Sikel chooses a zinc coat with presenceThe zinc coating thickness gauge by electrogalvanizing
At Sikel, steel strips susceptible to corrosion that are intended for the automotive industry are coated by an ultra-thin zinc layer. Sikel owes its existence to the ever longer guarantees against corrosion that automobile manufacturers grant their customers. Steel is particularly susceptible to damage by the nature elements. The ten-year guarantees against rust automobile manufacturers are now granting, indicate that their products are particularly well protected against corrosion. Steel manufacturers therefore subject their product, coiled cold rolled low-carbon steel, to a special treatment before car body components are pressed from it.
This treatment is called zinc-plating or galvanizing. At Sikel, the zinc coating is not applied by immersion in a bath of liquid zinc. Instead, a large amount of electrical energy is used. In practice, this means that the steel strip is passed between two anodes at a rate of 190 metres per minute, and the zinc layer is applied electrolytically. The thickness of the zinc layer is on average 7 microns (seven thousandths of a millimetre), and may vary according to the particular requirements of the customer. The principle of electrolysis can be exploited for ideal adaptation of the zinc layer. The steel can also be zinc-plated on one side only if desired by the customer. The zinc-plating process is fully automatic and computer-controlled.
The zinc coating thickness gauge
The Sikel production line incorporates a thickness gauge. This appliance enables fast and accurate measurement of the thickness of the zinc or zinc/nickel coating on the steel strip. Measurement is performed on-line, during the production process, without making contact with the steel strip and with a high level of accuracy and reliability.
Principle of measurement: X-ray fluorescence (XRF)
When an x-ray is applied to a metal, the elements of the metal react by transmitting energetic radiation. This is called the "photoelectric effect". The energy generated varies in value according to the element. The phenomenon of x-ray fluorescence is used to measure coating thicknesses.
Generation of XRF radiation by photoelectric effect
At Sikel, this fluorescence technology is used to measure firstly the total thickness of the coating (in the order of 0.005 mm), and secondly, its composition.
In the first case, we want to know the thickness of a zinc layer applied to a steel strip. The substrate (the steel strip) consists of one element, Fe, coated with a second element, Zn.
An X-ray with an energy level I generates a fluorescent beam with an energy level IZn and IS.
These values correspond to the fluorescent energy levels of zinc and iron respectively.
The value of IS drops with increasing coating thickness. This measurement principle is used chiefly on very thin coatings (less than 0.010 mm).
IZn rises with increasing coating thickness, and is used to measure a thicker zinc coating on steel strip. A coating thickness gauge thus comprises:
- a source of x-ray or gamma radiation;
- detectors to measure the energy levels IZn and IS.
The coated strip passes the thickness gauge immediately after the electrolytic zinc-coating process. Two measuring heads scan both the top and bottom edges of the strip. The measurement data are transferred to a process computer that computes the thickness and composition of the coating in the transverse axes and longitudinal axes of the strip.
The measurement is also used to control the galvanizing process itself.
> Link to configuration.
Integration of layer thickness measurement device in the production process