Design, manufacture, and testing of an integrated pH-potential measuring device for subsea metallic structures

Abstract

This study addresses a cultural heritage conservation challenge for underwater metallic wrecks. Left unattended, shipwrecks will eventually be consumed by corrosion, creating a cultural heritage conservation issue, and also an environmental problem, if the wrecks still carry payloads of oil or unexploded ordnance. This study used a structured approach to the design, fabrication, and testing of a handheld device that can be operated by underwater archaeologists and conservators for the in-situ corrosion monitoring of metallic underwater cultural heritage. By measuring the pH and the corrosion potential at various points around the surface of the wreck, the state of corrosion that the metal is in was assessed. The device was equipped with a pH measuring system comprised of a pH meter and probe, along with a corrosion potential measuring system, comprised of a multi-meter, a silver/silver chloride reference electrode, and a platinum electrode. While several examples of such a device are found in literature, there exists no off-the-shelf systems available for purchase nor any documentation which provides a structured approach to the design of such a device. The design methodology adopted for this study was the Pahl and Beitz systematic approach, with an implemented verification validation, and testing framework. The pressure vessel portion of the device was designed using a design by formulae approach that was complemented with Finite Element Analysis exercises conducted on each of the constituent parts of the vessel. Following the fabrication of the device, a hydrostatic pressure test was conducted to ensure it would withstand the 100 m maximum depth required by the stakeholders of the project (Heritage Malta). Tests were conducted to validate the effectiveness of the method of corrosion assessment, following which a pilot implementation of the device was conducted on the wreck of the X127 at the Lazzaretto Wharf on Manoel Island. The device was successfully able to take measurements of pH and corrosion potential, allowing the determination of the state of corrosion of the hull of the X127 by plotting the data on a eh-pH (Pourbaix) diagram of iron in seawater.