Corrosion is a major concern at the Kennedy Space Center in Florida due to the proximity of the center to the Atlantic Ocean and to salt water lagoons. High humidity, salt fogs, and ocean breezes, provide an ideal environment in which painted steel structures become corroded. Maintenance of painted steel structures is a never-ending process

Bastin, Gary L.

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2011 PRC! Pipeline Program Research Exchange Meeting 
Detecting Corrosion under Paint and Insulation 
By Gary L. Bastin, ASRC Aerospace, Kennedy Space Center Florida 
Detection of corrosion through layers of paint, while 
also pinpointing where corrosion is present using 
millimeter wavelength signals at 20 GHz. 
Corrosion is a major concern at the Kennedy Space 
Center in Florida due to the proximity of the center to 
the Atlantic Ocean and to salt water lagoons. High 
humidity, salt fogs, and ocean breezes provide an ideal 
environment in which painted steel structures become 
corroded. Maintenance of painted steel structures is a 
never-ending process. 
Historically, steel launch towers and related ground 
support equipment were scheduled for sandblasting 
every few years, to assess whether or not corrosion 
was forming under paint. This has been an expensive 
process, due both to the labor involved in assessing the 
degree of corrosion, as well as the down-time incurred 
while sandblasting and repainting structures. 
Recent developments in NASA's laboratories have led 
to the non-destructive evaluation and detection of such 
corrosion under paint, based upon a new technology 
for scanning and detecting such corrosion. This 
technology holds considerable promise for eliminating 
repetitive sandblasting and re-painting of ground 
support structures, while providing indications when it 
is actually time to sandblast and repaint such 
structures. 
The new technique relies on using two millimeter 
wavelength signals. An early laboratory prototype is 
shown here, and has been successfully used for 
assessing corrosion under paint and insulation non-
destructively in a laboratory environment. 
In operation, two signal generators provide two 
unequal level millimeter wavelength signals. On the 
output of one of these generators is an RF Isolator, that 
prevents intermodulation products from being 
produced within the output circuits of the two signal 
generators. The two millimeter wavelength signals are 
then combined in a 2-way power combiner and are fed 
into a dual-directional coupler. The through path 
output from the coupler, consisting of two signals at 
slightly different frequencies and signal levels, are fed 
into a dielectric lens antenna. The two signals are then 
focused onto the surface of the object being evaluated 
for corrosion using a dielectric lens antenna. 
In the presence of non-oxidized metal surfaces, only 
two incident signals are reflected back to the dielectric 
lens antenna, and into the dual directional coupler. 
The reverse direction sampled port of the coupler is 
then fed into a spectrum analyzer for further analysis. 
NDE/I CUP Test Set Laboratory Prototype with 
painted steel test exemplar 
However, in the presence of metallic oxides, that is, 
corrosion, a third signal is also produced. It is this 
third signal that provides the positive indication 
needed for detecting corrosion under paint. 
The fundamental theory of operation relies on the fact 
that metallic oxide layers provide a non-linear junction 
to incident millimeter wavelength signals. Since the 
non-linear junction is inherent as a bulk property of 
metallic oxide layers, .the presence of overlying 
insulation or other materials, such as pollen and dust, 
that do not themselves contain metallic oxides, causes 
no effect on the corrosion detection measurement. 
However, the presence of metallic oxide layers on the 
surface of a painted metal structure being evaluated 
provides the third signal that provides a positive 
indication in the presence of hidden corrosion. 
Three sets of test panels were used for testing the 
laboratory prototype. The first set of panels, painted 
white, were fabricated from galvanized steel panels, 
12" x 18" sheet metal, 26 gauge Z. Corrosion, 
resulting from exposure for 3 months to a sodium 
chloride and water solution, was present under the 
painted white surface. 
2011 PRCL Georgia Tech University Conference Center 
01-03 February 2011 
https://ntrs.nasa.gov/search.jsp?R=20110008110 2019-08-30T14:50:14+00:00Z
2011 PRC! Pipeline Program Research Exchange Meeting 
On the second set of panels, two sub-sets of panels 
with slight and nearly identical corrosion patterns to 
each other were created: One sub-set was cleaned 
before a protective coating system was applied, with 
only the corrosion pits remaining. The other sub-set 
was completely coated with an aluminum oxidation 
product. The aluminum test panels were constructed of 
0.063-inch-thick 2024 aluminum coated with Super 
Koropon® fluid-resistant interior primer standard, a 
common space flight vehicle structural configuration. 
One of the sub-set of aluminum panels is shown 
below, with the distinctive green Koropon® primer. 
NDE/I CUP Test Set Detecting Corrosion for 
Aluminum Panel 
Operational performance includes and surpasses the 
0.008-inches thick original paint thickness 
requirement, surpassing 12-inches of paint or 
insulation thickness, enabling unanticipated use of the 
new corrosion-detecting technique for use in situations 
beyond those originally desired such as for detecting 
corrosion under thick blankets of insulation for pipes 
and tubes, such as for cryogenic applications. 
Work is currently underway at Kennedy Space Center 
to miniaturize the technology. 
Positive indications of corrosion were detected for 
both steel and aluminum painted panels. An 
operational, proof-of-concept, CUP Test Set capable 
of detection and localization of corrosion under paint 
for painted steel and aluminum materials in laboratory 
environments has been built. 
Prior-year activities focused on development of the 
concepts necessary for developing an actual 
implementation. Subsequent work has taken this 
prior-year conceptual work into a real implementation, 
resulting in an operational, proof-of-concept, 
laboratory CUP Test Set. 
Measuring 3rd Order Passive Intermodulation 
Products (at F3) for Painted Steel with Corrosion 
under Paint 
;:tl 
: 
Measuring no 3rd Order Passive Intermodulation 
Products (at F3) for Painted Steel without Corrosion 
under Paint 
The theory of operation of this technology relies on 
Passive Intermodulation Products, which are a well-
known issue arising with multi-frequency 
communications systems. 1 When they arise, they 
often cause adjacent channel interference for tower-
mounted arrays of antennas supporting multiple 
communications services. 
I 
Lui, P.L., Passive intermodulation interference in communication 
systems, IEEE Electronics & Communication Engineering Journal, 
Vol. 2, No.3 , pp.109-11 8, Jun 1990. 
2011 PRCL Georgia Tech University Conference Center 
01-03 February 2011 
2011 PRC! Pipeline Program Research Exchange Meeting 
t i r H r i t 
F3 F1 F2 FA FB Fe 
Note the equal spacing, 8 , between the two input 
tones F I and F 2, as well as between each adjacent pair 
of tones, which collectively constitute all ofthe 
intermodulation products. That is to say, we can 
define the equal spacing between all tones as being the 
spacing of the two input tones, or of any other pair of 
adjacent tones, as: 
Inband third order passive intermodulation products 
are of the form: 
2F; -F; =F2 +(F2 -F;)=F2 +8 
2F; - F2 = F; - (F2 - F;) = F; - 8 
Now, consider the inband products at 
FB = F2 + 28 and at F; - 28. What products 
are these? 
They are the fifth order products, for: 
and 
F2 +28=(F; +8)+8 
=(F2 +(F2 -F;))+8 
=2F2 -F; +8 
= 2F2 - F; + F2 - F; 
=3F2 -2F; 
F; - 28 = F; - 2(F2 - F;) 
= F; - 2F2 + 2F; 
=3F; -2F2 
and, since the coefficients 3 and 2 sum to 5, in each of 
the two equations, this means these are fifth order 
products. 
Similarly, the product at Fe = F2 + 38 is one of 
the Seventh Order products, for: 
F2 + 3(F2 - F;) = F; + 3F; 3F; 
=4F2+3F; 
and the coefficients 4 and 3 add to 7, indicating that 
this is one of the 7th Order Passive Intermodulation 
Products. 
The hypothesis is that the products in the figure, seen 
with equal spacing, spaced alongside F I and F2, are the 
spectral indications of the odd-order Passive 
Intermodulation Products. 
To prove this hypothesis is true, consider the general 
case: 
F2 +n8=F2 +n(F2 -F;) 
= (n+l)F2 -nF; 
The Order of an intermodulation product is defined as 
the sum of the coefficients, that is: 
Order = (n + 1) + n = 2n + 1 
But, this value, of 2n + 1 is odd for all n 2:: I! SO, 
it is true that the products on the right hand side of the 
figure, equally spaced to the right ofF2, are only the 
odd products. 
Likewise, for the equally spaced tones on the left hand 
side of the figure, representing the lower frequency 
products: 
F; -n8=F; -n(F2 -F;) 
= F; -nF2 +nF; 
= (n+l)F; -nF2 
and, again, we define the Order of the product as the 
sum of the coefficients, that is: 
Order = (n + 1) + n = 2n + 1 
But, this value, of 2n + 1 is odd for all n ::; -1 ! 
So the hypothesis is true for the products on the left 
hand side ofthe FI and F2 pair that were shown 
previously in the figure. Since the hypothesis is true 
2011 PReL Georgia Tech University Conference Center 
01-03 February 2011 
2011 PRC] Pipeline Program Research Exchange Meeting 
for all the products to the right of the Fl and F2 pair, 
and is additionally true for all intermodulation 
products appearing to the left of the Fl and F2 pair, it is 
true across the entire spectrum. Hence, all the evenly-
spaced products in the spectral analyzer display, 
besides just the F 1 and F2 pair, are odd-order 
intermodulation products. 
The only passive intermodulation products seen in a 
narrowband system are odd-order products. They are 
also the only intermodulation products that matter for 
detecting corrosion using the technique described in 
this paper. In practice, only one of the two 3rd Order 
products need even be considered for the case of 
employing two-unequal level input tones for 
implementing an F 1 and F2 scanning signal pair of 
tones. Using unequal scanning tones is advantageous 
because although it causes one ofthe two 3rd Order 
Products to be of considerably lesser magnitude than 
the other 3rd Order Product, more importantly, the use 
of unequal tones significantly reduces the power 
consumption necessary for generating a response in 
the presence of corrosion by roughly Yz, since but one 
large signal interacting with a much smaller signal will 
suffice to generate the single 3rd Order Product 
necessary for detecting corrosion under paint. With 
power efficiencies running only a few percent to 
generate millimeter wavelength signals, eliminating 
the need to generate but one tone instead of two is a 
major advantage. 
Fortunately, the use of unequal tones does not change 
the spectral relationships in the frequency domain ; it 
only changes the magnitudes of the two odd order 
products of the same order, with the largest, most 
detectable signal, being found on the side where the 
largest of the F 1 and F 2 pair resides. Hence, for setting 
F], the lower frequency tone to, say, 15 dB above the 
power level ofF2, the only 3rd Order Product likely to 
be visible on the spectrum analyzer display will be on 
the lower frequency side of the F 1 and F 2 pair, offset 
by the same spacing, is , as exists between the F 1 and 
F2 pair, at F3• (There is another 3rd Order 
intermodulation product that exists on the high side, 
under the noise floor of the spectrum analyzer 
typically, unless resolution and video bandwidths are 
both reduced to very small values, and the sweep time 
is increased to a very long time period) 
The actual microscopic physics principles responsible 
for the presence of such non-linear passive 
intermodulation products are complex and poorly 
understood. Nonetheless, there have been attempts 
documented in the literature to understand the 
phenomenon and all of the following mechanisms are 
believed responsible for the generation of passive 
intermodulation products: 1 
1.) Electron tunneling and semiconductor 
action through thin oxide layers separating 
conductors at metallic contacts 
2.) Micro-discharges between micro-cracks 
and across voids in metals 
3.) Nonlinearities associated with dirt and 
metal particles on metal surfaces 
4.) High current densities at contacts 
5.) Nonlinear resistivity of carbon fibers 
6.) Nonlinear hysteresis effects in 
ferromagnetic materials 
For steel surfaces, all but number 5 are clearly at play 
for creating the passive intermodulation products 
needed for detecting corrosion under painted surfaces, 
as the carbon and iron that together collectively 
become steel are typically not arranged as fibers but as 
tiny domain areas when examined in cross-section -
however, on a small enough scale, even these tiny 
domains often do exhibit slight fibrous resemblances, 
hence, even number 5 could be slightly at play for 
steel materials. 
For aluminum surfaces, all but numbers 5 and 6 are at 
play, as there is typically no carbon, but only silicon, 
arsenic, and other elements in tiny amounts within 
commercially-produced aluminum. In either case, 
numerous mechanisms are available for generation of 
passive intermodulation products. Whatever the 
cause, it is the presence of these numerous 
mechanisms as well as perhaps other mechanisms that 
enable the detection of corrosion under paint, which, 
in its most simplest form, is often a manifestation of 
the presence of oxide (corrosion) layers on aluminum 
or steel. Likewise, the presence of corrosion on other 
metals could also presumably be detected, although 
the power levels required for detecting corrosion with 
other metals and metallic alloys might require higher 
power levels than has been found to be needed for 
detecting corrosion under paint for steel and 
aluminum. 
Contact: Dr. Gary L. Bastin, ASRC Aerospace, 
Mailstop: ASRC-48, Kennedy Space Center, FL 
32899 (Gary.L.Bastin@nasa.gov, (321) 867-9275) 
http://www.ustdc.com This ongoing research is 
sponsored by NA SA 's Kennedy Space Center. 
2011 PRCL Georgia Tech University Conference Center 
01-03 February 2011 
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Detecting Corrosion Under Paint and Insulation

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