Buhrow, J. W.

Calle, L. M.

Johnsey, M. N.

Jolley, S. T.

Li, W.

Pearman, B. P.

Zhang, X.

NASA Technical Reports Server

Recent Progress in the Development of a Multifunctional 
Smart Coating for Autonomous Corrosion Detection 
and Protection 
Research Team: W. Li, J. W. Buhrow, S. T. Jolley, X. Zhang, M. N. Johnsey, 
B. P. Pearman, and L. M. Calle
NASA Corrosion Technology Laboratory, Kennedy Space Center, FL, USA
Research Partner: M. Gillis, M. Blanton, J. Hanna, and J. Rawlins
The University of Southern Mississippi, Hattiesburg, MS, USA
Smart Coatings 2015, February 25-27, Orlando, Florida, USA
https://ntrs.nasa.gov/search.jsp?R=20150023601 2019-08-31T04:37:47+00:00Z
Outline
Introduction
Corrosion Protective Coatings
Electrochemical Nature of Corrosion
pH Change and Corrosion
pH Sensitive Microcapsules
Smart Coating Response to Corrosion
Encapsulation
Concept vs Reality
Hydrophobic Core Microcapsules
Hydrophilic Core Microcapsules
Microparticle
Corrosion Indication
Development and Optimization
Early Indication of Corrosion
Indication of Hidden Corrosion
Fluorescent Corrosion Indicating Coating
Corrosion Inhibition
Development and Optimization 
Corrosion Protection: Steel
Corrosion Protection: Al Alloys
Self Healing
Self Healing Approaches
Self Healing Coating: 2 Capsules
Self Healing Coating: 1 Capsule 
Summary
Acknowledgement 
Outline
Introduction
Outline
Corrosion Protective Coatings
Barrier (passive)
Barrier plus corrosion inhibiting components:
Sacrificial (zinc-rich primers)
Corrosion inhibitors (can have detrimental effects on the coating properties and the 
environment; most expensive additive; subject to progressively stricter environmental 
regulations)
Smart (active)
Electrochemical Nature of Corrosion
Overall Reaction:
2𝐻2𝑂 + 𝑂2 + 2𝐹𝑒 → 2𝐹𝑒
2+ + 4𝑂𝐻−
Anodic:
𝐹𝑒 → 𝐹𝑒2+ + 2𝑒−
Cathodic: 
2𝐻2𝑂 + 𝑂2 + 4𝑒
− → 4𝑂𝐻−
Elapsed Time: 0 hours
0.5 hours
1.5 hours
4.5 hours
3 days
pH change and Corrosion
pH-triggered Release Microcapsules
Microcapsule containing pH 
indicator (inhibitor, self healing 
agents) 
The shell of the microcapsule breaks 
down under basic pH (corrosion) 
conditions
OH-
OH-
pH indicator changes color and is 
released from the microcapsule 
when corrosion starts
pH Sensitive Microcapsule
Smart Coating Response to Corrosion
2. Corrosion inhibitors
3. Healing agents
1. Corrosion indicators
incorporated into
coating
Ruptured Microcapsule:
 indicates corrosion 
 protects metal from      
corrosion
 repairs damaged area
OH-
Corrosion 
causes capsule
to rupture
O2 + H2O
OH−
Fe2+ e-
mechanical 
damage causes 
capsule to rupture
Fe2+ e-
Microcapsule Response to pH Increase 
Encapsulation
Concept vs Reality
- Initial concept: a simple pH sensitive microcapsule
- Reality: wide range of active ingredients and various coating systems 
- Result: a portfolio of different control delivery systems to fit the needs
- pH sensitive microcapsules, pH sensitive microparticles, and inorganic microcontainers. 
- pigment-graded materials with good coating compatibility in free flowing powder forms. 
Hydrophobic Core Microcapsules
Interfacial polymerization of oil-in-water microemulsion procedure for making hydrophobic-core 
microcapsules. Oil is shown in yellow and water in blue.
Hydrophilic Core Microcapsules
Interfacial polymerization of water in oil microemulsion process for hydrophilic-core microcapsules. Oil 
is shown in yellow and water in blue.
Hydrophilic-core Microcapsules
SEM images of hydrophilic-core microcapsules
Microparticle Formation
water
surfactant prepolymer
Addition of active 
agents in solution
water-miscible 
solvent
inhibitor
Mixing  & Ouzo 
effect
Further 
Polymerization
Particle with 
inhibitor
Solvent Diffusion & 
polymerization
Corrosion Indicating Microparticles
SEM image of microparticles with color changing indicator (left) and with 
fluorescent indicator (right)
Microparticles with Inhibitors 
SEM and EDS of microparticles
with corrosion inhibitor
phenylphosphonic acid (PPA)
Inorganic Carriers
Microcapsules for Self-healing Coatings
Optical micrographs of spherical and elongated microcapsules for self-healing of 
mechanical scratches
Corrosion Indication
Development and Optimization: Indication
Oil core microcapsule 
formulation was 
developed by interfacial 
polymerization process.    
MFPTT microparticles 
with incorporated phph 
(30wt%) were 
synthesized. 
Oil core microcapsule 
containing corrosion 
indicator phph (2wt%) 
was synthesized. 
Oil core microcapsule 
formulation was 
modified to reduce the 
capsule size.
Water core microcapsule 
formulation was developed 
by interfacial polymerization 
process.    
Water core microcapsule 
formulation was modified 
to reduce microcapsule 
cluster formation.  The 
new formulation also uses 
environmentally friendly 
reagents.  
Water core  microcapsule 
formulation was optimized 
by using a water soluble 
wall forming prepolymer. 
The synthesis time was also 
reduced, and the 
microcapsule wall 
properties were improved.
Water core 
microcapsules 
containing corrosion 
indicator phph (5wt%) 
were synthesized.  
Oil core microcapsule 
formulation was modified 
to yield free flowing 
powder. Corrosion indicator 
microcapsules were 
prepared as free flowing 
powder. 
The pH sensitivity of the 
microcapsules was 
tested. 
Early Indication of Corrosion
Salt immersion test results of panels coated with a clear polyurethane coating loaded 
with 10% indicator microparticles in their core. The coating detects corrosion in the 
scribed area at a very early stage (30 seconds) before the appearance of rust is visible 
(2 hours).
Early Indication of Corrosion
Indication of Hidden Corrosion
Conceptual illustration of corrosion 
indication in structural bolts at the 
launch pad
Indication of 
hidden  corrosion 
by color change
Pad 39B MLP-1: Bolt from Victaulic joint 
on center upper shield
Hidden Corrosion Indication
System 
label
Metal 
Substrate
Coating systems
1 Zinc
galvanized
nut and bolt
Clear urethane coating containing 10%
phenolphthalein (phph) microcapsules.
2 Zinc
galvanized
nut and bolt
First coated with epoxy, then top coated
with clear urethane containing 10% phph
microcapsules.
3 Sand blasted
nut and bolt.
The ends of the nut and bolt were coated
with inorganic zinc coating; the entire nut
and bolt was coated with urethane
containing 10% phph microcapsules.
4 Sand blasted
nut and bolt
The ends of the nut and bolt were coated
with inorganic zinc coating. The entire nut
and bolt was coated with epoxy and then
top coated with a clear urethane
containing 10% phph microcapsules.
5 Zinc
galvanized
nut and bolt
The ends of the nut and bolt were coated
with urethane containing 10% phph
microcapsules.
6 Zinc
galvanized
nut and bolt.
The ends of the nut and bolt were coated
with epoxy and then top coated with
urethane containing 10% phph
microcapsules.
Coating systems used for hidden corrosion indication testing. 
Nut and bolt set up for crevice corrosion testing. The pictures show 
results after 600 hour of salt fog exposure
Indication of Hidden Corrosion
Corrosion Sensing through Fluorescence Change
- Color changing corrosion indicators: a simple approach
- Florescent corrosion indicators provide very sensitive detection at very low indictor 
particle loading (0.05-0.5%) in coatings.
Fl rescent Cor sion Indicati g Co ting
Corrosion Sensing through Fluorescence Change
Confocal scanning laser microscopy 2D images of fluorescent corrosion sensing 
coating on steel. Unexposed panel (left) and near scribe after 15 hours of immersion 
in 5% NaCl (right).
Fl rescent Cor sion Indicati g Co ting
Florescent Corrosion Indicating Coating
TECAN Scan Images 
TECAN reflectance fluorescence spectroscopy scanning composite image on the coated 
cold rolled steel panel during salt immersion exposure up to 5.5 hours, with an artificial 
defect in the middle. While the main event is at the defect sites, there are many other 
corrosion events occurring as early as 1 hour. 
Florescent Corrosion Indicating Coating
Fluorescence Corrosion Sensing
Fluorescence corrosion sensing coating for early corrosion detection.  TECAN scan 
image after 5.5 hours salt water immersion testing is in the middle, while a picture and 
optical microscopy images (100X) of the panel after 15 hours of salt water immersion 
testing are shown on the left and right. 
Florescent Corrosio  I dicating Coating
Laser Scannin Microscopy (LSM) confocal fluorescent microscope image
Florescent Corrosion Indicating Coating
Corrosion Inhibition
Oil-core microcapsule 
developed using 
interfacial 
polymerization
The pH sensitivity of 
the microcapsules was 
tested. 
Different inorganic 
inhibitors were 
encapsulated at different 
concentrations.  These 
microcapsules were then 
heat treated at different 
conditions to achieve 
various release rates. They 
were incorporated into 
coatings for testing. 
Water-core microcapsule 
developed using 
interfacial 
polymerization
Oil-core microcapsules 
with organic and some 
inorganic inhibitors, 
such as CeCl3, were 
synthesized.  
Water core microcapsule  
modified to reduce 
microcapsule cluster 
formation.  The new 
formulation uses 
environmental friendly 
reagents.  Various water 
soluble inhibitors were 
encapsulated: cerium 
nitrate, sodium molybdate, 
sodium phosphate, calcium 
metaborate, and phenyl 
phosphonic acid.      
Oil-core microcapsule s 
optimized  to reduce the 
emulsion stability for easy 
microcapsule separation.  
This process  yields  a free 
-flowing powder.  
Water core microcapsule 
synthesis  was optimized 
by using  a water soluble 
wall forming  pre-
polymer. The synthesis 
time was also reduced, 
and the microcapsule 
wall properties were 
improved.
Corrosion tests showed 
the need to control the 
permeability of the 
capsule wall to avoid the 
leaching of inhibitor into 
coating when the 
encapsulated inhibitor 
concentration is too high. 
To address this problem, 
MFPTT microparticles 
containing various 
corrosion inhibitors were 
synthesized.  
Development and Optimization: Inhibition
Improve Existing 
System: Inorganic 
Zinc
•Inorganic zinc is watched 
for environmental impact.
•Provides excellent 
protection when steel is 
sand-blasted to white 
metal - difficult for hard to 
access area.
•Inhibitor particles can 
Improve adhesion -
reduce cost and improve 
protection.   
Develop New System: 
Polymer Coating. 
•Environmental friendly 
corrosion protective 
coating through 
controlled releasing 
inhibitors.
•Partner with commercial 
companies for product 
development.
•Improved protection for 
steel. 
Next Generation: 
Waterborne
•While solvent based 
coatings still dominate 
heavy protection areas, 
the next generation will 
be solvent free 
systems: waterborne, 
powder coating.
•New particle formulas 
are being developed to 
target these systems. 
Corrosion Protection Function Development
Controlled release inhibitors have been used in three areas of coating development 
for steel protection: improved inorganic zinc, new Cr free organic coating, and 
effective solvent-free coatings. 
Corrosion Protection: Steel
with 
inhibitor
control
with 
inhibitor
control
Organic coating formulations being developed with industrial partners for steel 
protection.  Steel panels after accelerated cyclic corrosion testing (left), and coated 
steel panels being tested at beach atmospherical testing. 
Corrosion Protection: Steel
Control
with 
inhibitor
New inorganic delivery systems being developed (left) show great promise for 
improving corrosion protection of waterborne system (right). 
Corrosion Protection: Steel
Corrosion Protection: Al Alloys
 
Method System Al 7XXX (0.3% Cu) Al7XXX (0.6% Cu) Al7XXX (0.9% Cu) 
Visual 
Without 
Inhibitor 
   
With 
Inhibitor 
   
SEM 
(LABE) 
without 
inhibitor 
   
With 
inhibitor 
   
- Further testing and development to extend the protection benefits to Al alloys.
- Some encapsulated inhibitors proven to be effective for protecting Al alloy substrates as well. 
- The inhibitor particles will be used to develop Cr free paint for different Al alloys.  
Self Healing
Self Healing Self- Healing Approaches
Several self healing coating systems have been developed, including 1 capsule and 2 capsule healing 
systems, as well as self sealing system using flowable polymers. Elongated microcapsules were also 
developed for thin film applications.  So far, one capsule healing system performs the best and provides 
significant improvement for epoxy coating on steel substrate.   
Self Healing
Control and 2-Part siloxane capsule system 
(siloxane and tin catalyst),  blended into an 
epoxy primer coating, after 700 hrs of salt fog 
exposure testing. Coating thickness is about 
400µm and microcapsule content is 20 wt%. 
Siloxane (up) and tin catalyst (down) microcapsules
Self Healing Coating: 2 Capsules
Outline
Summary
The authors are developing a smart coating, based on pH-sensitive 
microcontainers, for early corrosion detection, corrosion inhibition, and self-
healing
The corrosion indicating function has been tested by incorporating encapsulated 
color changing and fluorescent indicators into coatings of interest. Salt immersion 
test results showed that the coating detects corrosion at a very early stage before 
the appearance of rust is visible. 
Salt fog test results showed the effectiveness of the encapsulated corrosion 
indicator in detecting hidden corrosion in an epoxy coating with urethane as a top 
coat.
Salt fog test results showed the effectiveness of an encapsulated corrosion 
inhibitor .
Salt fog test results showed the effectiveness of an encapsulated self-healing 
system.


Recent Progress in the Development of a Multifunctional Smart Coating for Autonomous Corrosion Detection and Protection

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