COrrosion Quantification Through Extended use of Lamb waves – COQTEL

To monitor corrosion damage, COQTEL relies on ultrasonic (US) waves generated and received by piezoelectric elements. Since this effect is reversible, these elements behave as sensors and actuators. US waves allow monitoring of corrosion damage either via an active interrogation (waves are emitted by some elements and the echoes generated by the damage are recorded by others) or passive (corrosion damage generates acoustic events during their development). A specific electronic hardware combining these two modes has been developed and validated on artificial hemispheric damage of 100 µm radius.

An electrochemical corrosion methodology based on the local input of corrosive solution via a very fine capillary and electric charge steering has been implemented on 316L steel and aluminium. This methodology allows the production of corrosion unit pieces of size that are correlated with the electrical charge injected at a predetermined location. Tests to characterize the fatigue behaviour of 316L steel were also conducted.

A specific test bed to study the correlation between corrosion and fatigue and to generate CC&F damage in a controlled manner is being established by combining these two approaches. Optical and electrochemical means are also used to monitor this damage in situ.

Corrosion is a major threat to the aviation industry, both in terms of safety and cost. Effective, versatile and affordable solutions are needed to monitor it. The COQTEL project demonstrated that ultrasonic Lamb waves are effective for monitoring corrosion pits when emitted and received by a sparse network of piezoelectric elements (PZTs). A solution based on a sparse PZT network to monitor the growth of corrosion pits of some µm on a 316L stainless steel plate and an aeronautical aluminum plate was developed during the project. Experimentally, the size of the corrosion pit is controlled electrochemically both by the voltage clamp and by the injection of a corrosive NaCl solution through a capillary at the desired location of the pit. In parallel, the growth of the corrosion pit is monitored in situ every 10 seconds by sending and measuring Lamb waves using a sparse network of 4 PZT bonded to the back of the plate to resist corrosion. As a reference, the volume of the corrosion pit is estimated as the dissolved volume balancing the electronic charges exchanged during the chemical corrosion reaction. The radius of the corrosion pit is also checked a posteriori with precision by an optical measurement. The experimentally measured Lamb wave signals are then post-treated to calculate a set of synthetic damage indices (SDIs). After size reduction steps, the SDI values obtained were found to be extremely well correlated with the radius of the corrosion. Using a linear model relating these SDI values to the radius of the corrosion pit, it is shown that a corrosion pit from 30 µm to 150 µm can be reliably detected, localized with an accuracy of 5 mm, and its future size extrapolated. Two independent steel experiments, two independent aluminum experiments, and one demonstrator experiment were conducted to ensure the repeatability of the proposed approach. Lamb waves generated by a sparse PZT array are therefore a reliable and efficient approach for monitoring the growth of micrometric-sized corrosion pits on aeronautical plates. If integrated into aircraft structures, such an approach could be a versatile and cost-effective alternative to current time- and labour-intensive non-destructive maintenance procedures.

Corrosion is one of the phenomena of damage to aeronautical structures. However, corrosion is usually coupled with fatigue phenomena that greatly influences the life of the part in question. A first perspective of the COQTEL project is thus to better understand the linkages and synergies existing between fatigue and corrosion and to be able to differentiate and follow these coupled fatigue-corrosion phenomena in aeronautical structures through ultrasonic monitoring.

In addition, the ultrasonic signals collected during the COQTEL project are extremely complex. On the one hand, the physics of Lamb wave propagation has been known since the beginning of the 20th century, and on the other hand, the current tools of artificial intelligence are capable of capturing complex physical phenomena. Another aspect of the COQTEL project is to define artificial intelligence architectures guided by physics for the study of waves in thin structures.

“In-situ monitoring of µm-sized electrochemically generated corrosion pits using Lamb waves managed by a sparse array of piezoelectric transducers” Ultrasonics, Volume 147, March 2025.

“COQTEL dataset: Corrosion Quantification Through Extended use of Lamb waves”, Zenodo, November 2024.

“COQTEL project dataset : Corrosion quantification trough extended use of Lamb waves” Data In Brief, , Volume 59, April 2025.

In the framework of the transition toward condition-based maintenance, connected objects deployment is reinventing monitoring processes. This is extremely relevant with respect to corrosion which represents a significant issue in aeronautics. In 2016 the annual corrosion cost for commercial aircraft fleet operated by European airlines was estimated to 2.2 B$. Anticipating corrosive conditions ahead of time is estimated to generate between 15% and 35% of cost savings. Specific coatings are used to prevent corrosion but remain limited in their ability to completely avoid structural corrosion specially in harsh operational environment. To detect such damages, nondestructive testing methodologies for corrosion consist in “on ground” regular visual inspection which does not allow detection of the corrosion damage premises, precise quantification of corrosion damage size, and prediction of the remaining useful life of associated parts. Adding on board native connectivity to aircraft metallic parts appears as the key technology to safely face this issue while minimizing costs and environmental impact. This goes through embedding ultrasonic sensors into aeronautic airframe parts and providing them with on board structural health monitoring algorithms and digital twins able to improve operational availability without compromising safety. COQTEL is the joint multidisciplinary effort of French experts in structural health monitoring (PIMM lab), ultrasonic hardware designing (CTEC SME), aerospace metallic parts coating (RESCOLL SME), and corrosion and fatigue modelling (I2M lab) to face this challenge. It proposes to move from classical “on ground” non-destructive testing to “on board” condition-based maintenance by embedding ultrasonic sensors in aircraft metallic parts and by developing and validating dedicated hardware, digital twins, and algorithms endowing airframe parts with built-in corrosion monitoring functionalities. The ambition of COQTEL is to detect the premises of corrosion, to quantify the size of in situ corrosion damages, and to be able to predict associated remaining useful life in order to participate in the revival of the aeronautical industry and the SMEs accompanying it by providing them with a proof of concept of the use of intelligent aeronautical structures for predictive maintenance.