Contents lists available at ScienceDirect NDT and E International journal homepage: www.elsevier.com/locate/ndteint A microwave non-destructive testing method for controlling polymeric coating of metal layers in industrial products A. Mazzinghi, A. Freni, L. Capineri ∗ DINFO, University of Florence, Via di Santa Marta 3, 50139, Florence, Italy ARTICLE INFO Keywords: Inspection systems Sensor Sizing Thickness Non-destructive testing Microwave sensing Periodic structures Oil industry Metal-plastic composites ABSTRACT The paper exploits the possibility of extracting the geometric properties of a polymeric coating of periodic metal layers used in industrial products from non-contact microwave backscattering measurements. Specifically, a flexible pipe having a periodically corrugated metal profile coated with a thick Polivinylidenedifluoride (PVDF) layer, suitable for oil industry applications, has been considered. The combined use of magnitude and phase of the reflected signal, acquired on a wide relative frequency band (∼15%), allows calculating the dielectric filling level and the period of each corrugation with accuracy better than 1/100 of the period length, 21 mm in our case. The results of an experimental campaign, carried out to estimate the accuracy achievable in the operational environment, are also shown. 1. Introduction Several industrial manufacturing processes of costly and high re- liability items involve the use of different materials. In general, such materials are combined to form a composite structure with dimensional and structural mechanical characteristics that must accomplish to tight standards for acceptability. Examples are composite structures made of metallic parts coated by polymeric layers. The coating, made of che- mically inert polymers, ensures protection against corrosion, while the metal structures ensure the mechanical resilience. Depending on the application, the metal layer can be flat or corrugated. In the latter case, the right adhesion and filling between the metal and the polymer coating can be lost, due to the variability of the manufacturing process. In the future perspective of the digital industrial manufacturing for optimizing the production costs [1], and for reducing the time to market, it is important to develop sensor systems that can provide real time measurements of those parameters useful to control the manu- facturing process. In this work, we have investigated non-destructive testing methods suitable for the analysis of the adhesion quality of a polymeric layer, with up to 20 mm thickness, deposited on a periodic metallic frame- work. The polymeric coating can be obtained with an extrusion process. As a reference example, we have considered a flexible pipe for gas or oil industry [2] as that shown in Fig. 1. In this case, to guarantee the flexibility of the pipe, the polymeric coating has not to fill the grooves in the interlocking profile (Fig. 1b top). In particular, when an air gap between the coating and the metallic corrugations is less than 0.5 mm then the polymeric coating is considered out of the specifications (see Fig. 1b bottom). An initial evaluation of common methods used for non-destructive investigation has been carried out taking in mind that the system has to be as simple as possible, real-time, and with no contact with the pipe. In fact, it will be manufactured in harsh environment and subjected to vibrations. The outcomes are reported in the following. Basically, there are two techniques suitable for the detection of anomalous polymer coating profile: electromagnetic methods related to the electrical im- pedance variations, and ultrasonic methods related to the acoustic impedance variations. The feasibility of the ultrasonic methods was here first considered, since the transducers and electronic front-end can be easily purchased with COTS (Commercial Off-the-Shelf compo- nents). The bottom profile of the polymer coating can be in principle measured by ultrasonic methods with high spatial resolution probes operating at high central frequency [3]. A series of preliminary tests with several contact probes, with operating frequency from 0.5 MHz to 10 MHz, placed over a layer of Teflon or PVDF, with thickness from 12 mm to 20 mm, has pointed out a very high attenuation, which is estimated around 20 dB/cm. The limited propagation velocity of the polymer coating (between 1700 m/s and 2200 m/s) allows enough axial resolution when operating at 1.5 MHz in pulse echo mode (λ u / https://doi.org/10.1016/j.ndteint.2018.12.003 Received 13 August 2018; Received in revised form 16 November 2018; Accepted 11 December 2018 ∗ Corresponding author. E-mail addresses: agnese.mazzinghi@unifi.it (A. Mazzinghi), freni@unifi.it (A. Freni), lorenzo.capineri@unifi.it (L. Capineri). NDT and E International 102 (2019) 207–217 Available online 15 December 2018 0963-8695/ © 2018 Elsevier Ltd. All rights reserved. T