Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Tim H. Muster et al.
RAPID SCREENING OF AGENTS FOR SELF
HEALING COATINGS
Tim H. Muster*, Penny Corrigan, Scott A. Furman, Simon Hardin, Tim G. Harvey, Anthony
E. Hughes, Natalie Sherman, Fiona H. Scholes and Paul White
CSIRO Manufacturing & Materials Technology
Private Bag 33, Clayton South MDC, Clayton 3169, Australia
Tel: +61 3 9252 6000
Fax: +61 3 9252 6253
Tim.Muster@csiro.au
The protection of metal-based systems against environmental damage can be achieved through the application of
organic paint systems which contain active inhibitors. The release of these inhibitors by diffusion of water into
the paint system at a defect releases the inhibitor and stops corrosion on the surface, thus forming a self-healing
system. Chromate, which is a widely used inhibitor, is targeted for removal from the market due to its toxicity.
Thus there is a need for inhibitors to replace chromate. In addition, opportunities exist to build other functions
into paint systems, such as sensing and damage healing capabilities.
In this article we present a rapid screening method for assessing the ability of chemical species to impact the
anodic and cathodic reactions that occur at metal surfaces. A simple electrochemical test is performed by
applying a fixed potential between two identical electrodes and measuring the current flow between them. The
present electrode configuration incorporates nine pairs of metallic wire types into a single electrode assembly:
pure aluminium, aluminium alloys 2024-T3 and 7075-T6, pure iron, mild steel, stainless steel 316, pure
magnesium, magnesium alloy AZ31, and pure zinc. The metals coexist in an electrolyte composition of interest
and the electrochemical response of each metal can be rapidly characterized. Current methodology enables
approximately 30 electrochemical experiments to be performed per hour, thus enabling data on the inhibitive
properties of reagents to be collected in a timely manner.
The rapid screening technique enables a comparison of the resistances against current flow in a corrosive
environment without and with the incremental addition of different chemical species. In addition, screening for
both anodic and cathodic inhibition is achieved simultaneously, and information regarding the concentration
dependent efficiency of corrosion inhibition can be monitored.
Keywords: Corrosion inhibitor, Rapid screening technique, electrochemical testing
1
Introduction
There are large costs associated with the corrosion of metals and related maintenance [1].
Metal corrosion is predominantly an electrochemical process that cannot be stopped, only
slowed, for example by the application of paint films. For electrochemical reactions to
proceed, the system requires an anode, a cathode, an electron conducting path (through the
metal) and an ionic conduction path (through the solution). If one of these processes is slowed
or eliminated, the electrochemical current becomes rate limited (i.e. corrosion is slowed).
Current paint technologies act in two key ways to slow corrosion. Firstly, they form a barrier
to stop water, salts and oxygen reaching the metal surface. Water and salts provide an ionic
conduction path between anodic and cathodic sites.
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Tim H. Muster et al.
Oxygen promotes increased cathodic reaction rates as it is readily reduced. The second way in
which paint films protect metals is their ability to contain corrosion inhibiting compounds,
such as chromate compounds. These compounds can be released from the paint film where
corrosive conditions exist and diffuse to active corrosion sites, where they form compounds of
low solubility and low conductivity that passivate the metal surface [2]. This inbuilt
mechanism for corrosion inhibition may be viewed as being a self-healing system.
There is considerable interest in the development of self-healing paint systems for the
protection of metals. The key reasons for this include:
•
•
•
•
the need to make structures last longer
the need to reduce maintenance costs,
the introduction of scratch resistant clear coats into the market that are able to heal
mechanical damages to a paint [3,4],
as noted, earlier the need to replacements for chromate pigments .
There are two clear ways to approach self-healing technologies with respect to paints (Figure
1): paints that act as a barrier and maintain their performance by mechanically healing
scratches and cracks, and paints that support self-healing agents or are able to be refreshed
with self-healing agents.
Since very few paint systems provide a perfect barrier to prevent corrosion, the importance of
developing new corrosion inhibiting chemistries is required. Existing chromate inhibiting
technologies work for a wide range of situations and attempts to date to find a direct
replacement have failed. Simple chromate-replacement technologies have generally relied
upon limiting either anodic or cathodic reactions, but not both. Therefore, present day
approaches aim to develop ‘multifunctional’ corrosion inhibitors, which have the ability to
passive both anodic and cathodic reaction sites at the metal surface. This article presents an
electrochemical testing methodology for the rapid screening of chemicals that may be
formulated into multifunctional corrosion inhibitor delivery vesicles, which are envisaged to
form the basis of a self-healing paint. Using a multielectrode configuration that is comprised
of nine different metals, a short electrochemical measurement is performed that is sensitive to
anodic and cathodic inhibition mechanisms.
2
Concept
Improvements in computational power and automation have led to the development of a range
of rapid screening and combinatorial techniques, chiefly aimed drug discovery [5]. This
enables a large number of experiments to be carried out and a wide array of variables to be
investigated. In the search for corrosion inhibitors, a number of traditional techniques have
been used such as, mass loss determination and electrochemical studies. These traditional
techniques generally require times of an hour to weeks to return information and are carried
out only on specific metals of interest.
Work by Taylor et al. [6] presented the use of multi-electrode arrays as a means to carry out
high-throughput testing on corrosion inhibitors. In their work, 50 different inhibitor
chemistries were formulated and placed them into 2 ml reaction wells.
2
© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Tim H. Muster et al.
Two identical aluminium alloy 2024-T3 wires were placed into each solution and a 100 mV
potential was applied to one wire with respect to the second wire. The second wire was held at
the open-circuit potential of a ‘control’. The current between each pair of electrodes was
logged and was used as a measure of the polarization resistance. Although the testing by
Taylor and Chambers was carried out over a nine-hour period, it was demonstrated that large
numbers of solutions could be tested simultaneously. The methodology presented in this
paper also uses the application of a 100 mV DC potential between two identical electrodes.
However, there are three key differences in the current work from that of Taylor and
Chambers:
(1) no reference electrode is used to fix electrode potentials
(2) several metals are investigated simultaneously in one solution
(3) experimental time is reduced.
Electrochemical methods are not usually carried out in the absence of a reference electrode.
However, for the rapid screening of multifunctional corrosion inhibitors the omission of the
reference electrode would appear to offer some benefits. Figure 2 demonstrates the currentpotential relationship upon the application of a potential across two electrodes. The solid lines
1c and 1a represent the equilibrium relationship of I vs E for a metal as probed by
potentiodynamic polarization experiments. 1c represents a cathodic polarization and 1a
represents an anodic polarization. The open-circuit potential (OCP) is the unpolarized
potential where the rate of cathodic surface reactions balances the rate of anodic surface
reactions. By applying a 100 mV potential between two identical electrodes one is polarized
to be anodic and the other cathodic. The current flow between the electrodes is determined by
the equilibrium of the anodic and cathodic surface reactions on both electrodes. The solid
horizontal arrow connecting curves 1c and 1a represents a 100 mV applied potential and its
position on the y-axis determines the magnitude of current flow. Where a cathodic corrosion
inhibitor is introduced the cathodic reactions are limited (curve 2c). The dotted gray
horizontal arrow shows that a reduction in the measured current is expected upon the
application of a 100 mV potential between two identical electrodes. In a similar fashion, an
anodic corrosion inhibitor would decrease the measured current upon the application of the
same potential (see curve 2a and the solid-dashed horizontal arrow). It should be noted that
changes in the position of the OCP (not shown here) will occur with changes in anodic or
cathodic reaction rates. The measurement of the current between the two identical electrodes
is therefore sensitive to both anodic and cathodic inhibition, which is important when
screening for multifunctional inhibitor formulations.
3
Experimental
3.1
Wire electrode assembly
Wire specimens according to Table 1 were used to construct electrode assemblies for the
rapid scanning of corrosion inhibitors. Wire samples of aluminium alloys specimens AA2024T3 and AA7075-T6 (electrodes 2 and 3) were obtained by machining from sheet specimens.
Magnesium alloy wires used for electrode no. 6 were machined to reduce the as-received
diameter of 3 mm down to a 2 mm diameter. Mild steel wire was abraded using 500 Grit SiC
paper to remove the copper coating.
3
© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Tim H. Muster et al.
All other wire specimens were used as received. Electrode wire specimens with lengths of 1.5
– 2 cm were attached electrically to polymer coated wires by tin solder (electrodes 4, 7, 9) or
silver-loaded epoxy (electrodes 1, 2, 3, 5, 6, 8). Each metal pair had one ‘green’ and one ‘red’
wire terminal. The electrodes were aligned parallel to each other and configured into
hexagonal arrays according to Figure 3. An epoxy resin typically used for preparing
metallographic samples (EpofixTM) was used to encapsulate the electrical connections and to
fix the electrodes at a separation of approximately 3.1 mm (centre-to-centre). In addition to
the multielectrode assembly, an AA7075-T6 electrode pair was prepared in the absence of all
other wires. This individual assembly was used to explore the experimental methodology for
corrosion inhibitor assessment.
Table 1: Wire specimens used in the construction of wire electrodes
Purity
Electrode
Wire
Diameter
Area
(wt %)
(mm)
(cm2)
1
Al
2.0
0.0314
99.999
2
AA2024-T3
2.0
0.0314
92.2
3
AA7075-T6
1.8
0.0254
89.1
4
Fe
2.0
0.0314
99.95
5
Mg
1.6
0.0201
99.9
6
AZ31 (approx.)
2.0
0.0314
~96
7
Steel, Mild
2.2
0.0380
98.44
8
Stainless 316
2.0
0.0314
9
Zn
2.0
0.0314
99.99+
3.2
Source
Goodfellow, U.K.
Kaiser Aluminum Corporation
Kaiser Aluminum Corporation
Goodfellow, U.K.
Goodfellow, U.K.
Goodfellow, U.K.
CIGWELD, Australia.
Goodfellow, U.K.
Goodfellow, U.K.
Electrochemical testing
All electrochemical testing was carried out using a 1270 Solartron potentiostat in conjunction
with a 1281 Solartron 8-channel multiplexer, which enabled switching between metal types.
The standard testing configuration involved coupling the counter and reference electrodes to
the ‘red’ terminal of the wire electrode, which performed the function of the anode in
solution. The working electrode was connected to the ‘green’ terminal of the wire electrode
(cathode in solution) and held at -100 mV versus the ‘red’ wire electrode. All experiments
were carried out using a stirred background electrolyte of aerated 0.1 M NaCl solution.
Aeration was achieved by bubbling high purity air through the bottom of the electrolyte.
Stirring was carried out using a magnetic stirrer operated at 150 rpm.
A series of simple electrochemical tests were carried out in order to fully understand and
develop the methodology used for corrosion inhibitor assessment. The influence of varying
the polarization potential (from -10 to -150 mV) was investigated for AA2024-T3 electrodes
with and without inhibitor (CeCl3). Also, the distribution of overpotential between anodic
and cathodic electrodes was investigated using AA7075-T6 wire specimens. For this a
combined open-circuit potential was monitored for 15 mins using a conventional threeelectrode setup (Pt gauss counter electrode and SCE reference electrode). The current
generated during a standard -100 mV polarization was recorded and then applied to the
individual ‘red’ and ‘green’ wire electrodes as a galvanostatic experiment. The galvanostatic
experiment was conducted using the conventional three-electrode setup and monitored the
potential versus the SCE electrode.
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Tim H. Muster et al.
3.2.1 Potentiostatic scans with a continuous dosing of corrosion inhibitor
Using the standard testing configuration, the electrode assembly was systematically immersed
into 200 ml of 0.1 M NaCl and allowed to equilibrate for 15 minutes. A -100 mV
potentiostatic scan was then performed for each electrode pair for a period of 60 seconds. 200
μl of 0.1 M inhibitor solution was then added to the 0.1 M NaCl background electrolyte and
allowed to equilibrate for 5 minutes before a second series of scans was commenced.
Two more scans were carried out after the further addition of 0.1 M inhibitor solution (2 ml
and 18 ml), allowing for 5 minutes calibration time after each corrosion inhibitor addition.
This gave current measurements for 8 different metals without inhibitor, and at 10-4 M, 10-3 M
and 10-2 M of inhibitor. The original NaCl concentration is reduced to approximately 0.091 M
after all the inhibitor additions.
4
Results
4.1
Establishing an appropriate interrogation time
A -100 mV potential was applied between the two like electrodes and the current exchange
was measured for periods of time up to 300 seconds. The measured currents (adjusted to
positive values for ease of presentation) appeared to be stable after 20 seconds and did not
change significantly up to 300 seconds. Therefore, the rapid screening of corrosion inhibiting
solutions can be done in 60 seconds per electrode type (Figure 4). A significant amount of
scatter was observed in the first 20 seconds of testing and therefore all high throughput
screening data was averaged in the time period from 20 to 60 seconds.
4.2
Potentiostatic scans with a continuous dosing of corrosion inhibitor
Previous studies focusing on the inhibition characteristics of chemicals have shown that
electrochemical/corrosion rates have been dependent upon the concentration of inhibitor [7].
For this reason a series of potentiostatic scans were performed with an incremental dosing of
the prospective inhibitor. Figure 5 shows the current density of various metals in 0.1 M NaCl
without inhibitor and then at three different inhibitor concentrations (increasing from left to
right).
Table 2 provides averaged current values during the continuous dosing experiment. Pure
aluminium, stainless steel and mild steel showed little change with inhibitor present. Currents
were significantly reduced for zinc and aluminium alloys 2024-T3 and 7075-T6. The
measured current for pure magnesium increased slightly upon the addition of CeCl3. An
identical experiment performed on an isolated AA7075-T6 electrode (no other metals in
solution) showed similar but more definite decreases in current through the addition of
inhibitor. Slight differences between multielectrode AA7075-T6 and isolated AA7075-T6 are
not surprising since the communal solution chemistry is almost certain to influence current
exchange densities.
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Tim H. Muster et al.
Table 2: Average current density determined for each potentiostatic measurement in Figure 5. *Experiment
performed on isolated AA7075-T6 electrode
I (μA cm-2)
Electrode
Al
AA2024-T3
AA7075-T6
AA7075-T6*
Mg
Steel, Mild
Steel, Stainless 316
Zn
4.3
No CeCl3
0.315
30.7
11.6
15.1
495
223
2.80
74.5
10-4 M CeCl3
0.317
14.3
6.73
1.38
606
207
2.45
69.9
10-3 M CeCl3
0.321
5.65
4.37
1.43
826
183
2.33
34.0
10-2 M CeCl3
0.320
7.04
4.89
1.35
751
198
2.67
37.8
Reproducibility of electrode preparation
The measured currents for freshly prepared electrodes surfaces (after 15 min equilibration in
0.1 M NaCl) are given in Figure 6. Changes in current of up to 60 % of the measured current
value were frequently obtained, suggesting that the response of the electrodes to the applied
potential was highly variable. It follows that the activity of each inhibitor was compared with
the performance in a uninhibited 0.1 M NaCl solution. The ability of reagents to limit current
flow was described by an inhibition index:
Inhibition index =
I no inhibitor
I inhibitor
The use of the inhibition index will be described in the following sections.
4.4
Distribution of overpotential
Since no reference electrode is used for potentiostatic testing, the overpotential existing on the
anode/cathode was explored using AA7075-T6. The solid black line in Figure 7 shows the
open-circuit potential of AA7075-T6 during a 15 minute equilibration period in the test
solution (aerated 0.1 M NaCl). A stable potential of approximately -0.724 VSCE was
maintained after approximately 500 s. Upon the application of a -100 mV potential between
the two AA7075-T6 electrodes, the current density equilibrated to approximately -1.6 × 10-5
A cm-2. This current density was then applied to the “anodic” and “cathodic” AA7075-T6
electrodes and their potentials measured. The dashed lines in Figure 7 show that the AA7075T6 electrode acting as the anode (Potential 2 during scan) achieves a potential of -0.716 VSCE,
which is only slightly positive (approximately 8 mV) to the open-circuit potential. The
potential of the cathodic AA7075-T6 electrode (Potential 1 during scan) was shifted negative
by over -90 mV to -0.85 VSCE. This data suggests that the application of the -100 mV during
testing has a more profound effect on the cathodic electrode than the anodic electrode. It
follows that the overpotential distribution is linked to the Tafel slopes of both anodic and
cathodic exchange current densities. Metals such as aluminium alloys have low
anodic:cathodic Tafel slope ratios (mv/decade) and therefore the cathodic electrode carries a
higher overpotential. In contrast, magnesium and zinc materials have increased
anodic:cathodic ratios, which results in a more even distribution of the overpotential between
the anodic and cathodic electrodes.
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
4.5
Tim H. Muster et al.
Multielectrode scans carried out at varied voltage
The influence of the polarization on current exchange was investigated by changing the
applied potential from -10 mV up to -150 mV. Figure 8 shows current data from five repeat
experiments. Currents measured at low polarization showed a decreased difference in current
between uninhibited and inhibited solution. Currents measured in 0.1 M NaCl solutions
without inhibitor showed significant scatter in measured current values. Significant current
reduction was noted when CeCl3 was added to solution, indicating that is performed
admirably as a corrosion inhibitor for AA2024-T3. Also, variations in the measured current
were minimal where CeCl3 was present in solution. The application of a -10 mV potential
between the two electrodes gave a positive current in one instance, indicating that natural
fluctuations in the potential of the alloy exceeded the applied overpotential. A -100 mV
appears to be an applied potential that can discern differences in inhibitive properties, yet does
damage the electrode pairs with high mass loss.
4.6
Inhibition factors for common salts
A series of continuous dosing experiments have been performed according to the standardized
procedures outlined above. Current data was analysed using a purpose built batch file
converter with inbuilt peak (outlier) removal. Figure 9 shows selected data from early studies
using the multielectrode technique. Aluminium alloys are inhibited by potassium chromate
and cerium salts. Note that the inhibition index can appear negative when inhibitor currents
are reduced to such low values that a sign reversal is possible due to scatter in the current
measurement. Therefore, negative values indicate that inhibition is significant. For
magnesium and magnesium alloys the most significant inhibition was achieved through the
addition of anionic phosphates, with cerium salts and potassium chromate showing no
significant inhibition. Iron and mild steel were best inhibited by potassium chromate and
somewhat by phosphate anions. One positive outcome from this work is the consistency in the
values of the inhibition index between similar metal types. That is, trends held between
AA2024-T3 and AA7075-T6, between pure Mg and Mg alloy AZ31, and also between pure
Fe and mild steel. These trends indicate variations in the mechanisms of inhibition, which can
then be explored in further detail using more precise test methods.
5
Conclusions
A high throughput method for screening chemicals for use in self-healing paint systems has
been presented. The methodology presented allows approximately 30 electrochemical
experiments to be performed per hour. The absence of a reference electrode in carrying out
potentiostatic testing enables both anodic and cathodic inhibition mechanisms to be screened
simultaneously. The use of an inhibition index, which normalizes current data against that of
an uninhibited solution, enabled reproducible analysis of potential inhibitor functionalities.
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[2]
[3]
Thompson, N., Yunovich, Dunmire, D. (2005) Proceedings of the 1st World Congress on Corrosion in
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Furman, S.A., Scholes, F.H., Hughes, A.E., Jamieson, D.N., Macrae, C.M., Glenn, A.M. (2006)
Corrosion Science, 48(7), 1827-1847.
Nissan Press Release (2005) http://www.nissan-global.com
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© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
[4]
[5]
[6]
[7]
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Bayer AG News Release (2005) http://www.news.bayer.com
Houston, J.G., Bankst, M. (1997) Current Opinion in Biotechnology, 8, 734-740.
Taylor, S.R. and Chambers, B.D. Proceedings of the 4th International Symposium on Aluminium
Surface Science and Technology, Beaune, France, May 2006.
Schweitzer, P.A. Corrosion inhibitors. In: Corrosion Technology, 11th ed., Corrosion Engineering
Handbook, New York (1996), 555-561.
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