Reader Asks About Measuring Coatings on Steel

We recently received an inquiry from a reader about measuring ATBC on steel in the 0.5 mg/ft range, and asking if online Infrared (IR) technology is capable of the measurement and if so, what size area this method covers.

ATBC (acetyl tributyl citrate) is a lubricating film — an oil — that is usually applied to minimize friction and scratches while handling and transporting steel sheets.

In our previous article, Online Measurement of Oil Coatings Improves Steel Component Manufacturing, we explained that to prevent unplanned line stoppages, lubricants are often applied in excess, reducing profitability, creating slip hazards in the coil storage areas and generating an additional waste stream to manage. Each of these scenarios can lead to production delays.

It makes sense that our reader is concerned about measurement of the ATBC. Online measurement of oil coatings helps metals component manufacturers eliminate costly delays in their process, save raw materials, reduce re-work, and improve product quality.

A non-contact metallic coating gauge that utilizes an online infrared (IR) light sensor is ideal for measuring hydrocarbon-based lubricants and offers a better solution to ensure that the critical oil layer is thick enough to guarantee consistent production, but not excessively thick to cause waste or pooling in the dies. By positioning an IR light source and detector on the same side of the coated product, a system of optics can be used to measure the intensity of a specific wavelength of reflected light relative to a reference wavelength. By comparing the ratio of the two intensities, a relative measurement of the coating thickness can be made. In general terms, the thicker the coating, the more absorption will occur at the measured wavelength in relation to the reference wavelength and the larger the observed ratio.

To answer the question, let’s clarify. The measure is 0.5 mg/ft² and the ATBC is being placed on a steel substrate. The piece that is missing is if the metal is cold rolled steel, aluminum, galvanized or some other material. The IR sensor should be able to measure it, but before purchasing a non-contact metallic coating gauge, you would want to confirm the application and even set up a test to measure some samples of various coating weights to determine the uncertainty in the measurement – keeping in mind that most weigh-strip-weigh methods will have a few mg/ft² error in their value. Typically when you want to measure to X, the instrument you select must be able to measure to 0.X (ten times better). To be absolutely sure, you might want to set up a test by sending the vendor some solution and bare samples, so they can make some measurements on freshly made samples.

A sensor that measures a wide spectrum of IR light is the most appropriate for those production situations that involve multiple coating/substrate compositions. For IR spot size, you should expect the area to be roughly 10mm x 20mm.

I hope this answers the reader’s question. If you have any questions, please post them below and we’ll do our best to answer them.

If you want to learn more about metal gauging technology and other products used in steel manufacturing, download our free ebook: A Practical Guide to Improving Steel Manufacturing Processes and Production Methods.