Abstract:
The coating monitoring system is based on electrochemical impedance spectroscopy (EIS). The system consists of one or more compact and rugged mini-potentiostat modules coupled to one or more electrodes mounted on top of the paint coating of the structure being monitored. The electrodes and modules can be coated with a topcoat if desired. Alternatively, they may be mounted only temporarily to the structure for spot inspection. They periodically report to a laptop. Communications may be implemented using a wireless protocol. The units may be battery powered with an estimated battery lifetime of up to ten years, depending on the frequency of measurement and interrogation Alternative power supplies may be used to replace or supplement the battery to allow extended battery lifetime. Moisture, humidity, or other sensors may be incorporated into the coating monitor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. 119(e) of prior U.S. Provisional application 61/185,835 filed on Jun. 10, 2009. 
    
    
     SEQUENCE LISTING 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     The invention involves a compact, rugged coating monitor. Corrosion is a wide-spread problem that affects nearly all industry and government sectors. A recent report determined that the direct cost of corrosion in the United States to be 3.1% of the Gross Domestic product (GDP) [Gerhardus H. Koch, Michiel P. H. Brongers, Neil G. Thompson. Y. Paul Virmani, Joe H. Payer, “Corrosion Costs and Preventive Strategies in the United States,” Report by CC Technologies Laboratories, Inc. to Federal Highway Administration (FHWA), Office of Infrastructure Research and Development, Report FHWA-RD-01-156, September 2001]. This corresponds to $300B annually or $1000 per person. This figure includes only the direct costs (e.g., corrosion prevention, corrosion inspection, and replacement or refurbishment of corroded structures). The indirect costs (e.g., lost productivity, taxes, and overhead) were conservatively estimated to be equal to the direct costs. 
     Paint coatings are the primary means of corrosion protection for most structures and they can be very effective. However, paint coatings are only temporary; they weather, absorb moisture, blister, become scratched or undergo other mechanical damage. Even fresh paint coatings can exhibit pinholes, holidays, or other coating defects that can adversely affect corrosion protection. Thus, there is a need for corrosion sensors to assess the health and effectiveness of paint coatings, especially on critical structures and equipment. 
     Although many corrosion sensors have been proposed and developed [G. D. Davis, C. M. Dacres, and L. A. Krebs, “In-Situ Corrosion Sensor for Coating Testing and Screening,”  Materials Performance  39(2), 46 (2000); G. D. Davis, C. M. Dacres, and L. A. Krebs, “EIS-Based In-Situ Sensor for the Early Detection of Coating Degradation and Substrate Corrosion,”  Corrosion 2000, Paper 275 (NACE, Houston, Tex., 2000); G. D. Davis, C. M. Dacres, and L. A. Krebs, “EIS-Based In-Situ Sensor for the Early Detection of Coating Degradation and Substrate Corrosion,”  Corrosion 2000, Paper 275 (NACE, Houston, Tex., 2000); J. Green, M. Jones, T. Bailey, and I. Perez,  Process Control and Sensors for Manufacturing , R. H. Bossi and D. M. Pepper, ed., (SPIE—The International Society for Optical Engineering, Bellingham, Wash., 1998), p. 28; V. S. Agarwala,  Corrosion 96, Paper 632, NACE, Houston, Tex., 1996; R. G. Kelly, J. Yuan, S. H. Jones, W. Blanke, J. H. Alor, W. Wang, A. P. Batson, A. Wintenberg, and G. G. Clemena,  Corrosion 97, Paper 294, NACE, Houston, Tex. 1996; J. Zhang and G. S. Frankel, in  Nondestructive Characterization of Materials in Aging Systems, MRS Symp. Series , Vol. 503, R. Crane, J. Achenbach, S. Shah, T. Matikas, P. Khuri, and L. Yakub, eds., (Materials Research Society, Warrendale, Pa., 1998), p. 15; R. E. Johnson and V. S. Agarwala,  Corrosion 97, Paper 304, NACE, Houston, Tex., 1997; L. D. Stephenson, A. Kumar, J. Hale, and J. N. Murray, “Sensor System for Measurement of Corrosion Under Coatings,”  Mater. Perf.  48(5) 36 (May 2009)], many are not suitable for monitoring coating health. A major disadvantage of many of these, such as galvanic couple sensors (U.S. Pat. No. 5,306,414, U.S. Pat. No. 5,437,773, U.S. Pat. No. 5,243,298, U.S. Pat. No. 6,809,506, U.S. Pat. No. 4,380,763, U.S. Pat. No. 6,683,463, U.S. Pat. No. 7,313,947, U.S. Pat. No. 5,338,432, and U.S. Pat. No. 5,310,470) and many fiber optic corrosion sensors (U.S. Pat. No. 5,299,271, U.S. Pat. No. 7,228,017), is that they are more properly considered corrosivity sensors; that is, they detect degradation of a sensor element and not degradation of the structure of interest. As such, they measure only how corrosive the environment is and provide no direct information on the condition of the coating or the structure. Furthermore, they are consumed and have a limited lifetime and can provide no information concerning any environmental degradation prior to installation. A second disadvantage of many sensors is that they need to be embedded into the structure. This limits them to new construction and poses important issues on the effect of the sensor on structure properties and data acquisition/transfer. These sensors cannot inspect existing structures and cannot be replaced if damaged or past their useful lifetime. The electrochemical impedance sensor approach developed here has neither of these critical disadvantages. The technology is suitable for determining coating health and detection of damage under paint coatings. 
     The coating monitor of the present invention is a compact and rugged integrated detection and reporting system that uses electrochemical impedance spectroscopy (EIS)-based corrosion sensors and mini-potentiostat elements. Corrosion sensors for military and civilian applications are most useful when the devices do not require invasion into the coating being monitored for protection or embedding within the structure. The coating monitor of the present invention uses conductive tape sensors to allow EIS measurements to be taken without remote electrodes. Using that engineering approach allows the coating monitor to be applied at times and in locations needed with the flexibility to remove it without decommissioning the structure or vehicle to which it is attached, or removing part of the monitored structure. Its small size and ruggedness are a huge commercial advantage. A patent search found a number of patent documents that approached some aspect of coating monitoring from an EIS sensor perspective. Documents of particular note include U.S. Pat. No. 7,477,060 (&#39;060 Yu, S. Y. et al) and published application US20080150555 (&#39;555 Wang, D. et al), which has its sensors on the surface or embedded in a flexible substrate integrated into the monitored structure, and U.S. Pat. No. 6,911,828 (&#39;828 Brossia, C. S. et al), which has arrays of sensor pins in contact with the structure&#39;s coating. Also of interest is U.S. Pat. No. 7,088,115 (&#39;115 Glenn, D. F. et al), for an EIS system designed specifically for uncured concrete. 
     With respect to the &#39;060 patent, the specification discusses the feasibility of attaching the sensors with adhesives but this feature is not included in the claims of the patent. In the published application, &#39;555, an optoelectronic backbone communicates data from the sensors to a control device. 
     Similarly, the &#39;828 patent describes a sensor array in the form of sensor pins in contact with the coating being monitored and a data interrogation device positioned in proximity to the sensor array. 
     The &#39;115 patent is directed specifically at detecting defects in uncured concrete before the curing process. It is much more limited in application than the &#39;060 or &#39;828 systems. 
     The DACCO SCI, INC., portfolio discloses permanent or handheld sensors that use EIS to detect moisture and other changes in coatings U.S. Pat. No. 5,859,537 (&#39;537 Davis, G. D. et al), U.S. Pat. No. 6,054,038 (038 Davis, G. D. et al), U.S. Pat. No. 6,313,646 (&#39;646 Davis, G. D. et al) and U.S. Pat. No. 6,328,878 (&#39;878 Davis, G. D. et al). The &#39;537 patent is directed to an in situ sensor suitable for coated metal structures. The sensor comprises conductive ink and is permanently applied to the topcoat being monitored. The &#39;038 patent teaches the corrosion sensor as a handheld device comprising a metal. The &#39;646 patent teaches the use of two hand-held electrodes to detect moisture absorption, corrosion, and adhesive bond degradation. The &#39;878 patent teaches the use of a pair of conductive foil adhesive tapes, one with a conductive adhesive and one with a nonconductive adhesive, to determine coating or substrate degradation. All of these inventions use a separate bench-top or similar-sized potentiostat to be connected and to acquire the EIS measurements; thus they are not suitable for remote or unattended operation. 
     An example of a prior art potentiostat would be the Gamry Reference 3000 potentiostat that is 20-cm×23-cm×30-cm and weighs approximately 6 kg.  FIG. 1  shows a block diagram of a generic potentiostat  10  comprised of an ac voltage generator  12 ; a galvanometer  14  to measure the current (magnitude and phase) induced by the said ac voltage; a means  16  to make electrical connection to the specimen being measured; a means  18  to make electrical connection to reference and counter electrodes immersed into an electrolyte along with the specimen; a means  20  to convert the current measurement into an electrochemical impedance measurement (magnitude and phase); and a means for input/output  22 . 
     SUMMARY OF THE INVENTION 
     A principal objective of the present invention is to provide a compact and rugged coating monitor. The entire coating monitor of the present invention can be attached to the coated structure and left in place for long periods of time. This permits the coating monitor to be secured to the coated structure wherein it can take and store measurement data regarding coating characteristics and transmit these data to a remote receiver at a convenient time. 
     However, it is not necessary to permanently secure the coating monitor to the structure. The compact nature of the coating monitor of the present invention permits it to be useful in situations where a permanent attachment to the structure is not desirable. For example, a permanently mounted monitor may not be desired for cosmetic reasons. Additionally, in cases such as a fluid flow environment, e.g., an airfoil surface, a permanent monitor could disturb the fluid flow and would not be desirable. In this embodiment, the coating monitor could be removably secured to the structure. A final advantage of this embodiment is the ability to collect coating measurement data from multiple locations on the structure or from multiple structures using a single coating monitor. 
     An additional objective of the present invention is to provide a coating monitor which can take and store information concerning the coating and also take and store other information of interest such as environmental information, battery life, and other parameters of interest. For example, the electrochemical impedance spectrum of a coated structure may depend on the humidity or surface wetness of the structure. A means to determine the humidity or surface wetness is desirable. 
     The present invention allows for broad applicability, flexibility in utilizing the coating monitor in various environments without structural compromise and the ability to inspect and evaluate the actual structure regardless of its size. This adaption includes the utilization of a widely accepted and recognized laboratory technique of electrochemical impedance spectroscopy for the investigation of coating deterioration and substrate corrosion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a block diagram of a generic potentiostat. 
         FIG. 2  shows a block diagram of the coating monitor. 
         FIG. 3  shows a first embodiment of the coating monitor involving a temporary attachment. 
         FIG. 4  shows an example of the electrochemical impedance spectrum of a coating as the coating is deteriorating. 
         FIG. 5  shows a second embodiment of the coating monitor involving a temporary attachment. 
         FIG. 6  shows a third embodiment of the coating monitor involving a temporary attachment. 
         FIG. 7  shows a blown-up view of area A of  FIG. 6 . 
         FIG. 8  shows a fourth embodiment of the coating monitor involving a permanent attachment. 
         FIG. 9  shows a fifth embodiment involving an application-specific integrated circuit version of the coating monitor. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  shows a block diagram of a coating monitor  24  comprising a circuit board potentiostat  26  comprising an ac voltage generator operating at one or more frequencies; a galvanometer to measure the current (magnitude and phase) induced by the ac voltage; a means  28  to make measure the current (magnitude and phase) induced by the ac voltage; a means  28  to make electrical connection to the substrate shown in subsequent figures; a means  30  to make electrical connection to the electrode(s) shown in subsequent figures; a means to convert the current measurement into an electrochemical impedance measurement (magnitude and phase); and a transceiver  32  for input/output. The coating monitor is powered by an AA or A battery  34 . The entire coating monitor is contained in a compact waterproof casing  36 . 
     Compact waterproof casing  36  has an interior recess with a predetermined length, width and height. As can clearly be seen from the showing of  FIG. 2 , the predetermined length of the interior recess of casing  36  is slightly more than the length of A or AA battery  34 . The predetermined width of the interior recess of casing  36  is also slightly greater than the length of A or AA battery  34  but somewhat greater than the predetermined length. As can be clearly seen from the showing of  FIGS. 3, 5 and 6-8 , the predetermined height of the interior recess of casing  36  is substantially less than the length of A or AA battery  34 . Both A and AA batteries have substantially the same length, approximately 50 mm, thus the predetermined length and width of the interior recess of casing  36  is approximately 50 mm. 
     The coating monitor of the present invention includes a potentiostat that is designed to provide only the most essential information to assess a coating&#39;s effectiveness and thus minimizes its size, weight, complexity, and power consumption. It comprises an ac voltage generator operating at one or more frequencies, preferably a plurality of frequencies below 100 Hz, more preferably a plurality of frequencies below 10 Hz; a galvanometer to measure the current (magnitude and phase) induced by the said ac voltage; one or more electrodes applied to or pressed against a coating; a means to make electrical connection to the substrate and to the electrode(s); a means to convert the current measurement into an electrochemical impedance measurement (magnitude and phase); and an optional clock to provide a time-stamp to the data. The coating monitor may comprise distinct components, a hybrid microcircuit or an application specific integrated circuit. The data may be stored internally to the coating monitor or wirelessly transmitted to a computer or other device. Alternatively the data may be outputted to a computer or other device in real time. The coating monitor may have internal battery power or may obtain power via a lead. Other power sources, such as solar cells, vibration energy scavenging, or the like may replace or supplement the battery. Additional sensors, such as moisture or humidity sensors, may be incorporated into the coating monitor. 
       FIG. 3  shows one embodiment of a coating monitor  24  attached with a suction cup attachment  38  to a coated structure  40 . Clearly, other attachment means could be used such as mechanical fastening means, magnets, adhesives, or other means. The coating  42  would typically be applied to the substrate  44  to prevent or reduce corrosion, degradation, or other deterioration. The coating may include paints, other polymeric films, appliqués or other peel-and-stick films, ceramic coatings, conversion coatings, anodized films, or other applied, grown, or deposited coatings. The substrate could be any material that is electrically conductive for example metal, carbon-filled polymers, metal-filled polymers or any other electrically conductive material. This substrate could be a component of almost any type of structure, for example bridges, airplanes, ships, ground vehicles, tanks, pipelines, buildings, towers, supporting members or the like. The substrate could also be laboratory test panels, coupons, samples, or similar items. 
     In the embodiment shown in  FIG. 3 , the coating monitor  24  applies the ac voltage between the substrate  44 , which is electrically connected to the coating monitor via lead  46  at a convenient connection point and one or more electrodes  48  and  50 . These electrodes are electrically connected to the coating and comprise a flexible, electrically conductive material, preferably metal and more preferably a metallic tape with an electrically conductive pressure sensitive adhesive. The data may be stored internally to the coating monitor or may be outputted to a computer or other device in real time via lead  52 . Alternatively and, in some cases preferably, the data may be wirelessly transmitted to a computer or other device. The coating monitor may have internal battery power or may obtain power via lead  52 . 
       FIG. 4  shows an example of the electrochemical impedance spectra of a painted substrate following different exposure times in an aggressive, corrosive environment. The coating monitor provides the electrochemical impedance at one or more frequencies as a function of time. As the coating degrades or deteriorates the electrochemical impedance at low frequencies decreases by several factors of ten. Because of this large change in the magnitude of the low-frequency impedance, these frequencies were chosen for the coating monitor. In the case of a coating defect  54  of  FIG. 3 , the electrochemical impedance can drop a similar amount with a very short exposure to the aggressive environment. The range of detection of coating defects  54  or deterioration by the coating monitor  24  depends on the coating&#39;s surface conductivity, which is commonly governed by the amount of humidity or moisture on the surface. The coating monitor  24  can provide a measure of the amount of humidity or moisture using several methods. One such method is to measure the electrochemical impedance between the two electrodes  48  and  50  instead of between one of the two electrodes  48  or  50  and the substrate  44 . Another method is to measure the humidity/moisture with a humidity/moisture detector  56  mounted on or near the coating monitor  24 . The electrochemical impedance of a coating is also a function of the coating thickness; accordingly, the results of the coating monitor may also be used to determine the thickness of the coating. 
       FIG. 5  shows a second embodiment of the coating monitor  24 . In this embodiment, the flexible electrodes  48  and  50  of  FIG. 4  are replaced by one or more probes  58  and  60  that are electrically connected to the coating monitor  24  via leads  62  and  64 . The probes comprise an electrically conductive material, preferably metal, which can be of any shape. The probes  58  and  60  may be held against the structure  40  by a variety of methods, including gravity based on the weight of the probes, electrically conductive adhesive applied to the bottom of the probes, adhesive tape applied across the probe and the structure, magnets, mechanical fasteners, suction cup apparatuses, or other means. A small amount of electrically conductive liquid may be used underneath the probes to facilitate good electrical connection to the surface of the substrate. Measurements using this embodiment proceed according to same procedure described above. 
       FIG. 6  shows a third embodiment of the coating monitor  24  in which the flexible electrodes  48  and  50  as shown in  FIG. 3  are replaced by one or more probes  66  that are not attached to structure  40  but are instead pressed against the structure  40  using springs  68  attached to the coating monitor. The probe(s) comprise an electrically conductive material, preferably metal, which can be of any shape. The springs provide electrical connection between the probe(s)  66  and the coating monitor  24 . Alternately, the springs may be replaced with a pad or other material or device that provides a force to hold the probe(s)  66  against the structure  40 . In this case, since the pad might not be a conductive material, the required electrical connections between the probe(s)  66  and the coating monitor  24  may be provided by wire leads (not illustrated). A small amount of electrically conductive liquid may be used underneath the probe(s) to facilitate good electrical connection to the surface of the structure. Measurements using this embodiment proceed according to the same procedure described above. This embodiment is useful for situations where it is not desirable to attach the probe to the structure. This situation could arise in an environment where the structure might be damaged by the attachment means. In addition, this embodiment would be useful in a fluid flow environment where anything attached to the structure would not be desired, for example on an airfoil. This embodiment also permits a single coating monitor to be used to take coating measurements in multiple locations on a structure or on multiple structures. 
       FIG. 7  shows an enlarged view of area A of  FIG. 6 . Coating monitor  24  is mounted on coated substrate  40 . Probe(s)  66  are pressed against the structure  40  by springs  68  attached to coating monitor  24 . Springs  68  provide an electrical connection between probe(s)  66  and coating monitor  24 . 
       FIG. 8  shows a fourth embodiment of the coating monitor  24  where it is permanently or semi-permanently attached to the structure  40  via brackets  70  and  72  and screws, rivets, or bolts  74 . Other attachment methods are also possible. Electrical connection from the coating monitor to the substrate  44  is provided by the brackets and screws, rivets, or bolts. The electrodes  76  and  78  may be similar to the electrodes  48  and  50  of  FIG. 3 . These electrodes comprise an electrically conductive material, preferably metal and more preferably a metallic tape with an electrically conductive pressure sensitive adhesive. Alternatively, electrodes  76  and  78  may be comprised of a conductive paint or ink or a more rigid material, such as electrodes  58  and  60  (as shown in  FIG. 5 ), or probe(s)  66  (as shown in  FIG. 6 ). Because the coating monitor  24  may remain attached to the structure  40  for an extended period of time, the electrodes  76  and  78  preferably comprise a corrosion-resistant material that will not corrode or degrade during the expected period of operation. Alternatively, the electrodes and the coating monitor  24 , brackets  70  and  72 , and screws  74  may be covered with paint or other coating for corrosion protection or cosmetic, camouflage, or other reason. If the coating monitor is powered by an internal battery instead of external power via lead  52 , a removable lid  80  may be provided for battery replacement. Alternative means of power may include solar cells, vibrational or other energy scavenging, or radio frequency induction to replace or supplement the battery. Battery lifetime depends on a number of factors including the frequency of measurement and the frequency of interrogation. Calculations assuming reasonable values for these parameters predict a battery lifetime of up to approximately ten years. Measurements using this embodiment, including humidity/moisture measurements proceed according to same procedure described above. 
       FIG. 9  shows a fifth embodiment of an application-specific-integrated-circuit version of the coating monitor. The hybrid-circuit and application-specific-integrated-circuit versions of the coating monitor provide significant reductions in size and power consumption. The coating monitor electronics  82  have been reduced in size to fit into the hollowed head  84  of a bolt. In this case, the battery (not shown) lies below the coating monitor electronics. Other configurations are also possible. The electrode  86  can be comprised of the materials of electrodes  48  and  50  (as shown in  FIG. 3 ) or  76  and  78  (as shown in  FIG. 7 ). Electrical connection to the substrate  44  is achieved via the threads of the bolt. An optional washer  88  allows the bolt to be tightened without damaging the electrode. In this configuration, the bolt head  84  provides electrical connection to the electrode and is electrically isolated from the threads. Other configurations are also possible. 
     The coating monitors may be provided with a unique number or indicator so that if a plurality of coating monitors are mounted onto a structure, readings can be obtained from each individually. Alternatively, they may be networked together so that readings from all of them may be obtained as a system. The coating monitor can be programmed to take measurements at a desired repeat frequency, for example hourly, daily, weekly, or monthly or whenever the monitor receives a signal from an inspector. 
     Operation: 
     An operator or inspector will attach, using either a temporary or (semi)permanent attachment, one or more coating monitor units to a structure. The inspector records the coating monitor identification number and location on the structure and then either begins the measurement or sets the measurement schedule. The measurement results are either transferred directly to a computer or other device or are stored until the coating monitor is interrogated. The electrochemical impedance measurement provides an indication of the health of coating or, in some cases, the coating thickness. A protective coating will exhibit a large electrochemical impedance at low frequencies while a poor coating or a good coating with a defect will exhibit a low-frequency electrochemical impedance several factors of ten smaller. Knowledge of the protectiveness of a coating can allow condition-based maintenance of a critical structure and help provide increased readiness and safety.