Anti-corrosion Coatings

Corrosion protection plays a large role in the automotive sector. To ensure the effectiveness of rust protection for bodywork and chassis, the thickness and composition of protection layers must be right; this is achieved, for example, with cathodic dip-paint (EPD) coating processes. FISCHER offers tailored measurement technology for the automotive industry.

Corrosion protection coatings

Application notes

Non-destructive measurement of CDP coatings inside hollow car body parts

In the automotive industry, the anti-rust warranty has become an important hallmark of car quality. Particularly in hard-to-reach areas such as the cross beams and pillars of the steel car body, specified tolerances for CDP coating thicknesses must be ensured, because in these areas, corrosion can proceed long undetected.

Modern car bodies are usually protected from rust with a primer coat applied by cathodic dip painting (CDP). Until now, it was not feasible to measure the coating thickness inside the hollow parts without destroying the entire car body. Common practice has been to dismantle random car bodies for spot-checking of these hidden areas. This approach carries a very high cost and makes systematic, real-time quality control more or less impossible.

For exactly this purpose, FISCHER has developed the V3FGA06H “cavity probe” which, due to its design, can be inserted into almost any opening in cross beams or pillars for accurate and non-destructive measurement of the CDP coating thickness – on the inside.

Fig.1: DUALSCOPE® FMP100 and cavity probe V3FGA06H for measurement of CDP coatings on car bodies

The magnetic inductive probe with its wear resistant tip has been optimized for all typical CDP coatings thinner than 25 μm (including Zn). It has a specially curved, slim shape and fits through small openings in the car body to measure the thickness of lacquer coatings in what was once inaccessible areas. Its small, flexible, attached head with three-point support guarantees precise positioning and repeatable measurements even inside the cross beams and on curved surfaces.

Fig.2: Insertion of the V3FGA06H probe into a hollow car body part to measure the CDP coating on the inside

The V3FGA06H probe works with FISCHER’s FMP series instruments, which are equipped with a USB port that makes it convenient to transfer measurement results to a computer for evaluation, recording and storage using FISCHER DataCenter software. A combination of the DUALSCOPE® FMP100 gauge and FISCHER DataCenter IP (Inspection Plan) allows the user to create inspection plans on a PC and transfer them back to the instrument, providing visually aided guidance though a defined measurement task.

With the cavity probe V3FGA06H and easy-to-operate FMP instruments from FISCHER the thickness of CDP coatings in difficult to access cavities such as inside door sills, brackets or reinforced parts can be measured quickly, accu­rately and, above all, without dismantling. Your local FISCHER representative will be happy to answer any questions you may have.

Measuring duplex coatings in the automotive industry

For automotive manufacturers, it is of increasing importance to control the quality of the paint processes in order to ensure effective corrosion protection while reducing costs. Especially in EDP (electrophoretic deposition) processes, this requires adherence to tight tolerance limits, which in turn, requires careful monitoring. But the measurement task itself should also be fast and cost-effective; the instrument must be able to measure lacquer atop various base materials (mainly galvanized steel or aluminum) without refitting or recalibration.

To measure lacquer coatings on both steel and aluminum with the same instrument and without having to change probes, it is necessary to utilize different methods. Furthermore, when the steel is galvanized, the Zn coating must be measured separately in order to determine the thickness of the lacquer coating above it. Especially for this purpose, FISCHER has developed the PHASCOPE® PMP10 DUPLEX and the ESG20 probe, uniting three measurement methods in one system for testing the thickness of each layer precisely and accurately.

Fig.1: PHASCOPE® PMP10 DUPLEX 

For a typical lacquer/Zn/Fe combination, the Zn layer is measured through the lacquer using the phase sensitive eddy current method (employing lift-off-compensation). At the same time, the overall coating thickness (Zn+ lac­quer) is measured using the magnetic induction method; the thickness of each individual layer is derived from this and displayed on the instrument.

For coating systems on aluminum, the amplitude sensitive eddy current method is used. A special conductivity compensation feature ensures that the various conductivities of different Al alloys exert no influence on the measurement of the lacquer layer. Without any additional calibration, the coating thickness can be measured precisely and reproducibly in one pass even when the substrate changes, as the probe automatically identifies what the base material is.

Fig 2: Typical measurement tasks in the automotive industry. MS = magnetic induction method, WS PS = phase sensitive eddy current method, WS AS = amplitude sensitive eddy current method

Especially with costly EDP coatings, very small tolerances – often of just ± 2 µm – must be met in order to ensure the minimum thickness for effective corrosion protection while minimising the amount of material consumed. This is why it is necessary to measure the Zn layer underneath the paint to determine the real thickness of the lacquer, since even minor variations in the Zn coating could erroneously indicate a (non-existent) flaw in the paint process. 

The PHASCOPE® PMP10 DUPLEX, together with the ESG20 probe, was developed especially to meet the requirements of the automotive industry. Its three different measurement methods make it possible to measure lacquer coatings precisely, in one operation, whether on galvanized steel or on aluminum. Your local FISCHER representative will gladly provide you with further information. 

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Fischer Technology Inc.
Windsor/United States

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Phone: (860) 683-0781
E-Mail: info@fischer-technology.com
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