Galvanic sensor for monitoring structural damage

In recent years there has been an increased research activity on aircraft health monitoring tools. Numerous studies have been carried out as a response to the demand for a better damage detection in inaccessible areas. In most of these studies, the hidden corrosion events are significantly related to the nature and magnitude of corrosive environment present in the localized hidden areas. Because of the difficulty in accessing hidden zones and the limitations of NDE currently used, primarily corrosion damages go undetected in the hidden areas during routine maintenance inspection. For old aircraft, the accumulation with time of even weak/least corrosive environments can produce serious structural damage, as demonstrated by enormous weight loss of material in some parts. In many critical aircraft parts the undetected corroded surfaces may lead to structural concerns acting as promoters and/or initiation sites of stress corrosion cracking and corrosion fatigue phenomena. In an earlier effort, galvanic probes were designed and employed to measure galvanic current using electrochemical technique to monitor the corrosivity of an environment on a continuing basis in inaccessible areas. Among them, the ICS (Intelligent Corrosivity Sensor) has been the most successful application: this probe is a galvanic device that uses the condensed moisture and the environment's pollutants as an electrolyte, generating a cell current that relates to the corrosivity of the condensed film. In the present work, a preliminary study has been done on modified ICS, to develop a probe for specifically measuring the corrosion structural part as in lap joints; i.e., probes were installed on the internal side of sandwich specimens made of Al 7075-T6 alloy. Preliminary results show that the sensor output from such tests can be used in the evaluation of hidden surface corrosion and serve as a meaningful tool to provide quite an accurate warning for subsequent inspection.