Measuring paint thickness on hot rolled steel

You got a tough one! I can only offer that you make an allowance for the mill scale and subtract it from your readings or destructively test your parts. Unless, you can perform a QC inspection before and after at your vendor's facility or ask them to provide the necessary inspection documentation and assume liability for out of spec paint film thickness. But, you may be already doing this. You didn't mention whether you were concerned about sheet, plate or shapes, nor did you mention the DFT that you were looking for. You must be aware that the surface of the mill scale is not very smooth (uniform). Nor did you mention that the steel is being pickled prior to paint. Just doesn't sound like you need much accuracy based on the material you're buying and, on that basis, I'd just make an allowance for the mill scale and be done with it because, if the paint and the paint film thickness is that important, you should be pickling the steel as part of the preparation process and painting it properly. Hope this helps.

Agreed. Hot rolled steel scale varies widely throughout a coil and from point to point because of variances in temperature during cool-down of the steel. This also results in the chemistry of the scale which can include a multitude of iron oxide chemistries. Pickling of the steel to remove scale from the hot-rolled product prior to painting would appear critical to obtaining good control of paint thickness, as well as paint adherence.

You might consider the optical interference based instruments by Lumetrics. They are quite versatile, non-destructive, and can detect individual layers. www.lumetrics.com.

jhammond

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www.patent-innovations.com

There shouldnt be any mill scale on the steel before they paint it, especially on a machine part where the paint thickness is in any way critical. You should spec at least a SSPC-SP6 Commercial Blast Clean finish before painting, maybe even a Near White Metal finish SP 10. just my $0.02.