Anti-corrosion coatings or double coatings, which consist of paint or plastic coatings applied to galvanized steel sheets, are often used in the manufacturing process. As with any anti-corrosion system, the thickness of the individual coats is an important quality factor for double-sided coatings. The operator can determine the individual coat thickness for a double-sided coating system. Anti-corrosion coatings or double coatings, which consist of paint or plastic coatings applied to galvanized steel sheets, are often used in the manufacturing process.
As with any anticorrosion system, the thickness of the individual coats is an important quality factor for double-sided coatings. For obvious reasons, not least because of the costs involved, thickness measurements should be non-destructive. The newly developed measurement method for these applications is adapted to several practical application examples.
There are various types of double-sided coatings, classified by their measurability:
Hot-dip galvanized steel components with a galvanized thickness of more than 80 microns, a unique zinc-iron diffusion zone and a paint coating with a typical thickness of more than 85 microns. These mainly occur in the field of steel structures.
Version with galvanized layer thickness between 0 and 10 microns, electrodeposited or deposited in a dipping bath, coating thickness up to 150 microns. They are mainly used in body building.
Coating systems on pipes such as brake lines, building exterior metal panels, items such as shopping carts or household appliances with a zinc thickness of up to about 30 microns, and organic paints or plastics with a thickness typically less than 200 microns.
A coating consisting of an organic layer and a zinc-iron or zinc-nickel alloy layer. Due to the higher cost of zinc alloy coatings, these coatings are mainly found in the automotive industry on components with a high risk of corrosion.
If the zinc coating is uniform in thickness, subsequent coating thicknesses can be measured using conventional magnetic induction coating thickness measuring instruments.
Select measurement method
Since these biphasic systems are a combination of organic and metallic coatings, typical measurement methods for measuring multi-coated systems are excluded since they cannot measure purely organic coatings. Thus, economical non-destructive and user-friendly instruments combine magnetic induction and eddy current testing methods.
Using the magnetic induction method according to the new ASTM Practice D 7091 or DIN EN ISO 2178, it is possible to measure the coating of all non-magnetic coatings applied to ferrous metals such as steel and iron. Non-ferromagnetic non-conductive coatings on non-ferrous metals can be measured using the ordinary eddy current method according to ASTM Practice D 7091 and DIN EN ISO 2360.
For example, if the latter can be considered as the base material, it should be possible to measure paint coats on intermediate zinc coats. Unfortunately, the conductivity of hot-dip galvanized or electrodeposited zinc coatings is only 7 megaohms per meter (MS/m), and copper is 58 MS/m. At a measuring frequency of about 20 megahertz (MHz), which can be achieved at reasonable expense, zinc coatings will need to have a paint thickness of at least 80 to 100 microns if thickness fluctuations do not have an effect on the measured values . Group 1 Only duplex systems fall into this category.
In a modified version, the phase-sensitive eddy current method can be used to measure metallic coatings on essentially any substrate material.
One application is the measurement of non-ferrous coatings on ferromagnetic steel. With a suitable instrument design, this measurement can be performed even under non-conductive coatings.
The electrical signal originating from the probe can be viewed as a vector having variable magnitude and phase angle or phase. With a two-sided coating system, a change in zinc thickness causes a change in the magnitude and phase of this vector, whereas an increase or decrease in paint thickness basically only affects the magnitude.
By using suitable instrument electronics capable of evaluating the probe signal based on its magnitude and phase, it is possible to determine zinc thickness based on phase, independent of the paint thickness on top of it. The magnitude of the zinc thickness-induced phase change can be optimized by an appropriate choice of eddy current measurement frequency for the zinc thickness range in question.
Since double-sided coating is mathematically a system of equations with two unknowns, the solution requires two independent equations. The first is represented by the phase-sensitive eddy current method, while the second is obtained only by conventional magnetic induction methods. As mentioned earlier, this method can be used to measure the overall thickness of bifacial systems. If the individually measured zinc thickness is subtracted from this measurement, the resulting value is the thickness of the organic coating.
As a third unknown, the equivalence between the measured values of the duplex coating and magnetic properties different from the substrate material or the pure zinc coating occurs.
The phase-sensitive eddy current method also requires that the metal coating to be measured has a sufficiently high electrical conductivity so that eddy currents can be generated.
Unfortunately, due to their crystalline structure, said zinc coatings exhibit electrical conductivities significantly less than 1 MS/m. To be able to perform coating thickness measurements on the basis of phase measurements, frequencies in excess of 10 MHz are required - which is not achievable in typical coating thickness instruments.
Large components that are often encountered in practical measurements combine the magnetic induction method with the phase-sensitive eddy current method. If no additional action is taken by the operator, after the probe is placed, measurements are taken consecutively both ways. Measurement data is appropriate. Therefore, the probe needs to contain the measurement systems required by both methods. The instrument measures paint thickness independent of zinc thickness. The repeatability of zinc thickness measurement is as low as tens of nanometers, and the accuracy of paint thickness measurement is less than 0.5 microns.
car color coating
The use of galvanized steel sheets in the automotive industry is increasing. Sheet metal suppliers apply zinc coatings by galvanic deposition or in a zinc dipping bath. If the zinc coating is uniform in thickness, subsequent coating thicknesses can be measured using conventional magnetic induction coating thickness measuring instruments. Simply subtract a constant value from the actual reading.
This uniformity of zinc thickness is usually given within batches on supply sheets that still need to be formed . When forming body parts, flow or even scraping of the zinc coating can occur in areas of severe bend radii, and this can vary in thickness from 3 and 9 microns, and occasionally the coating is completely removed.
A similar situation is encountered when repairing areas of the body with coating defects due to sanding and subsequent repainting of the defective areas. Also in this case, if a conventional paint coating thickness measurement system is used, the zinc coating may also be worn away, resulting in a significant reduction in coating thickness. This is not only problematic for the inspection of finished painted bodies, but also critical for the quality monitoring of electrocoats, since this thickness is usually only about 20 microns, measured by a reduced thickness of a coating of 5 or 6 microns The importance of thickness defects in zinc coatings cannot be overemphasized.
To reduce vehicle weight, the use of aluminum is becoming increasingly popular for non-safety-related parts of the human body. The measuring instrument is therefore also equipped with a conventional eddy current channel to measure the paint layer thickness on these components according to the standard.
Sheet metal suppliers apply zinc coatings by galvanic deposition or zinc dipping baths.
No action is required from the operator, even without knowing which parts are made of steel and which are made of aluminum, the instrument automatically selects the desired measurement method - double-sided or eddy current - as soon as the probe touches the substrate and Storing the paint thickness data in the same application allows a simple evaluation of the paint thickness distribution regardless of the type of metal sheet in this way.
Application examples for double-sided coatings on pipes or wires can be found in brake and fuel lines in the automotive industry and other pipes exposed to high corrosion loads or in the manufacture of shopping carts made of wire mesh. In most cases, the double-sided coating consists of a relatively thick zinc coating of about 25 to 30 microns and a paint or plastic coating of typically 20 to 100 microns.
It is also important here to measure both coatings separately, since they have different tasks to perform and the quality depends on the individual thicknesses. Particularly advantageous is the measurement method for such products in which the two coatings are applied in a continuous production process, as is the case for brake or fuel lines.
In such applications, the plastic coating is applied to the coiled tubing immediately after electrogalvanizing. Intermediate inspections of zinc coatings represent a cost-intensive interruption of the production process.
The introduced measurement method provides a user-friendly and reliable method of thickness measurement for double-sided coating systems on steel, which may consist of zinc coatings on which paint or plastic coatings are applied. Due to the large measurement range of these two coatings, the method is suitable for a wide range of applications.
tech tips
By using suitable instrument electronics capable of evaluating the probe signal based on its magnitude and phase, it is possible to determine zinc thickness based on phase, independent of the paint thickness on top of it.
The magnitude of the phase change induced by the zinc thickness can be optimized by proper selection of the frequency of eddy current measurements for the zinc thickness range in question.
Large components that are often encountered in practical measurements combine the magnetic induction method with the phase-sensitive eddy current method.
