Corrosion detecting and monitoring method and apparatus

A nondestructive method and apparatus for optical detection and monitoring corrosion in structures normally inaccessible to light and observation. An optical fiber coated with a corrosion sensitive compound is embedded in the structure. Tapped Bragg gratings of different Bragg periods are spaced along the fiber and refract a narrow bandwidth component of a broad beam light pulse transmitted through the fiber. Due to corrosion, the refracted components are reflected by the compound and their amplitudes are detected and displayed for each narrow bandwidth.

BACKGROUND OF THE INVENTION 
The present invention relates generally to corrosion and material damage 
detection; and more particularly to a nondestructive method and apparatus 
for optical detection and monitoring corrosion and material degradation in 
metal and composite structures which are generally inaccessible to light 
and visual inspection. 
A heavy toll on material and maintenance costs on military as well as 
commercial aircraft can be attributed to the severity of the environment 
in which they operate. Under the influence of environmental effects or 
corrosion, susceptibility to stress-corrosion cracking and corrosion 
fatigue of critical aircraft structural materials (steels and aluminum) 
increase by a factor greater than ten and significantly reduce the useful 
life of aircraft. Even newer aircraft which use advanced materials such as 
graphite/epoxy composites are susceptible to such effects. Frequently, 
they are detected too late for any simple measure to be taken to repair 
damaged parts. Occasionally, if the corrosion or environmental effects 
were not discovered in time, the results could be catastrophic. 
As current fleets of aircraft age without new aircraft entering a fleet 
inventory, the degrading effects of corrosion become more critical in 
terms of maintenance, readiness and safety. Flying aircraft near their 
expected useful life might actually be well beyond their safe life. Due to 
limited resources, some aircraft are not retired at their original 
expected lives but are reconditioned to fly beyond that time. 
Consequently, frequent inspections, preventive maintenance, and repairs 
require older aircraft to be removed periodically from service for costly 
and extended periods of time. In many cases, it is necessary to remove the 
aircraft's skin to access parts for inspection further adding to cost and 
down-time. 
All of these considerations indicate that early detection and 
quantification of corrosion is extremely important, especially for 
carrier-based Navy aircraft which are exposed at sea to extremely 
corrosive environments. 
Extensive studies in the area of corrosion detection and prevention have 
been carried out in the laboratory. In connection with these studies, 
electrochemical sensors (current) and optical (color) sensor for detecting 
early signs of corrosion have been investigated. 
Electrochemical sensors are either incorporated in coatings or installed in 
a structure to produce signals when there is corrosion or damage, and 
before the effects become too severe. The sensing elements are bimetallic 
galvanic ultrathin-film devices fabricated on a polymeric film to generate 
a current when exposed to moisture. 
Optical sensors, on the other hand, require reduction-oxidation (redox) 
chemicals which produce a change in an optical property such as a color or 
fluorescence when exposed to visible or ultraviolet light. However, the 
choice of inspection sites in structures for optically detecting corrosion 
is greatly limited because they must be accessible to both light and 
observation at the sensors. For instance, optical changes produced by 
redox reactions and/or corrosion in lap joints, under protective coatings 
or paint and on the backside of aircraft skin are particularly difficult 
to observe. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a 
nondestructive method and apparatus for optical detection and monitoring 
of corrosion and environmental effects in structures which are normally 
inaccessible to light and observation. 
Another object is to provide an optical sensor which will detect corrosion 
in lap joints, under paint and on the backside of the skin of an aircraft 
or like structure. 
Still another object is to provide an corrosion monitoring system in which 
a single optic fiber can be installed in critical aircraft structures to 
provide early detection and cumulative quantification of corrosion at 
multiple sites within the structure. 
A still further object is to provide a sensing element which is small, 
lightweight, immune to electromagnetic interference and corrosion, and 
which can be easily embedded in or surface mounted on a structure. 
Briefly, these and other objects and novel features of the invention are 
accomplished in a corrosion detection and monitoring system utilizing an 
optical fiber coated with a corrosion sensitive compound suitable for 
embedment in structure such as lap joints, under paint primers and 
topcoats, and on the backside of aircraft skin. Tapped Bragg gratings of 
different Bragg periods formed in the optical fiber at spaced intervals 
each tap off a unique "signature" having a narrow wavelength component of 
a broad beam light input pulse transmitted through the optical fiber. The 
tapped components scatter light into an optically sensitive compound at 
respective grating sites, and any change in a specified optical property 
of the coating, such as color or fluorescence, due to corrosion in the 
structure causes a fraction of the scattered light components to be 
reflected and returned by the gratings through the optic fiber in the 
opposite direction as the input signal. A two-way light coupler in the 
optical fiber diverts a portion of the reflected components for detection 
and display of the signature components from each grating as a function of 
intensity and cumulative corrosion. 
Other objects, advantages and novel features of the invention will become 
apparent from the following detailed description of the invention when 
considered in conjunction with the accompanying claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawings wherein like reference characters denote like 
or corresponding parts throughout the several views, FIG. 1 shows an 
optical corrosion detecting and monitoring system, indicated generally by 
the numeral 10, as applied to a steel lap joint M. A light source 12 
intermittently transmits a broad bandwidth light pulse L.sub.1 in an 
optical fiber 14, which passes the frequencies unaltered through a two-way 
optical coupler 16, such a 3 dB coupler, to a series of tapped Bragg 
gratings A, B, C, D, E and F formed at selected spaced intervals in a 
distal segment of fiber 14 for embedment at sites of interest between 
opposed members M.sub.1 and M.sub.2 of a typical lap joint M with 
fasteners 15. A light absorber 17 at the distal end of optical fiber 14 
prevents feedback or straying of any residual input signal. 
As shown in FIGS. 2 and 3, optic fiber 14 is coated with a layer of 
compound 18 of a color responsive reduction/oxidation (redox) chemical 
either neat, or micro-encapsulated into a sparingly soluble methyl 
cellulose polymer and then silanized to protect the capsule walls from 
interaction with the solvents used in the coating foundation. Compound 18 
is then completely surrounded by a non-continuous (perforated) metallic 
reflector 20 and finally the interstices are filled with a polyurethane to 
provide an environmental barrier coating 22. Compound 18 responds to 
corrosion due to changes in the environment and manifests itself as a 
change in color or fluorescence. The following table lists examples of 
redox color-responsive chemicals found suitable for detecting corrosion in 
alloys containing iron. 
TABLE I 
______________________________________ 
Redox 
Poten 
Indicator Compound Color Change tial 
______________________________________ 
1,10 Phenanthroline 
Red to Faint Blue 
1.06 
5-Nitro-1,10-Phenanthroline 
Red to Faint Blue 
1.25 
Ferrous Sulfate 
2,2-Bipyridyl Ferrous Sulfate 
Red to Faint Blue 
0.97 
Ruthenium Tripyridyl Nitrate 
Yellow-Colorless 
1.25 
Phenyl-2-Pyridyl Ketoxime Iron 
Red to Colorless 
N.A. 
______________________________________ 
The chemicals trigger a color response when in contact with ionic species 
Fe(II) which are the first ions produced during corrosion of steel. For 
instance, the transmission spectrum of Ferrorin (1,10 
Phenan-throline-iron) is a function of the concentration of iron ions. 
Small amounts of the ions in a solution of this type can substantially 
increase the absorption behavior of the solution. As a result of a 
reduction-oxidation (redox) reaction, a clear coloration change is 
manifested from colorless (faint blue) to intense red or purple. The 
maximum absorption for this complex is in a narrow band region and in the 
visible spectrum, i.e. approximately 520 nm wavelength. Thus, the 
intensity of the color produced decreases as a function of corrosion. 
A different corrosion-sensing scheme may be required for aluminum alloy 
structures because some of the chemicals do not form a colored complex 
with Al(III) ions. For this case, Columbia blue and fluorescein exhibit 
fluorescence chemicals when coupled with the aluminum ions exhibit 
fluorescence under ultraviolet light exposure. 
Optical fiber 14 comprises a core 14a with an outer cladding 14b. Both the 
core and cladding are preferably made of silica, but a germanium (Ge) 
dopant is added to the silica core 14a to provide a slightly larger index 
of refraction. The difference in refraction indices confines the light 
input pulse L.sub.1 to the core region. 
The segment of core 14a in FIG. 2 schematically shows tapped Bragg grating 
A, and is representative of the construction of the other gratings B-F. 
The grating is made by placing the optical fiber at an approximate angle 
of 45.degree. from the plane of an interference region of two orthagonal 
coherent light energy excimer laser beams thus forming a plurality of 
alternate planes of high and zero field intensities h and l slanted 
45.degree. from the axis of core 14a. The fields of high intensity h 
induce a change in the refraction index in core 14a due to small changes 
in the bonding properties of the Ge dopants present. As a result, periodic 
variations in the refraction indices are impressed along core 14a for each 
of gratings A-F. 
The slanted intensity fields h and l in the interference region of grating 
A give off a spectral component L.sub.a of input signal L.sub.1 
approximately 90.degree. from the axis of core 14a in a scattered fan-like 
pattern through cladding 14b and corrosion sensitive compound 18 to 
reflector 20. Depending upon the number of high intensity fields h in the 
interference region of each grating, the magnitude of change in refraction 
index from one grating to the next, and the spacing of the high intensity 
fields, a portion of spectral components L.sub.a -L.sub.f tapped at each 
grating will be specularly returned by reflector 20 through the grating to 
two-way coupler 16 at a discrete spectral or "signature" component. The 
amplitude of each component is an indication of the amount of corrosion 
present adjacent to the respective gratings in lap joint M. 
Reflector 20 may be omitted when compound 18 by itself provides sufficient 
corrosion response for detection. 
The untapped portion of input signal L.sub.1 represented by signal L.sub.2, 
continues through grating A, to grating B where a different spectral 
component L.sub.b is tapped off and returned as described for component 
L.sub.a. The remaining untapped portions of signal L.sub.2 pass through 
gratings C, D, E, and F, each time tapping off and returning a spectral 
component L.sub.c, L.sub.d, L.sub.e or L.sub.f. 
Each tapped Bragg grating in optical fiber 14 is made with a different 
Bragg period .LAMBDA..sub.B so that they have unique "signature" narrow 
band widths that can be interrogated with a single broadband input pulse. 
The wavelength of the light pulse for the maximum sensitivity can be 
optimized by choosing the appropriate Bragg grating wavelength. 
The number of tapped Bragg gratings placed on optical fiber 14 depends on 
the bandwidth of the input pulse and the bandwidth of each scattered 
spectral component. If the spectral bandwidth of the input pulse is 
sufficiently large compared to the bandwidth of the scattered spectral 
components, many unique "signature" Bragg gratings can be placed on a 
single optical fiber enabling interrogation of each individually. In the 
illustrated embodiment there are six gratings A-F each having a scattered 
spectral bandwidth of 200 nm, thereby requiring a broadband input pulse 
L.sub.1 of white light with a bandwidth of 1200 nm. Other factors 
considered when fabricating the gratings are described in a paper 
incorporated by reference herein by I. Perez, V. Agarwala and W. R. Scott 
entitled Bragg Grating Corrosion Sensor presented on Jul. 31, 1995 at the 
Proceedings of the 19th Progress in Quantitative Nondestructive 
Evaluation, Iowa State University, July-August 1994. 
The spectral components L.sub.a -L.sub.f tapped and reflected at each of 
gratings A-F are diverted by two-way optical coupler 16 to a spectrum 
analyzer 26 which transforms the light signals to an electrical output 
signal. The amplitude of each unique "signature" component from analyzer 
26 appears at a display 28 as a measure of the respective gratings. FIG. 4 
represents a typical display of three reflected narrow bandwidth 
components of each grating A-F measured at different intervals of time 
t.sub.1, t.sub.2 and t.sub.3. A chart recorder 30 connected to an output 
of analyzer 26 displays the cumulative corrosion at gratings A, B and C 
over the interval between times t.sub.1 and t.sub.3. 
Some of the many advantages and novel features of the invention should now 
be readily apparent. For example, the invention as herein described and 
claimed provides a nondestructive method and apparatus for detecting and 
monitoring the insidious effects of corrosion in structures which are 
normally inaccessible to observation. A single optic fiber can be 
installed in critical aircraft structure with tapped Bragg gratings 
located along the length of the fiber for detecting and measuring 
corrosion at multiple sites within the structure. The sensing element is 
very small, light weight, immune to electromagnetic interference and 
corrosion, and can be easily embedded or surface mounted on structure 
subject to corrosion. 
It will be understood, of course, that various changes in the details, 
materials, steps and arrangement of parts, which have been herein 
described and illustrated in order to explain the nature of the invention, 
may be made by those skilled in the art within the principle and scope of 
the invention as expressed in the appended claims.