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What does 'mill finish' of aluminum mean?

I am working in building curtain wall area. We use aluminum with different finishing: mill finish, anodize, Alodine, powder coating.... My questions are: What is 'mill finish'? When material extruded from die a coating of oxide is building-up while cooling. So it is not 'mill finish' but something like anodizing. What's the big differences(durability, easy sanding,against corrosion) between this oxide coating and anodizing? I don't know how long this coating forms? And how thick it will be? Another question is the 'mill finish' of extrusion is different of the 'mill finish' of aluminum panel? I will use answer to solve silicone adhesion problem.

Thanks,

Robin Ng
Engineer - Toronto, Ontario, Canada
2004

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"Mill Finish" is the natural appearance of the aluminum as it comes from the rolling mill (Alcoa, Reynolds, Kaiser, or Alcan) or the extrusion mill. It is "as is" with no external mechanical or chemical finishing. Extruded metal is considered "mill finish". All aluminum has an oxide of some varying thickness. Anodizing is a very heavy controlled oxide. Anodizing is an electrolytically formed and controlled heavy oxide 0.0003 inches thick on up the 0.002 and on up. Mill Finish is a very lightly oxidized film and will wipe off with your bare finger and immediately form. Rolled sheet would probably have a thinner oxide than hot extruded aluminum. If you want to bond silicone adhesive then you want a phosphate etch, or a light chromate or a thin (.0004 anodize in either chromic, sulfuric, or phosphoric.) If you anodize, do not seal the coating and be sure your anodizer knows you do not want it sealed.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

2004