Volume 2 Paper 6
The Corrosion Resistance of the Different States of Polyaniline Compared with Strontium Chromate in Powder Coating Epoxy on Mild Steel
M.M..Attar, J.D. Scantlebury and J. Marsh
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JCSE Volume 2 Paper 6
Submitted 31st October 1999
The Corrosion Resistance of the Different States of Polyaniline Compared
with Strontium Chromate in Powder Coating Epoxy on Mild Steel
*M.M..Attar, J.D. Scantlebury and J. Marsh
*Polymer Engineering Department, Amirkabir University of Technology,
P.O.Box 15875-4413, Tehran, Iran
Corrosion and Protection Centre, UMIST, P.O.Box 88, Manchester M60 1QD
This study compares polyaniline (PANI) at three levels of protonation 0%,
42% and 50%, with strontium chromate as anticorrosion pigments for
use as primers for mild steel. The binder used is an epoxy powder and
pigments were incorporated during the extrusion stage of powder
manufacture. Application was by electrostatic spray, and curing was for 5
minutes at 180°C giving a final film thickness of 22 μm. A 250 μm
hole was made in some of the panels by a laser. Assessment was by a
standard cathodic disbonding test -1000mV SCE in 3% NaCl, an ASTM
B117 hot salt spray test and a wet dry cyclic test based on Artificial
Manchester Rain Water. In all cases not surprisingly the chromate
system performed well. In the atmospheric tests, the 0% protonated PANI
was seen to be as good as the strontium chromate. In the cathodic
disbonding test, the 0% PANI performed poorly. Possible explanations for
this are given in the text.
§2 Previous work by the same authors has looked at dispersion of protonated
polyaniline, PANI, (42% and 50% protonated ,) in a solvent borne epoxy
system. This produced severe flocculation in the final film and further study
on these systems was abandoned. When attempting to use powder techniques it
was found that the problem of flocculation disappeared and this paper reports
data on the powder coating system. The purpose of this research is to examine
the corrosion resistance of the different states of PANI in comparison with
strontium chromate in a powder epoxy coating, under three conditions of
exposure; namely full immersion in 3%w/w NaCl solution at –1000 mV(SCE), hot
salt spray testing (ASTM B117-95) and a wet-dry cyclic test employing
artificial Manchester rain water .
§3 Keywords: Poly-aniline, Powder epoxy coatings, Strontium chromate,
Corrosion prevention, Cathodic disbonding, Accelerated testing
§4 Preparation of Powder Epoxy Paint
Different states of PANI(0%, 42% and 50% protonated) were synthesised based
on the oxidation reaction using ammonium persulphate. Protonation was
completed using hydrochloric acid producing the chloride salt.
§5 The powder epoxy was produced according to the following process:
All materials including epoxy resin, hardener, flowing agent and pigment
(different states of polyaniline, strontium chromate) were premixed in 2%w/w
concentration, 5 min 1800 rpm, in order to prepare for the next stage of
extrusion which is a homogenous blend of ingredients. Mixed materials were
extruded so that the pigment blends and disperses more efficiently. The
extruded polymer was chipped using a Cumberland Granulater and granulated and
ground to a fine powder by a mini-kek pin disk mill. The final epoxy powder
paint was sieved obtain small particles less than 75 microns in size.
§6 The powder coatings were applied to a thickness of 22±
2 microns using an electrostatic spray gun. The curing schedule used for all
powder coated panels was 15 minutes at 180 °C.
§7 Types and formulation of the powder epoxy paint are as follows:
0% Protonated PANI
42% Protonated PANI
50% Protonated PANI
Epikote 3003 = Epoxy resin
Epikote 3003 -FC-A-10 = Epoxy resin + 10% flow control agent
Epikote 107FF = Accelerated dicyandiamide curing agent
§8 Cathodic disbondment test
The samples were prepared with 2 weight % concentrations of the pigments. A
single holiday of 250 microns was made in the coating of each panel using a
laser (Quanty Ray-Laser, Model GCR-150-20 NBI YAG ). Each sample was coated
with a 3:1 mixture of beeswax and colophony resin rounds the edges leaving a
central area of 36 cm2 unmasked. The sample, a saturated
calomel reference electrode with a Luggin agar gel bridge and a
platinum counter electrode were dipped in 1 litre 3%w/w NaCl solution. The
working electrode, counter electrode and reference electrode were connected to
the terminals of a potentiostat, Thomson model Ministat 253 and maintained at
-1000 mV (SCE). The disbonded area was calculated by weighing tracing paper of
an area equal to the separated layer of coating.
§9 Accelerated testing
Two accelerated atmospheric corrosion tests were performed:
Firstly a 1000h 5% NaCl continuous salt spray test according to ASTM:
B117-95. Both scribed and unscribed samples were tested. Various degradation
features were selected and assigned a number between 1 and 6 based on visual
observation where the higher the number indicates the more corrosion. Specific
features include; corrosion product at the edge, corrosion products at the
scribe, blistering size and extent, black spots not at scribe and edge. Five
samples were tested for each situation and the numbers were added. Thus the
number for best is 6 and worst is 30.
Secondly, a 2000h wet dry cyclic test was performed using Artificial
Manchester Rain Water. Because of the benign nature of the environment only
scribed samples were tested. Cabinet conditions were 2h at 250C
with spray followed by 2h at 350C without spray. Visual assessment
was carried out as previously described except no blistering took place and
was therefore not considered.
§10 Results and Discussion
Figures 1 shows the rates of disbonding over 15 days for the five systems.
Clearly the greatest disbonding occurs with the 0% protonated PANI, the least
disbonding with the strontium chromate. Further, the strontium chromate system
is the only system to exhibit a delay time before disbonding. The rest of the
samples are not too dissimilar. The lack of alkali resistance of the 0%
protonated PANI , is the probable explanation of the poor performance of
this system. The resistance of the chromate system may be explained by a
variety of mechanisms including enhanced adhesion , resistance to cathodic
reduction , improved cross linking of the adjacent coating .
§11 Figure 1: Cathodic disbonding as radial delamination versus time for
various coating samples at -1000 mV (SCE) in 3% NaCl.
§12 Table 1: corrosion performance of the scratched samples after a 1000h
continuous hot salt spray test
Corrosion at edge
Corrosion at scribe
Size/extent of blistering
Isolated black spots
§13 Table 2: The same exposure conditions as Table 1 except that the panels
were unscribed and an assessment of general corrosion is made.
Corrosion at edge
Size/extent of blistering
§14 Table 3: Visual assessment of the various systems after 2000h wet-dry
Corrosion at edge
Corrosion at scribe
Isolated black spots
Examination of the salt spray data produces the following interesting
§15 To achieve a similar degree of degradation, the test period for the cyclic
test had to be twice as long as the hot salt spray test. Even so, the degree
of discrimination is somewhat less in the cyclic test.
§16 The two protonated PANI samples behave significantly worse than the rest of
the samples including the blank in both the cyclic and continuous tests.
§17 Behaviour of the exposed steel adjacent to the coating is an important
feature of an anticorrosion system, i.e. at an edge or a scribe. With the
systems tested, the 0% protonated PANI provides a significant improvement
compared with the blank and the two protonated forms and in certain situations
is comparable or even better than the conventional strontium chromate pigment.
§18 However, the cathodic disbonding experiments provide completely contrary
data. The 0% protonated PANI is now the system with the highest rate of
cathodic disbonding; i.e. the lowest performing system. It is necessary
explain why under free corrosion this system is good and at a relatively
negative potential this system is the worst.
§19 It is likely that the poor performance is associated with the oxygen
reduction reaction which is accelerated at negative potentials. One suggestion
is that the 0% protonated PANI is in contact with the electroactive steel
surface and it catalyses the peroxy intermediates that are a feature of the
oxygen reduction reaction. A second suggestion is that this particular PANI is
unstable at the alkali pH’s generated by the cathodic reduction reaction. A
third suggestion is that this PANI accelerates the oxygen reduction reaction
in general. At this stage it is not possible to be more definitive about
which mechanism is most likely.
§21 Unlike solvent-borne epoxy binders, 0%, 42% and 50% protonated PANI may be
incorporated into powder epoxy systems, provided the PANI is added during the
extrusion stage of manufacture of the powder coating.
§22 The coating containing 0% protonated PANI showed the good protective
properties against a corrosive environment including a conventional hot salt
spray test and a more unconventional wet dry cyclic test Performance was
assessed visually and seemed as good as a more traditional anti-corrosion
pigment containing strontium chromate.
§23 In a cathodic disbonding test, the 0% protonated PANI came out worst.
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