In-Situ Sensor for Early Detection of Corrosion

An in-situ corrosion sensor capable of detecting and monitoring corrosion of aircraft or other structures from the earliest stages of deterioration has been developed. Two versions are available: a permanent, attached sensor and a hand-held, portable probe. The sensor utilizes electrochemical impedance spectroscopy (EIS), an established technique for investigating corrosion and coating degradation during immersion in electrolytes in the laboratory. Identical results were obtained from the sensor and conventional, remote electrode immersion measurements. The sensor extends the applicability of the EIS technique to arbitrary painted metal structures under diverse conditions, including field or service use and accelerated testing. Potential applications include corrosion monitoring of critical structures to enable condition-based maintenance and direct correlation of material deterioration during accelerated testing and use in the field.

Keywords: corrosion, sensor, electrochemical impedance spectroscopy, coating, paint, degradation, monitor, condition-based maintenance

1.0 Introduction

The cost of corrosion is difficult to determine, but is known to be very large. Studies by the National Bureau of Standards (NBS) estimated that overall corrosion costs in the United States are 4.2% of the Gross National Product (GNP)[1] or $290 billion in 1996. Military and commercial aircraft are aging and projected retirements are being extended. In 1993, 2660 Air Force planes, or 57% of the fleet, were over 20 years old [2]. Some military transport planes are expected to be flying until they are 80 years old. It is estimated that corrosion costs are $4 billion annually for military aircraft systems and $9 billion for commercial and private aircraft [3].

The cost of repairs, maintenance, and replacement is a direct cost. The loss of lives and readiness are extremely important indirect costs which cannot be assessed in dollar amounts. According to Hoeppner [4], corrosion-related accidents resulted in 11 fatalities for the military and 70 fatalities for the civilian air fleet over a recent 17 year period. More recently, the National Transportation Safety Board concluded that undetected corrosion was the cause of a commuter aircraft crash that killed ten people in 1995. In 1997, a 35-year old Russian passenger jet broke apart in flight with 50 deaths. Early reports indicated that extensive corrosion was responsible.

One means to prevent catastrophic failure and to increase the lifetime of hardware and structures and to get the most out of stretched maintenance budgets is to track corrosion from its early stages so that its progression can be monitored and predicted. Consequently, maintenance could be performed on a needs basis when it is relatively inexpensive before deterioration becomes critical and expensive remedial action is needed. The maintenance schedule would be a proactive one rather than one that reacts to failures.

In this paper, we report on an in-situ corrosion sensor capable of detecting coating deterioration and substrate corrosion underneath a paint coating [5]. Based on electrochemical impedance spectroscopy (EIS), the sensor is sensitive to early stages of material degradation before any visual damage is present. It allows quantification of deterioration of the actual structure of interest during both ambient service conditions and laboratory accelerated exposures. Consequently, it is applicable for

Two versions of the in-situ sensor have been developed: a permanently attached sensor and a portable hand-held sensor. The former is suitable for corrosion inspection in inaccessible areas or for automatic or semi-automatic inspection of a particular area. The hand-held sensor is suitable for spot inspection where a permanent sensor is not desired or had not been previously placed.

Electrochemical impedance spectroscopy has previously been used to detect coating degradation on steels and other metals in the laboratory [6-8]. Good correlation has been reported between short-term EIS measurements and long-term coating performance during immersion in different electrolytes, demonstrating the technique's predictive capabilities [9-12]. However, these applications involved immersion of the specimen and use of external reference and counter electrodes. Although limited success has been reported on the use of flat cells or similar apparatus for detection of corrosion in the field [13-15], they require a smooth, flat surface to obtain a seal when using a liquid electrolyte (Table 1). Even the use of gels, sponges, and other solid or semisolid electrolytes can be messy and require access to the location being inspected. Such cells can only detect corrosion directly under the electrolyte.

In-Situ Corrosion Probe

Suitable for field use or laboratory test chamber/ immersion
Permanent electrode is available for inaccessible regions
Hand-held sensor is available for structures/ areas without a permanent sensor
Arbitrary structure configuration
Easy set-up/inspection
Inspection can detect corrosion over large area

Conventional EIS

Requires immersion or clamp-on liquid cell
Cell requires accessible, flat, horizontal area, messy electrolyte, and remote electrodes
Set-up is time consuming and must be performed for each measurement
Corrosion is detected only directly under cell
Can result in artifactual degradation.

The in-situ corrosion sensor offers a quantitative measure of incipient corrosion in the field unlike other corrosion monitors (Table 2). The first field corrosion sensors simply measured the time of wetness of a surface - a useful parameter, but only one factor controlling corrosion. An early in-situ sensor provided some of the advantages of the current permanent sensor, but required vacuum deposition of the sensor electrode thus restricting it to small test panels or components [16-18]. Other sensors determine the corrosion rate of the sensor itself using coated optical fibers, mass gain/loss, or electrochemical measurements on witness electrodes that are part of the sensor package [19,20]. Because of differences in corrosion susceptibility of the witness electrode material(s) and the actual structure and differences in the microenvironment of the sensor and the structure, the corrosion rates measured by the witness sensors may or may not correlate with corrosion rates exhibited by the structure being inspected. Traditional nondestructive evaluation (radiography, ultrasonics, thermography) has required extensive structural degradation - loss of material, blistering delamination of paint, coatings, or of the structure itself. In some cases, these techniques are better suited for detection of strain or cracks (other serious problems) instead of corrosion. As such, they are complementary to the in-situ corrosion sensor. Many of these procedures either require sophisticated equipment or highly trained personnel or do not measure the corrosion of the structure of interest. In the case of x rays, significant safety issues arise with the use of ionizing radiation.

In-Situ Corrosion Probe

Measures corrosion of actual structure
Sensitive to early stages of corrosion/degradation
Very sensitive to moisture intrusion into bondline
Relatively inexpensive instrumentation
Monitors electrochemical process (corrosion) directly

Other Corrosion Sensors

Time of wetness monitors
Corrosion of sensor itself
- Material differences
- Environmental differences
Require significant loss of material
Require delamination or blistering

The in-situ corrosion probe 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. Alternatively, by detecting corrosion from the very early stages and allowing a quantifiable comparison between field or service degradation and that observed during accelerated testing, the sensor should be valuable during coating development.

2.0 Experimental

A variety of different paints and other coatings have been evaluated primarily on aluminum, although steel substrates have also been used. Electrochemical impedance spectroscopy measurements were made using the substrate as the working electrode and the sensor as both the counter and reference electrodes. Most of the data were acquired using a EG&G Princeton Applied Research potentiostat Model 273 with an EG&G lock-in amplifier Model 5210 with Model 398 version 1.10 software. Potentiodynamic polarization measurements (Model 352 SoftCorr II software) were also obtained to determine corrosion rates of selected samples. The remainder of the data were acquired by a Gamry Model CMS100/105/300 portable corrosion measurement system. This unit is more compact than traditional bench-top units and would be suitable for inspection in the field. The specimens were exposed to several different exposure conditions. These are given in Table 3 along with relevant instrumentation.

3.0 Results and Discussion

To establish the validity of the in-situ sensor, EIS measurements were made with the sensor and conventional three-electrode EIS on the same or identical specimens. Figure 1 shows that the impedance spectra obtained using the in-situ sensor and the conventional three remote electrodes in a flat cell are virtually identical. Similar comparisons have been obtained with other coatings and metals. For the hand-held sensor, measurements were taken on painted specimens with and without a scratch to simulate a defect in the coating (Figure 2). Each of three variations of the hand-held probe give results very similar to the conventional three-electrode measurements. Each measurement very clearly reflects the presence of a gross defect such as a scratch.

EIS and the in-situ sensors detect much more subtle degradation than a scratch that exposes bare metal. Figure 3 shows that initially the coated metal demonstrates capacitive behavior with very high impedance at low frequencies. As the coating degrades during immersion, its resistance decreases (as modeled in an equivalent circuit) and the impedance become independent of frequency at low frequencies.

The low-frequency response can be tracked as a function of exposure. Figure 4 gives the near-DC impedance of an epoxy-coated aluminum immersed in hot water for approximately six months [21]. The data clearly show corrosion to occur in three stages for this system:

In the later stage, the corrosion products erupted through the epoxy coating and allowed the electrolyte unimpeded access to the metal. The relative times and impedance decreases for the different stages will depend on the quality and chemistry of the coating, the metal underneath and any surface treatment, and the exposure conditions. For example, Figure 5 demonstrates that a waterborne coating is not as effective as an epoxy polyimide coating. The impedance of the waterborne coating drops approximately one order of magnitude more than the organic solvent coating, reflecting the greater moisture uptake. Partially as a result of the increased moisture concentration at the interface, the incubation period is shorter and active corrosion of the substrate occurs sooner.

One of the principal advantages of the in-situ corrosion sensor is its ability to monitor corrosion under a variety of conditions in addition to immersion for which conventional remote electrode EIS measurements are possible. As a result, material degradation in accelerated laboratory conditions can be directly compared to degradation occurring in the field or under different conditions. Figure 6 presents sensor results for identical aluminum panels exposed to different environments. As one would expect, no material degradation or other change was observed in specimens stored in a refrigerator or inside under ambient conditions. The specimen kept in a humidity chamber (50°C, 98% RH) showed a steady drop in impedance until the coating reached equilibrium with absorbed moisture. The panel exposed near a busy highway in Maryland in winter/spring show little degradation during mostly dry periods, but show a response similar to that of the humidity exposed specimen during extended periods of rain. However, this decrease in impedance, representing moisture absorption, is largely reversible and once the paint dries, the impedance increases to near the original value. In this relatively short field exposure, we cannot rule out a small permanent impedance decrease that might reflect the beginning of irreversible damage. However, the majority of the changes are reversible and appear to mirror changes occurring in the humidity chamber, but at a much slower rate.

One issue concerning the validity of the permanent sensor measurement is the stability of the sensor itself during long-term or aggressive exposures. If the sensor electrode corroded or otherwise deteriorated during the exposure, the measurements might be indicative of the sensor and not of the structure of interest. Several different electrode materials were evaluated using different accelerated testing, including exposure to salt fog for more than 1000 hours. In this test, the electrodes showed no signs of loss of integrity or adhesion to the specimen. In fact, the experiment as ended after the glyptal coating used to protect the back and sides of the specimens began to fail. Validation of the in-situ sensor was achieved by the similarity of measurements taken under a variety of exposure conditions using the permanent sensor, conventional remote-electrode EIS, and the hand-held probe which is not exposed to the aggressive environments.

To correlate sensor results with corrosion rates, polarization resistance measurements were taken in conjunction with the EIS sensor measurements on scribed, painted aluminum panels immersed in water. The results are shown in Figure 7. For this material system and exposure conditions, the corrosion rate remains very low, aside from a quick oxidation of the scribed area during the very initial exposure, until the log-frequency impedance decreases to approximately 10⁵ . However, once the impedance approaches 10⁴ , the corrosion rate increases dramatically. A similar correlation was obtained using ellipsometry, the oxide film in the scribed region remained relatively thin until the third stage of the corrosion behavior. Subsequently, the thickness of the oxide coating increased as corrosion products were formed.

Figure 7. Left: Low frequency impedance and instantaneous corrosion rate as functions of immersion time for epoxy coated aluminum. The sharp initial decrease in impedance results from the scribe which has the effect of by-passing the moisture absorption stage of Figure 4. Right: The instantaneous corrosion rate as a function of the low frequency impedance.

The distance from which the in-situ sensor can detect corrosion or a coating defect depends on a number of factors, most notably the surface conductivity of the coating. As a result, it can be controlled. Under appropriate conditions, the in-situ sensor can distinguish between good and poor areas of the surface. However, increasing the surface conductivity of the specimen can substantially increase the detection range of a sensor. This effect is illustrated in Figure 8 and Figure 9. The current path from the sensor to the substrate will vary, depending on various conditions. If the surface resistance is very high, the dominant path will be directly across (through) the coating. In this case, detection of degradation will be localized. On the other hand, if the surface resistance is relatively low, the current can proceed along the surface until it reaches a defect or other localized region of low coating resistance. If an equivalent circuit analysis were being performed, this would be represented by an additional RC branch. As a result, the detection area of the sensor can be controlled. If a wide detection range is desired, for example, if the inspector wants to take a quick look to see if any corrosion is present, wetting the surface with a conductive fluid will allow global inspection with a minimum of sensors. On the other hand, if it is desired to localize the degradation, inspection of a dry surface will allow the area of deterioration to be pinpointed.

A field demonstration was performed to illustrate the use of the sensor under typical depot conditions on a C-135 aircraft (Figure 10). The measurements verify that EIS measurements can be readily obtained from different areas of an airplane using the hand-held sensor in a depot. No special preparation or facilities were needed. Furthermore, the sensor could readily distinguish between good primer and deteriorated primer, which exhibited a decrease in the low-frequency dependence of at least two orders of magnitude. Measurements on other aircraft components showed a similar correlation between the EIS signal and extent of corrosion or other degradation.

Figure 10. (Left) EIS spectra taken from three areas of a C-135 aircraft at Tinker AFB. (Right) Engineer performing measurements with hand-held sensor.

4.0 Summary and Conclusions

An in-situ corrosion sensor has been developed that extends the applicability of electrochemical impedance spectroscopy (EIS) to monitor material degradation beyond laboratory immersion studies. Two versions of the sensor are available: a permanent electrode that is especially suited for inaccessible areas or for areas for which a corrosion history or database is desired and a portable, hand-held electrode that is especially suited for areas where a permanent sensor is not desired for aesthetics or other reasons or where only a one-time measurement is needed. Both versions provide EIS measurements identical to conventional remote electrode procedures when tested in immersion. The ability of the in-situ sensor to monitor material deterioration from the very initial stages in accelerated tests, such as salt fog or humidity, and during actual field service enables two applications: 1) in-situ corrosion monitoring of critical structures to allow condition-based maintenance and reduce the likelihood of unforeseen corrosion-induced failure and 2) enhanced coating development with the quantitative comparison of degradation occurring during accelerated testing and system service use.

5.0 Acknowledgments

We would like to acknowledge the technical assistance of D. Stuart, B. Taggart, and P.L. Whisnant over the course of this investigation. We would like to express our appreciation of Courtaulds Aerospace for painting some of the specimens and of Don Nieser and Dick Kinzie for arranging field demonstrations at Tinker AFB and Robbins AFB, respectively. This effort was funded by AFSOR under SBIR contract F49620-95-C-0040.

6.0 References

[1] J.A. Payer and G.M. Ugiansky, "Impact of the NBS - Batelle Cost of Corrosion Study in the United States," Proc. Corrosion, Symp. Int. Approaches to Reducing Corrosion Costs (NACE, Houston, 1986).

[2] C.I. Change, "Aging Aircraft Science and Technology Issues and Challenges and USAF Aging Aircraft Program," in Structural Integrity in Aging Aircraft AD-47 ed., C.I. Chang and C.T. Sun, (ASME, New York, 1995), p. 1.

[3] "Economic Effects of Metallic Corrosion in the United States, A 1995 Update," Batelle Report to Specialty Steel Industry of North America, April 1995.

[4] D.W. Hoeppner, L. Grimes, A. Hoeppner, J. Ledesma, T. Mills, and A. Shah, "Corrosion and Fretting as Critical Aviation Safety Issues: Case Studies, Facts, and Figures from US Aircraft Accidents and Incidents," Proc. 18^th Symp. Inter. Comm. Aeronautical Fatigue (Melbourne, Australia, May 1995), p. 87.

[5] G.D. Davis and C.M. Dacres, "Electrochemical Sensors for Evaluating Corrosion and Adhesion on Painted Metal Structures," patent pending.

[6] F. Mansfeld, "Recording and Analysis of AC Impedance Data for Corrosion Studies: I. Background and Methods of Analysis," Corrosion 37, 301 (1981).

[7] M. Kendig, F. Mansfeld, and S. Tsai, "Determination of the Long Term Corrosion Behavior of Coated Steel with AC Impedance Measurements," Corros. Sci. 23, 317 (1983).

[8] M. Kendig and J. Scully, "Basic Aspects of the Application of Electrochemical Impedance for the Life Prediction of Organic Coatings on Metals," Corrosion89 Paper 32, NACE (1989).

[9] J.R. Scully, "Electrochemical Impedance of Organic-Coated Steel: Correlation of Impedance Parameters with Long-Term Coating Deterioration," J. Electrochem. Soc. 136, 979 (1989).

[11] J.N. Murray and H.P. Hack, "Long Term Testing of Epoxy Coated Steel in ASTM Sea Water Using EIS," Corrosion90 Paper 140, NACE 1990.

[12] J.A. Grandle and S.R. Taylor, "Electrochemical Impedance Spectroscopy of Coated Aluminum Beverage Containers: Part 1 - Determination of an Optimal Parameter for Large Sample Evaluation," Corrosion 50, 792 (1994).

[13] A. Zdunek and X. Zhan, "A Field EIS Probe and Methodology for Measuring Bridge Coating Performance," 4^th World Congress on Coating Systems for Bridge and Steel Structures, Steel Structures Painting Council, St. Louis, MO, February 1995.

[14] K. Homma et al., "Utilization of Electrochemical Impedance Techniques to Estimate Corrosion Damage of Steel Infrastructure," Corrosion Forms and Control for Infrastructure, ASTM STP 1137, Victor Chaker, ed., ASTM, Philadelphia, 1992.

[15] P.C. Su, O.F. Devereux, and W. Madych, "Impedance Imaging for Prediction and Detection of Airframe Corrosion," Structural Integrity in Aging Aircraft, AD-Vol. 47, C.I. Chang, and C.T. Sun, ed., American Society of Mechanical Engineers, 1995.

[16] T.C. Simpson, P.J. Moran, W.C. Moshier, G.D. Davis, B.A. Shaw, C.A. Arah and K.L. Zankel, "An Electrochemical Monitor for the Detection of Coating Degradation in Atmosphere," J. Electrochem. Soc. 136, 2761 (1989).

[17] T.C. Simpson, P.J. Moran, H. Hampel, G.D. Davis, B.A. Shaw, C.A. Arah, T.L. Fritz, and K.L. Zankel, "Electrochemical Monitoring of Organic Coating Degradation during Atmospheric or Vapor Phase Exposure," Corros. 46, 331 (1990).

[18] T.C. Simpson, H. Hampel, G.D. Davis, C.O. Arah, T.L. Fritz, P.J. Moran, B.A. Shaw, and K.L. Zankel, "Evaluation of the Effects of Acidic Deposition on Coated Steel Substrates," Prog. Organic Coatings 20, 199 (1992).

[19] V.S. Agarwala, "In-Situ Corrosivity Monitoring of Military Hardware Environments," Corrosion96 Paper 632, NACE 1996.

[20] R.G. Kelly, S.H. Jones, W. Blanke, J. Aylor, and A. Batson, "Embeddable Microinstruments for Corrosion Monitoring," Corrosion97 Paper 294, NACE 1997.

[21] G.D. Davis, P.L. Whisnant, and J.D. Venables, "Subadhesive Hydration of Aluminum Adherends and its Detection by Electrochemical Impedance Spectroscopy," J. Adhes. Sci. Technol. 9, 433 (1995).

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