Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a process of post
rinsing conversion layers which have been formed on
surfaces of aluminum or aluminum alloys by a treatment with
solutions based on titanium, zirconium and/or hafnium ions,
and to the use of that process for a pretreatment preceding
a painting, film laminating or adhesive coating operation.
The production of conversion layers on surfaces
of aluminum or aluminum alloys, particularly as a
pretreatment before the application of an organic coating,
is industrially performed on a large scale. Such conversion
layers will prevent or oppose the occurrence of corrosion
and under an organic coating will improve the bond strength
of the coating which has been applied. Whereas said
requirements are sufficiently met, as a rule, by the widely
employed chromate layers, the production of chromium-free
conversion layers is increasingly preferred for reasons of
work place hygiene and environmental protection or because
the treated material is intended to be used for specific
purposes, e.g., for packaging foodstuffs. The treating
solutions for forming chromium-free conversion layers are
generally adjusted to an acid pH value and they contain,
e.g., titanium, fluoride, phosphate and tannin (U. S. Patent
4,017,334) or zirconium, fluoride and boron (U. S. Patent
3,964,936). A treating solution which contains hafnium and
fluoride has been described in French Patent Specification
2,417,537.
The properties of the conversion layers which
have been formed can further be improved by a succeeding
passivating aftertreatment. For the reasons stated, it is
preferred to effect the aftertreatment with chromium-free
treating solutions, which contain organic active
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substances, as a rule. Such an aftertreating solution based
on polyvinylphenol has been described in US-A 4,376 000.
But that solution will pollute the sewage, particularly
because there is a large demand for oxygen required to
decompose the organic component.
It is an object of the invention to provide for
the passivating post rinsing of chromium-free conversion
layers on surfaces of aluminum or aluminum alloys a process
which does not have the disadvantage of post rinsing
solutions that contain organic components as regards the
need for a treatment of the waste water but will result in
an improvement of the corrosion resistance and the bond
strength of paint at least to the same degree.
The object is accomplished in that the process
which is of the kind described first hereinbefore is
carried
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out in such a manner in accordance with the invention that
the conversion layers are aftertreated with aqueous
solutions which contain aluminum, zirconium and fluoride
and have been adjusted to a pH value <_ 5.
More particularly, the present invention proposes
the a process of aftertreating conversion layers which have
been formed on surfaces of aluminum or aluminum.alloys by a
treatment with solutions containing titanium, zirconium
and/or hafnium ions, characterized in that the conversion
layers are aftertreated with aqueous solutions which
contain aluminum, zirconium and fluoride and have been
adjusted to a pH value <_ 5; and wherein the Al: Zr: F mole
ratios of the aqueous solutions have been adjusted to (0,15
to 8) . 1: (to 52).
The process in accordance with the invention can
be used for a passivating aftertreatment of conversion
layers that have been formed on surfaces of aluminum or
aluminum alloys, which materials may consist of massive
aluminum or massive aluminum alloy or of objects which have
been plated therewith, e.g., by hot dip coating. Suitable
elements which can be alloyed with aluminum are
particularly silicon, manganese, magnesium zinc and copper.
The conversion layers formed on such surfaces by
a treatment with solutions based on Ti, Zr and/or Hf
distinguish in that the thickness of the layer is
distinctly below 1 Vim, that the layers are amorphous at
least in part, and that they do not contain chromium. In
addition to titanium, zirconium and/or hafnium ions the
treating solutions employed to form the conversion layers
contain additional film-forming and/or pickling components,
such as fluorides, phosphates and compounds of boron, and
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optionally contain passivating components, such as tannins.
Suitable treating solutions have been described in the
patent specifications mentioned herein before.
The surface on which the conversion layer has
been formed is suitably rinsed with water.
The post rinsing solution may be applied, e.g.,
by dipping, flooding, spraying or roller coating.
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A preferred feature of the invention resides in
that the post rinsing solution employed contains aluminum,
zirconium and fluoride in a total A1 + Zr + F concentration
between 0.1 and 8 g/l, preferably between 0.2 and 5 g/l.
The Al . Zr . F mole ratios should desirably be adjusted to
(0.15 to 8) . 1 . (5 to 52), particularly to (0.15 to
2.0) . 1 . (5 to 16). In a particularly preferred
embodiment of the process the A1 . Zr . F ratios in the
post rinsing solution are (0.15 to 0.67) . 1 . (5 to 7).
According to a further desirable feature of the invention
the pH value is adjusted to 2 to 5.
The post rinsing solutions used in the process in
accordance with the invention contain, inter alia, acid
aluminum fluoride zirconates and in case of a surplus of
aluminum additionally contain other salts of aluminum, such
as fluorides, terafluoroborates, nitrates. They may be
produced, e.g., in that zirconium metal or zirconium
carbonate is initially dissolved in aqueous hydrofluoric
acid to form complex fluorozirconium acid. Aluminum metal
or aluminum hydroxide or an aluminum salt, such as the
nitrate, fluoride, tetrafluoborate, formate, acetate, is
then added, preferably in dissolved form, and is optionally
dissolved. A possible slight cloudiness of the solution
will not adversely affect its effectiveness. Whereas the
preparation is preferably effected on the described route,
the solutions may alternatively prepared in a different
manner.
The pH value of the solution is preferably
adjusted with rations of volatile bases. These include
particularly ammonium, ethanolammonium and di- and
triethanolammonium. The adjustment particularly to
CA 02041891 2000-09-27
relatively high pH values in the stated pH range and to
relatively high concentrations in the range stated for the
total concentration of A1 + Zr + F may result in a
cloudiness of the solution but this will not adversely
affect the effectiveness of the process.
According to a further desirable feature of the
invention the surfaces provided with a conversion coating
are aftertreated with an aqueous solution which
additionally contains at least one of the anions benzoate,
caprylate, ethyl hexoate, salicylate, preferably in a total
concentration of 0.05 to 0.5 g/l. This will further
increase the bare corrosion protection. The anions may be
introduced by means of the corresponding acids or their
salts.
According to a further desirable feature of the
invention the post rinsing solution is applied for between
about 1 and 120 seconds, particularly for between 1 and 30
seconds: The solution may be applied at a temperature
between 20°C and about 80°C. Temperatures between 20 and
50°C are preferred.
Deionized water or low-salt water is preferably
used to prepare the post rinsing bath. Water having a high
salt content is less suitable for the preparation of the
bath.
After the passivating aftertreatment the surface
may be dried, e.g., on the air or in an oven and optionally
may be rinsed before with deionized water. According to a
preferred embodiment of the invention the surface which has
been subjected to the passivating aftertreatment is
subjected to an accelerated drying, e.g., with hot air or
by infrared radiation.
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The process in accordance with the invention
serves primarily to pretreat surfaces of aluminum or
aluminum alloys before they are painted, laminated with a
film or coated with an adhesive. Suitable organic coating
materials include, e.g., polyesters, silicone-modified
polyesters, polyvinylidenefluorides, acrylates, epoxides,
epoxy-phenol resins, plastisols or organosols (e.g., or PVC
or acrylates).
The advantage afforded by the process in
accordance with the invention resides particularly in that
the bond strength of the organic films on the metallic
substrate is increased, as is reflected, e.g., by
correspondingly good results achieved in the T-Bend Test
(ISO 1519-1973) or in the Feathering Test (bond strength of
paint during an opening of can closures). Besides, the
corrosion resistance of the organic films to a corrosive
action, is also increased, e.g., in a test in which an
aqueous condensate is formed under constant air conditions
(DIN 50 017 KK) or a sterilization test. The process in
accordance with the invention also increases the corrosion
resistance of the surface which has not been provided with
an organic coating, as is reflected, e.g., in a test for
well water blackening (no blackening in boiling water).
The process in accordance with the invention will
be explained more in detail and by way of example with
reference to the following example.
Example
To form a conversion coating on cleaned and
degreased aluminum sheets they were dipped for l0 seconds
into a solution which was at a temperature of 50°C and had
the following composition:
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Ti 0.17 g/1
F 1.24 g/1
P205 0.09 g/1
NH4 0.91 g/1
tannin 0.11 g/1
Na 0.003 g/1
biocide 0.10 g/1
Thereafter the sheets were rinsed with water and
were then subjected to a passivating aftertreatment. To
that end the sheets were dipped into the post rinsing
solution for 5 seconds and were subsequently squeeged for a
removal of surplus solution. After a drying at 60°C in an
oven operated with circulating air for 0.5 minute the
sheets were painted with a two-layer coating that is
approved for use with food-stuffs. The first layer of said
coating consists of an epoxyphenol resin and the second
layer consists of an organosol. The coating had a total
thickness between 10 and 15 ~,m.
Circular blanks 60 mm in diameter were
subsequently blanked from the treated sheets, which had a
thickness of about 0.25 mm, and the blanks were deep drawn
to form cups which were 26 mm in diameter and had a height
of 25 mm.
Said cups were subjected to a sterilization test,
in which they were exposed in a pressure container to the
action of an aqueous solution of 3% common salt, 1% citric
acid and 0.5% lactic acid at 121°C for 40 minutes. The
defects subsequently detected on the cups (delamination of
paint, blisters) were rated with reference to a scale from
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1 (delamination of the paint throughout the shell of the
cup = unusable) to 15 (no paint defect = excellent.)
To prepare the post rinsing solution to be
employed, 1.6 g/1 and 3.2 g/1 respectively of an aqueous
concentrate, which contained 0.855% by weight Al and 8.62%
by weight Zr and 10.7% by weight F, was diluted with
deionized water to yield post rinsing solution A and post
rinsing solution B.
Both solutions were adjusted with ammonia to a pH
value of about 3.6.
For comparison, an aftertreatment was effected
with a polyvinylphenol solution having a concentration of
0.6 g/1 and a pH value of about 5 (post rinsing solution
C) .
The post rinsing solutions employed in the
process in accordance invention had the following
with the
compositions:
Post rinsing solution A: Post rinsing
solution
B:
A1 0.014 g/1 A1 0.028 g/1
Zr 0.14 g/1 Zr 0.28 g/1
F 0.17 g/1 F 0.34 g/1
NH4 0.016 g/1 NH4 0.03 g/1
Test results obtained in sterilization test
(Rating scale: 1 = unusable to 15 = excellent)
Post rinsing
Rating
solution
A (invention) 11
B (invention) 10
C (invention) 6
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A comparison of the tabulated data reveals that
the process in accordance with the invention produces
distinctly better results than the comparison example using
an post rinsing solution based on polyvinylphenol.
