1. Technical Paper
Anticorrosive test
The fast lane to failure
Cyclic impedance test gives rapid characterisation of coating breakdown
* Corresponding author: José Javier Gracenea* useful technique here [1-8], although long periods are
José Javier Gracenea Maria José Gimeno needed (days, weeks and sometimes months) to per-
Mediciones y Corrosión SL Julio José Suay form this type of test and obtain good results.
+ 34 964 387-389
josejgracenea@medco.es
Two different tests of anticorrosive properties (salt Combined AC/DC impedance
fog spray and a cyclic impedance test called ACET)
have been used to evaluate two-layer coating
test gives fast results
systems for aeronautical application and establish Hollaender et al. [9-11] developed a rapid method for
a correlation between them. The ACET technique testing coated metals in food packaging, consisting of
provides good correlation to salt spray, faster a combination of DC and AC measurements (the AC/DC/
evaluation and more details of the mode of coating AC procedure) which has been successfully adapted and
breakdown. used in liquid paints applied to steel substrates [12-14].
After an initial AC measurement, the test sample is sub-
P
rediction of the anticorrosive properties of paints jected for a short time to a constant cathodic voltage (DC)
is one of the most important lines of research in producing a stress on the sample and, following that, an
organic coatings. Due to the complexity of the cor- AC spectrum is recorded again.
rosion and degradation processes in organic-metal sys- The change in the characteristics of the impedance spec-
tems, the main routines for assessing interactions and trum can be attributed to coating deterioration (pore for-
materials have been developed experimentally by us- mation) and a delamination process in the metallic sur-
ing different exposure processes and techniques for the face due to hydrogen and OH- production (if a cathodic
measurement of properties. reaction takes place).
The main types of exposure process are accelerated The new ACET (Accelerated Cyclic Electrochemical Tech-
ageing tests (e.g. salt fog spray) and natural exposure nique) procedure developed by Medco [15-19] is based
experiments (which must be planned for long durations on the Hollaender method but incorporates two principal
and are very expensive to run). innovations. First, a long potential relaxation period was
Salt fog spray is one of the longest-established evaluation introduced after the polarisation, where the potential and
techniques for which different international standards intensity are recorded against time, data that is used to
(ISO 9227, ASTM B117) have been developed. Neverthe- evaluate the adhesion of the coatings to the substrate.
less, this technique is very subjective and does not give Second, new numerical methods were developed to
quantitative information about the corrosion processes or establish correlations between ACET and traditional salt
an interpretation of the overall process itself. spray resistance results.
It is still necessary to develop techniques to measure the In this work ACET and salt fog spray were used to study
anticorrosive properties in a direct sense, thus various the anticorrosive properties of the paint systems de-
electrochemical techniques have been used to evalu- scribed below. The validity of the new technique has
ate the protective performance of organic coating/metal also been examined as a useful method for determining
systems. The application of electrochemical impedance the anticorrosive properties of paints in very short times,
spectroscopy (EIS) to coated metals has proved to be a comparing it with other different evaluation procedures
(EIS and salt fog spray).
Coating systems and test procedures
Two epoxy-polyurethane two-coat systems were chosen
for the comparative tests. Both systems were solvent-
based, with a high solids epoxy primer containing Cr(VI)
anticorrosive pigments. The samples were cured at room
temperature for 21 days. The total thickness of the paint
systems was in the range 50-60 µm.
The accelerated salt fog spray test was performed in
accordance with ASTM B 117-85/ISO 9227. In this test a
“Anticorrosive cross is cut along the coating through to the bare metal.
Coatings” Samples are collected after different periods of time in
Jörg Sander the test and evaluated up to a maximum of 3000 hours
www.european- exposure.
coatings.com/books After each collection, the samples were dried; blister-
Figure 1: Representation of the ACET test schedule ing, corrosion and delamination were measured after
84 European Coatings J OURNAL 03 l 2011 www.european-coatings.com

2. Technical Paper
Anticorrosive test
24 hours of ambient relaxation time. Delamination was presence of the electrolyte, with production of iron ox-
evaluated after applying 30 mm wide tape to one arm of ides and hydroxides.
the cross and peeling off the paint with it. On the other hand, the forced polarisation means that
The ACET procedure (Figure 1) is based on the applica- the double layer in the interface is disturbed and needs
tion of a stress (cathodic polarisation) to a coated sample, to be reorganised, which is reflected in the variation of
then measuring the impedance (EIS) of the system after the potential at the relaxation process. At the same time,
it. This stress/impedance sequence is repeated several the different ions inside the coating will leave it, produc-
times until the system is substantially damaged. In order ing charge equilibration and reorganisation of the poly-
to obtain more information about the coating itself and meric molecule dipoles, also producing a variation in the
the whole system, a depolarisation step can be included potential.
for a given period (relaxation time). Thus the system is degraded by the loss of adhesion
An initial EIS measurement gives an idea of the imped- (formation of H2), the pore opening by the incoming of
ance of the system (using Bode plots and modelled char- the different species from the electrolyte, and the forma-
acteristic parameters and the cycle shown in Figure 1). tion of corrosion products by electrochemical processes.
These processes can be followed by the results obtained
Underlying corrosion in the relaxation process.
theory summarised
A cathodic reaction of water hydrolysis occurs
when the potential is more negative than -1.0
V relative to a saturated calomel electrode [27]. EXCELLENCE FOR YOUR PLANT
The test technique is based on the influence
that this has on coating adhesion because of
the formation of H2 (gas) and OH-. The evolution
of H2 will increase local delamination (Figure 2)
giving rise to the failure of the coating system
(reflected in the variation of the impedance).
When the cathodic reaction stops and H2 pro-
duction has taken place, the normal electro-
chemical corrosion of the system occurs in the
Results at a glance
Although the neutral salt spray test has
been established for many year in evalu-
ating anticorrosive behaviour, it gives no Complete plant for the
production of water base
information on the underlying corrosion paints and additives
processes, and testing times for high per-
formance coatings can be quite long.
COMEC has been producing machines and systems for the chemical
An alternative process known as ACET and paint industry for over 35 years, designing specific solutions for all
(Accelerated Cyclic Electrochemical Tech- customer needs.
nique) has been developed, which produces The only STANDARD way: QUALITY first!
very rapid breakdown of coatings under Comec machines manage every step of the production process:
cathodic polarisation and also provides raw materials and additives STORING and DOSING, MIXING, GRINDING,
some information on different modes of FILTERING, PAINT METERING, PACKING and PALLETIZING.
coatings failure.
COMEC offers the best safety level guarantee for
A comparison between two polyurethane
inflammable products processing.
coating systems showed a good correlation
SYSTEMS AND MACHINES CERTIFIED FOR ZONE 0
between salt spray and ACET test cycles.
Ex II 1/2 Gc II B T4 mark in accordance with the
The time required for the evaluation was
certificate issued by notified body ICEPI 06 ATEX 03C021
3000 hours in the salt spray but less than
24 hours in the ACET test.
In the cases studied, poor salt spray be-
haviour is correlated with a strong change HALL 6 - STAND 253
in the impedance value at low frequencies
during the ACET test cycles. www.comecsrl.it
www.european-coatings.com 03 l 2011 European Coatings J OURNAL 85

3. Technical Paper
Anticorrosive test
the left, where it can be seen that the impedance varia-
tion is smaller than in the plot on the other side.
In order to understand these figures, it is necessary to
look at the theoretical basis underlying this ACET meth-
od. The cathodic polarisation applied to the coated metal
can cause two processes to occur in the paint.
Firstly, the introduction and passage of different cations
(H+, Na+, and so on) from the electrolyte through the
paint due to the negative potential imposed in the me-
tallic substrate. This can produce a concentration of posi-
tive charges in the coating that must be neutralised by
a balancing entry of anions (like Cl-) The passage of ions
(which can also be hydrated) through the coating can
cause its deterioration and the formation of pores.
Secondly, the cathodic reaction that can take place in the
metallic surface depending on the level of negative po-
larisation and the type of electrolyte [27] is shown in this
Equation:
2H2 O (l) + 2e - → H2 (g) + 2 OH – (1)
Figure 2: Processes occurring during the ACET test schedule The cathodic reaction will take place first if the elec-
trolyte is able to pass through the coating and reaches
the interface. This depends on the properties of the film
(permeability to ions, adhesion to substrate, existence
The ACET procedure measures the quality of the coat- of local film delamination, susceptibility of the coating
ing and its adhesion through the study of the resistance to form cracks because of its high rigidity, etc.) and, of
that the system offers to its degradation by the cathodic course, the applied cathodic voltage.
polarisations. In this study the cathodic polarisation was Obviously, the higher the quality of the primer (low per-
carried out for 20 minutes at a constant voltage of -4 V. meability and high ductility) the lower will be the prob-
Following that, the relaxation time was for 3 hours, and ability of the electrolyte reaching the interface, and of
finally an EIS was applied under the stated conditions the cathodic reaction taking place. The deterioration of
. The test sequence was repeated six times (around the coating due to cathodic polarisation can be caused
24 hours of testing) and was fully automated in Zahner primarily by the film delamination process at the metallic
equipment. interface produced by the cathodic reaction (evolution of
H2), although the passage of ions can also exert a de-
Results evaluated in terms grading effect.
of cathodic reactions Relaxation potential reveals
The ACET test was carried out on aeronautical epoxy-
details of film breakdown
polyurethane systems. Figure 3 shows the Bode plots If it is possible to detect whether the cathodic reactions
of these coatings, EP-PU1 (left) and EP-PU2 (right). The have taken place during polarisation, this information
worst behaviour is seen on the right-hand side. could be used to learn a little more about the perform-
This is characterised by a strong change in the impedance ance and quality of the paint. One possible way of de-
value at low frequencies during the cycles of the ACET tecting the existence of H2 (g) and OH- production (lead-
technique. Systems with better responses are shown on ing to more delamination) at the interface is to study the
evolution of the open circuit potential after polarisation
Figure 3: Bode
plots for aeronauti-
cal epoxy-poly-
urethane systems,
EP-PU1 (left) and
EP-PU2 (right);
paramters: ACET:
-4 V, 20 min, 25
min EIS, 3 h relax
86 European Coatings J OURNAL 03 l 2011 www.european-coatings.com

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5. Technical Paper
Anticorrosive test
Figure 4: Relaxation
plots for aeronauti-
cal epoxy-poly-
urethane systems
EP-PU1 (left) and
EP-PU2 (right); pa-
rameter: ACET: -4 V,
20 min, 25 min EIS,
3 h relax
the poor quality of the coating. Probably if longer relaxa-
Figure 5: Neutral tion times were used, new relaxation processes could be
salt spray test observed.
results showing The relaxation time will give an idea of the evolution with
good performance time of the system after the applied stress until it again
of EP-PU1 (left) and reaches the steady state (graphics Erelaxation = f(time)).
severe blistering Figure 5 shows salt fog spray test results. The system
of EP-PU2 system EP-PU1 (left) achieved 3000 h of exposure while EP-PU2
(right) after 3000 (right) was damaged by blistering. Thus, it is clear that a
hours correlation exists in this case between the established
salt fog test and the much faster ACET procedure.
(during the relaxation time). When cathodic polarisation REFERENCES
is stopped the coated metal potential will relax showing
two possible spectra. [1] Mansfeld F., Jnl. Applied Electrochem., 1995, Vol 25, pp 187ff.
In the first case, if cathodic reactions were taking place [2] Bierwagen G. P., Jnl. Coat. Tech., 1992, Vol 64, pp 71ff.
(H2 production), the potential would have a quick relax- [3] Liu B. et al, Acta Physico-Chimica Sinica, 2001, Vol 17, pp 241ff.
ation, normally around -1 V [27] (with small variations [4] Skerry B. S., Chen C-T., Ray C.J., Jnl. Coat. Tech., 1992, Vol 64, pp 77ff.
depending on the coating), which corresponds to the [5] Gwori S., Balakrishnan K., Prog. Org. Coat., 1994, Vol 23, pp 363ff.
termination of the reaction and, afterwards, a second re- [6] Selvaraj M., Guruviah S., Prog. Org. Coat., 1996, Vol 28, pp 271 ff.
laxation that corresponds to ions and electrolyte leaving [7] Hernández L. S., del Amo B., Romagnoli R., Anti-Corrosion Methods
the coating and possibly the formation of a new double and Materials, 1999, Vol 46, pp 198ff.
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In any case, the cathodic reaction will produce the entry [9] Hollaender J., Ludwig E., Hillebrand S., Proc. 5th International
of electrolyte through the coating and the production of Tinplate Conference, London, 1992, pp 300ff.
H2 (g) and OH- at the metal/coating interface. The time [10] Hollaender J., Schiller C. A., Strunz W., Food additives and contami-
needed for this electrolyte and the ions to leave the nants, 1999, Vol 14, No. 6-7, pp 617ff.
film will therefore be higher because they have to pass [11] Hollaender J., Schiller C.A., Strunz W., Proc. EIS 2001, Marilleva-
through the entire primer film. Italy, 2001.
Alternatively, if no cathodic reactions have taken place, [12] Rodriguez M.T. et al, Prog. Org. Coat., 2004, Vol 50, pp 68ff.
there would be a single relaxation process that corre- [13] Suay J. J. et al, Prog. Org. Coat., 2003, Vol 46, pp 121ff.
sponds to ions and electrolyte leaving the primer or to [14] Rodriguez M.T. et al, Prog. Org. Coat., 2004, Vol 50, pp 123ff.
the reconfiguration of the polymer dipoles. This relaxa- [15] García S. J., Suay J., Prog. Org. Coat., XX (2009), Prog. Org. Coat. 66
tion will take place over longer times as ions and electro- (2009), p. 306.
lyte penetrate deeper into the film, but they will probably [16] García S. J., Suay J., Prog. Org. Coat., 2007, Vol 59, pp 251-258.
need less time than in the first case described above. [17] García J. et al, Prog. Org. Coat., 2007, Vol 60, pp 303-311.
Figure 4 shows the potential relaxation versus time of the [18] García S. J., Suay J., Prog. Org. Coat., 2006, Vol 57, pp 273–281.
different epoxy-polyurethane systems after five cathodic [19] Rodriguez M. T. et al, Prog. Org. Coat., 2004, Vol 50, pp 123-131.
polarisations. The sample EP-PU1 with the best behav- [20] Lee S. S. et al, Prog. Org. Coat., 1999, Vol 36, pp 79ff.
iour shows only one relaxation at high potential which [21] Mansfeld F., Electrochim. Acta, 1993, Vol 38, No 14, pp 1891-1897.
indicates that during the cathodic polarisation there was [22] Amirudin A., Thierry D., Prog. Org. Coat., 1995, Vol 26., pp. 1-28.
no hydrogen production because of its good anticorro- [23] Walter, G. W. J.nl. Electroanal. Chem., 1981, Vol 118, pp 259-273.
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The sample EP-PU2 shows a relaxation at -1 V that cor- [25] Gamry Instruments, “Electrochemical Impedance Spectroscopy
responds to the ending of hydrogen production, because Primer”, www.gamry.com.
the cathodic reaction of electrolysis could take place due [26] Tang, N., Ooij W. J., Górecki G., Prog. Org. Coat., 1997, Vol 30, pp 255ff.
to the presence of water in the interface as a result of [27] Leidheiser H., Jnl. Adhesion Sci. Tech., 1987, Vol 1, pp 79ff.
88 European Coatings J OURNAL 03 l 2011 www.european-coatings.com

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