Mill=Scale Removal

[dian]

i know what happens on steel, still trying to find out what happens on mill scale. does this look somehow phosphated? i cannot grasp the the idea that ironoxide (fe2o3, fe304, feo, whatever) gets phospated or even "passivated". would that mean it gets sealed by the phosphate? or reacts with it (to what)?

[J Tiers]

The color of the iron phosphate is pretty similar to the scale, so it might even be replacing the scale.. If you have been "simmering" the steel in phosphoric, there is for sure a coating of phosphate on it. It will develop in cold acid in under an hour on bare steel.

[dian]

o.k., im interested in this because im still looking for an easy way to handle mill scale. we might be using "passivate" i a different way here.

so can you explain what happens when you you apply phosphoric to steel with mill scale on it?

(as mentioned iv had a plate simmering in 20% phosphoric for several hours and nothing happend to the scale, at least i thought so.

[J Tiers]

I have noticed that paint sticks to mill scale pretty well. Sometimes the mill scale does not stick so well, however, I have seen it pop off for mo particular reason, other than maybe new rust getting under it.

Mill scale is really just rust.... black rust, not red rust. If conditions change, it can turn to red rust (different oxidation state) or proceed as black rust. Whatever the reason, it seems that it may not stick well, and then the paint comes off.

Since much of the time, it works fine painted, I assume that it is environmental conditions that affect the performance. I think that in many cases, water got through the paint due to cracks or scrapes, and that started the problems.

[nickel-city-fab]

so phosphoric will get under the scale and produce a phophate coat on the metal there?

It doesn't need to get under it. It is possible to have more than one oxygen atom bound to iron, and that is what is happening with the phosphates, and with the old-time slow rust blueing.

[Doozer]

If water will rust steel that has mill scale on it
then phosphoric acid will passivate steel with mill scale on it.
Or is there something I am not taking into account ? ? ?

-D

[dian]

so phosphoric will get under the scale and produce a phophate coat on the metal there?

[nickel-city-fab]

Doesn't matter if there is scale on the material, phosphatizing will still work:

"Ferrous materials, including steel, may be somewhat protected by promoting oxidation ("rust") and then converting the oxidation to a metalophosphate by using phosphoric acid and further protected by surface coating. As the uncoated surface is water-soluble, a preferred method is to form manganese or zinc compounds by a process commonly known as parkerizing or phosphate conversion."

from Wiki:https://en.wikipedia.org/wiki/Passivation_(chemistry)

On a somewhat related note, rust bluing is an old, old technique: allow the part to rust under controlled conditions, then boil it in hot water. Repeat this a few dozen times. You get a nice velvet black coating which retains oil. The boiling converts red rust (FeO2) to black iron oxide (FeO3) The technique is commonly used in protecting shotgun barrels (a subject I have been researching lately)

[Doozer]

Mill scale is an oxide, a ceramic. It is just along for the ride.
But you knew that. And you knew that I knew that.
You know the passivation was for the base metal.
Do you think I just walked out from behind the barn
with shlt on my boots ? ? ? Play with someone else.
Maybe Jerry needs a playmate.

-D

[dian]

All "proper" painting means is you follow the spec. temp, humidity, thickness, surface prep etc. We do a fair bit of sand blasting, to whatever profile the spec calls for, however I can tell you >95+ of large fabrications, cranes and equipment are painted direct to the mill scale. You would see more blasting on structural shapes than plate and sheet, where the mill scale is hard smooth and durable

no idea about cranes, but i would guess that a different primer must be used on mill scale.

also, from my experience, phosphoric creates a phosphate layer on steel but does nothing to mill scale. how would iron oxide get "passivated".

[nickel-city-fab]

Nope, not Watson-Bowman-Acme. It was my father's own business, Superior Contractors.
We had a shop in Tommy Price Trucking's building on Mystic St, off Hopkins, near SouthPark
and Tift streets. We operated it for 7 years, but NYS was always late on the budget and it
was difficult to have payroll for the men when no payments were coming in. We had a stud
welder that was 1800 amps. Would weld a 1" stud down in 1/2 a second. Later after we
moved out of Price's building, a CNC outfit moved in there. Just last year, I saw Cash Cunningham
auctioning all the machines off. Nice stuff to. Even a HBM or 2 as I recall.

--Doozer

Cool that's some good info. FWIW there is another bridge shop in Sanborn that hires sporadically http://www.nbrfab.com/
pretty sure they used to be Great Lakes Metal. Core Welding is still out that way, and Buffalo Gear but I don't see much activity around their shops in the last ~20 yrs. I'm curious to find out what kind of gear machines they have.

[Doozer]

Nope, not Watson-Bowman-Acme. It was my father's own business, Superior Contractors.
We had a shop in Tommy Price Trucking's building on Mystic St, off Hopkins, near SouthPark
and Tift streets. We operated it for 7 years, but NYS was always late on the budget and it
was difficult to have payroll for the men when no payments were coming in. We had a stud
welder that was 1800 amps. Would weld a 1" stud down in 1/2 a second. Later after we
moved out of Price's building, a CNC outfit moved in there. Just last year, I saw Cash Cunningham
auctioning all the machines off. Nice stuff to. Even a HBM or 2 as I recall.

--Doozer

[nickel-city-fab]

I don't think the phosphoric acid just sprayed on removes any mill scale,
it just passivates the surface.

-Doozer
--D

100% correct, the phosphates are used extensively to blacken guns in order to protect the machined surfaces. It's just porous enough to retain oil. Manganese Phosphate or Zinc Phosphate. There's actually a mil-spec for the process (fairly simple). The same process is often used to blacken and protect tools -- such as hex keys, impact sockets, etc.

BTW a large fab sop was hiring all last year for a expansion joint welder. It's prolly your old employer now owned by BASF. I have zero interest in welding 2" plate with flux core unless their bennies package is huge. (defined pension, 100% medical)

I regular see their ads online in the local area.

[JRouche]

LOL you're 80 yrs old ??? j/k

Hahaa. Sure feel like it sometimes. No, Im young, well relative to all the old farts here JR

[Doozer]

I agree. For a customer, sandblast or similar prep for sure.
I used to be in the bridge expansion joint business. The state had a
spec on that for sure. We would usually send stuff out for blasting
and then sometimes galvanizing.
But for stuff around my shop, I have had really good luck with just
wire brushing structural steel, and spraying on phosphoric acid,
and when it is dry, brush painting with Rustoleum oil paint, no primer.
I don't think the phosphoric acid just sprayed on removes any mill scale,
it just passivates the surface.

-Doozer

BTW- Our bridge joint shop was an old Westinghouse building, with
a 20 ton crane. Riveted construction, don't know the name.
It was 3 phase motors with fluid coupling drive. No brakes or they
did not work. To stop it, you had to reverse the motor. Thankfully
the hoist had a brake. --D