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Title: Multi-Modal Monitoring System of Shaft and Protective Coating Integrity 

Authors: Fritz Friedersdorf, Kevin Farinholt


Maintaining the integrity of ship propulsion shafts is a critical concern for the US Navy in order to provide full mission capability of fleet assets. Fleet-wide maintenance experience presently dictates drydocking ships at ten year intervals for full shaft removal and inspection to determine whether the glass-reinforced plastic (GRP) covers protecting the shaft-sleeve interface have successfully prevented seawater intrusion and consequential shaft pitting/fatigue damage. Often, the inspection reveals no water ingress or shaft damage, meaning that the sleeve was removed before the end of its useful operating life. Conversely, some inspections have revealed significant damage, indicating cover material failure well before the end of expected design life. The Navy has identified a strong need for an in situ method to monitor shaft health at more frequent intervals to better manage risk while optimizing propulsion shaft life. With this enhanced capability, shaft maintenance would be driven by shaft and GRP barrier condition instead of a calendar basis, thereby reducing the number of costly drydock cycles over a ship’s lifetime. Likewise, more frequent monitoring would reduce the risk of catastrophic shaft failure between inspection cycles, were the coating and sleeve materials to fail prematurely. To meet this critical need, Luna is developing an embedded, multi-modal distributed sensing platform to provide in situ assessment of shaft and barrier coating condition.

Key Points:

Technique utilizes electrochemical impedance spectroscopy (EIS) measurements
• The complex electrical impedance can be related to the physical state of an electrochemical system
• EIS is sensitive to changes in the lowest impedance path between two electrodes
• Specific ranges within the frequency spectrum correspond to different physical attributes of a system, including: electric double layer, solution / substrate resistivity, as well as others
• Concept has been demonstrated in laboratory using noble metal and corrosion resistant electrodes

Embedded electrochemical impedance spectroscopy system
• A first generation prototype has been designed that provides sufficient frequency and impedance resolution for EIS measurements at a critical location along the shaft length
• Prototype has been validated against a commercial potentiostat over a 0.1-100k Hz frequency range
• Circumferential ring electrodes are monitored by Luna’s lost cost, low power smart sensor platform
• The brassboard prototype is compatible with open standards for communication
• Electronics design is evolving into a flexible circuit that can be embedded within the GRP layup
• Low-power wireless energy transmission is being investigated to power hardware beyond the lifespan of typical battery technologies

Long-term testing of the EIS measurement system in a relevant environment
• Small scale laboratory tests have demonstrated performance on three inch steel test shafts subject to varying degrees of pre-corrosion prior to the application of a protective covering
• Testing has transitioned to a 14 inch diameter steel shaft that is more representative of small scale propulsion shafts used by the Navy.

An embedded sensing system that provides multi-frequency electrochemical impedance spectroscopy measurements has been developed. The system is designed to monitor the hydration state and presence of active corrosion using circumferential ring electrodes embedded within the four layer protective coating. A brassboard prototype has been developed and is being evaluated under relevant environment conditions. Laboratory experiments have demonstrated the capabilities of this system on 3 inch diameter steel shafts, and have transitioned to a 14 inch test shaft. Preliminary results indicate good correlation between the prototype system and a commercial potentiostat. Next stage developments will transition from the present rigid circuit board design to a thin flexible electronic system that can be embedded within the coating lay-up for long term evaluation.