Molecular sieve abstract
Metal surfaces are protected against corrosion by a coating of
molecular sieve, notably a zeolite or a phosphate-containing molecular
sieve, rendered substantially non-porous by the retention (or addition)
of a pore-filling member inside the voids of the molecular sieve
crystal structure. Pore-filling agents convenient for use include
species typically used as structure-directing agents in the synthesis
of zeolites and other molecular sieves. A further aspect of the
invention is a method of protecting a metal surface from corrosion
by crystallizing a molecular sieve in situ on the metal surface.
Molecular sieve claims
What is claimed is:
1. A composition of matter comprising: a substrate of a metal that
is susceptible to corrosion upon atmospheric exposure; and a corrosion-resistant
coating on the surface of said substrate, said coating comprising
a molecular sieve with a pore-filling member retained in its crystal
structure, said pore-filling member being of sufficient molecular
size and in sufficient quantity to render said molecular sieve substantially
non-porous.
2. A composition of matter in accordance with claim 1 in which
said molecular sieve is a zeolite.
3. A composition of matter in accordance with claim 2 in which
said zeolite has a silicon:aluminum atomic ratio of at least about
20:1.
4. A composition of matter in accordance with claim 2 in which
said zeolite has a silicon:aluminum atomic ratio of at least about
50:1.
5. A composition of matter in accordance with claim 2 in which
said zeolite has a silicon:aluminum atomic ratio of at least about
90:1.
6. A composition of matter in accordance with claim 2 in which
said zeolite is aluminum-free.
7. A composition of matter in accordance with claim 2 in which
said zeolite has a topology substantially equal to that of a member
selected from the group consisting of MFI, MEL, MTW, and MTN.
8. A composition of matter in accordance with claim 2 in which
said zeolite has a topology substantially equal to that of MFI.
9. A composition of matter in accordance with claim 1 in which
said molecular sieve is a phosphate-containing molecular sieve selected
from the group consisting of aluminophosphates, silicoaluminophosphates,
metal-containing aluminophosphates, and metal-containing silicoaluminophosphates.
10. A composition of matter in accordance with claim 1 in which
said pore-filling member is an alkylammonium cation.
11. A composition of matter in accordance with claim 1 in which
said pore-filling member is a quaternary ammonium cation having
a molecular weight of at least about 70.
12. A composition of matter in accordance with claim 1 in which
said pore-filling member is a tetraalkylammonium cation in which
each alkyl group contains from 1 to 4 carbon atoms.
13. A composition of matter in accordance with claim 1 in which
said pore-filling member is a quaternary ammonium cation selected
from the group consisting of tetramethylammonium, tetraethylammonium,
tetrapropylammonium, tetrabutylammonium, benzyltrimethylammonium,
and benzyltriethylammonium ions.
14. A composition of matter in accordance with claim 1 in which
said pore-filling member is a tetrapropylammonium ion.
15. A composition of matter in accordance with claim 1 in which
said corrosion-resistant coating has a thickness of from about 0.3
micron to about 300 microns.
16. A composition of matter in accordance with claim 1 in which
said corrosion-resistant coating has a thickness of from about 5
microns to about 100 microns.
17. A composition of matter in accordance with claim 1 in which
said substrate is a member selected from the group consisting of
aluminum-containing metals, iron-containing metals, and zinc-containing
metals.
18. A composition of matter in accordance with claim 1 in which
said substrate is an aluminum alloy.
19. A process for applying a corrosion-resistant coating to the
surface of a metal that is susceptible to corrosion upon exposure
of moisture, said process comprising forming on said surface a layer
of molecular sieve with a structure-directing agent retained in
its crystal structure, said structure-directing agent being of sufficient
molecular size and in sufficient quantity to render said molecular
sieve substantially non-porous.
20. A process in accordance with claim 19 comprising forming said
layer on said surface by: (a) applying to said surface a liquid
comprising said molecular sieve suspended in a curable organic vehicle;
and (b) curing said vehicle to solid form.
21. A process in accordance with claim 20 in which said curable
organic vehicle is a polyurethane.
22. A process in accordance with claim 20 in which said molecular
sieve is a zeolite.
23. A process in accordance with claim 19 comprising forming said
layer on said surface by: (a) immersing said surface is an aqueous
solution of a molecular sieve-forming mixture under conditions that
will cause formation of said molecular sieve from said solution,
said molecular sieve-forming mixture containing a structure-directing
agent of sufficient molecular size and in sufficient quantity to
render said molecular sieve thus formed substantially non-porous;
and (b) drying said molecular sieve without removing said structure-directing
agent therefrom.
24. A process in accordance with claim 23 in which said molecular
sieve is a phosphate-containing molecular sieve selected from the
group consisting of aluminophosphates, silicoaluminophosphates,
metal-containing aluminophosphates, and metal-containing silicoaluminophosphates.
25. A process in accordance with claim 23 in which said molecular
sieve is a zeolite and said molecular sieve-forming mixture is a
mixture of a silicate compound, an aluminate compound, a base, and
a quaternary ammonium hydroxide having a molecular weight of at
least about 70.
26. A process in accordance with claim 25 in which said zeolite-forming
mixture is a mixture of a tetraalkylorthosilicate, an aluminate,
a base, and a tetraalkylammonium hydroxide having a molecular weight
of at least about 70 and said conditions of step (a) comprise a
temperature of from about 80.degree. C. to about 200.degree. C.
27. A process in accordance with claim 25 in which said zeolite-forming
mixture is a mixture of tetraethylorthosilicate, an aluminate, a
base, and tetrapropylammonium hydroxide, and said conditions of
step (a) comprise a temperature of from about 150.degree. C. to
about 200.degree. C.
28. A process in accordance with claim 23 in which said conditions
of step (a) comprise a temperature of from about 80.degree. C. to
about 200.degree. C.
Molecular sieve description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to corrosive metals and to means for treating
such metals to render them resistant to corrosion.
2. Description of the Prior Art
Metal corrosion is a widespread problem throughout the industrialized
world, causing losses amounting to several percent of the gross
domestic product of the typical industrialized country. Many types
of metals are susceptible to corrosion, with aluminum alloys being
prominent examples. The protection of metals against corrosion is
generally achieved by applying a coating to the exposed surface
of the metal to serve as a physical barrier between the metal and
the environment. Organic and inorganic coatings have been used,
as well as coatings of metals that are themselves non-corrosive.
Inorganic coatings and certain metal coatings such as electroplated
hard chrome generally offer the highest wear resistance. The typical
inorganic coatings are chemical conversion coatings, glass linings,
enamels and cement. Chemical conversion coatings are produced by
intentionally corroding the metal surface in a controlled manner
to produce an adherent corrosion product that protects the metal
from further corrosion. Examples are anodization, phosphatization,
and chromatization. Disclosures of conversion coatings are found
in: Aizawa, K., et al. (Henkel Corporation), U.S. Pat. No. 5904785
issued May 18 1999 Rodzewich, E. A., et al., U.S. Pat. No. 5801217
issued Sep. 1 1998 Zheng, H., et al., U.S. Pat. No. 5807430
issued Sep. 15 1998 Isobe, Y., et al., U.S. Pat. No. 55500288
issued Mar. 19 1996 Miller, R. N., et al., U.S. Pat. No. 5399210
issued Mar. 21 1995
Glass linings are used in process industries such as the pharmaceutical
industry, breweries, and food plants where there is concern over
contamination of the product. Enamel coatings, notably porcelain
enamel, are used on appliances and plumbing fixtures. Portland cement
coatings have been used to protect steel and cast-iron water pipes.
To be effective in protecting the underlying metal from corrosion,
the coating must be both resistant to chemicals and impervious to
gases and liquids. At present, enamels are the only inorganic coatings
that meet these requirements. The application of enamel coatings
requires a firing temperature in excess of 500.degree. C., however,
which renders enamels unsuitable for certain metals, notably aluminum
alloys. Accordingly, the most effective inorganic coatings for aluminum
alloys at present are chromate conversion and chromic acid anodization
coatings. Unfortunately, the application of these coatings involves
the use of hexavalent chromium, a carcinogenic substance. This raises
environmental hazards and concerns of worker safety, and for this
reason many of these coatings have had to be replaced.
Molecular sieves are porous inorganic materials that have uniformly-sized
pores whose diameters are either in the micro-size (less than 2
nanometers) or meso-size (2 to 50 nanometer) range. Among the molecular
sieves, the best known are zeolites, which are members of a family
of minerals known as tectosilicates and are typically microporous
crystalline materials with high internal surface area formed by
an open, three-dimensional framework consisting of tetrahedral SiO.sub.4
units linked together by shared oxygen atoms. Many naturally occurring
zeolites exist and many synthetic zeolites have been prepared and
are commercially available. In conventional zeolites, a number of
the SiO.sub.4 units in the framework are replaced by AlO.sub.4 units
which bear a negative charge which is neutralized by a cation. The
choice of the cation influences the properties and use of the zeolite.
Many zeolites are synthesized with a high ratio of SiO.sub.4 units
to AlO.sub.4 units, and some are synthesized with no AlO.sub.4 units
at all. The three-dimensional lattice structure, or "topology,"
differs among the various zeolites, and the pore size and pore volume
likewise differ accordingly. Zeolites are most commonly used as
catalysts and separation media, in each case in the form of a powder
composite in which the zeolite is combined with binders. Zeolites
have also been disclosed for use as polycrystalline thin films on
porous ceramic and metal substrates for membrane and membrane reactor
applications. In all of the uses of zeolites to date, the characteristic
or quality of the zeolites that has made them useful has been their
uniform microporosity.
An additional class of molecular sieves is that of phosphate-containing
molecular sieves. In these non-zeolitic molecular sieves, many,
if not all, of the SiO.sub.4 groups are replaced by PO.sub.4 groups
as the major structural components of the lattice structure, and
the resulting structures offer further variations in the topology
relative to zeolites and a wider range of pore structure. These
phosphate-containing materials are often used for the same ion-exchange,catalysis,
and absorption functions as the zeolitic molecular sieves.
SUMMARY OF THE INVENTION
It has now been discovered that corrosion resistance can be imparted
to a metal surface by coating the surface with a substantially nonporous
molecular sieve. The nonporous character is achieved by incorporating
within the voids of the molecular sieve crystal a filler molecule
(which may be referred to as a "pore-filling member")
such as those typically used as structure-directing agents in the
manufacture of synthetic zeolites and phosphate-containing molecular
sieves. The coating is readily applied without the need for extreme
conditions and, once applied, the coating displays excellent cohesion
to the metal surface. In addition, both the coating method and the
coating itself are ecologically harmless. The coating provides corrosion
protection in both acidic and basic environments and in environments
where aggressive pitting occurs, such as those in which chloride
and cupric ions are present. The coating also withstands both thermal
and mechanical stresses.
This invention resides both in molecular sieve-coated metal substrates,
regardless of how the coating is applied to the substrate, and also
in particular methods for applying a corrosion-resistant coating
on metal substrates by crystallizing a molecular sieve directly
on the substrate. Even though molecular sieves are typically sought
for their porosity, this invention demonstrates the unusual discovery
that a non-porous molecular sieve has utility that is derived at
least in part from its lack of porosity, both inter-crystal and
intra-crystal. Utility has thus been achieved by eliminating what
has traditionally been considered one of the most significant and
valued qualities of molecular sieves.
These and other features, advantages, and embodiments of the invention
will become apparent from the description that follows.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an X-ray diffraction pattern of a zeolite coating formed
in accordance with this invention on an aluminum alloy metal surface.
FIGS. 2a and 2b are micrographs of a zeolite coating formed in
accordance with this invention on an aluminum alloy surface, FIG.
2a showing a top view and FIG. 2b showing a side view of the coating.
FIG. 3 is a set of DC polarization curves comparing an aluminum
alloy surface coated with zeolite in accordance with this invention
with other aluminum alloy surfaces, after exposure to different
corrosive media.
FIG. 4 is a set of AC impedance curves comparing an aluminum alloy
surface coated with zeolite in accordance with this invention with
other aluminum alloy surfaces, after exposure to different corrosive
media.
DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS
As indicated above, the term "molecular sieve" is used
herein to denote any of the known inorganic materials that possess
uniform pores with diameters in either the micro (<2 nm) or meso
(2-50 nm) size range. Preferred molecular sieves are those classified
as zeolites and those classified as phosphate-containing molecular
sieves.
Zeolites suitable for use in this invention include any zeolites
that contain pore filler species occupying the openings in the zeolite
crystal structure. The species known in the art as "structure-directing
agents" are particularly convenient for this purpose since
they are commonly used in the preparation of synthetic zeolites.
Accordingly, the most appropriate zeolites are synthetic zeolites,
whose structure, properties, and methods of manufacture are known
among those skilled in the art.
Zeolites with silicon-to-aluminum atomic ratios as low as 1.0 can
be used in the practice of this invention. In certain environments,
however, notably those in which the metal surface has greater susceptibility
to corrosion, zeolites with higher silicon-to-aluminum atomic ratios
are preferred. In these environments, preferred zeolites are those
in which the silicon: aluminum atomic ratio is at least about 20:1
more preferably at least about 50:1 and most preferably at least
about 90:1. Zeolites that are alumina-free can be used as well.
Further preferred zeolites are those whose topology is limited to
relatively small pores, such as those of sodalite-type zeolites
and pentasil-type zeolites. Pentasil-type zeolites whose pores are
in the form of small intersecting channels are particularly preferred.
Using the three-letter code of the International Zeolite Association
(http://www.iza-online.org/), some of the preferred zeolite structures
(followed in parentheses by their industry names) are those of MFI
(ZSM-5), MEL (ZSM-11), MTW (ZSM-12), and MTN (ZSM-39). Zeolites
having topologies that are substantially the same as the topologies
of these four zeolites are preferred for use in this invention.
By "substantially the same" is meant that at least a majority
of the crystal structure is identical, and that the pore arrangement
and size is approximately equal (i.e., within about 20%).
The topology of a given zeolite is conventionally identified by
the X-ray diffraction pattern of the zeolite, and X-ray diffraction
patterns of the zeolites given above are known and available in
the literature for comparison. For example, the X-ray diffraction
patterns and methods of preparation of some of these zeolites are
found in the patent literature as follows: MFI (ZSM-5): U.S. Pat.
No. 3702886 Robert J. Argauer et al., Nov. 14 1972 MEL (ZSM-11):
U.S. Pat. No. 3709979 Pochen Chu, Jan. 9 1973 MTW (ZSM-12):
U.S. Pat. No. 3832449 Edward J. Rosinski et al., Aug. 27 1974.
The disclosures of each of these patents are incorporated herein
by reference.
Phosphate-containing molecular sieves include aluminophosphates
(commonly referred to in the industry as "AlPO.sub.4 "
or "AlPO4"), silicoaluminophosphates (commonly referred
to as "SAPO"), metal-containing aluminophosphates (commonly
referred to as "MeAPO" where the atomic symbol for the
metal is substituted for "Me"), and metal-containing silicoaluminophosphates
(commonly referred to as "MeAPSO"). Aluminophosphates
are formed from AlO.sub.4 and PO.sub.4 tetrahedra and have intracrystalline
pore volumes and pore diameters comparable to those of zeolites
and silica molecular sieves. Similarly to the zeolites, phosphate-containing
molecular sieves that are suitable for use in this invention are
those that contain pore-filling members in the openings throughout
the crystalline structure, and the same "structure-directing
agents" that serve this function in zeolites do so in phosphate-containing
molecular sieves. Examples of known phosphate-containing molecular
sieves that are commercially available (from UOP LLC, Des Plaines,
Ill., USA) and useful in the practice of this invention are those
sold under the following names: AlPO4-5 AlPO4-8 AlPO4-11 AlPO4-17
AlPO4-20 AlPO4-31 AlPO4-41 SAPO-5 SAPO-11 SAPO-20 SAPO-34 SAPO-337
SAPO-35 SAPO-5 SAPO-40 SAPO-42 CoAPO-50
The compositions, physical characteristics, properties, and methods
of preparation of phosphate-containing molecular sieves are known
to those skilled in the art and disclosed in readily available literature.
The following United States patents, each of which is incorporated
herein by reference, are examples of these disclosures: Wilson,
S. T., et al., U.S. Pat. No. 4310440 (Union Carbide Corporation),
issued Jan. 12 1982 Lok, B. M., et al., U.S. Pat. No. 4440871
(Union Carbide Corporation), issued Apr. 3 1984 Patton, R. L.,
et al., U.S. Pat. No. 4473663 (Union Carbide Corporation), issued
Sep. 25 1984 Messina, C. A., et al., U.S. Pat. No. 4554143 (Union
Carbide Corporation), issued Nov. 19 1985 Wilson S. T., et al.,
U.S. Pat. No. 4456029 (Union Carbide Corporation), issued Jan.
28 1986 Wilson, S. T., et al., U.S. Pat. No. 4663139 (Union Carbide
Corporation), issued May 5 1987
The pore-filling agent for any of the molecular sieves addressed
by this invention can be any species that will remain in the molecular
sieve structure and reside in the pores, occupying sufficient pore
volume to reduce the porosity of the molecular sieve substantially
to zero. In the practice of this invention, "substantially
non-porous" means that the pore volume as measured by nitrogen
porosimetry is negligible, and no water (or at most an amount that
is insufficient to cause noticeable corrosion) can penetrate the
coating. Chemical species that are typically used as structure-directing
agents in synthesizing zeolites for other uses can be used here.
Prominent examples are alkylammonium cations, notably quaternary
ammonium cations having molecular weights of at least about 70.
Specific examples are tetramethylammonium, tetraethylammonium, tetrapropylammonium,
tetrabutylammonium, benzyltrimethylammonium, and benzyltriethylammonium
ions. Tetraalkylammonium cations in which each alkyl group contains
from 1 to 5 carbon atoms are particularly preferred. A prominent
example of an alkylammoniun cation is the tetrapropylammonium ion.
Other examples of pore-filling members are tri-n-propylamine and
quinuclidine.
The amount of pore-filling member used in any particular embodiment
of this invention will depend on the nature and the porosity of
the molecular sieve, and will be that amount that is sufficient
to fill the pores and thereby render the molecular sieve substantially
nonporous. The appropriate amount for any particular molecular sieve
will generally be the amount used in the published method for manufacturing
the molecular sieve (as referenced in the patents cited above) and
will be readily apparent to those skilled in the art.
The thickness of the molecular sieve coating may vary depending
on the uses contemplated for the metal surface and on the environment
to which the metal surface will be exposed during use. In most cases,
the appropriate thickness will be in the range of from about 0.3
micron to about 300 microns, preferably from about 5 microns to
about 100 microns.
While methods of forming the coating are disclosed in the patent
references cited above, the coatings can generally be applied either
by depositing pre-formed molecular sieve material over the metal
surface or by crystallizing the molecular sieve in situ on the surface
from an aqueous solution of molecular sieve-forming materials. Since
zeolites for example are compatible with certain organic vehicles,
notably urethane-based paints and resins, the pre-formed zeolite
can be applied as a mixture with the organic vehicle. The vehicle
may also contain pigments or other components for decorative purposes,
and can be applied by brushing, dipping or spraying. Once applied,
the vehicle is either cured or evaporated, depending on the vehicle
itself, leaving a solid coating containing the zeolite. The same
procedure can be used with non-zeolitic molecular sieves.
A preferred method of forming the molecular sieve coating is by
immersing the metal surface in an aqueous solution of molecular
sieve-forming materials, and doing so under conditions that will
cause the materials to crystallize into the appropriate molecular
sieve structure. Molecular sieve-forming materials are known in
the art and cited in the patents referenced above. For zeolites,
preferred materials are mixtures of a silicate compound, an aluminate
compound, a base, and a quaternary ammonium hydroxide having a molecular
weight of at least about 70. Within this class of mixtures, a further
preferred subclass are those that contain a tetraalkylorthosilicate,
a base, an aluminate compound, and a tetraalkylammonium hydroxide.
The immersion temperature and time can vary, and those that will
result in, a zeolite coating of a particular thickness will be readily
apparent to those skilled in the art or readily determined by routine
experimentation. In most cases, the appropriate temperature will
be within the range of from about 80.degree. C. to about 200.degree.
C., preferably from about 150.degree. C. to about 200.degree. C.
For phosphate-containing molecular sieves, the typical molecular
sieve-containing mixture will contain a reactive source of phosphate
(such as P.sub.2 O.sub.5), alumina (Al.sub.2 O.sub.3), and water,
plus a pore-filling agent, all at appropriate proportions selected
to give the desired atomic ratios. When the inclusion of an additional
metal, such as iron, magnesium, manganese, cobalt, or zinc, is desired
in the molecular sieve, the metal may be introduced in the molecular
sieve-forming mixture in the form of the metal salt, oxide, or hydroxide.
Examples are iron oxide; magnesium acetate, bromide, chloride, sulfate,
iodide, or nitrate; manganous acetate, bromide, or sulfate; cobalt
chloride hexahydrate, sulfate, or acetate; cobaltous iodide, sulfate,
bromide, or chloride; and zinc acetate, bromide, formate, iodide,
or sulfate heptahydrate.
This invention is applicable to the treatment of any metal surface
that is otherwise susceptible to corrosion upon exposure to the
atmosphere. Examples are ferrous metals and aluminum-containing
metals. Aluminum alloys are of particular interest.
The following examples are offered for purposes of illustration,
and are not intended to limit the invention.
EXAMPLE 1
A clear aqueous solution was prepared by dissolving aluminum foil
in aqueous sodium hydroxide, then adding tetrapropylammonium hydroxide
(TPAOH) and tetraethylorthosilicate (TEOS) dropwise with constant
stirring, the proportions being such that the resulting aqueous
solution had the composition 0.16 TPAOH: 0.64 NaOH: 1 TEOS: 92 H.sub.2
O: 0.0018Al. A plate of aluminum alloy Al-2024-T3 measuring 2.5
cm.times.5 cm was fixed vertically in a Teflon-lined Parr autoclave,
and the solution was added to immerse the plate. The sealed autoclave
was maintained at 175.degree. C. Within 12 to 24 hours, a high-silica
MFI coating 35 microns in thickness had formed on the plate. A similar
coating was applied using a solution that did not include the aluminum
or the sodium hydroxide (0.32 TPAOH: 1 TEOS: 165 H.sub.2 O). The
sealed autoclave was maintained at 165.degree. C., and within 100
minutes a pure silica MFI coating 0.5 micron in thickness had formed
on the plate. Similar coatings were obtained when the plates were
positioned horizontally or tilted. In each case, the plates upon
removal from the autoclave were rinsed with tap water, then dried
under ambient conditions.
The structure of the 35-micron film was confirmed by X-ray diffraction
as that of MFI (ZSM-5). The X-ray diffraction pattern is shown in
FIG. 1. Micrographs were also taken by a scanning electron microscope
after etching away part of the coating with hydrofluoric acid to
expose the cross section of the coating. FIG. 2a is a top view micrograph
of the coating, while FIG. 2b is a side view of the coating showing
the cross section. The micrographs show that the coating is dense,
polycrystalline, and highly intergrown with no intercrystal porosity.
EXAMPLE 2
This example compares the behavior of various surfaces upon exposure
of these surfaces to different corrosive media. The surfaces include
an aluminum alloy surface coated with zeolite in accordance with
this invention, aluminum alloy surfaces coated by other methods,
and an uncoated aluminum alloy surface.
The first comparison was among DC polarization curves taken on
each surface. These curves are shown in FIG. 3 in which the numbered
curves refer to the aluminum alloys either uncoated or coated with
different coatings and the lower-case letters refer to the corrosion
media, as follows:
Coatings: 1. No coating: bare Al-2024-T3 2. Anodization coating
on Al-5005 anodized sheet obtained from Southern Aluminum Finishing,
Atlanta, Georgia, USA - 18-micron thick coating applied by anodization
with sulfuric acid followed by sealing with hot water (Al-5005 was
used in this test since it has much better corrosion resistance
than Al-2024-T3) 3. Painted chromate conversion coating obtained
from Triple AAA Plating, Inc., Bells, Texas, USA; the paint used
was floss polyurethane oil, supplied by The Thompson Minwax Company,
Memphis, Tennessee, USA 4. Zeolite coating as prepared in Example
1 35 microns in thickness Corrosive media: a. 0.5M aqueous H.sub.2
SO.sub.4 immersion time 20 minutes b. 0.5M aqueous H.sub.2 SO.sub.4
immersion time one day c. 0.5M aqueous NaCl/HCl + 0.26 g/L CuCl.sub.2
(pH = 1) immersion time one day d. 0.1M aqueous NaOH, immersion
time one day for samples 3 (painted chromate conversion coating)
and 4 (zeolite coating), and 20 minutes for samples 1 (bare aluminum)
and 2 (anodization coating) due to the high rate of chemical dissolution
of bare and anodized aluminum in basic solution
The DC polarization tests were conducted at 25.degree. C. with
a sweep rate of 1 mV/s, using a saturated calomel electrode and
a Solartron 1287 potentiostat. FIG. 3 shows that in all cases the
polarization current of the zeolite coating (curve 4 on each section
of the Figure) approaches the detection limit of the potentiostat
(10.sup.-10 amps) causing a noisy signal to appear on the chart.
Comparison of the curves in section "a" of the Figure
reveals that the zeolite coating exhibited greater corrosion resistance
during twenty minutes of sulfuric acid exposure than both the painted
chromate conversion coating and the anodization coating. The superiority
is even more pronounced upon exposure to sulfuric acid for one day
(section "b"). Note that the behavior of the anodization
coating after one day's exposure was closer to that of the uncoated
sample. The zeolite coating (curve 1) was unaffected by the added
exposure time. Although not shown in the figure, it was also determined
that the polarization current of the zeolite coating did not change
even after immersion for ten days in the same medium.
The corrosion medium containing the chloride and cupric ions (section
"c" of the Figure) was used since these ions are known
to facilitate pitting corrosion. Comparison of the curves in this
section shows that the zeolite coating gave superior performance
relative to the painted chromate conversion coating and performed
even better relative to the anodization coating. Although not indicated
in the figure, the anodization coating experienced serious pitting
within one hour. In the basic solution (section "d"),
the anodization coating was completely damaged within twenty minutes
while the polarization current of the zeolite coating increase only
slightly after one day of immersion.
Another comparison was obtained by measuring AC impedance. The
same four aluminum samples were exposed to the same four corrosion
media, and the AC impedance was measured at 25.degree. C. in an
open AC circuit at an amplitude of 10 mV using a saturated calomel
electrode, a Solartron 1260 impedance analyzer, and a Solartron
1287 potentiostat. The results are shown in FIG. 4 which indicates
that in all corrosion media, the zeolite coating exhibited a higher
impedance than both the anodization coating and the bare aluminum
sheet, consistent with the results from the DC polarization studies.
EXAMPLE 3
This example illustrates the high resistance of the zeolite coatings
of this invention to thermal stresses.
A zeolite-coated aluminum plate (35-micron coating thickness) prepared
as described in Example 1 was placed in an oven at 240.degree. C.
for twenty minutes and then quenched in cold water at 20.degree.
C. This treatment was repeated to achieve a total of six heating
and cooling cycles. Inspection of electron micrographs of the coating
revealed no cracks, and corrosion resistance tests performed as
described in Example 2 indicated that the thermal cycling had caused
no change in the corrosion resistance.
The same zeolite-coated plate was also placed in a freezer at -70.degree.
C. for twenty minutes, followed by warming to room temperature for
sixty minutes, for a total of six cycles. Again, the coatings showed
no change in corrosion resistance.
Finally, the zeolite-coated plate was heated in an oven at 240.degree.
C. for one day, and again showed no change in corrosion resistance.
These tests indicate that the zeolite coating is superior to the
literature-reported behavior of chromate conversion coatings and
comparable to anodization coatings.
EXAMPLE 4
This example illustrates the high resistance of the zeolite coatings
of this invention to mechanical stresses.
Aluminum samples coated with a 35-micron thick zeolite coating,
prepared as in Example 1 were tested for bending stress on a standard
three-point bending apparatus. The apparatus was a single-column
Universal Testing Machine, obtained from INTRON Inc., Canton, Mass.,
USA. The distance between the two supporting points on the apparatus
was 3.2 cm, and a sample having a width of 2.5 cm was used. In the
test, the aluminum sheet was bent downward between the two supporting
points, and for downward displacements of up to 6 mm no cracking
or peeling of the coating was observed. Corrosion resistance tests
as described in Example 2 were performed on the samples after bending,
and for downward displacements of up to 1 mm, no decrease occurred
in the corrosion resistance of the coating. These results were similar
to those obtained with anodized aluminum alloy 5005.
Impact tests were performed by striking the coated samples with
hammers having including ball-, flat- and cone-shaped heads. Light
impacts generated a noticeable dent mark on the coating but no cracks,
the size of the mark being comparable to the size of the hammer
head. When heavy impacts were applied, the coating at the impacted
area formed small cracks, but no damage was observed in the areas
surrounding the impacted area.
Cutting tests were also performed by use of a scissors. The result
was only slight and localized cracking (within about 200 microns
of the cutting edge.
When the zeolite coating was manually polished to a shiny luster
with abrasive paper (1200 and 2000 grit silicon carbide), none of
the coating peeled off. Abrasion tests were then performed using
an automatic Buehler laboratory polisher with a 2-micron diameter
alumina suspension, a 150 cm/sec linear velocity, and a 13.1 kPa
load. The abrasion rate under these conditions was 6.5 microns per
hour for the zeolite coating, as compared to 26 microns per hour
for the anodization coating.
The foregoing is offered primarily for purposes of illustration.
Further variations, modifications, and substitutions of the materials,
procedures, and operating conditions that still fall within the
scope of the invention will be readily apparent to those skilled
in the art. |