In-Situ Corrosion Sensor

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Summary

Comparison of Corrosion Sensors

Comparison to Conventional EIS

Potential Applications

Discussion

Summary

• The patented in-situ sensor (additional patents pending) uses electrochemical impedance spectroscopy (EIS) to monitor degradation of structures in the laboratory and in the field.
• The measurements obtained with the sensor are identical with those obtained using conventional remote electrodes. This assures that the methodology and analyses established for the conventional EIS measurements are suitable for the corrosion sensor results.
• Two sensor versions are available: a permanent, attached sensor and a portable hand-held sensor. The incorporated sensor is especially suited for areas that are not readily accessible or that require frequent inspection/monitoring. The hand-held sensor is suitable for spot inspection and areas where a permanent sensor is not desired for cosmetic, aerodynamic, or other reasons. Identical results are obtained with the two sensor versions.
• Sensor EIS data correlate very well with corrosion rate measurements.
• The corrosion sensor is suitable for use in a variety of environments, including immersion, salt fog, humidity, and aggressive atmospheres as well as ambient service environments.
• The sensor is suitable for a variety of metal substrates and coating chemistries. In particular, it has been demonstrated on aluminum and cold rolled, electrogalvanized, and galvanealled steel substrates and on epoxy, polyamide, urethane, and alkyd primers/paints and epoxy adhesives. No limitation in the substrate or coating is anticipated provided the substrate is conductive and the coating is non-conducting.
• Differences in relative coating effectiveness are easily observed.
• The sensor detects very early stages of paint degradation and corrosion before any visual indications. This detection of corrosion before any structural damage has occurred is important. It potentially allows the health of the painted structure to be monitored so that maintenance can be scheduled on a condition or needs basis instead of a fixed time interval basis.
• Sensor measurements during a 5-month cyclic test have been correlated with amount of corrosion at the end of the test. This test has, in turn, been correlated with performance in the field.
• The sensor is capable of detecting coating defects away from the sensor itself. Laboratory measurements show that defects can be found up to fifteen feet from the sensor. This detection range can be engineered by controlling the wetness of the coating surface.
• Other applications for the sensor are being developed. These include:
  • Moisture detection in composites
  • Early detection of adhesive bondline degradation.

Comparison of Corrosion Sensors

DSI Corrosion Sensor Monitors electrochemical process (corrosion) directly Measures corrosion of actual structure Sensitive to early stages of corrosion/degradation Very sensitive to moisture intrusion into bondline or composite Relatively inexpensive instrumentation Detection range is controllable from very local to several feet Suitable for one-time spot inspection or long-term monitoring

Other Corrosion Detection Techniques Conventional NDE Require significant loss of material (>5%) or presence of corrosion product Require delamination or blistering Corrosivity Sensors Measure corrosion of sensor itself, not structure of interest Material differences Environmental differences Require extended exposure time for sensor to corrode

Comparison to Conventional EIS

DSI Corrosion Sensor Suitable for field or test chamber or immersion exposures Permanent electrode for inaccessible regions or portable electrode for convenient areas Arbitrary structure configuration Detection area is controllable and can be large

Conventional EIS Requires immersion or clamp-on liquid cell Clamp-on cell requires accessible, flat, smooth, horizontal area and messy electrolyte Coating is monitored only where exposed to the electrolyte Coating can suffer artifactual damage from the electrolyte

Potential Applications

• Aircraft
• Ships
• Highways/Bridges/Tunnels
• Storage Tanks
• Reaction Vessels
• Pipelines
• Buildings

Discussion

Electrochemical impedance spectroscopy (EIS) has previously been used to detect coating degradation on steels and other metals in the laboratory. Very good correlation has been reported between short-term EIS data (low frequency [near-dc] impedance and break-point frequency) and long-term coating performance in sea water immersion, demonstrating the technique's predictive capabilities. Furthermore, aluminum alloys with common aircraft coatings have been examined in salt water by EIS with excellent correlation with visual evidence of corrosion. However, each of these applications involved immersion of the specimen and/or use of external electrodes. Such procedures are not suitable for detecting corrosion on structures in the field.

The in-situ sensor offers a quantitative measure of incipient corrosion unlike other corrosion monitors that simply measure the time of wetness or the corrosion rate of the sensor itself or that require external electrodes and a bulk electrolyte medium. As such, it is directly applicable to detect the degree of coating degradation and the amount of substrate corrosion of real structures. Because it detects the very early stages of corrosion, it provides a warning before structure degradation occurs. Thus preventative maintenance can be scheduled in time to forestall corrosion damage.

Two versions of the sensor have been developed:

• A portable hand-held instrument that a technician would press or hold against the structure.
• A painted sensor that would be applied to the structure permanently and then monitored at appropriate intervals with a technician equipped with a portable computer. Alternatively, the sensor(s) could be continuously monitored by a dedicated computer.

When exposed to moisture, either by immersion or high humidity, the low-frequency impedance of the paint/metal system decreases by one or more orders of magnitude. Further decreases in the impedance occur when the substrate undergoes active corrosion. These decreases are readily measured by the prototype sensor. It has been demonstrated on painted steel and aluminum coupons. Identical results can be obtained using external electrodes or the sensor in immersion control studies that validated the design and operation of the prototype sensor. The sensor results have also been verified and correlated with ellipsometry, potentiodynamic polarization, and long-term visual observations.

The in-situ corrosion sensor measures the impedance spectrum of a metal/coating system as a function of accelerated exposure. This impedance spectrum can be modeled as an equivalent electric circuit, such as the one shown in Figure 1.

Figure 1. Schematic representation of the sensor on a coated metal structure and an equivalent circuit model with elements that correspond to the coating (red) and coating/metal interface (blue).

Initially, the coating resistance is very high so that the system acts as a capacitor with the log of the impedance being proportional to the log of the frequency (Figure 2). As the coating degrades and substrate corrosion occurs, the resistance of the coating decreases and the low-frequency impedance then becomes independent of frequency.

Figure 2. Impedance spectrum for painted aluminum following immersion for different periods in salt water.

The impedance spectrum can interpreted to precisely determine the stage of corrosion the metal/coating system is experiencing as illustrated in Figure 2. The data have shown that the painted metals degrade with a distinct signature. There are definite corrosion stages corresponding to water uptake by the polymer, corrosion incubation, and intense corrosion activity. Furthermore, differences are readily observed between coatings of different effectiveness. As shown in Figure 3, the waterborne coating does not provide as good a protection of the substrate as does the polyamide coating - it exhibits greater uptake of moisture and the time prior to active substrate corrosion is less.

Figure 3. Impedance versus immersion time for waterborne and epoxy polyamide paints on two different alloys of aluminum immersed in salt water.

The use of ellipsometry and DC potentiodynamic measurements have verified the results obtained from EIS. Figure 4 shows the correlation between corrosion rates and low-frequency impedance as measured by the sensor. For this system, corrosion rates are small until the low-frequency impedance decreases to below 10⁵ ohms.

Figure 4. Low-frequency (low frequency) impedance as determined by the corrosion sensor and instantaneous corrosion rates as determined by potentiodynamic polarization as a function of time for painted aluminum immersed in salt water.

The predictive ability of the sensor is illustrated in Figure 5. A variety of different coatings and substrates were exposed to cyclic immersion/drying/humidity conditions that have been correlated with service conditions. A very good correlation has been obtained between the time required for the low-frequency impedance to decrease to 10⁷ and the amount of corroded area on the specimen, as determined by a modified ASTM D1654 procedure. The cold-rolled steel specimen lying above the curve experienced corrosion beginning at an unprotected edge and rapidly progressed underneath the paint. The sensor detected the corrosion once it began.

Figure 5. Corroded area after 100 cycles of a cyclic corrosion test as a function of time for the low frequency impedance to drop to 10⁷ W.. Four different substrates with different coatings were tested.

Two versions of the corrosion sensor have been developed that are suitable for monitoring different structures. One is an incorporated electrode that is permanently attached to the structure. This version is especially useful for inaccessible locations. Wires are then routed to a convenient connection point. The other version is a portable hand-held electrode that is pressed against the structure during the measurement. This version could be used whenever a permanent electrode had not been incorporated into the structure. It is also suggested for certain porous coatings. Laboratory measurements have indicated that the two sensor configurations give identical results. In both cases, measurements can be obtained using commercial portable equipment.

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19 March, 2003

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