Technology for the noncontact and nondestructive coating weight measurement of metallic coatings on metal strips. This technology provided by Thermo Radiometrie enables the online measurement during the production process.

Principle:

Coating weight measurement of thin metallic coatings applied to steel strips can be achieved by using X-ray fluorescence (XRF) radiation. Generation of X-ray fluorescence radiation is achieved by utilizing the photoelectric effect - Fig. 1.

The coated steel strip is exposed to a primary beam of photon radiation. This photon radiation can be gamma rays or X-rays, having sufficiently high energy to stimulate excitation and emission (fluorescence) of X-rays at critical energies which are characteristic of the component elements in the substrate and/or coating materials. The excitation of iron atoms in a steel strip leads to emission of fluorescence radiation with an energy of 6.4 keV (1 kilo electronvolt = 1.6?10^-16 J). If the steel strip is coated by another material, this 'iron fluorescence' radiation is attenuated whilst passing through the coating, as seen in Fig. 2. If the coating weight increases, less radiation emitted by the steel will pass through the coating. An ionization chamber measures the intensity of the remaining iron fluorescence radiation.

In the case of a galvannealed coating (zinc/iron alloy) on steel strip the angular distribution of the emitted X-ray fluorescence radiation is taken into account. The mathematical comparison of the measurement values taken at different angles provides precise determination of the iron content and of the total coating weight.

Each element has characteristic fluorescence energies (K-alpha and L-alpha), and associated absorption-edges (electron binding energy K, L) at slightly higher energies. In practice, the K series are normally used in coating weight gauges. The primary radiation beam must have some component energies which are higher than the Kab absorption-edge energy of the element required to fluoresce at its corresponding K-alpha energy.

The XRF measurement principle can be used for coating weight measurement of the following metallic coatings on steel strips:

-Zinc
-Iron content in galvannealed coatings (% Fe)
-Zinc-/aluminum alloys
-Zinc-/nickel
-Aluminum
-Tin
-Nickel
-Copper
-Chromium
-Terne

Typical primary sources for X-Ray Fluorescence method:

Am-241
X-ray

Detectors for X-Ray Fluorescence method:

Typically two or more differential ionization chambers are used. The number of the ionization chambers depends on the application. Detectors and source are installed within one housing.