Among the most common principles to measure any parameter is to look for physical and chemical phenomena that are reproducible and with a behavior that is easy to implement numerically. The most popular example is that of the different expansion in the heat of the mercury and the crystal that gave rise to the standard thermometer.
This same idea is what guides many other methods of measuring physical and chemical variables. In our field, we will be based on the different characteristics between the coating and the substrate.
One of the most common cases is the thickness measurement of coatings that prevent corrosion of ferrous materials. In this case, the main characteristic that distinguishes the coating from the substrate is magnetism. The magnetic induction method (UNE EN ISO 2178) uses an instrument with an electromagnetic sensor that will generate more or less signal in contact with the sample depending on the greater or lesser thickness of the generally non-magnetic coating (zinc, chromium, paint, …). ) on the magnetic ferric substrate.
For the measurement of thicknesses of magnetic coatings (nickel, ..) on ferrous bases, the method of phase variation of the eddy currents is used (UNE EN ISO 21968) that takes advantage of the different electrical conductivity of the coating and the substrate.
For non-magnetic metallic substrates (aluminum, copper, brass, etc.) we will distinguish when the coating is or is not conducive of electricity.
In this second case, the physical phenomenon that distinguishes us from substrate coating is conductivity. Again, an instrument with an electromagnetic sensor induces Foucault currents in the substrate that are attenuated the greater the non-conductive coating. The Foucault currents method (UNE EN ISO 2360) is commonly used to measure lacquer thicknesses on metals or anodizing layers, which although have a metallic composition are not electrical conductors.
For the first case, metallic coatings on non-ferrous metallic substrates, one of the phenomena used again to measure the thickness is the different electrical conductivity of the substrate and the coating by the phase variation method of the eddy currents (UNE EN ISO 21968).
Among these methods of measurement that use a magnetic sensor is to discuss the case of measurement of a magnetic coating (nickel, iron, …) on a non-magnetic substrate (metallic or not) in which our magnetic sensor measures the thickness of the layer or less interaction with it depending on its thickness through a Hall effect sensor. It is the so-called magnetic method (UNE EN ISO 2178) that should not be confused with the magnetic induction method already mentioned.
The methods described above, although of great precision, have a series of limitations linked mainly to the geometry of the pieces to be measured. The response of the electromagnetic sensors is not the same when faced with a plane measuring point that curves or with a more or less conductive or more or less magnetic substrate. Calibration of equipment with specific patterns corrects these limitations.
So far we have described measurement methods based on electromagnetic sensors. However, there, is a type of coatings that by their specific application process generate multilayer metallic coatings in pieces of complex geometry or very low thickness coatings. Electromagnetic methods can not generate sufficiently precise measurements or distinguish the thicknesses of the different metallic layers. The method of anodic dissolution or coulometry (UNE EN ISO 2177) allows differently measuring the thickness of this type of coatings.
Its principle of measurement is based on inverting the process by which the coating is generated. If the coating has been generated from a metal anode that dissolves in a bath and adheres to the material to be coated, which acts as a cathode, coulometry is a measurement method that consists of a small amount of electrolyte being placed in contact with the sample coating to be measured by applying a current intensity in the opposite direction, so that the coating of the sample, which is now the anode, dissolves constantly and is therefore measurable.
In the case of various metallic layers, the process is repeated with each of them in a manner with specific electrolytes.
The need for a method of measuring thicknesses of coatings, fast, reliable, without geometric dependence of the pieces and the disadvantage of manipulation and destruction of the colorimetry sample l,ed to the use of this technology, the X-ray fluorescence (UNE EN ISO 3497), as a method for measuring coating thicknesses.
The principle of its operation is based on the fact that a certain element that receives an X-ray emission generates a specific emission of photons that distinguishes it univocally from the one that would generate the same emission on the rest of the elements.
From this principle we only need a good sensor that can discriminate the spectrum of the emitted photons and their emission power, to distinguish the present elements and their quantity.
This technology allows measuring multilayer metallic coatings simultaneously, both in the micrometric and nanometric range and in the case of alloys its composition
Its only limitation is that plastic coatings (paints, lacquers, etc.) cannot be measured unless they have a high metallic presence in their composition; but for these cases, the methods explained at the beginning of this article already provide a solution.
The popularization of all types of electronic devices due to the low prices of its components and mass production has, in many cases, led to the banalization of the manufacture and use of measurement instrumentation. Many times we are not aware that when we measure 15 microns of zinc on an iron plate, we actually measure only 0.015 mm, a minimal and hidden magnitude and not even the usual 300 microns of paint that can be found in any object of street furniture are also perceptible to our sight. For this reason, the measurement of this small but crucial parameter that is the thickness of a coating should not be trivialized and should be carried out with the appropriate and precise instrumentation.