Volume 2 Paper 25
Using Electrical Impedance Spectroscopy (EIS) to Support Conventional Artificial Perspiration Testing
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JCSE Volume 2 Paper 25
Submitted 13th September 1999, published for public review 29th October 1999
Using Electrical Impedance Spectroscopy (EIS) to Support Conventional
Artificial Perspiration Testing.
CAPCIS Ltd, CAPCIS House, 1 Echo Street, Manchester M1 7DP.
Electrical impedance spectroscopy (EIS) was compared with the conventional
testing regime for examining the resistance to attack by artificial
perspiration of a number of water based finishing lacquers. The lacquers were
used as finishing treatments for lacquer coated steel shopping trolleys. EIS
was found to be superior to the conventional regime (exposure followed by
abrasion). EIS was able to provide quantitative data about the degradation
process, whereas the conventional regime could only provide a pass/fail
result. EIS was also able to differentiate between the performance of
different lacquers capable of obtaining a pass result in the conventional
§2 Keywords: EIS, Artificial Perspiration, Lacquer Coatings, Zinc
§3 Resistance to perspiration exposure is especially important for items
fabricated with the requirement of being physically handled. A prime example
is shopping trolleys. In Britain, shopping trolleys are usually constructed of
lacquer coated zinc electroplated steel. The lacquer used is usually
colourless, transparent and approximately 5m m
thickness. One of the largest problems associated with this product is
discoloration and zinc loss of the handles and top rail. This leads to an
unattractive dull grey/black appearance, as opposed to the initial bright
metallic finish. In today’s world, maintaining the appearance of the product
can be as important as maintaining its function.
§4 One testing method used to examine this problem is exposure to artificial
perspiration. Examples are the ANSI artificial perspiration test for
coating degradation, and the Volvo leaching test for chromate loss from
surfaces. This article describes the use of electrical impedance spectroscopy
(EIS)[3,4,5] to support conventional artificial perspiration exposure testing.
The substrate examined is zinc electroplated steel coated with a variety of
thin water based finishing lacquers.
§5 Three lacquer systems are discussed. These are representative of the
different responses obtained from a number of water based finishing lacquers
from a variety of sources. The lacquers have been designated as A, Band C.
§6 Samples of lacquer coated zinc electroplated steel (samples taken from
shopping trolleys in the vicinity of the top rail/handle or testing panels of
dimensions 6cm by 10cm) were examined using the ANSI artificial perspiration
exposure test. This consists of exposing the samples to synthetic
perspiration for four 15 minute cycles, with hardness testing of the lacquer
coating by abrasion with a 2B pencil (prepared as described in the standard)
after each exposure cycle. If the coating can be removed, this is defined as a
test failure. The cycle after which the failure occurred is also noted. The
formulation of the ANSI artificial perspiration solution is given in Table 1.
§7 Samples were also examined using EIS spectroscopy, with testing undertaken
after 0, 15, 30, 45, and 60 minutes of exposure. The samples were abraded
using a 2B pencil after each 15 minute exposure period, before EIS
measurements were taken. The instrument used was a single channel ACM autoAC
DSP (1996 model). The frequency range examined was 30 KHz to 0.1 Hz., and the
potential perturbation used was � 10mV around the
§8 The high frequency coating response was determined from a Nyquist
plot[3,4,5], and the coating resistance calculated. All samples were examined
in triplicate. Values varied within each sample group by up to �
30% with respect to the mean value.
§9 The results of the conventional test are given in Table 2. The thin and
colourless nature of the lacquer coatings made it extremely difficult to
visually determine the failure point of the test.
§10 Figure 1 is an example of the Nyquist response obtained from lacquer C
after 60 minutes of exposure. The high frequency coating response is clearly
differentiated from the lower frequency response associated with the charge
transfer processes associated with substrate corrosion[3,4,5].
§11 Figure 2 shows the coating response of zinc electroplated steel coated with
the three lacquers, A, B and C. Lacquer A shows no degradation during the
test. Lacquer B shows no penetration over the time period of the test, but
clearly shows coating degradation. Lacquer C shows a sudden drop in coating
resistance during the test, indicative of penetration of the pencil through to
the metallic substrate.
§12 Figure 1 shows an example of an EIS Nyquist response obtained from lacquer
C after 60 minutes of exposure. The response covers the frequency range of
30KHz down to 1Hz. Two semicircles are clearly visible. The high frequency
semicircle, with Z imaginary max occurring at a value of about 10,000Hz, can
be attributed to the response of the lacquer coating[3,4,5]. The lower
frequency circle, with Z imaginary max at about 100Hz, can be attributed to
the corrosion of the substrate, presumably the zinc electroplate film. A value
for the coating resistance can be calculated from the first semicircle.
§13 From the coating resistance EIS results presented in Figure 2, it can be
noted that the three lacquer coatings discussed here produce different modes
of behaviour. As stated previously, these are typical examples from a larger
group of lacquers. The three modes of behaviour are:
1. Lacquers showing little or no degradation (reduction in coating
resistance) with exposure time. This is shown by lacquer A, where little
or no decline from the initial coating resistance of about 4000W.cm2.
Lacquers showing a steady decline in coating resistance with exposure,
but no sharp drop to a very low value. This is shown by lacquer B, where a
decline in coating resistance from 2000 W.cm2
to approximately 450 W.cm2 is
observed. The environment is progressively degrading the coating,
indicating that the coating would be unsuitable for use in conditions
where contact with perspiration would occur. However, no penetration of
the 2B pencil through to the substrate occurred, and the lacquer would
have passed the standardised conventional test.
Lacquers which show a rapid decline in coating resistance to a value
below 100 W.cm2 after a given period
of exposure, associated with coating degradation, and penetration of the
2B pencil through to the substrate. An example is lacquer C. After 45
minutes of exposure a significant decline in coating resistance can be
observed. This is followed by a fall to less than 100
W.cm2 after 60 minutes of exposure as the 2B pencil
penetrates through the lacquer coating.
§14 Examining the conventional test results (Table 2), it can be noted that
there are substantial problems associated with using this test to examine thin
colourless transparent finishing lacquers. The first problem is that the test
failure definition is based on the visual observation of coating removal. We
have found that with the lacquer types examined it was usually impossible to
detect this. Secondly, the test regime is a pass/fail test. It is not
generally possible to differentiate between different "pass"
coatings. Finally, the test is a qualitative test, in that a judgement of
coating softening is entirely dependant on the person undertaking the test as
they abrade the coating with the 2B pencil.
§15 The use of EIS as a supporting technique provides substantial advantages. A
failure, associated with penetration of the 2B pencil through to the metal
substrate is immediately apparent, with the coating resistance dropping to a
low level, usually less than 100W.cm2.
This can be seen in Figure 2, for lacquer C after 60 minutes of exposure. The
second advantage over the conventional technique is that it enables
differentiation between coatings that would be defined as passing the
conventional test. The conventional test could not differentiate between
lacquers A and B. However, EIS can clearly show that lacquer A has a lower
initial porosity (higher initial coating resistance) than B. Also, lacquer B
degrades over the lifetime of the test, whereas lacquer A does not.
EIS is a useful supporting technique for use with artificial
EIS was better able to determine the failure point of the test, in
comparison to the conventional testing regime.
EIS was able to differentiate between different lacquer coatings that
would be defined as passing the artificial perspiration exposure test.
Both the initial porosity and the resistance to degradation of the coating
could be determined quantitatively.
ANSI/BHMA A156.2-1983 9.7.5.
Volvo Leach Test, STD 5732, 105.
G.W. Walter, J. Electroanal. Chem., 118 (1981) 259.
G.W. Walter, Corrosion Sci., 26 (1986) 681.
F. Mansfeld, Electrochim. Acta, 35 (1990) 1533.
§16 We would like to thank Buko Ltd (Glenrothes, Scotland) for the support and
funding contributed to this project.
Table 1 ANSI artificial perspiration solution.
* because of the nature of the lacquer systems examined it
was very difficult to confirm a failure visually.
§17 Table 2 Conventional ANSI artificial perspiration
exposure test results.
§18 Figure 1 EIS response for zinc electroplated steel coated with lacquer
C, after exposure to ANSI artificial test solution for 60 minutes.
§19 Figure 2 Mean values of coating resistance against exposure time for the
three lacquer coatings A, B and C, examined in triplicate.