Transactions of the IMF
The International Journal of Surface Engineering and Coatings
ISSN: 0020-2967 (Print) 1745-9192 (Online) Journal homepage: https://www.tandfonline.com/loi/ytim20
Porosity Measurements on Coatings
P. Leisner & M. E. Benzon
To cite this article: P. Leisner & M. E. Benzon (1997) Porosity Measurements on Coatings,
Transactions of the IMF, 75:2, 88-92, DOI: 10.1080/00202967.1997.11871149
To link to this article: https://doi.org/10.1080/00202967.1997.11871149
Published online: 08 May 2017.
Submit your article to this journal
Article views: 55
View related articles
Citing articles: 1 View citing articles
Full Terms & Conditions of access and use can be found at
https://www.tandfonline.com/action/journalInformation?journalCode=ytim20
LABORATORY
TECHNIQUES
TUTORIAL 6
P. Leisner and
M. E. Benzon
Department of Manufacturing
Engineering, Technical University of
Denmark, DK-2800 Lyngby, Denmark
P. Leisner and M. E. Benzon, Trans IMF,
1997, 75(2), 88
88
Porosity Measurements
on Coatings
INTRODUCTION
A porosity test expresses either the number of pores in a coating or the area of
substrate exposed through pores in the
coating. The function of a porosity test is
to examine the quality of an applied coating. It is usually a destructive test.
There are often very different demands
on bulk and surface properties of functional parts. Usual demands on the bulk
material are: high strength, low weight,
low price, high workability, whereas typical demands on the surface are: high corrosion and wear resistance and decorative
appearance. The most beneficial way to
fulfil the different demands is often by
depositing a metallic coating on a suitable
bulk material.
To maintain a high quality product during the time of its use, it is often important that the substrate is perfectly sealed
by the coating. The appearance of pores
in the coating, reaching the substrate,
could introduce serious corrosion attack
limiting the effectiveness of the coating. If
the coating is more noble (cathodic) than
the substrate the unfavourable anodecathode area ratio will cause serious
localised attacks on the substrate (Figure
1). Even though the coating is not
attacked it might be discoloured by corrosion products, or more seriously, it might
peel from the substrate due to the formation of voluminous corrosion products
under the coating.
If, on the other hand, the coating is the
least noble of the two metals, then it may
act as a sacrificial coating protecting the
substrate (Figure 2), but the pores will
grow and the coating will eventually be
destroyed.
It is clearly an advantage to avoid or at
least to reduce the porosity level in coatings. To do that, it is necessary to consider
the origin of pores. The substrate itself
can contain the source of the porosity e.g.
if it has non-metallic inclusions such as
carbides or slag (Figure 3). The coating
will not deposit on the inclusion, but grow
over it. Depending on the nature of the
deposition process the inclusion might be
sealed, but often a tiny pore will be left.
The same could be the result of insufficient pretreatment leaving areas of surface soiling, oil or oxide films. Pores can
also be caused by sharp topographical
changes resulting from damage or a too
aggressive
mechanical
pretreatment
(Figure 4). A well prepared substrate is
sometimes insufficient, particles or hydrogen bubbles might be included in the coating during the electroplating process
(Figures 5 and 6). This can be avoided by
filtration and agitation, respectively.
Finally, porosity can occur as a consequence of the nature of the deposited
coating. Many metals which have deposition processes accompanied by substantial
hydrogen gas evolution, contain tensile
stress, which often results in the formation
of cracks (Figure 7). Since, it is often
impossible to avoid this crack formation, a
crack-free under-coating can be used to
prevent pores from reaching the substrate.
The aim of this paper is to give an
overview of the many methods of testing
for porosity.,. in coatings and to discuss
whict9..:J!l~ds are recommendable for
different combinations of substrates and
coating. An overview is shown in Table 1.
METHODS OF TESTING FOR
POROSITY
Chamber tests
In chamber-type tests the test samples are
exposed to well-defined atmospheres. In
spray tests a known solution is sprayed.
into the chamber (without spraying
directly on the samples) forming a fog
environment from which droplets are
allowed to settle on the exposed surfaces.
In gas tests a well-defined gas and moist
atmosphere produce the conditions in
which condensate forms on the exposed
surface of test specimens. Substrate metal
revealed at pores is expected to demonstrate its presence by discoloration/dissolution.
a
Neutral salt spray test (NSS) 1
In the NSS test the solution is 50 g/1 NaO
at neutral pH and the temperature is kept
at 35°C. Number and distribution of corrosion failures are evaluated at selected
exposure times typically ranging from 2 to
1000 hours. The NSS test is recommended
for many combinations of substrates and
coatings.
Acetic acid salt spray test (AASS)l
The ASS test is very similar to the NSS
test. The only difference is that the pH of
the solution is adjusted to 3.1-3.3 with
glacial acetic acid which increases the
severity of the test. The ASS test is in particular recommended for decorative
chromiurnlnickel(/copper) coatings, but it
is also suitable for other coatings and
anodised aluminium (see Table 1).
Copper accelerated acetic acid salt spray
test (CASS) 1
The CASS test is similar to the ASS test,
but 0.2 gil of CuCI 2 has been added to the
solution increasing the corrosivity still furTrans IMF, 1997, 75(2)
ther. The CASS test is recommended for
chromium/nickel(/copper) coatings and
anodised aluminium.
Coating (e.g. Ni)
Substrate (e.g. Fe)
Figure 1. Corrosion anack on a substrate with a more noble coating. The current flow is indicated by
the arrows.
Coating (e.g. Zn)
Substrate (e.g. Fe)
Figure 2. Corrosion anack on a coating deposited on a more noble substrate. The current flow is indi·
cated by the arrows.
Table 1. Porosity tests recommended in the literature for different substrate-coating combiutions. Chromium/nickel means a chromium coating over a nickel coating on the substrate.
Substrate
Coating
Porosity test
Aluminium and
aluminium alloys
Chromium/nickel
Chromium/nickeUcopper
Aluminium oxide
Nickel
NickeUphosphorus
ASS, CASS. CORR or SD
ASS, CASS. CORR or SD
ASS
Alizarin
Alizarin
Coper and
copper alloys
Chromium/nickel
Gold
ASSorCASS
Nitric acid vapour, moist S02,
sulphur or electrography
Sulphur
Moist S02or electrography
Moist S02
Moist S02 or sulphur
MoistS02
MoistS02
Nickel
Platinum group metals
Silver
Tin
Tin-nick~!
Tin-lead
1
Tin
Nitric acid vapour, moist S02or
electrography
Moist S02or electrography
Moist S02
Silver
Gold
Platinum group metals
Nickel
Tin
Moist S02, sulphur or electrography
Moist S02or electrography
Sulphur
Sulphur
Steel
Cadmium
Chromium
Nickel-phosphorus
Tin
Tin-lead
Tin-nickel
Tin-nickeUcopper
Zinc
NSSor ASS
SD, water, ferroxyl or copper
electroplating
ASS, CASS, SD, CORR or water
ASS, CASS, SD, CORR or water
Modified ferroxyl or electrography
SD, ferroxyl or water
NSS, ASS, SD or water
NSS, ASS, SD, ferroxyl, water or
electrography
NSS, ASS, SD, ferroxyl or water
SD, moist S02test, ferroxyl or water
SD, moist S02test or water
SD, moist S02test or water
SD, moist S02test or water
NSSor ASS
Chromium/nickeUcopper
ASS, CASS, SD or CORR
Nickel
Gold
Platin~up metals
Chromium/nickel
ChromiumlnickeUcopper
Copper
Gold/nickel
Lead
Nickel
Zinc alloys
TRillS lMF, 1997, 75(2)
Saline droplets corrosion test (SD )2
In the SD test a solution of artificial sea
water is sprayed directly on the test specimens leaving a clearly defined droplet pattern on the surface. The specimens are
then placed in a humidity chamber
(85-95% relative humidity) at room temperature. The specimens are evaluated at
selected exposure times typically ranging
from 2 to 1000 hours. If necessary the
specimens are re-sprayed during the test
to maintain the droplet pattern. The SD
test is suitable for detecting pores in coatings cathodic to the substrate and for testing conversion coatings such as chromate
and phosphate coatings.
Corrodkote corrosion test (CORR)3
In the CORR test the specimens are
painted with a corrosive slurry containing
copper nitrate, iron(m) chloride and
ammonium chloride before being placed
in a humidity chamber (80-90% relative
humidity, 38°C). 16 hours in the chamber
represents one cycle. If more cycles are
applied the specimens are cleaned and the
preparation repeated. The CORR test is
recommended for chromium/nickel(/copper) coatings.
Moist S02 test
The moist so2 test exists in several variants4-7. In all cases the specimens are
placed in a moist so2 atmosphere, but the
so2 concentration and the temperature
varies. The so2 atmosphere can either be
formed by injecting so2 gas directly into
the chamber or by reaction between sulphuric acid and sodium thiosulphate in
the chamber. The duration of the tests
varies and depends on the so2 concentration. It is typically from 8 hours up to several days. The moist so2 test is not
recommended for coatings anodic to the
substrate.
Nitric acid vapour test 1
The specimens are exposed to a chamber
containing 70% HN0 3• After 75 minutes
the specimens are removed and dried in
an oven for 30 minutes and the number of
pores and blisters per area unit are
counted. The test set-up is very simple
and can be done using ordinary laboratory
glassware. This test is recommended for
gold coatings on nickel, copper and copper alloy substrates. Figure 8 shows porosity in a gold coating on brass after testing.
Sulphur tests
Like the nitric acid vapour test the sulphur test is suitable for gold coatings,
but on silver as well as copper and copper alloy substrates. The specimen is
exposed to a chamber containing two
reservoirs: one for water and one for
sublimed sulphur. By heating the chamber to 60°C for some hours pores
become visible by the formation of silver
or copper sulphide.
89
Exposure to liquid environments
Ferroxyl test
The ferroxyl test is specific to the porosity
testing of coated steel substrates. Filter
paper impregnated with a chloride contaimng gel is wetted in a chloride solution
a•·d pressed on to the specimen 9•10• After
10 to 30 minutes the paper is removed
and introduced into a hexacyanoferrate(m) containing solution. Iron(II)
ions that have contaminated the filter
paper through pores will then react with
the hexacyanoferrate to form Prussian
blue. This gives a clear image of the
porosity of the coating.
Since the filter paper has to be pressed
on to the surface in the ferroxyl test, it is
restricted to more or less planar surfaces.
In a variant of the method the specimen is
immersed directly into a solution containing both chloride and hexacyanoferrate(m) for 30 seconds 10• After rinsing
and drying, blue spots are visible at pore
sites.
at different temperatures in aerated
water. In the boiling water test, the specimen is immersed in boiling water for 30
minutes 11 • In the hot water12 and the aerated water11 tests the parameters are
85°C/60 minutes and room temperature/4
hours, respectively. After drying the specimen is examined for rust spots.
Copper electroplating13
This method is used for the determination
of the presence of cracks and pores in
hard chromium coatings on steel. When
copper is deposited from an acid copper
bath at low current density (0.3 A/dm2)
for approximately 1 minute, copper in
deposited at sites where thtstrate is exposed through I,__. the
chromium coating. Thus, inspection by
microscope can be achieved quite easily.
A similar immersion copper process
exists, this is used for checking for porosity of phosphate coatings on steel.
Alizarin test 11
Water tests
For testing the porosity of coatings on
steel substrates there are a number of
water-based tests which can be carried out
The alizarin test is specific for aluminium
and aluminium alloy substrates. After the
specimen has been exposed to a 10%
NaOH solution for a few minutes and
treated with alizarin, pores are visible
red spots.
Electrochemical tests
Common to all the electrochemical tests is
that the specimen is designated an anode
in an electrochemical cell. The coating
should be immune to the electrolyte even
when an anodic potential is applied. Only
the substrate metal exposed to the elec·
trolyte through the pores corrodes. These
tests have been primarily developed for
porosity testing of precious metal coatings.
J;'fectrographic tests
m paper electrographic testing, a paper
soaked in a suitable electrolyte is compressed between the specimen (which is
connected to an anode) and a cathode14·15·16. After current has been passed
for a short time the paper is removed and
developed in an indicator containing solution. An image of the distribution of pores
will then appear on the paper.
Since it is necessary to press the paper
against the specimen, paper electrography
is limited to very simple geometries. To
compensate for this lack of flexibility, an
electrographic test in gelled media has
Figure 3. Electroplating over an inclusion resulting in a pore.
Figure 4. Electroplating over a scratch leaving a pore.
Figure 5. Hydrogen bubble trapped in the coating leaving a pore.
Figure 6. A particle incorporated in the cvating resulting in a pore.
90
Trans IMF, 1997, 75(2)
been developed 17. In this case the soaked
paper has been substituted by a gel containing both the electrolyte and the indicator. When the test is carried out in a
glass container, pores become visible as
coloured spots in the gel. This can be carried out on very complex shaped specimens.
Other electrochemical tests
Figure 7. Under coating corrosion on a steel substrate initiated through cracks in the chromium coating (coating thickness: 20 J..Lm).
In addition to the electrographic methods,
a number of variants of electrochemical
tests based on measurements from electrochemical cells exist. Notter and Gabe•s
have reviewed the electrochemical tests
and divided them into three categories:
(a) Measurement of the corrosion potential, giving information on the ratio of
coating to substrate areas, (b) measurement of corrosion current and (c) measurement of polarisation resistance; (b)
and (c) giving information on the area of
substrate exposed. Combining method (a)
and (c) information can be given on the
porosity of non-conducting coatings over
a metal coated substrate 19. All these tests
are relatively complicated compared to
the information that can be generated
from them and this information rarely
reveals the number of pores. This makes
them less attractive from a practical point
of view and they will therefore not be discussed in further detail here. Information
on those tests is available in the literature1g...24.
Field exposure tests
Field exposure tests are the most realistic,
but also the most time consuming test
methods. In these tests samples are
exposed to environments similar or even
identical to in-service conditions. Outdoor test sites are usually placed in surroundings that can be characterised as
industrial, marine, domestic or rural.
There are also examples of accelerated
field exposure tests, where samples placed
on an out-door test site are periodically
sprayed with sea water25.
Field corrosion tests are usually not carried out as specific porosity tests, but a
consequence of the testing is the generation of information about the porosity.
Figure 8. Porosity in a gold coating on brass after testing in the nitric acid test (21 5x).
Trans IMF, 1997, 75(2)
Inspection
After most tests the test specimen is
rinsed with water, sometimes combined
with a gentle mechanical action. Some
tests require drying in an oven. In other
cases drying with compressed air or natural evaporation is sufficient.
In cases where it is difficult to distinguish a corroded steel substrate from a
corroded copper under coating the specimen can be exposed to ammonia fumess.
This changes the colour of the copper corrosion products to blue, enabling a clearer
determination of the corroded steel.
In the chamber tests and the field exposure tests inspection is carried out with the
unaided eye. The pore density and/or the
area ratio of pores to sound material are
rated according to the international standards ISO 146226 and ISO 454()27, respectively. In both standards, samples are
compared with maps covered with spots
of a well-defined density or area ratio.
91
Coatings that have been exposed to liquid media including the electrographic
methods are usually inspected through a
magnifying glass or an optical microscope
for determination of pore density and size
of pores. Chromium coatings can be
placed under the microscope either asplated or after copper electroplating for
determination of the crack density,
expressed as cracks per em counted along
a line on the surface 28 • This procedure can
also be applied on other coatings forming
micro- or macro-cracks.
For a more detailed investigation of
corrosion in and around pores, cross-sections can be made and studied in the
microscope (Figure 5). The Scanning
Electron Microscope can also be used for
studying details before and after exposure
to a corrosive medium.
CONCLUSIONS
It is clear that among the many porosity
tests some will be more suitable than others for a given situation. Table 1 has been
prepared to help in the process of selecting the right test method depending on
coating and substrate. Often there are several alternative tests to choose between.
In the process of selecting one of these
tests several questions should be taken
into consideration:
Which evaluation criteria are relevant?
92
Can the test be carried out at adequate
speed?
o Are the necessary facilities available for
the test?
The last question is important since
some tests need special designed test
apparatus, e.g. the spray chamber tests,
whereas other tests can be carried out
using existing facilities in an ordinary laboratory.
o
REFERENCES
1. International
Standard
ISO
9227
(1990).
2. International Standard ISO 4536
(1985).
3. International Standard ISO 4541
(1978).
4. International Standard JSO 6988
(1985).
5. International Standard ISO 2179
(1972).
6. ASTM Standard G 87-84 (1984).
7. ASTM Standard B 735-84 (1984).
8. Annual Book of ASTM Standards
Vol 03.04 (1983).
9. International Standard ISO 4526
(1984).
10. International
Standard
ISO
4527
(1985).
11. ASTM Standard B 733-86 (1986).
12. ASTM Standard B 689-81 (1981).
13. International
(1984).
Standard
ISO
6158
14. ASTM Standard B 741-85 (1985).
15. H. W. Hermance and H. V. Wadlow,
Electro-Spot Testing and Electrography, ASTM Spec Tech Publ No 98
(1949).
16. H. J. Noonan, Plating 53(4), (1966),
461-470.
17. F. V. Bedetti and R. V. Chiarenzelli,
Plating 53(3) (1966) 305-308.
18. I. M. Notter and D. R. Gabe, Corr Sci
34(5) (1993) 851-870.
18. I. Notter and D. R. Gabe, Trans lnst
Metal Finish 68 (1990) 59-64.
20. S. C. Shome and U. R. Evans, J
Electropl Depos Tech Soc 27 ((1951)
45.
21. R. A. Ehrhardt, Proc Amx Electropl
Soc 47 (1960) 78.
22. M. Clarke and S. C. Britton, Trans
lnst Metal Finish 36 (1959) 58.
23. M. Clarke and J. M. Leeds, Trans lnst
Metal Finish 43 (1965) 50.
24. R. J. Morrissey, J. Electrochem Soc
117 (1970) 742.
25. P. M~ller and P. Leisner, Proceedings
of Sur/Fin '96, American Electropl;t~nd
Surface Finishers Society,
~rodo (19960), PP 885-890.
26. International Standard ISO •1462
(1973).
27. lntefllational Standard ISO 4540
(198t>).
28. International Standard ISO 6158
(1984).
Trans IMF, 1997, 75(2)