How Naval Forces Combat Ship Deck Plate Corrosion with Pulsed Eddy Current

This blog post is provided by Eddyfi Technologies, an ASNT Affiliate Partner.

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by Gabriel Cyr

Introduction: The Cost of Covert Corrosion in Naval Ships

With advances in nondestructive testing having helped many other industries with their goals for asset life extension, it is understandable why naval forces also continue to explore options for cost efficiencies. The reality of the associated costs and time consumed procuring new ship vessels has turned the initiative to extending the service life of existing ships in order to continue achieving strategic objectives. A real danger at sea is contained within these naval fleets: undetected corrosion left to compromise the structural integrity of marine ship decks.

Counterproductive to the cost-saving efforts afforded by service-life extension programs, unforeseen repairs that are required as a result of deck plate corrosion has the naval industry seeking a better solution to enable proactive versus reactive maintenance schedules. Current inspection methods used for the detection and assessment of corrosion in ship deck plates disrupt not only cost-efficient operations but also the ability to achieve the maximum time a vessel is therefore able to perform its mission-critical duties.

Water ingress is the culprit that encourages corrosion to form under anti-skid coatings, ceramic tiles, vinyl, and other flooring used on ship decks. Areas with tile delamination are presumed to indicate corrosion under the flooring. Unfortunately, simply relying on experience-based observations and tools instead of quantitative measurements may potentially result in missed calls or false positives. Moreover, delamination is not always present in areas where corrosion beyond accepted thresholds runs unbridled and poises to inflict disaster if left unchecked for too long. Alas, there is no way to truly confirm the presence of corrosion and assess the extent of the damage without undertaking the costly work of removing ship deck plates and performing ultrasonic testing thickness measurements in the vulnerable spaces like galleys, laundry rooms, and wash areas. It is usual for a typical compartment to take the equivalent of one week to accommodate this inspection process performed with two workers. With the knowledge that relying on visual cues is no guarantee of protection and unnecessarily removing what may be perfectly fine flooring simply is not a worthwhile compromise, the naval industry has started to analyze inspection best practices elsewhere.

How Pulsed Eddy Current Can Help Extend Fleet Life

Pulsed eddy current, or PEC, is the answer for detecting corrosion under insulation, a problem faced by the energy sector and mitigated for decades with this proven nondestructive testing inspection technique. Acting as a screening tool that eliminates the need for removing insulation materials, a PEC system (Figure 1) uses a magnetic pulse to energize the bulk of carbon steel components and cut the pulse sharply to produce eddy currents in the test part. The inspection system effectively listens to the decay of these eddy currents and analyzes the signal to establish the remaining wall thickness.

Although PEC generates eddy currents into a component, it is very different technology from traditional eddy current survey tools. And while standalone training and certification are available to better enable greater efficiencies in production by the crew, the leading PEC system on the market requires no specialty training like conventional eddy current or electromagnetic testing Levels I, II, or III certification to operate.

A limitation commonly associated with PEC is its cross-sensitivity to both wall loss and variations in material properties. In the past, there have been occurrences where due to this limitation, features such as heat-affected zones, residual magnetization, or localized changes in properties have led to false positives (Figure 2) where wall loss would show in the PEC data but none would be present in the material. Recent developments in PEC technology enable the PEC examinator to discriminate most types of false positives due to changes in material properties from true wall loss indications.

There are obvious similarities between corrosion under insulation, or CUI, and deck plate corrosion assessments that have driven further investigation by navies worldwide. A commercially available solution with conventional PEC probes demonstrated its ability to overcome the challenge of inspecting ship deck plates without removing the tiles or even requiring surface preparation ahead of assessment. Few ship compartments are likely to have significant damage. PEC discriminates between sound and problematic areas with a higher degree of confidence than traditional surveys, reducing the number of compartments that require tile removal many times over.

These self-run ship deck plate assessments can be performed while the navy ship is in service for months or even years before it enters a shipyard for a preplanned midlife refit or life-extension program modernization. The objective inspection data collected with a simplified workflow and one-click calibration process can be used to better plan and schedule maintenance and repairs, drastically reducing costs and service disruptions. Minimizing the unforeseen with earlier assessments, PEC inspection solutions give back control to schedule maintenance work based on a real need, preventing rising costs from the discovery of corrosion during refits.

Case Study

There are both naval and maritime applications for PEC inspections including ship deck inspections and ship hull corrosion assessment. The nonintrusive inspection solution is a game changer for naval forces under extraordinary pressure and recognizing this opportunity, industry leaders collaborated with the Royal Canadian Navy to conceive a dedicated solution that reduces overall corrosion management costs.

A unique technology, pulsed eddy current array, or PECA™, pulses and listens with multiple PEC sensors simultaneously, and it is this advanced technology that enabled the development of a specialized ship deck plate inspection solution with unmatched productivity and sensitivity. The purpose-built PECA probe is a custom array sensor with seven channels and 7-inch (178-millimeter) coverage. The increased coverage is key for optimal productivity on decks, and the sensor was designed with the ship environment in mind.

The tiles or other decking materials add liftoff between the probe and the inspected surfaces. Each PEC probe is optimized to work with a certain range of liftoff and plate thicknesses. In the case of the specialized array probe, it was optimized for low liftoff applications, from 0 in. (0 mm) up to 1 in. (25 mm) on most deck plates. There is no minimum tile thickness for the inspection to be performed, but liftoff variations must remain within 50% of the nominal value to ensure optimal detection and sizing. Other PEC probes are available for applications where more liftoff or thicker plates are present.

It may take a full week to remove and reapply tiles in a suspicious but sound compartment measuring 10 × 10 ft (3 × 3 m) following traditional surveys. The same scan made by PECA technology takes only one hour with a team of two technicians. One will operate the probe while the other operates the instrument. The typical probe speed is up to 5 in. (130 mm) per second on 3/16-inch (5-millimeter) thick plates and will vary with deck plate thickness, desired resolution, and flooring thickness. The PECA technology allows five or six compartments to be scanned in a single workday—significantly higher inspection productivity.

The solution was first qualified on a ship deck mock-up featuring machined defects to assess the performance of the system. With satisfactory results achieved, the solution was tested and then deployed more widely for the survey of in-service military vessel ship decks. As part of ongoing refits, PEC was used to scan the decks alongside conventional surveys. Follow-ups were completed where PEC found corrosion, confirming the superior detection and accuracy of the system.

For example, a corrosion repair had been performed a few years before as indicative with the newer tiles seen in Figure 3. A quick PEC scan showed that corrosion had formed in the insert plate and required intervention. The tiles were removed and both visual and ultrasonic testing inspections confirmed exactly what PEC had indicated, both generalized corrosion and pitting colonies through the flooring in many compartments.

In another instance, scans were performed in a washing area where two regions of wall loss were identified based on the PEC data. Following the PEC scans, the ceramic tiles were removed for further assessment, confirming the calls made by the PEC inspection team (Figure 4). Generalized corrosion with some deeper pits and holes were assessed and required repair.

The ship deck plate corrosion solution is actively deployed by the Royal Canadian Navy and has been tested by multiple NATO navies including the French Navy. The results and cost-saving consequences continue to create waves in the naval industry today.

“The disruption and costs arising from finding corrosion in a single compartment during refit are more than the costs of the [PEC] system,” said René Blais, Halifax Class Platform Manager, Royal Canadian Navy.

Conclusion: No Surprise Attacks

In an industry where the top priority is security and defense, unplanned fleet repairs and their associated costs cannot afford to impact operations. Pulsed eddy current deck surveys have proven a key element in reducing the life-extension costs of aging ships with a better understanding of specific repair and maintenance work required before the start of docking work periods. An advanced, proven, and unique technology, sophisticated PEC inspection tools reliably expose any undercover corrosion activity. PECA technology remains on guard for defects to help ensure continued safeguarding of our seas.

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Gabriel Cyr, PEC Product Manager, Eddyfi Technologies, Quebec, Canada, info@eddyfi.com. www.eddyfitechnologies.com/lyft