TECHNOLOGY AREAS: Information Systems, Ground/Sea Vehicles, Materials/Processes

ACQUISITION PROGRAM: OHIO Replacement Program, PEO SUB, PMS397, ACAT I

OBJECTIVE: The objective is a paint coating condition or "health" monitoring system that can signal without tank entry by an inspector when a coating actually needs opening, gas-free engineering and physical entry for inspection and possible repair and can provide diagnostic assistance in assessing with a high degree of confidence whether or not a coating needs to be replaced.

DESCRIPTION: The coatings of concern act principally as corrosion barriers for steel (and other metals) in an aggressive aqueous environment such as seawater or wastewater. They act to isolate the substrate metal from the corrosive service environment. As long as it is intact and undegraded, the corrosion barrier coating prevents the electrochemical corrosion reaction to which the metal would otherwise be subject from taking place by blocking electrical charge transfer, insulating the metal surface.

The desired coating health monitoring system must meet the following installation and service requirements:
1. Coating condition sensor element for unattended containment within inaccessible, sealed, air-tight, steel tank.
2. Condition of coating inside must be indicated or sensed outside the tank.
3. Signal emissions outside the tank wall are subject to restrictive limits.
4. Sensing element must be robust enough in the service environment to function reliably for the intended life (or planned maintenance cycle) of the coating (minimum of five years in full or intermittent seawater immersion).
5. System must indicate simply and reliably that the coating condition is "good" (no need to enter and inspect the coating) or "failing" (and in need of inspection and possible repair / replacement at the earliest opportunity) and do so regardless of the state of the coating, wet or dry.
6. The system must provide more definitive information as to the extent and distribution of the coating degradation if the "failing" indication is given and the tank entered for coating inspection.

The state-of-the-art technologies available to address this need include a number of laboratory and fielded experiments and trials that apply electrochemical protocols and instrumentation for electrochemical impedance spectroscopy (EIS) and or electrochemical noise analysis (ENA) which essentially measure the electrical impedance across the coating at sensored locations. As a coating ages and degrades, its impedance may drops from Gig ohms to less than a Megohm. Coating electrical resistance-base sensor systems have seen some promising field applications where structure access was not limited and / or test time was short relative to maintenance cycle. However, they have been used mostly in laboratory coating test panel environments where detailed visual inspection criteria still dominate. The method needs further development to establish the failure state and calls for interpretation by trained technicians.

Another very promising system is the collateral use of a tank's cathodic protection (CP) system to indicate by its protective current requirement the health of the coating. But this is a global, overall coating condition indicator, it will not apply unless the CP anodes are immersed or at least correlated with the liquid level and it may not respond to even significant localized coating degradation.
It may be possible to integrate EIS-type (and/or other) local sensors with CP current monitoring to attain the desired system.

There are other new technologies that might serve as a basis for a sensor. Examples could be measuring changes in some coating structural (decrease in extent of polymer cross linking or increase in porosity) or chemical aspect (that could be sensed by a fiber optic probe such as degree of hydration and/or acidity) of the coating as it ages and degrades.

PHASE I: Preliminary design of a desired coating monitoring system requires proof of sensor function on epoxy coated steel panel in natural or artificial seawater; preliminary sensor/ system definition of coating "failure"; proof in principle of integratability of number, type and distribution of sensors required for accurate reproducible indications; and demonstrate delivery of sensor signal to and interpretation by system external to sealed steel container.

PHASE II: Demonstrate sensor / system performance and robustness in service-like conditions such as sensors on a range of coated steel panel sizes and coating thickness in alternate immersion and/or salt spray cabinet testing, 1000 hours or more.

PHASE III: Shipboard tank installation demonstration experiment to show shipboard tank applicability and advantage of sensor-based tank CBM over conventional tank scheduled maintenance.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: While the submarine tank coating monitoring problem is particularly challenging, it is by no means unique and once a system is developed it could meet the maintenance planning and decision needs of a wide variety of military vessel, marine transport, port facility, industrial, and municipal tank, pipe, and other structures reliant on organic coating health for corrosion resistance.
Another key advantage of developing this coating monitoring system rests on the data it generates to assist in coating failure diagnosis which may lead to coating formulation and application improvements.

REFERENCES:
1. John N. Murray, "Electrochemical Test Methods for Evaluating Organic Coatings on Metals: An update. Part III. Multiple Test Parameter Measurements," Reprinted from Progress in Organic Coatings, 31, 1997, pp 375-391.

2. John N. Murray, "Evaluation of Electrochemical Noise to Monitor Corrosion for Double Hull Applications," Technical Report, NSWCCD, CARDIVNSWC-TR-61-94/29, August 1994.

3. R.L. Ruedisueli and B.D. Layer, "Practical Application of ECN / EIS for In-Service Coatings Assessment," NACE 2002 Conference Proceedings.

4. R. L. Ruedisueli and R. A. Hays, "Corrosion Sensors in Navy CHT Tanks", NACE 2003 Conference Proceedings.

KEYWORDS: organic paint coatings; steel tanks; corrosion; electrochemistry; impedance spectroscopy; total internal reflectance spectrometry; ultra-sonic inspection;

** TOPIC AUTHOR (TPOC) **

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