Patent Publication Number: US-10761009-B2

Title: Manufacture electrodes for electrochemical monitoring

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of application Ser. No. 15/355,604, filed Nov. 18, 2016. The above referenced application is incorporated herein by reference in its entirety. 
    
    
     FIELD 
     Aspects of the present disclosure generally relate to material systems and methods for determining operational performance of material systems. 
     BACKGROUND 
     Spanning the lifetime of operation, an aircraft will experience repeated and harsh conditions resulting in degradation of component parts of the aircraft. Such degradation may take the form of, for example, corrosion and subsequent metal fatigue and fracture. Corrosion can contribute to a decrease in the integrity and strength of aircraft components. More specifically, a material system, such as an aircraft component, includes a fuselage or skin panels, a coated lap joint between two metal panels, or a wing-to-fuselage assembly on the exterior of an aircraft. Material systems may corrode over time due to exposure to mechanical and chemical stresses during use of the aircraft. Before a material is determined to be suitable for use as an aircraft material system, it may be desirable to determine the material system&#39;s propensity to corrode. However, performance of aircraft material systems, such as panels, during actual, real world use of the aircraft seldom correlates with corrosion testing data. 
     Furthermore, a corrosion testing procedure of a material system comprises spraying the material system with a salt solution in a chamber. Assessment of the extent of corrosion of the material system involves stopping the corrosion procedure and removing the material system from the chamber for visual inspection to determine the extent of corrosion. 
     Therefore, there is a need in the art for material systems, apparatus, and methods for controlled and accurate exposure and corrosion detection for determining operational performance of material systems. 
     SUMMARY 
     In one aspect, a material system comprises a metal substrate and a first coating layer disposed on the metal substrate. A first electrode is directly disposed on the first coating layer, and a second electrode is disposed on the metal substrate. 
     In another aspect, a method for determining material performance comprises flexing a material system and detecting impedance of the material system with an electrochemical impedance spectrometer. The material system comprises a metal substrate and a first coating layer disposed on the metal substrate. A first electrode is directly disposed on the first coating layer, and a second electrode is disposed on the metal substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to aspects, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical aspects of this present disclosure and are therefore not to be considered limiting of its scope, for the present disclosure may admit to other equally effective aspects. 
         FIG. 1  is a top sectional view of an apparatus for accelerating and controlling the corrosion-related failure modes of a material system, according to an aspect of the disclosure. 
         FIG. 2  is a side sectional view of an apparatus for accelerating and controlling the corrosion-related failure modes of a material system, according to an aspect of the disclosure. 
         FIG. 3  is a top sectional view of an apparatus for accelerating and controlling the corrosion-related failure modes of a material system, according to an aspect of the disclosure. 
         FIG. 4  is a perspective view of a flexer configured to perform cyclic flexing, according to an aspect of the disclosure. 
         FIG. 5  is a side view of a material system, according to an aspect of the disclosure. 
         FIG. 6  is a side view of a material system, according to an aspect of the disclosure. 
         FIG. 7A  is a plan view of a material system according to an aspect of the disclosure. 
         FIG. 7B  is a plan view of a material system according to an aspect of the disclosure. 
         FIG. 7C  is a plan view of a material system according to an aspect of the disclosure. 
         FIG. 8  is a graph of impedance data of a material system comprising an interdigitated electrode pair of Example 1 disposed onto a coating material. 
         FIG. 9  is a graph of impedance data of a material system comprising electrodes of Example 2, according to an aspect of the present disclosure. 
         FIG. 10  is a graph of impedance data of a scribed material system comprising electrodes of Example 3, according to an aspect of the present disclosure. 
         FIG. 11  is a graph of impedance data of a material system comprising electrodes of Example 4, according to an aspect of the present disclosure. 
         FIG. 12  is a graph of impedance data of a material system comprising electrodes of Example 5, according to an aspect of the present disclosure. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. The figures are not drawn to scale and may be simplified for clarity. It is contemplated that elements and features of one aspect may be beneficially incorporated in other aspects without further recitation. 
     DETAILED DESCRIPTION 
     The descriptions of the various aspects of the present disclosure have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the aspects disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described aspects. The terminology used herein was chosen to best explain the principles of the aspects, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the aspects disclosed herein. 
     Aspects of the present disclosure generally relate to material systems and methods for determining operational performance in situ of material systems. A material system can be a component of an aircraft and typically comprises a substrate, such as a metal, and one or more coatings, such as an epoxy, disposed on the substrate. One or more electrodes, such as a pair of electrodes, are disposed on or within a surface of the material system to provide electrochemical detection of operational performance, e.g. corrosion, of the material system. Determining operational performance of a material system can be performed in a lab setting or on an aircraft by an operator or manufacturer before, during (in situ), or after the material system has been exposed to flexing and/or moisture treatment. For example, a material system is in electrical communication with a spectrometer to provide impedance data of one or more surfaces of the material system to assist in determination of the operational performance of the material system during flexing and moisture exposure of the material system. 
     Apparatus 
     A material system, such as a panel, may have one or more surface layers such as a surface finish, a primer, and/or a top coat. Corrosion may occur at one or more of these layers in use due to mechanical and chemical stresses. Material systems, apparatus, and methods of the present disclosure provide in situ electrochemical monitoring of impedance to determine corrosion in a setting that mimics the corrosion experienced by a material system in actual use conditions. The material system is subjected to mechanical as well as chemical stresses without degradation of the electrochemical monitoring system. Material systems, apparatus and methods of the present disclosure provide electrochemical monitoring of impedance to determine corrosion at one or more of a material system surface, a finished surface, a primer surface, and/or a top coat surface. 
       FIG. 1  is a plan view of an apparatus  100  for accelerating and controlling the corrosion-related failure modes of a material system, according to an aspect of the disclosure.  FIG. 2  is a side perspective view of apparatus  100  of  FIG. 1 . One or more components of apparatus  100  are made of materials that show resistance to a corrosive environment, such as an environment containing a salt fog. As shown in  FIGS. 1 and 2 , apparatus  100  includes an enclosure  160  having one or more fog nozzles  102  (one shown) disposed therein and configured to spray a treating liquid, such as a salt fog, in the enclosure  160 . A fixture support is disposed in the enclosure to support a material system for exposure and flexing therein. Apparatus  100  includes a liquid reservoir  104  to supply a treating liquid to fog nozzle  102 . Fog nozzle  102  may be a nozzle, such as an atomizing nozzle, a nozzle calibrated for air consumption, BETE full cone nozzle, hollow cone nozzle, fan misting nozzle, tank washing spray nozzle, NASA Mod1 nozzle for water spray atomization and droplet control, Q-Lab OEM fogging nozzle, Cool Clean ChilAire Lite spray applicator nozzle, or combinations thereof. Fog nozzle  102  can be made of materials such as hard rubber, plastic, or other inert materials. 
     The fixture support comprises jaws  124   a - e  configured to flex a material system. Plate  146  is configured to support jaws  124   a - e . In at least one aspect, plate  146  comprises a mounting plate disposed on an I-Beam grate. Plate  146  is positioned between fog nozzle  102  and jaws  124   a - 124   e  (as shown in  FIGS. 1 and 2 ), allowing treating liquid to enter the enclosure without directly impinging upon a material system held by one or more jaws  124   a - e . This configuration mimics general humid atmospheric conditions, as compared to direct rainfall onto an aircraft material system. Alternatively, jaws  124   a - e  may be positioned between fog nozzle  102  and plate  146  (this configuration not shown), providing direct flow of treating liquid toward a material system held by one or more jaws  124   a - e . This configuration mimics direct rain fall or aerosol deposition onto an aircraft material system. Fog nozzle  102  may be configured for flow angle adjustment, allowing flow of treating liquid at one or more angles relative to a material system surface. In at least one aspect, a material system surface may be parallel to a principal direction of flow of liquid through apparatus  100 , based upon the dominant surface being tested, which reduces liquid collection on a material system during corrosion testing performed in apparatus  100 . In such aspects, fog nozzle  102  may be directed or baffled so that the liquid does not impinge directly on a material system. (Fog nozzle  102 , a vent  122 , a motor  126 , an outer enclosure  136 , and legs  148   a - f  are shown as dashed lines in  FIG. 1  to indicate that these parts are located behind a plate  146  in the aspect shown in  FIG. 1 ). 
     A fog pump  108  is configured to assist flow of a liquid from liquid reservoir  104  to fog nozzle  102  via first fluid line  106  and second fluid line  110 . First fluid line  106  couples liquid reservoir  104  at a first end with fog pump  108  at a second end to provide liquid communication of liquid reservoir  104  with fog pump  108 . Second fluid line  110  couples fog pump  108  at a first end with fog nozzle  102  at a second end to provide liquid communication of fog pump  108  with fog nozzle  102 . 
     A compressed air source  112  and bubble tower  114  are configured to provide humidified air to fog nozzle  102 . In at least one aspect, a pressure in the enclosure may be regulated to mimic the pressure experienced by an aircraft at various altitudes during real world use. Accordingly, compressed air source  112  is configured to flow air at a pressure ranging from about 2 pounds per square inch (PSI) to about 50 PSI, from about 5 PSI to about 30 PSI, from about 12 PSI to about 18 PSI. In these ranges, lower pressure values mimic pressures experienced by an aircraft at higher altitudes while higher pressure values mimic pressures experienced by an aircraft at lower altitudes and closer to sea level. Air may include a mixture of gases similar to that found in an ambient atmosphere, for example, comprising about 78% N 2 , about 21% O 2 , and about 0.039% CO 2 , among other gases. Third fluid line  116  couples bubble tower  114  at a first end with fog nozzle  102  at a second end to provide air and liquid communication of bubble tower  114  with fog nozzle  102 . A compressed air line  118  couples compressed air source  112  at a first end with bubble tower  114  at a second end to provide air communication of compressed air source  112  with bubble tower  114 . Bubble tower  114  may contain a liquid, such as water, to provide initial humidification or additional humidification to air flowed from compressed air source  112  via compressed air line  118 . 
     A vent  122  may be coupled with the first chamber wall  130 , a second chamber wall  132 , or a third wall  152  ( FIG. 2 ) to provide pressure regulation inside of apparatus  100 . A heater  120  may be provided and configured to heat the inside of apparatus  100  such as enclosure  160 . Heater  120  may be disposed adjacent to a first wall  130  of apparatus  100  and coupled with third wall  152  ( FIG. 2 ). Heater  120  may be adhered to third wall  152  by any suitable adherent, such as rivets. Heater  120  may be coupled with and controlled by controller  138 . 
     Fixture support is configured to support and flex a material system positioned in the enclosure for testing. Jaws  124   a ,  124   b ,  124   c ,  124   d , and  124   e  are configured to flex a material system, such as a panel, a coated lap joint between two metal panels, a wing-to-fuselage assembly, or combinations thereof. The material system may be an aircraft material system, such as a panel, such as a skin or fuselage flat panel. The material system may have a width that is, for example, about 4 inches, and a length that is for example, about 6 inches to about 14.5 inches. The fixture support may flex a material system to a strain ranging from about 0.05% to about 50%, about 0.1% to about 30%, about 0.3% to about 5%, such as about 0.37%. 
     Fixture support comprising one or more jaws  124   a - e  is configured to grip and release a material system. Jaws  124   a - e  are configured to flex a material system from a first starting position to a fully or partially flexed second position. Jaws  124   a - e  are configured to flex a material system from a first position to greater than 0° to about 180° from the starting position, such as about 5° to about 90°, such as about 5° to about 45°, during a flexing process. Jaws  124   a - 124   e  may be the same size or different sizes. For example, jaw  124   a  may be the same size as jaw  124   b , but be a different size than jaw  124   d  (as shown in  FIG. 1 ). Furthermore, jaws  124   a - 124   e  may be positioned from one another by a distance that is the same or different than a distance between a different pair of jaws  124   a - e . For example, a first distance between jaw  124   a  and  124   b  may be different than a second distance between jaw  124   d  and  124   e . Various jaw sizes and various distances between jaws provide, for example, simultaneous testing of different sized material systems, such as panels, during an exposing and flexing process within apparatus  100 . In at least one aspect, one or more of jaws  124   a - e  comprises steel. In at least one aspect, one or more of jaws  124   a - e  is anodized. In at least one aspect, one or more of jaws  124   a - e  comprises an inert material such as hard rubber and/or plastic. In at least one aspect, jaws  124   a - e  are configured to support a material system, such as a panel, from about 15° and about 30° relative to a first wall  130  and/or second wall  132 , which reduces liquid collection on a material system during corrosion testing performed in apparatus  100 . In at least one aspect, jaw  124   a  is configured to grip a material system at a first end of the material system and jaw  124   b  is configured to grip the material system at a second end of the material system. In at least one aspect, jaws  124   a - e  are configured to flex a material system simultaneously or alternatively. 
     A motor  126  operates jaws  124   a - e . Inlet tube  128  is coupled with motor  126  at a first end and coupled with first wall  130  at a second end for providing cooling material, such as air, to motor  126 . Outlet tube  134  is coupled with motor  126  at a first end and coupled with first wall  130  at a second end for removing hot air exhaust from motor  126 . Outer enclosure  136  surrounds motor  126  to enclose and protect the motor from liquid emitted from fog nozzle  102  or any other liquid present inside of apparatus  100 . Jaws  124   a - e  are supported by plate  146 . Plate  146  is supported by legs  148   a ,  148   b ,  148   c ,  148   d ,  148   e , and  148   f . Legs  148   a - f  are coupled with plate  146  at a first end and a chamber wall, a rack  150   a , or a rack  150   b  at a second end. 
     Apparatus and material systems of the present disclosure include one or more electrodes, such as one or more pairs of electrodes. An electrode may be coupled with a substrate (to form a material system) and subsequent use of apparatus  100  to test operational performance of the material system. During flexing, the center portion of the material system will experience more strain than the edges of the material system. Accordingly, a pair of electrodes disposed on the same side of the material system provides detecting impedance across the same side of the material system. 
     As shown in  FIGS. 1 and 2 , apparatus  100  includes electrode pairs  156  and  158 . Although electrode pairs are shown in  FIGS. 1 and 2 , in an alternative aspect, apparatus  100  comprises single electrodes. Electrode pair  156  is configured to couple with a first side of a material system (not shown), and electrode pair  158  is configured to couple with a second side of the material system (not shown). Electrodes can be made of conductive epoxy, gold, silver, copper, platinum, palladium, or mixtures thereof. Preferably, at least one electrode is conductive epoxy, such as the electrodes of pairs  156  and/or  158 . In at least one aspect, conductive epoxy is conductive silver epoxy. Electrode pair  156  is coupled with spectrometer  164  via electrical line  162  to provide electrical communication between electrode pair  156  and spectrometer  164 . Furthermore, electrode pair  158  is coupled with spectrometer  164  via electrical line  166  to provide electrical communication between electrode pair  156  and spectrometer  164 . Electrical lines  162 ,  166  can be insulated wire (e.g., insulated steel wire) or wire having insulated conductive tape. Electrode pair  156  is configured to couple with a first side of a material system, and electrode pair  158  is configured to couple with a second side of the material system, as described in more detail below. In at least one aspect, spectrometer  164  comprises a potentiostat, galvanostat, and/or zero-resistance ammeter. Spectrometer  164  can be an electrochemical impedance spectrometer, such as a Reference  600  supplied by Gamry Instruments or a VMP  300  supplied by Bio-Logic Science Instruments. When coupled with a material system, electrodes (e.g., electrode pairs  156  and  158 ) detect an electrical signal from the material system and transmit the electrical signal to a spectrometer, such as spectrometer  164 . Spectrometer  164  is configured to interpret the electrical signal to provide electrical data, such as impedance, regarding the condition, such as corrosion, of the material system. Electrochemical impedance is usually measured by applying an AC potential to an electrochemical cell and then measuring the current through the cell. The response to this [sinusoidal] potential is an AC current signal. This current signal can be analyzed as a sum of sinusoidal functions (a Fourier series). Electrochemical impedance is normally measured using a small excitation signal. This is done so that the cell&#39;s response is pseudo-linear. In a linear (or pseudo-linear) system, the current response to a sinusoidal potential will be a sinusoid at the same frequency but shifted in phase. EIS data are typically analyzed in terms of an equivalent circuit model. Echem Analyst [a Gamry software product] finds a model whose impedance matches the measured data. 
     Parts of apparatus  100  described herein may comprise materials that are suitably inert to conditions within apparatus  100  during a cyclic flexing fog spray process. Suitably inert materials may include plastic, glass, stone, metal, rubber, and/or epoxy. 
     Apparatus  100  may be controlled by a processor based system controller such as controller  138 . For example, the controller  138  may be configured to control apparatus  100  parts and processing parameters associated with a cyclic flexing fog spray process. The controller  138  includes a programmable central processing unit (CPU)  140  that is operable with a memory  142  and a mass storage device, an input control unit, and a display unit (not shown), such as power supplies, clocks, cache, input/output (I/O) circuits, and the like, coupled to the various components of the apparatus  100  to facilitate control of a cyclic flexing fog spray process. Controller  138  may be in electronic communication with, for example, outlet tube  134 , vent  122 , heater  120 , and/or jaws  124   a - e.    
     To facilitate control of the apparatus  100  described above, the CPU  140  may be one of any form of general purpose computer processor that can be used in an industrial setting, such as a programmable logic controller (PLC), for controlling various chambers and sub-processors. The memory  142  is coupled to the CPU  140  and the memory  142  is non-transitory and may be one or more of readily available memory such as random access memory (RAM), read only memory (ROM), floppy disk drive, hard disk, or any other form of digital storage, local or remote. Support circuits  144  are coupled to the CPU  140  for supporting the processor in a conventional manner. Information obtained from cyclic flexing fog spray processes with apparatus  100  may be stored in the memory  142 , typically as a software routine. The software routine may also be stored and/or executed by a second CPU (not shown) that is remotely located from the hardware being controlled by the CPU  140 . The memory  142  is in the form of computer-readable storage media that contains instructions, that when executed by the CPU  140 , facilitates the operation of the apparatus  100 . The instructions in the memory  142  are in the form of a program product such as a program that implements the method of the present disclosure. The program code may conform to any one of a number of different programming languages. In at least one aspect, the disclosure may be implemented as a program product stored on computer-readable storage media for use with a computer system. The program(s) of the program product define functions of the aspects (including the methods described herein). Illustrative computer-readable storage media include, but are not limited to: (i) non-writable storage media (e.g., read-only memory devices within a computer such as CD-ROM disks readable by a CD-ROM drive, flash memory, ROM chips or any type of solid-state non-volatile semiconductor memory) on which information is permanently stored; and (ii) writable storage media (e.g., floppy disks within a diskette drive or hard-disk drive or any type of solid-state random-access semiconductor memory) on which alterable information is stored. Such computer-readable storage media, when carrying computer-readable instructions that direct the functions of the methods and apparatus of the present disclosure, are aspects of the present disclosure. 
       FIG. 3  is a plan view of an apparatus  300  for accelerating and controlling the corrosion-related failure modes of a material system, according to an aspect of the disclosure. As shown in  FIG. 3 , the components of apparatus  300  are the same as the components shown in apparatus  100  of  FIG. 1 , except that motor  126  is not encompassed by outer enclosure  136  (outer enclosure  136  is not present), motor  126  is located external to first chamber wall  130 , and inlet tube  128 , and outlet tube  134  are not present. Motor  126  is coupled with first chamber wall  130 . In at least one aspect, motor  126  (coupled to first chamber wall  130 ) translates bending motion inside the chamber via a screw, such as a ball screw, Acme screws, Lead screws, Roller screws, and screw mount, or an axle, passing into the chamber (not shown). A screw maintains spacing between stationary block  406  and mobile block  404  during flexing. 
       FIG. 4  is a perspective view of a flexer  400  configured to perform cyclic flexing, according to an aspect of the disclosure. Flexer  400  may be located inside of a material performance chamber, such as apparatus  100  as described for  FIG. 1 . As shown in  FIG. 4 , flexer  400  includes a mobile block  404  and a stationary block  406 . Stationary block  406  is mounted to plate  146  by mounting bolts  410   a  and  410   b . Mobile block  404  is slidably disposed on plate  146  adjacent to stationary block  406 . Linear displacement between stationary block  406  and mobile block  404  is maintained by guide rods  408   a  and  408   b . Guide rods  408   a  and  408   b  may comprise stainless steel, high density polypropylene, high density polyethylene, chromium, such as an Armology coating, and combinations thereof. Each of guide rods  408   a  and  408   b  is coupled with stationary block  406  and mobile block  404 . Guide rods  408   a  and  408   b  are parallel to one another. Stationary block  406  and mobile block  404  mount or otherwise support jaws  124   a - f . Stationary block  406  mounts a first side of jaws  124   a - f  in a stationary position during flexing, while mobile block  404  mounts a second side of jaws  124   a - f  and allows movement of the second side of jaws  124   a - f  during flexing. Stationary block  406  and mobile block  404  may comprise high density polyethylene. During flexing, the mobile block is shifted laterally relative to the stationary block from a starting point to an end point resulting in flexing of the material system positioned in jaws  124   a - f . The starting point, end point, and shift distance may be controlled by a user of flexer  400  based on test fixture mechanical boundary limits, mechanical stop blocks, or fixture driving system software controls. Each of stationary block  406  and mobile block  404  includes a base and a plurality of stanchions extending from the base perpendicular thereto. Each stanchion may be rectangular or angled to provide an angle on which each of the jaws  124   a - f  can be mounted. Jaws  124   a - f  are hinged, which allows bending of a material system while compressing the material system. For example, jaws  124   a - f  may be mounted on one side of each stanchion and disposed at an angle from, for example, 15° to 30° relative to a line perpendicular to the base. Other angles relative to perpendicular are contemplated to achieve a desired testing condition for a material panel. The angle of jaws  124   a - f  determines the angular position of the material system. In at least one aspect, a material system is disposed at an angle from, for example, 15° to 30° relative to a line perpendicular to the base. In at least one aspect, jaws  124   a - f  are non-conductive and non-metallic so as to have little or no galvanic effect on the material system. One or more of jaws  124   a - f  may comprise high density polyethylene, commercial grade Titanium (II) with polyethylene insert, sacrificial 316SS with polyethylene insert, or combinations thereof, which prevents (partial or complete) galvanic corrosion of the jaws and material systems during testing. One or more of jaws  124   a - f  may comprise a sleeve cover comprising, for example, polyethylene, which further prevents galvanic corrosion of the jaws and material systems during testing. Electrode pair  158  of apparatus  100  is directly disposed on a first side of a material system disposed in jaw  124   f , and electrode pair  156  (not shown) of apparatus  100  is disposed on a second side of the material system opposite the first side. A non-conductive protective coating  412  (shown as a transparent coating for simplicity) is disposed on electrode pairs  158  and  156  to protect the electrodes from a corrosive environment while apparatus  100  is in use, e.g. testing the material system. Non-conductive protective coatings include non-conductive epoxy, tape, adhesive, sealant, or mixtures thereof. In at least one aspect, non-conductive epoxy is a 2-part waterproof epoxy. Although  FIG. 4  shows electrodes directly disposed on a material system disposed in jaw  124   f , it is to be understood that material systems (and apparatus) of the present disclosure embrace aspects where one or more electrodes and/or electrode pairs are directly disposed on material systems disposed in one or more of jaws  124   a - e , and the electrodes and/or electrode pairs can be in electrical communication with spectrometer  164  via electrical lines similar or identical to electrical lines  162 ,  166 . Such aspects provide electrochemical in situ monitoring of a plurality of material systems within a material performance chamber, such as apparatus  100 . Furthermore, a non-conductive protective coating, such as coating  412 , can be disposed on said electrodes and/or electrode pairs. 
     In at least one aspect of the present disclosure, a material performance chamber contains more than one flexer  400 . In at least one aspect where a material performance chamber contains more than one flexer  400 , guide rods  408   a  and  408   b  extend through multiple flexers  400 . 
     A flexer, such as flexer  400 , provides variable displacement of a mobile block and material systems at variable frequencies that are adjustable in real-time. A flexer also provides for application of tension and compression to a material system. 
     Material Systems 
     In at least one aspect, a material system is a metal panel that can be flat and can be coated. The material performance of the flat panel is tested by cyclically flexing the material system while exposing the panel to at least a cycle of salt fog. Before, during, and/or after exposure and flexing, the material system is assessed for corrosion onset, rate of propagation, and performance. 
     In at least one aspect, a material system comprises a substrate having two flat metal panels connected, joined, welded, bonded, or fastened together using metallic fasteners, screws, bolts, or other hardware, before being exposed to at least a cycle of salt fog. 
     In at least one aspect, a material system comprises a mechanical joint or knuckle joint that may be made of metallic or composite materials and coated before being exposed to a cyclic salt fog and/or before being assessed for corrosion onset, rate of propagation, and performance. 
     In at least one aspect, a material system comprises a structural system replicative of aircraft components, representing a side-of-body joint, a stringer-to-fuselage assembly, a fuselage panel, or wing spar-to-fuselage assembly. The produced assemblies may be actuated or flexed while being exposed to at least a cycle of salt fog before/while being assessed for corrosion onset, rate of propagation, and performance, as described herein. 
       FIG. 5  is a side view of a material system  500  depicting material system  500  comprising a conductive metal substrate  502  and a coating layer  504  disposed on substrate  502 . Metal substrate  502  can be made of titanium, aluminum, copper, or alloys thereof. Metal substrate  502  may be coated with one or more primers, such as a chromated primer, surface finishes and/or top coats. For example, coating layer  504  can be made of chromated primer, epoxy primer, urethane primer, or mixtures thereof. Electrode  506  (which can be part of an electrode pair such as electrode pair  158 ) is directly disposed on coating layer  504  and is a reference electrode. Electrode  508  (which can be part of an electrode pair such as electrode pair  156 ) is disposed on the metal substrate  502  and is a working electrode. In at least one aspect, an insulating adhesive, such as non-conductive epoxy, is disposed between electrode  508  and metal substrate  502 . For spectroscopic measurements during testing, the working electrode  508  is adhered to conductive metal substrate  502 , and an electrical signal is sent through the working electrode (or pair of electrodes). The signal then moves through coating layer  504  and is received by electrode  506  (of electrode pair  158 ) and transmitted to spectrometer  164 . In an alternative aspect, working electrode  508  and reference electrode  506  is each disposed (e.g., directly disposed) on coating layer  504 . 
     In aspects where a coating layer, such as coating layer  504 , is made of an epoxy and an electrode disposed on the coating layer is made of a conductive epoxy, it has been discovered that the epoxy materials of the coating layer and the electrode absorb to one another. Use of an adhesive to adhere the two materials together is optional such that the electrode is directly disposed on the coating layer. In such aspects, a surface of the coating layer can be lightly abraded, followed by applying the electrode directly to the abraded surface. This “like-on-like” interaction between coating layer and electrode improves compatibility of the interface of the electrode and coating layer. The improved compatibility between the electrode and coating layer improves thermal and mechanical properties between the coating layer and the electrode. Conventional electrodes are adhered to a substrate surface with non-conductive adhesives. These adhesives interfere with the electrical communication of the electrode and a substrate such as a coating layer, yielding inaccurate spectroscopic data. With use of such adhesives, the electrical properties of the material system are being affected by a component (the adhesive) that is not a component of a material system that would be used in commercial applications. The adhesive causes a sharp gradient in mechanical, chemical, and thermal performances of the material system where the electrode is located. The improved compatibility between electrodes and coating layers of material systems of the present disclosure provides homogeneity between the electrodes and coating layers yielding reduced noise observed in a spectroscopic signal. 
     As a comparative example to material systems having epoxy electrodes, a material system having metal electrodes deposited onto a coating layer was tested. Electrochemical monitoring of the material system having metal electrodes deposited onto a coating layer provided an EIS spectrum showing only an “air” curve, indicative of an insufficient interaction between the metal electrodes and the coating layer. As used herein, “air curve” indicates an open-lead experiment. This experiment records an EIS spectrum with no cell attached. The spectrum from an open-lead experiment looks very much like a noisy spectrum for a parallel RC network. So, when an air curve is observed in the data, the leads from the spectrometer are not making electrical contact with the coating, and an EIS spectrum of the open air (i.e. an “air curve) is being collected. 
     Furthermore, it has been discovered that the thickness of electrodes of a material system can affect spectroscopic results of electrochemical monitoring. Electrodes of the present disclosure, such as electrodes  506  and  508  of electrode pairs  156  and/or  158 , can have a thickness of about 12 micrometers (μm) or less. Electrodes having a thickness of about 12 μm or less provide flexibility of the electrodes disposed on and/or within a material system and provide material systems operable to have an electrode disposed on one or more layers of the material system for more accurate electrochemical monitoring of each of the one or more layers of a material system. In at least one aspect, electrodes of the present disclosure have a thickness of from about 1 μm to about 12 μm, such as from about 2 μm to about 11 μm, such as from about 3 μm to about 10 μm. In at least one aspect, a coating layer of the present disclosure has a thickness of from about 1 μm to about 500 μm, such as from about 2 μm to about 250 μm, such as from about 3 μm to about 100 μm, such as from about 4 μm to about 15 μm. Furthermore, the reduced size of the electrodes of the present disclosure provides smaller/thinner electrical wires (coupled with the electrodes at a first end and a spectrometer at a second end) to be used for material systems of the present disclosure, as compared to traditional electrical wires that are too large to be embedded within layers of a multilayered material system. 
     In comparison, an electrode having a thickness of 13 μm or greater (such as interdigitated electrodes) is more rigid than thinner electrodes and tends to disconnect from the material system during flex testing. The rigidity of thick electrodes hinders the electrode&#39;s ability to conform to a surface of the material system. Furthermore, if a conventionally thin coating layer (such as an assembly primer, interior primer, fuel tank primer) is disposed on an electrode, electrodes having a thickness of 13 μm or greater tend to create a defect in the overlying layer and the defect is then accentuated over the course of flex testing. Furthermore, some conventional electrode designs involve drilling through the substrate to embed electrodes within a layer. Such embedded electrodes have similar drawbacks as described for thick electrodes. 
     In at least one aspect, electrodes of a material system of the present disclosure are offset from one another. For example, as shown in  FIG. 5 , electrodes  506  and  508  are offset from one another by a distance (d). Offsetting the electrodes of material systems of the present disclosure reduces moisture effects because an electrical signal flows where the electrons have the least resistance. If the electrodes are not offset from one another, then the area under the reference electrode is shielded from absorbing electrolyte from moisture. As moisture content within a coating increases (e.g., in the cracks/crevices) during testing, the accuracy of electrical data is improved because of the relatively high dielectric constant of water and saline as compared to the dielectric constant of most intact coatings. Preferably, the electrodes themselves are protected from moisture or the electrical signal may be inaccurate. Protecting an electrode from moisture may be accomplished by sealing an electrode with a protective coating, such as a non-conductive epoxy. 
       FIG. 6  is a side view of a material system  600 , according to an aspect of the present disclosure. As shown in  FIG. 6 , material system  600  is a multilayered material system comprising metal substrate  502 , a first coating layer  602 , and a second coating layer  504 . Electrode  608  (which can be of an electrode pair) is disposed on metal substrate  502  and is in electrical communication with a spectrometer, such as spectrometer  164 , via electrical line  610 . Furthermore, electrode  604  (which can be of an electrode pair) is disposed on first coating layer  602  and is in electrical communication with a spectrometer, such as spectrometer  164 , via electrical line  606 . A protective coating (not shown), such as coating  412 , can be disposed on one or both of electrodes  608  and  604  before depositing a subsequent coating layer onto the electrodes and substrate. As shown in  FIG. 6 , electrodes  608  and  604  are internal to (e.g., embedded) the material system. Internal electrodes provide in situ electrochemical monitoring of individual layers of a material system at a coating/substrate interface of a multilayered material system to determine corrosion. In at least one aspect, an insulating adhesive, such as non-conductive epoxy, is disposed between electrode  608  and metal substrate  502 . 
     As shown in  FIG. 6 , electrode  608  and electrode  508  are offset by a distance (d 1 ). Electrode  508  and  506  are offset by a distance (d 2 ). Electrode  506  and electrode  604  are offset by a distance (d 3 ). (d 1 ), (d 2 ), and (d 3 ) are sized to prevent polarizing an electrode, which would otherwise move away from the pseudo-linear portion of a voltage-current response curve. In at least one aspect, (d 1 )=(d 2 )=(d 3 ). In at least one aspect, (d 1 ), (d 2 ), and/or (d 3 ) is between about 0.3 cm and about 10 cm, such as between about 0.5 cm and about 3 cm, for example about 1 cm. 
     Furthermore, varying the surface area of a surface of an electrode that contacts an underlying surface affects the electrochemical interaction of the electrode with the underlying surface. One way to take advantage of varying the surface area for a desired application is to vary the shape of one or more electrodes because, other parameters being equal, different shapes result in different surface areas of a contact surface of the electrode, as explained in more detail below. Electrodes of material systems of the present disclosure can have a variety of shapes. For example, an electrode of the present disclosure is square shaped. Alternatively, an electrode of the present disclosure has a shape selected from circular, star, rectangular, or polygonal, such as pentagonal, hexagonal, heptagonal, or octagonal. Furthermore, electrodes of the present disclosure may have one or more spokes extending (e.g., outwardly) from the shape. 
     An electrode of the present disclosure has a surface area (including spokes if present) that contacts an underlying layer (i.e., a contact surface area) that is suitable for a desired application. In at least one aspect, an electrode has a contact surface area from about 0.2 cm 2  to about 10 cm 2 , such as from about 0.5 cm 2  to about 5 cm 2 , such as from about 1 cm 2  to about 2 cm 2 . The overall shape, spokes, and surface area can affect electrochemical monitoring methods for a particular testing application of the present disclosure. 
     Each of  FIGS. 7A, 7B, and 7C  is a plan view of a material system according to an aspect of the present disclosure. As shown in  FIG. 7A , material system  500  (of  FIG. 5 ) comprises electrode pair  158  comprising electrodes  506  having a circular shape. Protective coating  412  is disposed on electrodes  506 . In at least one aspect, protective coating  412  is also disposed on electrical wire  166  (not shown) to further protect wire  166  during flexing and/or salt fog exposure. As shown in  FIG. 7B , material system  700  comprises electrode pair  706  comprising electrodes  702  having a rectangular shape. Each of electrodes  702  is disposed on material layer  704 . Protective coating  720  (shown as transparent for clarity) is disposed on electrodes  702 . In at least one aspect, protective coating  720  is also disposed on electrical wire  708  (not shown) to further protect wire  708  during flexing and/or salt fog exposure. Each of electrodes  702  can be in electrical communication with a spectrometer via electrical line  708 . As shown in  FIG. 7C , material system  710  comprises electrode pair  714  comprising electrodes  712  having a pentagonal shape. Each of electrodes  712  has five spokes  712   a  extending outwardly from the pentagonal shape of electrodes  712 . Each of electrodes  712  is disposed on material layer  718 . Protective coating  722  (shown as transparent for clarity) is disposed on electrodes  712 / 712   a . In at least one aspect, protective coating  722  is also disposed on electrical wire  716  (not shown) to further protect wire  708  during flexing and/or salt fog exposure. Electrodes  702  can be in electrical communication with a spectrometer via electrical line  716 . 
     Fabricating Material Systems 
     Fabricating a material system of the present disclosure can include lightly abrading an area of a coating layer that the electrode will be applied to. The abraded area can be cleaned with any suitable solvent and allowed to dry. Fabricating further includes disposing an electrode onto a coating layer, such as an abraded area of the coating layer. An end portion of insulation of an electrical wire, such as wire  166 , can be removed to form an exposed portion of the electrical wire. The exposed portion is then contacted with an electrode, followed by application of non-conductive tape and/or a protective coating, such as protective coating  412 . 
     Electrodes (and coating layers) of the present disclosure may be disposed on a metal substrate or layer by any suitable deposition process. Deposition processes include screen printing and 3D printing. In addition, photolithography may be applied to a coating layer followed by deposition of an electrode into the photolithographed region of the layer. 
     An electrode, for example, may be deposited using any suitable screen printing apparatus supplied, for example, by ASM Assembly Systems of Munich, Germany. Screen printing can be performed using a screen having one or more openings shaped with the desired geometry for electrode formation. A deposition material may be placed onto a portion of the screen and then squeegeed across the opening with a squeegee. More specifically, the screen is located over and just above the surface to be printed so that ink can be accurately deposited in the desired position. The mesh of the screen is brought into contact with the surface by the squeegee as it is moved across the screen. Ink is pushed into the open area forming the pattern and the surplus is removed by the edge of the squeegee. The mesh should peel away from the surface immediately behind the squeegee, leaving all the ink that was in the mesh deposited on the printing surface. The screen can then be lifted clear. The recommended screen tension is the tension necessary to stretch the mesh sufficiently to cause the screen to peel away from the substrate after printing but not be stretched so much that damage occurs. The applied tension depends on the screen material, e.g. the extension used for nylon meshes is typically 6% and for polyester 3%. It is normal practice for the squeegee to be held at a 45° angle relative to the frame area. 
     An electrode, for example, may be deposited using any suitable 3D printing apparatus supplied, for example, by nScrypt, Inc. of Orlando, Fla. The nScript apparatus dispenses a conductive ink, e.g. DuPont CB230 silver-coated copper conductive ink or DuPont CB028 flexible silver ink, at a material flow rate that is adjusted by backpressure on the nozzle. The speed of the nozzle movement while patterning is constant, and the backpressure of the material in the nozzle is directly proportional to the flow rate. The nScrypt printing apparatus has a range of backpressures from 0 psi to about 30 psi. For the deposition of conductive ink onto coated panels, 18 psi backpressure can be used, which corresponds to a flow rate of about 0.052 grams/minute. After deposition of electrodes with the nScript apparatus, ink is baked for a fixed time at an elevated temperature to facilitate curing, e.g. 170° C. for 30 minutes. 
     A coating layer, for example, may be photolithographed using any suitable photolithography apparatus. Electrodes formed by photolithography are typically interdigitated electrodes. 
     Suitable interdigitated electrodes can be obtained from, for example, Synkera Technologies, Inc. of Longmont, Colo. or Micrux Technologies, S.L. of Oviedo, Spain. 
     Testing Methods 
     A material testing process such as a cyclic flexing fog spray process, for example, within apparatus  100 , may be performed by exposing a material system, such as a panel, to a treating liquid, such as a salt fog, and flexing the material system. The exposing may be performed for from about 1 hour to about 4500 hours, such as about 200 hours to about 2000 hours, such as about 500 hours to about 1000 hours. Exposing a material system to a treating liquid for about 1 hour mimics, for example, salt fog exposure experienced by the material system as part of an aircraft in an arid climate. Exposing a material system to a treating liquid for about 4500 hours mimics, for example, salt fog exposure experienced by the material system as part of an aircraft in a very humid climate or a moderately humid climate for a prolonged period of time. The liquid may contain water that is reagent grade water. The liquid may be a salt solution. The salt solution may comprise sodium chloride. The salt solution may contain about 2 parts sodium chloride in 98 parts water to about 6 parts sodium chloride in 94 parts water, such as about 5 parts sodium chloride in about 95 parts water. The liquid, such as a salt solution, may contain less than about 0.1% of bromide, fluoride and iodide. The liquid, such as a salt solution, may contain less than about 1 ppm, such as about 0.3 ppm, by mass of copper. The liquid, such as a salt solution, might not contain anti-caking agents, as such agents may act as corrosion inhibitors. Material systems which may be tested include, for example, aircraft panels which may form the skins or fuselage of an aircraft, a coated lap joint between two metal panels, a wing-to-fuselage assembly, and combinations thereof. The liquid may be atomized to form the treating liquid, such as a salt fog, that may have a pH ranging from about 3 to about 11, such as about 5 to about 8, such as about 6.5 to about 7.2. pH may be measured using a suitable glass pH-sensing electrode, reference electrode, and pH meter system. It may be desirable to adjust the pH of the treating liquid. For example, a treating liquid having a low pH may mimic a polluted atmosphere containing acid rain and the like. Furthermore, pH of the liquid that is atomized into the treating liquid may be adjusted to recalibrate the liquid during an exposing process. pH may be adjusted by, for example, addition of hydrochloric acid (HCl) to decrease the pH or addition of sodium hydroxide (NaOH) to increase the pH. The liquid, such as a salt fog, may be flowed at a rate of about 0.5 milliliters per hour (mL/h) to about 5 mL/h per 80 cm 2  of horizontal collection area, such as about 1 mL/h to about 2 mL/h per 80 cm 2  of horizontal collection area. In at least one aspect, a material system, such as a panel, may be flexed by a fixture support using one of jaws  124   a - e  or by a plurality of jaws  124   a - e . Flexing may be performed at varying frequencies to mimic the effect of mechanical stresses for corrosive conditions experienced by an aircraft material system under real world conditions. For example, a material system may be flexed at a frequency from about 0.1 Hertz (Hz) to about 150 Hz, about 0.1 Hz to about 100 Hz, about 0.1 Hz to about 60 Hz. Furthermore, the greater the curvature of a flexed material system, the greater the degradation to the material system using apparatus and methods of the present disclosure. For example, a flat panel having a length of 6 inches may be gripped by two jaws with a distance of 6 inches between the two jaws. The panel may be flexed at a rate of 0.33 Hz during exposure to a salt fog solution. In another example, a flat panel having a length of 7.5 inches may be gripped by two jaws also having a distance of 6 inches between the two jaws. The panel may be flexed at a rate of 0.33 Hz during exposure to a salt fog solution. The panel having a length of 7.5 inches has an increased curvature and undergoes increased degradation as compared to the panel having a length of 6 inches under otherwise identical conditions. Without being bound by theory, mechanical stresses that give curvature to a material system result in cracking of the material system which permits access of corrosive fluid, such as a salt fog, into a crack of the material system. After entering a crack of the material system, corrosive fluid may further enter between various additional layers (such as an underlying coating layer), if present. Accordingly, corrosive fluid may cause corrosion of the material system and/or one or more of the additional layers of the material system. Such conditions mimic the conditions experienced by an aircraft material system, such as a panel, during real world use. 
     In at least one aspect, an exposure zone, such as an enclosure  160  of apparatus  100 , may be maintained at a temperature ranging from about −196° C. to about 100° C., −50° C. to about 95° C., 0° C. to about 50° C., such as about 33° C. to about 37° C., for example about 35° C., during the exposing of a material system to a treating liquid (such as a salt solution atomized into a salt fog), and/or the flexing the material system. The temperature may be monitored by a recording device or by a thermometer (not shown) that can be read from an outside surface of apparatus  100 . In at least one aspect, exposing a material system, such as a panel, to a liquid, such as a salt fog, and flexing the material system may be performed concurrently. In at least one aspect, exposing a material system, such as a panel, to a liquid, such as a salt fog, and flexing the material system may be performed sequentially. In at least one aspect, a material system may be exposed to a salt fog and flexed concurrently as well as sequentially, which provides recreation of an irregular or variable flight-specific strain profile that may be experienced by a material system in service. In at least one aspect, exposing a material system to a liquid and/or flexing the material system may be interrupted to visually inspect, rearrange, or remove the material system, and/or replenish a solution, such as a solution in liquid reservoir  104 . 
     Before, during (in situ), and/or after flexing and spraying, the impedance of one or more layers of the material system can be measured using an electrochemical impedance spectrometer. Electrochemical impedance spectroscopy (EIS) provides in situ measurements of impedance of one or more layers of the material system. The measurements provide information for determining coating properties, such as coating degradation, corrosion at the substrate/coating interface, and absorbed moisture over a period of time. Electrochemical impedance spectroscopic processes of the present disclosure can be performed at an excitation potential of from about 5 mV to about 150 mV, such as about 10 mV to about 20 mV. Electrical frequencies for EIS may be from about 0.1 Hz-10,000 Hz, such as from about 1 Hz to about 5,000 Hz, such as from about 1 Hz to about 100 Hz, such as about 0.01 Hz to about 10 Hz, or from about 100 Hz to about 4,000 Hz. In at least one aspect, EIS is performed continuously at a set interval and fixed frequency from about 0.5 Hz to about 100 Hz, such as from about 1 Hz to about 10 Hz. 
     In the following examples, a material system measuring 3.75 inches wide by 14.5 inches long was secured by two jaws in a fixture support in the device described in  FIG. 1 . While flexing the panel at about 1 Hz, the panel was exposed to a sodium chloride salt fog (pH 6.8) for several days. 
     Example 1: Material System Having Conventional Interdigitated Electrodes 
     Using conventional interdigitated electrodes, resistance is typically measured between the metal interdigitated electrode and underlying substrate as they corrode. In that case, thicker electrodes work better because the electrode is corroded during the process. If the electrode is too thin, the electrode will corrode away over time during testing. 
     For this example, interdigitated electrodes were secured to a top surface and bottom surface of a coated aluminum coupon using double sided tape. The electrodes were masked with plater&#39;s tape. A chromated primer coating was applied to the coupon (including the electrode-areas). The plater&#39;s tape was removed once the coating had cured. Wires were soldered to the electrodes. The electrodes and wire were then insulated with a 2-part epoxy and allowed time to cure to form the completed material system. Corrosion testing was performed within a cyclic corrosion chamber as described above using ASTM B117. EIS was performed at 150 mV excitation potential, 10 Hz-10,000 Hz frequency range, and was performed continuously at a set interval and fixed frequency of 1 Hz. 
       FIG. 8  is a graph of impedance data of a material system comprising interdigitated electrodes of Example 1. The material system was exposed to salt fog for 5 days. As shown in  FIG. 8 , impedance decreases over time upon moisture ingress into the material system. It was observed that the interdigitated electrodes cause a defect zone in the coating. Furthermore, the values of the data observed are indicative of the actual impedance of only the coating disposed over the electrodes, area surrounding the electrodes and coupon (due to the application of double sided tape for adhering the electrodes to the coupon). 
     Example 2: Material System Having Thin Circular Electrodes Formed by a Dispensing 
     Gun 
     A bottom and top surface of a coated aluminum coupon was lightly abraded, and cleaned with a solvent and allowed to dry. Electrodes were formed on the abraded areas using a conductive silver epoxy to a thickness of less than about 12 μm that was applied to a surface using a dispensing gun. Each electrode had a contact surface area of 1 cm 2  and a circular shape. Approximately 0.5 cm of insulation was stripped from a thin gauge wire, and the wire was taped to the coupon so that the exposed piece of wire was laying flat across the electrode application site. A 2-part waterproof epoxy was used as a protective coating to electrically and physically isolate the electrodes from the environment to form the completed material system. Corrosion testing was performed within a cyclic corrosion chamber as described above using ASTM B117. EIS was performed at 150 mV excitation potential, 10 Hz-10,000 Hz frequency range, and was performed continuously at a set interval and fixed frequency of 1 Hz. 
       FIG. 9  is a graph of impedance data of a material system comprising thin electrodes of Example 2, according to an aspect of the present disclosure. The material system was exposed to salt fog for 4 days. As shown in  FIG. 9 , impedance decreases over time upon moisture ingress into the material system. Unlike coupons having conventional interdigitated electrodes, the electrodes did not release from the coupon surface in the absence of adhesive during testing and did not substantially corrode during testing. 
     Example 3: Scribed Material System Having Thin Circular Electrodes Formed by a 
     Dispensing Gun 
     A bottom and top surface of a scribed coated aluminum coupon was lightly abraded and cleaned with a solvent and allowed to dry. Electrodes were formed on the abraded areas to a thickness of less than about 12 μm using a conductive silver epoxy that was applied to a surface using a dispensing gun. Each electrode had a contact surface area of 1 cm 2  and a circular shape. Approximately 0.5 cm of insulation was stripped from a thin gauge wire, and the wire was taped to the coupon so that the exposed piece of wire was laying flat across the electrode application site. A 2-part waterproof epoxy was used as a protective coating to electrically and physically isolate the electrodes from the environment to form the completed material system. Corrosion testing was performed within a cyclic corrosion chamber as described above using ASTM B117. EIS was performed at 150 mV excitation potential, 0.1 Hz-10,000 Hz frequency range, and was performed continuously at a set interval and fixed frequency of 1 Hz. 
       FIG. 10  is a graph of impedance data of a scribed material system comprising thin electrodes of Example 3, according to an aspect of the present disclosure. The material system was exposed to salt fog for 4 days. As shown in  FIG. 10 , impedance decreases over time upon moisture ingress into the material system. Unlike coupons having conventional interdigitated electrodes, the electrodes did not release from the coupon surface in the absence of adhesive during testing and did not substantially corrode during testing. 
     Example 4: Material System Having Embedded Thin Rectangular Electrodes Formed by Screen Printing 
     Rectangular electrodes were screen printed to a thickness of less than about 12 μm onto a top surface and bottom surface of an anodized and painted aluminum coupon. The electrodes were fabricated from Ag-530 ink manufactured by Conductive Compounds. Wire leads were soldered to the electrodes, and the electrodes and wire leads were sealed with non-conductive epoxy. A boric sulfuric acid anodized (BSAA) primer coating was applied to the coupon (including the electrode areas) to form the completed material system. Corrosion testing was performed within a cyclic corrosion chamber as described above using ASTM B117. EIS was performed at 150 mV excitation potential, 10 Hz-10,000 Hz frequency range, and was performed continuously at a set interval and fixed frequency of 1 Hz. 
       FIG. 11  is a graph of impedance data of a material system comprising thin electrodes of Example 4, according to an aspect of the present disclosure. The material system was exposed to salt fog for 5 days. Flat cell (FC) measurements and sensor (S) measurements were performed. Flat cell measurements involve the use of a specialized electrochemical cell filled with salt solution. This creates a “bulk electrolyte” on top of the coating. With the embedded sensor measurements, we are conducting EIS in a salt spray chamber which mimics atmospheric exposures. Salt fog leaves thin films of electrolyte on the coated surface. Thin films and bulk electrolytes have different diffusion properties, which can impact corrosion kinetics and absorption of moisture into the coatings. This can cause slight differences in the EIS spectrum. Thus, the EIS spectra from the sensors are compared to those in a flat cell because flat cell techniques are standardized. The feasibility and novelty of performing EIS is demonstrated with embedded electrodes by showing that these sensors give nearly-equivalent data as a flat cell experiment without having to use a specialized test cell or take the articles out of the chamber for analysis. 
     As shown in  FIG. 11 , impedance decreases over time upon moisture ingress into the material system. Unlike coupons having conventional interdigitated electrodes, a defect zone in the coating was not formed (1) in a material system having thin electrodes formed by screen printing and (2) in the absence of adhesive between the electrodes and the coupon surface. 
     Example 5: Material System Having Thin Electrodes Formed by 3D Printing 
     A chromated primer coating was applied to a coupon, followed by an epoxy coating deposited on the chromated primer. The epoxy coated aluminum coupon was not abraded, and two electrodes were disposed onto a top surface of a coupon and two electrodes were disposed onto a bottom surface of the coupon, each electrode disposed to a thickness of less than about 12 μm using an nScrypt 3D printer. Electrode material was Dupont CB230 ink, which is a silver coated copper conductive material. Wire leads were soldered to the electrodes. The electrodes and leads were sealed with non-conductive epoxy to form the completed material system. Corrosion testing was performed within a cyclic corrosion chamber as described above using ASTM B117. EIS was performed at 150 mV excitation potential, 10 Hz-10,000 Hz frequency range, and was performed continuously at a set interval and fixed frequency of 1 Hz. 
       FIG. 12  is a graph of impedance data of a material system comprising thin electrodes of Example 5, according to an aspect of the present disclosure. The material system was exposed to salt fog for 5 days. Flat cell (C4 FC) measurements and sensor (C4 S) measurements were performed. As shown in  FIG. 12 , impedance decreases over time upon moisture ingress into the material system. Unlike coupons having conventional interdigitated electrodes, the electrodes did not release from the coupon surface in the absence of adhesive during testing and did not substantially corrode during testing. Unlike coupons having conventional interdigitated electrodes, a defect zone in the coating was not formed (1) in a material system having thin electrodes formed by 3D printing and (2) in the absence of adhesive between the electrodes and the coupon surface. 
     Material systems, apparatus and methods of the present disclosure provide a controlled salt fog environment and monitoring of material performance, such as corrosion, on a variety of material systems, such as aircraft material systems, such as panels, coated lap joints between two or more panels, wing-to-fuselage assemblies, or combinations thereof. Material systems, apparatus and methods of the present disclosure provide an ability to replicate in-service, real-world failure modes and mechanisms in a controlled exposure environment. 
     Mechanical flexing of a material system in an apparatus of the present disclosure may result in increased corrosion of a material system. The compounding effects of mechanical and chemical stresses combine to induce corrosion, which more accurately replicates corrosion experienced by a material system, such as an aircraft panel, in a real-world environment. Accordingly, material systems, methods and apparatus of the present disclosure more accurately simulate the corrosion observed with aircraft material systems during real-world use of the aircraft. Material systems, methods and apparatus of the present disclosure allow for testing corrosion of stand-alone material systems and the interfaces between coating layers, which more accurately represents the corrosion experienced by material systems, such as panels, during actual use of the material systems as part of an aircraft. Material systems, methods and apparatus of the present disclosure further provide re-creation of irregular flight-specific strain profiles so that improved predictive as well as forensic investigations of aircraft material systems may be performed. 
     Material systems, methods and apparatus of the present disclosure provide electrochemical monitoring of a coating during outdoor exposure, accelerated testing in an environmental chamber, and electrochemical monitoring of material systems while in use (e.g, in situ). In situ electrochemical monitoring provides assessment of the integrity of a material system without visual inspection of the material system and does not require stoppage of a flexing and/or salt fog exposure of the material system. Material systems of the present disclosure further provide thin electrodes which can be located on an outer surface of a material system or embedded within the material system (e.g., disposed between two layers). Such material systems reduce or eliminate defect zones in a coating disposed on the electrode-area of the material system. Furthermore, material systems of the present disclosure further provide reduced or eliminated adhesive use between the electrode and an underlying substrate which provides accurate electrochemical data from a spectrometer during testing. Material systems of the present disclosure further provide electrodes with reduced or eliminated corrosion during testing. Material systems of the present disclosure further provide electrodes that can be shaped to provide a controllable contact surface area for desired electrochemical applications. 
     While the foregoing is directed to aspects of the present disclosure, other and further aspects of the present disclosure may be devised without departing from the basic scope thereof. Furthermore, while the foregoing is directed to material systems, such as aircraft material systems, such as panels, coated lap joints between two or more panels, and wing-to-fuselage assemblies, aspects of the present disclosure may be directed to other material systems not associated with an aircraft, such as a multicomponent material system used in aerospace, automotive, marine, energy industry, and the like.