Nondestructive Characterization of Corrosion Protective Coatings on Aluminum Alloy Substrates
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67 Mat. Res. Soc. Symp. Proc. Vol. 591 0 2000 Materials Research Society
substance. Future research concentrates on finding substitute inhibitors with little or no environmental pollution [2]. The primer coating acts as a physical barrier between substrate and environment. Hard thermosets, especially epoxy resins are used providing good adhesion to the substrate material and are capable to withstand mechanical loading during service. Furthermore, epoxies are highly chemical resistant and can contribute to corrosion protection. The topcoat's main purpose is to provide protection against erosion and mechanical abrasion. Thus, both sufficient flexibility and hardness are needed. It is also used to achieve a decent cosmetic appearance. Furthermore, the topcoat material must be water-repellent and UV radiation resistant. Certain polyester-urethanes (PUR) meet all these requirements the best [2]. Topcoat (PUR Elastomers) approx.. 50 gim wear protection water-repellant Ultraviolet (UV) radiation resistance cosmetic appearance
Primer ( hard epoxy resin) approx. 20 pim "*corrosion protection "•good adhesion to substrate "*withstands mechanical loading
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Chromate Conversion Coating improves corrosion protection of natural A120 3 by adding corrosion inhibitors • self-healing Figure 1: Schematics of coating system In the scope of this work a series of Al 2024-T3 specimens with different epoxy primerthickness have been examined. Thus, we first concentrate on a single layer system. Scanning Acoustic Microscopy The characterization of multi-layered structures using Scanning Acoustic Microscopy is classically performed by time-of-flight spectroscopy in the pulsed regime. The method is very easy to apply. An ultrasonic wave generated by a piezoelectric transducer is sent into the test material; the same probe detects the reflected waves [3,4]. The coupling between transducer and sample is achieved by submerging both in a water bath (figure 2). In this experiments a focused 140 MHz; was used. Focussing and high frequency of the excited wave and the scanning technique provide the possibility of high-resolution measurements (approx. 10 pim). The acoustic wave pulse travels through the different material as indicated in figure 2. The time difference between the detection of two reflected waves, the so-called time-of-flight, is measured. For known sound velocity, the thickness can be determined with equation 1. dct = I v•,tof
(1)
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Employing own measurements on specimens with known thickness we determined the sound velocity of epoxy to be 2250 m/s and that of 2024-T3 Al-alloy to be 6240 m/s.
water
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Figure 2: Schematics of pulse/echo scanning acoustic microscopy Continuous Acoustic Waves
In our experiment a network analyzer was used to generate, receive and analyze the continuous acoustic waves. A sample and transducer combination was connected to the network analyzer. The instrument treats the sample-transducer combination as an electrical network. One of the parameters of the network, S i1, defined a
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