Near-Field Microwave Detection of Corrosion Precursor Pitting under Thin Dielectric Coatings in Metallic Substrate

Abstract

Detection of corrosion precursor pitting on metallic surfaces under various coatings and on bare metal is of keen interest in evaluation of aircraft fuselage. Near-field microwave nondestructive testing methods, utilizing open-ended rectangular waveguides and coaxial probes, have been used extensively for detection of surface flaws in metals, both on bare metal and under a dielectric coating. This paper presents the preliminary results of using microwave techniques to detect corrosion precursor pitting under paint and primer, applique and on bare metal. Machined pits of 500 um diameter were detected using open-ended rectangular waveguides at V-Band under paint and primer and applique, and on bare metal. Using coaxial probes, machined pits with diameters down to 150 um on bare metal were also detected. Relative pit size and density were shown on a corrosion-pitted sample using open-ended rectangular waveguides at frequencies of 35 GHz to 70 GHz. The use of Boeing's MAUS™ 1 scanning systems provided improved results by alleviating standoff variation and scanning artifact. Typical results of this investigation are also presented.

Non-destructive techniques for corrosion detection: A review

“…With the decrease in aperture dimensions and maintaining a low frequency, Hugues et al demonstrated an increased special resolution on aluminum substrates. 182,184 Kharkovsky et al, 185 using focused and synthetic imaging techniques, focused on three recent applications of microwave and millimeter-wave NDT to detect disbonding between carbon fibre within a concrete structure, corrosion detection on aluminum and steel substrates as well as the flaws in insulation foam.…”

“…Hugues et al 182 demonstrated the detection of defects using Ka-Band (26.5–40 GHz) with V-band (50–75 GHz). His work also compared the effect of changing the bandwidth from Ka-Band to U-Band (40–60 GHz) and from V-Band to W-band (75–110 GHz).…”

“…His work also compared the effect of changing the bandwidth from Ka-Band to U-Band (40–60 GHz) and from V-Band to W-band (75–110 GHz). 182 Each band corresponds to an aperture opening with a higher band frequency, that is, the aperture opening is smaller. This method has been used to detect defects such as scratches, cracks and pitting.…”

“…With the decrease in aperture dimensions and maintaining a low frequency, Hugues et al demonstrated an increased special resolution on aluminum substrates. 182,184…”