Corrosion-protective coatings, or duplex coatings, consisting of a paint or plastic coating applied to galvanized steel sheet, are frequently used in manufacturing.
As with any corrosion protection system, the thickness of the individual coatings is an important quality factor in duplex coatings. For obvious reasons, not least as a result of the costs involved, the thickness measurement should be non-destructive. A newly developed measuring method for such applications is suitable for several practical application examples.
There are various types of duplex coatings, which are classified with regard to their measurability:
- Hot-dip galvanized steel components with a zinc coating thicker than 80 microns, distinctive zinc-iron diffusion zones and a paint coating with a typical thickness of more than 85 microns. These are found primarily in the field of steel construction.
- Versions with zinc thicknesses of between 0 and 10 microns, deposited galvanically or in a dip bath, and paint thickness of up to 150 microns. They are found primarily in vehicle body manufacturing.
- Coating systems on piping such as brake lines, metal sheets for building facades, items such as shopping carts or household appliances with zinc thicknesses of up to about 30 microns and an organic coating of paint or plastic with thicknesses of typically less than 200 microns.
- Coatings consisting of an organic layer and a zinc-iron or zinc-nickel alloy layer. Because of the higher cost of the zinc alloy coating, these are found primarily on components in the automotive industry with a high risk of corrosion.
Selecting the Measuring MethodSince these duplex systems are a combination of organic and metallic coatings, the typical measuring methods for the determination of multi-coating systems are ruled out because they are incapable of measuring the pure organic coatings. Thus, economical nondestructive and user-friendly instruments incorporate the magnetic induction and the eddy current test methods.
Using the magnetic induction method according to the new ASTM Practice D 7091 or DIN EN ISO 2178, all coatings made of nonmagnetic coatings applied to ferrous metals such as steel and iron can be measured. Using the common eddy-current method according to ASTM Practice D 7091 and DIN EN ISO 2360, nonmagnetic, nonconductive coatings on nonferrous metals can be measured.
For example, one should be able to measure a paint coating on an intermediate zinc coating if the latter can be seen as the base material. Unfortunately, hot-dip galvanized or galvanically deposited zinc coatings exhibit an electrical conductivity of only about 7 megasiemens per meter (MS/m), copper has 58 MS/m. At a measurement frequency of about 20 megahertz (MHz), which can be achieved with justifiable expenditure, the zinc coating would need to have a thickness of at least 80 to 100 microns if thickness fluctuations are not to have an influence on the measured value for the paint thickness. Only duplex systems in group one fall in this category.
In a modified version, the phase-sensitive eddy current method can be used to measure metallic coatings on basically any substrate material.
One application is the measurement of nonferrous metal coatings on ferromagnetic steel. With a suitable instrument design, such measurements are possible even under an electrically non-conductive coating.
The electrical signal originating at the probe can be viewed as a vector with a variable magnitude and phase angle, or phase. With a duplex coating system, a variation in the zinc thickness causes a change in the magnitude and phase of this vector, while an increase or decrease in the paint thickness essentially affects only the magnitude.
By using suitable instrument electronics that are capable of evaluating the probe signal according to its magnitude and phase, the zinc thickness can be determined based on the phase, independent of the paint thickness on top of it. The size of the phase variation caused by the zinc thickness can be optimized through a suitable selection of the eddy current measuring frequency for the zinc thickness range in question.
Because duplex coatings are, mathematically speaking, equation systems with two unknowns, the solution requires two equations that are independent of one another. The first is represented by the phase-sensitive eddy current method, while the second is obtained simply by the conventional magnetic induction method. As mentioned, this method can be used to measure the overall thickness of the duplex system. If the separately measured zinc thickness is subtracted from this measurement, the resultant value is the thickness of the organic coating.
The equivalence between the measurement of duplex coatings and the magnetic properties that are different from those of the substrate material or of the pure zinc coating occur as a third unknown.
The phase-sensitive eddy current method also requires a sufficiently high electrical conductivity of the metal coating that is to be measured in such a way that eddy currents can develop.
Unfortunately, the zinc coatings described exhibit an electrical conductivity of significantly less than 1 MS/m because of their crystalline structure. In order to enable coating thickness measurement to be carried out on the basis of the phase measurement, frequencies of more than 10 MHz are required-these cannot be achieved in typical coating thickness instruments.
The large components that often are present in practical measurement combines the magnetic induction method with the phase-sensitive eddy current method. Without additional action by the operator, measurements are made with both methods in succession after placement of the probe. The measurement data is then appropriately. Accordingly, the probe must contain the measurement systems required for both methods.
Automotive Body PaintingGalvanized steel sheet use is increasing in the automotive industry. The sheet metal supplier applies the zinc coating either by galvanic deposition or in a zinc dip bath. If the zinc coating thickness is uniform, the thickness of the subsequently applied paint coating can be measured using a conventional magnetic induction coating thickness measurement instrument. Simply deduct a constant value from the actual reading.
This uniformity of the zinc thickness is typically given within a batch on the supplied sheet that still has to be formed. When the body parts are formed, flowing or even scraping off of the zinc coating may occur in the areas of severe bending radii, and this may vary thickness by 3 and 9 microns, and occasionally remove the coating altogether.
Similar situations are encountered when repairing a body area that has coating defects because of sanding and subsequent re-painting of the defective area. In this case too, the zinc coating may be sanded away as well, leading to an apparent reduction in the paint thickness if the conventional paint coating thickness measurement system is used. This is not only problematic for the inspection of the finished painted body, but also critical in the quality monitoring of a cataphoretic paint because that thickness is typically only about 20 microns, a defect in the coating thickness measurement of 5 or 6 microns through a reduced zinc coating becomes critical.
In order to reduce vehicle weight, the use of aluminum is becoming increasingly popular for non-safety-relevant components of the body. Therefore, the measuring instrument also is equipped with a conventional eddy current channel to measure the paint thickness on these parts according to a standard.
Without any action by the operator, who may not even know which parts are made of steel and which parts are made of aluminum, the instrument automatically selects the required measuring method-duplex or eddy current-immediately after the probe touches the substrate and stores the paint thickness data in the same application, in such a way that a simple evaluation of the paint thickness distribution is possible regardless of the type of sheet metal.
Application examples of duplex coatings on pipes or wires can be found in brake and fuel lines in the automotive industry, as well as in other pipes that are exposed to high corrosion loads or in the manufacture of shopping carts made of wire mesh. In most of these cases, the duplex coatings consist of a relatively thick zinc coating of about 25 to 30 microns and a paint or plastic coating of typically between 20 and 100 microns.
Here too, it is important to measure the two coatings separately because they need to fulfill different tasks, and the quality is dependent on the individual thicknesses. Particularly advantageous is the measuring method for such products in which the two coatings are applied in a continuous production process, as is the case with brake or fuel lines.
In such applications, the plastic coating is applied immediately after electro-galvanization on the continuous pipe. An intermediate inspection of the zinc coating means a cost-intensive interruption of the production process.
The measuring method introduced provides a user-friendly and reliable means of nondestructive thickness measurement for duplex coating systems on steel and iron that may consist of a zinc coating with a paint or plastic coating applied on top of it. Due to the large measurement range for both coatings, this method is suitable for a broad range of applications.
- By using suitable instrument electronics that are capable of evaluating the probe signal according to its magnitude and phase, the zinc thickness can be determined based on the phase, independent of the paint thickness on top of it.
- The size of the phase variation caused by the zinc thickness can be optimized through a suitable selection of the eddy current measuring frequency for the zinc thickness range in question.
- The large components that are often present in practical measurement combine the magnetic induction method with the phase-sensitive eddy current method.