Note: Descriptions are shown in the official language in which they were submitted.
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Anti-corrosion Pigments
The present invention relates to anti-corrosion pigments based on clays.
Chromate (VI) based anti-corrosion pigments are widely used in primer systems
for
the protection of aluminium surfaces. However, the well-documented toxic and
carcinogenic nature of Cr(VI) means that there is a requirement for effective,
environmentally friendly alternatives. Considerable research is presently
underway to
elucidate the pathways by which water soluble Cr(VI)-oxo species inhibit
corrosion
activity on precipitation hardened aluminium aerospace alloys such as AA2024-
T3, in
order that other inhibitors exhibiting some, if not all, of the attributes of
Cr042- can be
developed.
It has been demonstrated that canon exchange pigments based on naturally
occurring
bentonite clay can efficiently inhibit corrosion driven delamination on
galvanised
steel surfaces. The bentonite ion exchange matrix provides a delivery system
for
inhibitor ions such as rare earth or alkali earth metal cations, which are
only released
when electrolyte is encountered on a corroding metal surface.
We have now found that an anionic clay can be used to generate anti-corrosion
pigments capable of delivering anionic inhibitor species onto corroding metal
surfaces.
According to the invention there is provided an exchangeable anion-bearing
hydrotalcite powder as an anti-corrosion pigment.
The invention also comprises anti-corrosion formulations containing an
exchangeable
anion-bearing hydrotalcite.
The invention also comprises protective coatings incorporating an exchangeable
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anion-bearing hydrotalcite powder as an anti-corrosion pigment.
Hydrotalcite type compounds refer to synthetic lamellar mixed hydroxides
represented by the general formula [M2z+I_x M33+x(OH)2]X+ ~A"-. nH20], where
M2 is
a divalent metal, M3 is a trivalent metal and A is an anion.
The hydrotalcites (HTs) have a layer-type structure similar to brucite
(Mg(OH)Z). Iso-
morphic substitution of divalent metal ions (e.g. Mg2+) with cations of higher
charge
(such as A13+) results in positively charged layers. Electroneutrality is
preserved by
anions (typically carbonate) located within the hydrated interlayer regions.
Any metal with the appropriate valency and suitable properties can be used and
the
preferred metal M2 is magnesium and the preferred metal M3 is aluminium.
Hydrotalcite materials have widespread use in the field of catalysis. Recent
developments in HT based catalysts for a range of organic and inorganic
reactions are
comprehensively reviewed by Vaccari the reference below.
It is well known that hydrotalcite and hydrotalcite like compounds (HTLs) will
decompose in a predictable manner upon heating and that, if the heating does
not
exceed certain temperatures, the resulting decomposed materials can be
rehydrated
(and, optionally, resupplied with various anions, e.g., C03=, that were driven
off by
the heating process) and thereby reproduce the original, or a very similar,
HTL
compound. The decomposition products of such heating are often referred to as
"collapsed," or "metastable," hydrotalcite-like compounds. If, however, these
collapsed or metastable materials are heated beyond certain temperatures
(e.g., 900°
C.); then the resulting decomposition products of such hydrotalcite-like
compounds
can no longer be rehydrated and, hence, are no longer capable of forming the
original
hydrotalcite-like compound.
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Such thermal decomposition of hydrotalcite-like compounds has been carefully
studied and fully described in the academic and patent literature. For
example,
Miyata, "Physico-Chemical Properties of Synthetic Hydrotalcites in Relation to
Composition," Clays and Clay Minerals, Vol. 28, No. 1, 50-56 (1980), describes
the
temperature relationships and chemical identity of the thermal decomposition
products of hydrotalcite in the face of a rising temperature regime.
HTs containing these anions are referred to as exchangeable anion-bearing HTs.
The HT matrix can be used as a generic support material for a wide range of
potential
inhibitor anions, which has the added benefit of enabling the performance of
such
species to be compared under identical conditions; this enables the HT anion-
bearing
pigment to be significantly more versatile than existing pigment technologies
based
on sparingly soluble salts. Any suitable anion can be used; the preferred
anions are
anions with corrosion inhibitor properties, oxidising agents or bases and
preferably
are capable of becoming strongly adsorbed at metal surfaces and (hydr)oxide
covered
metal surfaces and are capable of forming sparingly soluble precipitates with
metal
cations.
Preferred anions include a transition metal oxyanion, ~a group (III) oxyanion,
a group
(IV) oxyanion, a group (V) oxyanion, a group (VI) oxyanion, or a group (VII)
oxyanion e.g. nitrate (N03-), nitrite (NOz-), chromate (Cr04z-), dichromate
(CrzO~z-),
phosphate (P043-), carbonate (C03z-) bicarbonate (HC03-), molybdate (Mo04z-)
permanganate (Mn04z).
The anti-corrosion pigments can be made from commercially available
hydrotalcite
(HT) powder by a process comprising an initial heat treatment stage, followed
by re-
hydration in an aqueous inhibitor anion solution to produce the anion exchange
anti-
corrosion pigment. The hydrotalcite clays which can be used as the anion
exchange
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matrix include commercially available magnesium aluminium hydroxy-carbonate
powders.
Preferably the hydrotalcite clay is calcined (heated in air) to temperatures
between
200°C and 600°C to produce an amorphous mixture of metal
hydroxides. The
amorphous mixture of hydroxides is then cooled to room temperature and
rehydrated
using an aqueous solution containing the desired exchangeable anion. It is
thought
that the rehydration acts to restore the lamella structure, generating
positively charged
magnesium-aluminium hydroxide (Brucite) layers with the exchangeable anion in
the
interlayer spaces.
The pigments of the present invention can be used in a suitable carrier to
form an
anti-corrosion surface coating such as a primer or paint to cover the surface
to be
treated. Any of the typically used carriers can be used depending on the
application.
Preferably the composition is applied as a film and such films can be formed
based
on resins or polymers such as polyvinyl butyral (PVB) films, tall-oil modified
polyester solutions etc.
It is a feature of the present invention that the HT based anion exchange
pigments can
provide an effective delivery system for dispensing inhibitor anions on
corroding
aluminium surfaces and each hydrotalcite-based pigment can produce a profound
inhibition of filiform corrosion (ffc) attack when compared to control samples
coated
with un-pigmented PVB.
The invention is illustrated in the examples.
Examples
Model systems consisting of non-pretreated AA2024-T3 aluminium alloy
substrates
coated with polyvinyl butyral (PVB) films, in which fixed volume fractions of
HT
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based pigments are dispersed, were tested. Following initiation by applying
aqueous
HCl onto a penetrative coating, the performance of inhibitor pigments
dispersed in
polyvinyl butyral (PVB) films was quantified by means of repeated in-situ
scanning
of a fixed sample area using a scanning Kelvin microprobe apparatus. The
preparation is shown schematically in the drawings together with results.
Refernng to the drawings:
Figure 1 is a schematic illustration showing the two stages of HT pigment
preparation
along with the structural transformations involved.
Figure 2 is a schematic diagram showing (a) sample preparation and (b)
experimental
procedure.
Figure 3 shows (a) the quantity of Cr042- released from Cr042- exchanged
calcined
HT, on dispersing lg of HT in 100m1 of O.1M aqueous solutions of Na2C032-,
NaOH
and NaCI and (b) the quantity of Cr042- released from 1 g of Cr04z- exchanged
calcined HT following successive re-dispersions in 100 ml aliquots of 0.1 M
aqueous
Na2C03.
Figure 4 is photographic images showing 1 cm2 areas surrounding a penetrative
scribe
on PVB coated AA2024-T3 aluminium alloy samples after a period of 7 days
following ffc initiation using aqueous HCI, when maintained at a constant
relative
humidity of 93%. The appearance of a sample coated using un-pigmented PVB is
2 5 shown in (a), while samples coated with carbonate, nitrate and chromate
exchanged
HT-containing films at a pigment volume fraction (~) of 0.2 are given in (b),
(c) and
(d) respectively.
Figure 5 shows the quantification of the average corroded sample area for
AA2024-
T3 samples coated using HT pigmented and un-pigmented PVB films after a period
of 7 days following ffc initiation.
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Figure 6 shows FFC delaminated area, quantified by repetitive scanning of a
fixed
sample area by means of a SKP apparatus, plotted as a function of time for PVB
coated AA2024-T3 samples in the presence and absence of HT inhibitor pigment.
Key: (a) unpigmented PVB, (b) C032--HT, (c) N03--HT, and (d) Cr042--HT at
pigment volume fractions of 0.2.
1. Pigment preparation
Hydrotalcite (HT) powder (Mg6A12(OH)16C03.4H20) was obtained from the Aldrich
Chemical Company. Prior to ion exchange, the powder was heat treated at
450°C for
3h, whereupon the layered structure of the hydrotalcite collapses with the
evolution of
C02 and water. lOg of the resultant powder, consisting of an amorphous mixture
of
magnesium and aluminium hydroxides, was then dispersed in 100 cm3 of aqueous
0.1
mol dm 3 solutions of either NaN03, Na2C03 or Na2Cr04 and stirred for 3h. The
rehydration of the heat-treated HT leads to the reconstruction of the layered
structure,
which is accompanied by the intercalation of anions from solution. The two
stages of
anion exchange process are represented schematically in Figure 1. After anion
exchange, the HT powder was exhaustively washed by repeated cycles of
centrifugation and re-dispersion in fresh distilled water until sodium ions
could no
longer be detected in the supernatant when using flame ionisation testing.
Finally the
powders were allowed to dry in air and pulverised to give a particle size of
<20~m
diameter.
Details
1. Hydrotalcite (HT) powder (Mg6A12(OH)16C03.4H20) was calcined at
450°C for
3h, whereupon the layered structure of the hydrotalcite collapsed with the
evolution
of COZ and water. The resultant powder (lOg), consisting of an amorphous
mixture of
magnesium and aluminium hydroxides, was then dispersed in 100 cm3 of aqueous
0.1
mol dm 3 solutions of NaN03 and stirred for 3h. The rehydration of the heat-
treated
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HT produced a reconstruction of the layered structure and anion (N03-)
incorporation.
After anion incorporation, the HT powder was exhaustively washed by repeated
cycles of centrifugation and re-dispersion in fresh distilled water until
sodium ions
could no longer be detected in the supernatant when using a flame emission
test.
Finally the powders were allowed to dry in air and pulverised to give a
particle size of
<20pm diameter. A polyvinyl butyral (PVB) solution was prepared in ethanol
(15%
w/w) and sufficient hydrotalcite powder added, in the form of an ethanolic
slurry, to
give a pigment volume fraction of 0.15 in the final (solvent free) coating.
The
components were mixed thoroughly using a high shear blender to produce an anti-
corrosion paint. The hydrotalcite pigmented PVB solution was bar cast on to a
clean
metal surface and allowed to dry in air. This procedure gave an anti-corrosion
coating
(paint film) with a dried film thickness of 30 ~m as measured using a
micrometer
screw gauge.
2. Hydrotalcite (HT) powder (Mg6Al2(OH)16C03.4H20) was calcined at
450°C for
3h. The resultant powder (lOg), consisting of an amorphous mixture of
magnesium
and aluminium hydroxides, was then dispersed in 100 cm3 of aqueous 0.1 mol dm
3
solutions of Na2C03 and stirred for 3h. The rehydration of the heat-treated HT
produced a reconstruction of the layered structure and anion (C03z-)
incorporation.
2 0 After anion incorporation, the HT powder was exhaustively washed by
repeated
cycles of centrifugation and re-dispersion in fresh distilled water until
sodium ions
could no longer be detected in the supernatant when using a flame emission
test.
Finally the powders were allowed to dry in air and pulverised to give a
particle size of
<20pm diameter. A tall-oil modified polyester solution was prepared in 1:1
ethanolaoluene (15% w/w) and sufficient hydrotalcite powder added, in the form
of
an ethanolic slurry, to give a pigment volume fraction of 0.2 in the final
(solvent free)
coating. The components were mixed thoroughly using a high shear blender to
produce an anti-corrosion paint. The hydrotalcite pigmented polyester solution
was
bar cast on to a clean metal surface and allowed to dry in air. This procedure
gave an
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anti-corrosion coating (paint film) with a dried film thickness of 30 pm as
measured
using a micrometer screw gauge.
3. Hydrotalcite (HT) powder (Mg6A12(OH),6C03.4H20) was calcined at
450°C for
3h. The resultant powder (lOg), consisting of an amorphous mixture of
magnesium
and aluminium hydroxides, was then dispersed in 100 cm3 of aqueous 0.1 mol dm
3
solutions of ammonium molybdate ((NH4)2Mo04) and stirred for 3h. The
rehydration
of the heat-treated HT produced a reconstruction of the layered structure and
anion
(Mo042-) incorporation. After anion incorporation, the HT powder was
exhaustively
washed by repeated cycles of centrifugation and re-dispersion in fresh
distilled water
until ammonia could no longer be detected in the supernatant by alkalisation
and
testing with damp litmus. Finally the powders were allowed to dry in air and
pulverised to give a particle size of <20~m diameter. An ethyl cellulose
solution was
prepared in 1:1 ethanol: toluene (15% w/w) and sufficient hydrotalcite powder
added,
in the form of an ethanolic slurry, to give a pigment volume fraction of 0.15
in the
final (solvent free) coating. The components were mixed thoroughly using a
high
shear blender to produce an anti-corrosion paint. The hydrotalcite pigmented
ethyl
cellulose solution was bar cast on to a clean metal surface and allowed to dry
in air.
This procedure gave an anti-corrosion coating (paint film) with a dried film
thickness
2 0 of 30 pm as measured using a micrometer screw gauge.
2. Sample preparation
All tests were carried out using AA2024-T3 aluminium alloy (composition by
weight
: 0.5% Si , 0.5% Fe, 3.8 - 4.9% Cu, 0.3 - 0.9% Mn, 1.2 - 1.8% Mg, 0.1% Cr,
0.25%
Zn) supplied by BAE Systems Ltd. Samples were cut into 35 mm square coupons
and
abrasively cleaned using an aqueous slurry of 5 pm polishing alumina, followed
by
degreasing in acetone. Poly vinyl butyral (PVB) solutions were prepared in
ethanol
(15% w/w) and any required amount of exchanged hydrotalcite powder added as an
ethanolic slurry. The components were mixed thoroughly using a high shear
blender.
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Pigmented PVB solutions were bar cast on to the clean sample surface as shown
in
Figure 2(a) and allowed to dry in air. This procedure gave a dried film
thickness of 30
~m as measured using a micrometer screw gauge. Filiform corrosion was
initiated by
applying a 1 pl volume of aqueous HCl (0.5 mol dm-3) along the length of a 10
mm
defect, scribed in the centre of the coated sample using a scalpel blade (see
Figure
2(b)). In each case it was ensured that the direction of the scribe was normal
to that of
substrate extrusion. After allowing any excess water to evaporate, the sample
was
placed in the environmental chamber of the scanning Kelvin microprobe (SKP)
apparatus. Repetitive scans were carried out every 4 hours on a 1 cm2 area of
the
coated AA2024-T3 alloy sample encompassing the scribe, using a data point
density
of 10 points per mm and a mean probe to sample height of 100 microns. In
separate
experiments, duplicate samples were initiated by the same procedure and placed
in a
separate environment chamber maintained at a constant relative humidity of
93%.
After a period of 1 week following initiation, these samples were removed and
photographic images of any corroded regions were recorded.
Full details of the SKP instrument used in this work can be found in reference
3
below. The scanning reference probe consisted of a 125 ~m diameter gold wire
vibrated along its long axis and normal to the sample surface with amplitude
40 ~m
and frequency 280Hz. The sample to be scanned was held in a thermostated
stainless
steel environment chamber. A constant relative humidity of 93% was achieved by
the
presence of a reservoir containing saturated Na2S04.1OH20 (aq), maintained at
a
temperature of 20°C. Prior to commencement of a scanning experiment,
the SKP was
calibrated in terms of electrode potential using the established procedure of
reference
3. This involved measuring the free corrosion potential, E~°n (vs. SCE)
and the Kelvin
potential, EKp simultaneously for a series of couples (Ag/Ag+, Cu/Cu2+,
Fe/Fez+ and
Zn/Zn2+). For this purpose a series of calibration cells, were prepared by
machining
wells (8mm diameter, lmm deep) in discs of the relevant metal (15 mm diameter,
5
mm thick). These wells were then filled with a 0.5 mol dm 3 aqueous solution
of the
respective metal chloride salt (0.5 mol dm 3 nitrate salt in the case of Ag).
The
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influence of the PVB coating on calibration was taken into account by placing
a free-
standing PVB film in contact with the meniscus of the calibration electrolyte,
allowing the electrolyte, PVB film and SKP chamber atmosphere to become
equilibrated, and then measuring EKP at the PVB-air interface.
Results
3. HT anion exchange capacity
The anion exchange properties of both calcined and as-received hydrotalcite
was
conveniently studied by spectrophotometric means for samples treated with
aqueous
Cr042- solutions. The UV-visible absorbance of the yellow coloured chromate
anion
at a ~,m~ of 370 nm was used to quantitatively analyse the exchangeable Cr(VI)
content of the HT pigments prepared. As-received commercially available HT
powder does not readily disperse in aqueous media and a chromate containing 50
vol% ethanol/water was used for this material. Attempts to exchange Cr042-
into the
un-calcined HT matrix proved largely unsuccessful and as a consequence, less
than
0.01 mmol of exchangeable Cr(VI) per gram of Cr042'-treated starting material
could
be detected in back exchange experiments employing O.lmol dm'3 Na2C03 aqueous
2 0 solutions.
However, following calcination at temperatures of 400°C or higher,
treatment of the
HT powder with aqueous Cr042- containing solutions leads to a significant take-
up of
Cr(VI) resulting in a yellow colouration of the processed HT powder. An
optimum
calcination temperature of 450°C, giving maximum anion sorption
capacity was used.
Back exchange using a 1 g quantity of powder dispersed in 100 cm3 of 0.1 mol
dm'3
Na2C03 (aq), showed an exchangeable Cr(VI) content of approximately 0.3
mmol/g.
Figure 3(a) shows a comparison of Cr(VI) exchange efficiency in the presence
of
various commonly encountered anions at the same concentration. The effect of
successive back exchanges carried out in aqueous carbonate solution for the
same lg
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quantity of Cr(VI)-HT is given in Figure 3(b). It may be seen that the
majority of ion
exchange occurs within the initial back exchange. The total exchange capacity
of the
Cr(VI)-HT powder, over 4 successive back-exchanges using C032' anions, was
calculated to be 0.7 mol equivalents per gram of pigment, corresponding to
0.35
mmol/g of exchangeable Cr042'.
4. Anti-corrosion performance of HT pi.~ments
The typical appearance of coated AA2024-T3 samples maintained at 93% rh for a
period of 7 days following ffc initiation using aqueous HCl are shown in
Figure 4.
The marked extent of ffc attack observed for the un-pigmented coating is
consistent
with results presented previously for the same model system (References 1, 2).
However, the three samples illustrated in Figures 4(b), (c) and (d), coated
with PVB
films incorporating C032', N03' and Cr04Z' exchanged HT pigments respectively,
show markedly reduced susceptibility to ffc attack. The use of image analysis
software permits quantification of the sample area affected by ffc activity
and the
average results obtained for duplicate test samples coated with the four
different PVB
based films are summarised in Figure 5. Further details of parameters such as
the
average number of filaments per sample are average filament extension and
width is
2 0 provided in Table 1. Although the presence of exchangeable nitrate ions
appear to
more efficiently inhibit ffc when compared to carbonate and a corroded sample
area
basis, there are a significantly greater number of what appear to be filament
initiation
sites. N03' ions appear to markedly limit the size of filaments, though not
their
number, while the presence of C032- acts in the reverse fashion, limiting the
number,
2 5 though not the size of any filaments. As expected, the incorporation of
chromate ions
profoundly inhibits any ffc activity, and the anti-corrosion performance of
the Cr(VI)-
HT pigment is comparable with SrCr04 under the same conditions. However, it
should be noted that the actual Cr042- content of the HT pigment coating,
calculated
on a molar basis, is about 30 times less than for the same polymer system
containing
30 SrCr04 at similar pigment volume fractions. Obviously this has important
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environmental connotations with regard to the long term release of Cr(VI)
species
from anti-corrosion coatings based on these materials.
5. SKP anal si~p~~ment efficiency
Repeated scanning of AA2024-T3 samples coated with unpigmented PVB films and
with PVB films containing the 3 different types of HT based pigments at a
fixed
volume fraction of 0.2, was used to generate a series of time lapse animations
showing dynamic changes in local free corrosion potential (Eon). A methodology
for
quantifying the time-dependent progress of ffc filament populations by
comparing the
spatial distribution of E~o,.r values between successive SKP scans has been
described
in detail in references 22 and 23. Using this approach, the effect of HT
pigmentation
on ffc delamination kinetics is sl-~own in figure 6. The advantage of using
the SKP
technique to quantify delaminated area in terms of Eon, in preference to
visual
inspection methods, is that some areas of underfilm corrosion may be obscured
by the
inclusion of pigment. It may be seen from Figure 6 that for an unpigmented PVB
coating the ffc delaminated area growth is linear with respect to time.
However, for
each HT pigment type, an initial increase of delaminated area with time over
the first
24 h after initiation, is followed by a marked reduction in delaminated area
growth for
further periods of up to 4 days. As observed previously in Figures 4 and 5,
the SKP
analysis confirms that the efficiency of ffc inhibition increases with the
type of
exchangeable anion in the order CO32- < N03-< Cr042-. The potential mode of
operation of an anion exchanged HT inhibitor pigment on a corroding aluminium
surface is thought to proceed in two stages. The first involves the initial
removal of
Cl- ions at the initiation stage and the subsequent release of inhibitor ions
into the
electrolyte covered bare metal defect. The second mode of inhibition occurs in
the
underfilm region when the initiating electrolyte has penetrated the metal-
coating
interface. Upon contact with the chloride-containing electrolyte within a
filament
head, inhibitor anions are released directly into the head regions, along with
concomitant removal of Cl- from the underfilm electrolyte.
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Conclusion
The examples studied the efficiency of carbonate, nitrate and chromate
exchanged HT
pigments in forestalling filiform corrosion on PVB coated AA2024-T3 aluminium
alloy surfaces by a combination of scanning Kelvin probe (SKP) and visual
examination techniques. Samples were initiated by placing a known quantity of
aqueous HCl on to a penetrative coating defect and maintained at a constant
relative
humidity of 93% for periods of up to 1 week.
As can be seen the incorporation of the HT based pigments profoundly inhibits
the
progress of ffc activity for up to 7 days following initiation. SKP
quantification of the
growth of delaminated area with time reveals that for each inhibitor ion, an
initial
increase in delaminated area over an initial 24h period is followed by a
marked slow-
down in rate for the remaining experimental duration. The efficiency of the
inhibition
of ffc is dependent upon the nature of the exchangeable anion present and
increases in
the order CO32~ < N03-< Cr042-.
Table 1: Characteristics of ffc behaviour observed for PVB coated AA2024-T3
samples in the presence and absence of HT inhibitor pigments.
Inhibitor Average Average Number of Corroded
filament filament filaments area (mmz)
extension width (mm)
(mm)
none 2.92 0.93 10 35.32
carbonate 1.63 0.8 4 7.95
nitrate 0.68 0. S 2 12 4.66
chromate 0.9 0.71 1 1.45
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