I finally got around to it, surprised to see so far so good.



http://www.picklex20.com/

 

[GHarman]

A36 is an ASTM designation. In theory, it means the yield stress of the steel is at least 36000 psi. Modern A36 usually has a higher yield. For example, this steel is actually stamped "A38737", which implies (to me) a yield stress of 38737 psi.



I didn't think this was welded tubing, I thought it was extruded. I never really considered what the seam is (I guess I assumed it was a "seam" left by the extrusion mold). Geez, I never really thought about it before. Someone more knowledgeable should be able to answer. [Why can't I find ASTM grade A36 in Machinery's Handbook? )



Interesting read. Interesting read.



I think I'll just leave the coating (whatever it is) in place and grind around the areas to be welded.



The upper right side in the picture and down the right side isn't rust - that's where I've ground it for welding. The remainder (inside and out) is in its as-delivered state.



Sorry about the crappy picture; the colors just don't look right.



Ryan

Hey Bud, you can find A-36 on page 6. 1-15 in the "Procedure Handbook of Arc Welding" from Lincoln Electric Company. You are correct as to the yield strength. The A-36 designation covers carbon steel shapes, plates , and bars of structural quality for use in bridges, buildings, and for general structural purposes. Strength requirements are 58,000-80,000 #/ sq. in. tensile strength.

I have welded alot of the tube that you are working on. That is indeed mill scale. If you really need to remove it use a radial flap disc on your grinder. It will cut that stuff off and leave a nice finish. If you use the disc flat it wont remove much, Just polish. If you use the flap disc aggressively at an angle It will really cut. As was recommended earlier try it on a piece of scrap.

Seamless tube is more expensive than welded tube. I dont know where you get your grinding discs or what size you use. There is also a fiber disc grinding wheel designed for rust removal but they dont last long on a large project and are, comparatively very expensive. What welding process are you using? There is another product you can try, Naval Jelly ( not for belly buttons ), you can get this in small quantities and it will remove oxides from steel. The steel will also form light rust if you do not treat it right away. I hope this helps? GregH

 

[rbattelle]

What welding process are you using?

Arc (stick) welding with 7018. I have used Naval Jelly for rust removal in the past, and it didn't occur to me that it might also be effective on mill scale.



As I mentioned, I decided to just lightly grind the scale away around the areas to be welded and leave it alone everywhere else.



Terrywerm - this is definitely not cold formed steel. It's hot roll. But the colors in the photo are deceiving.



Ryan

 

[GHarman]

Sounds like a plan. I'm headed over to my shop today. A little lathe work, as soon as I can get the place warmed up. Have FUN! GregH