ABSTRACT

A combination of material coating and cathodic protection systems are used to inhibit corrosion of steel tanks and enclosures on Navy ships. The state of these coating systems is often assessed through visual inspection of the confined space by human inspectors that enter tanks while vessels are in depot. Use of an automated or in-situ inspection method has the potential to reduce operating and maintenance costs by informing maintainers of specific tanks or enclosures that require further inspection or repair through condition based assessments. One such technology has been developed, using electrochemical sensors and stochastic models to detect the onset of damage and estimate the extent and location within an enclosed volume. This coating condition monitoring (CCM) system is composed of a network of sensor nodes that measure environmental conditions and electrochemical parameters to evaluate the health of coating and substrate for steel plates and tanks. Laboratory testing has shown that the technique works for 3 dimensional tanks treated with ultra-high solids protective coatings, and to length scales of at least 5.5 meters. A prototype of the sensors and embedded electronics has been built and tested in a physical scale model tank, and is currently being scheduled for relevant environment testing on a near full-scale test tank in open seawater. Performance of the prototype system under laboratory conditions will be presented, along with preliminary results from relevant environment testing on a representative ballast tank model.

INTRODUCTION

The inspection of tanks and enclosures is of particular interest to the U.S. Navy, Coast Guard, and many commercial ship owners and operators. Traditional methods such as visual inspection, the use of corrosion coupons, and some mature non-destructive evaluation (NDE) techniques are often used to detect the presence and extent of corrosion; however such techniques are often labor intensive, and can be limited by physical restrictions that prohibit access to certain areas. Accordingly, there is a desire to transition to embedded technologies that provide monitoring capabilities that can capture the onset, and monitor the evolution of damage in an autonomous manner. Several online technologies have been developed for corrosion monitoring in tanks, structures, pipelines, and other industrial applications.2-17 Techniques specific to the Navy have also advanced, using potential and anode current measurements, optical techniques, as well as electrochemical impedance spectroscopy to assess corrosion within holding tanks.18-20 While the majority of these systems can detect the presence of damage, there remains a significant need for systems that provide 1) improved sensitivity to coating defects, 2) direct measurements over large areas, 3) defect location, and 4) quantification of the severity of damage.

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