Combination of Galvanic Interaction and Acoustic Emission Measurements on Damaged Painted Al Alloys: Towards Instrumented Witness Coupons for Outdoor Corrosion Testing
Authors: Oltra Roland , Vuillemin Bruno , Larsen Rudy 
Published: November 2022

After real environmental exposures, the presence of corrosion damages on aircraft structures may be detected by means of non-destructive techniques (NDT). However, corrosion initiation and mechanism behind the defect cannot be distinguished by currently used physical methods. Therefore, aircraft industry needs sensors able to detect both corrosion initiation and its propagation.
Few years ago, a new test sample design providing accelerated response during atmospheric corrosion tests was reported by G.S. Frankel research group [[1],[2]]. A painted Al alloy panel, uncoated through-hole noble fasteners and scribes, was specially designed to measure the galvanic current between fasteners and scribes during exposure in a salt fog chamber using a zero-resistance ammeter.
But galvanic current measurement remains an electrochemical technique which could be difficult to be implemented in environments expected to be encountered in service, even if some attempts were described in literature [[3]]. More robust NDT methods, like acoustic emission was suggested to evaluate corrosion damage on Al alloys [[4]] but their application has not been yet transferred to instrumented witness coupons.
Recently a European program [[5]] was launched to validate the application of ultrasonic corrosion sensors for real time detection of early stages of localized corrosion on Al alloys. Following our previous work on laboratory-scale tests based on mass transport control from inhibitors leaching from primer exposed at scribes [[6]], this talk describes the laboratory validation of acoustic emission combined with galvanic corrosion current measurements to evaluate the role of depletion of available corrosion inhibitors in the paint on scratched plates, mimicking riveted aircraft panels, exposed to chloride environments, in various conditions, i.e. immersion and salt spray conditions.

[1] C.A. Matzdorf, W.C. Nickerson,B.C. Rincon Troconis,G.S. Frankel, Longfei Li, R.G. Buchheit, Galvanic Test Panels for Accelerated Corrosion Testing of Coated Al Alloys: Part 1—Concept, Corrosion, 69 (2013) 1240-1246.

[2] Z.Feng and G.S. Frankel, Galvanic Test Panels for Accelerated Corrosion Testing of Coated Al Alloys: Part 2—Measurement of Galvanic Interaction, Corrosion,70 (2014) 95-106.

[3] Z. Feng, G.S. Frankel,W.H. Abbott,C.A. Matzdorf, Galvanic attack of coated al alloy panels in laboratory and field exposure, Corrosion, 72 (2016) 342-355.

[4] A. Prateepasen, C. Jirarungsatian, Implementation of acoustic emission source recognition for corrosion severity prediction, Corrosion, 67 (2011), 056001/1-056001/11.

[5] Early detection and progress monitoring and prediction of corrosion in aeronautic Al alloys through calibrated Ultrasonic-CorROSion Sensor application, EU H2020 Grant agreement ID: 864905, https://cordis.europa.eu/project/id/864905

[6] R. Oltra and F.Peltier, Laboratory-scale testing of the anti-corrosion effectiveness of a primer coating on a bare AA2024 surface, Surface Interface Analysis, 48 (2016) 775–779.