Mill=Scale Removal

[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.

[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]

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]

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)?