Mill Scale ?

[wetwhopper]

O.K. I was enjoying reading some of the threads here and the term "mill scale" keeps coming up. I looked it up but still don't understand it other than it's not a beneficial thing for aluminum or any kind of metal. Can someone put it in laymen terms so the likes of me (challenged ) can understand it? Lol!

[welder]

Mill scale happens to FERROUS metals [ Steel ] and it happens during the HOT ROLLING PROCESS , it's a very thin layer of Iron Oxides that has to be removed before painting or welding .

Aluminum Alloy does not have that problem , it's a NON FERROUS metal .

Are you talking about ....MILL FINISH ?

[wetwhopper]

Was reading a thread by Katoh and ran into the term ...

[Katoh]

wetwhopper
Kevin Morin was talking about mill-scale on new aluminium sheets, apparently its The Shiny Film on the a new sheet and you need to remove it prior to welding and putting the vessel in service. I hope Kevin Chimes in to give a better description.

Katoh

[welder]

Here is a good link to help explain a few things.
http://www.millerwelds.com/resources/ar ... de-graphic

The term "MILL SCALE" is used more with MILD STEEL , not saying it couldn't happen with AL. most of the problem with Alloy is OXIDIZATION .

If you want to read about "mill scale" just Google it.

[wetwhopper]

Katoh and Welder, "thanks!" I get it. I figure better to ask ?'s and know what everyone's talking about...

[kmorin]

Mill scale or mill finish either term can be used to describe the coating that comes from the mill on aluminum sheet. Its shiny and is not (chemically the same as) the aluminum and if not removed will cause (facilitate/participate/enable/interact) corrosion of the underlying aluminum.

Look up "galvanic differential" or galvanic corrosion, then the idea will be more clear. There is a slight galvanic difference between the alloy under the mill scale/finish and the parent/native/final/underlying metal.

Next look up poultice corrosion, this is also facilitated/enabled/created/assisted by the mill scale/finish on marine aluminum alloy sheets/plates. This is where water is kept in contact with the sheet in a tight space and the oxygen goes out of the water so it becomes acidic and then speeds the surface corrosion using the scale/finish to enable the water retention and the reaction. Similar 'cells' in descaled panels show almost no trace of the level of corrosion.

This scale/finish is most often removed with acid, or (correctly selected) SS wire wheels, or 'buffering' (Scotchbrite TM type) pads or sand blasting.

Many boat builders and manufacturers leave the scale on and their boats show the results in a year or two, while 30 year old boats that have been descaled/etched to remove the finish/scale are clean.

This layer also retains atmospheric moisture (dew, condensation, humidity) so much it has to be removed to weld or the weld will have so many porosity gas bubbles it will be structurally unsound. [Many 'sound' MIG welds have some root porosity] However, since this is only needed in the weld zones, and is not always done there either, the remaining surface may or may not have this 'finish' removed, but it is best practice to remove this manufacturing byproduct, just like it is in steel sheet plate, prior to painting, or even if the boat will remain bare metal finish.

cheers,
Kevin Morin

[NTGeorge]

As always, you are an incredible wealth of knowledge Kevin.