U.S. patent number 3,632,447 [Application Number 05/039,523] was granted by the patent office on 1972-01-04 for metal-treating process.
This patent grant is currently assigned to Hooker Chemical Corporation. Invention is credited to Joachim Albrecht, Hans Hansen, Dieter Oppen, Werner Rausch, Peter Schiefer.
United States Patent |
3,632,447 |
Albrecht , et al. |
January 4, 1972 |
METAL-TREATING PROCESS
Abstract
A method for treating steel, galvanized steel, zinc and aluminum
sheet and strip with an HF-CrO.sub.3 containing solution wherein
the solution is applied so that no more than about 50 percent of
the applied solution is returned to the treating bath and about
0.1-6 grams/square meter HF and about 0.06-3 grams/square meter
CrO.sub.3 are retained on the metal surface to give a pickling
effect or metal removal of about 0.1 to 2 grams/square meter.
Inventors: |
Albrecht; Joachim (Frankfurt/M,
DT), Hansen; Hans (Rendel B. Badrilbel,
DT), Oppen; Dieter (Weiskirchen B. Offenbach,
DT), Rausch; Werner (Stierstadt/TS, DT),
Schiefer; Peter (Frankfurt/M, DT) |
Assignee: |
Hooker Chemical Corporation
(Niagara Falls, NY)
|
Family
ID: |
5735215 |
Appl.
No.: |
05/039,523 |
Filed: |
May 21, 1970 |
Foreign Application Priority Data
|
|
|
|
|
May 24, 1969 [DT] |
|
|
P 19 26 769.7 |
|
Current U.S.
Class: |
134/3; 134/41;
134/28 |
Current CPC
Class: |
C23G
1/02 (20130101); C23C 22/73 (20130101) |
Current International
Class: |
C23G
1/02 (20060101); C23C 22/73 (20060101); C23g
001/08 () |
Field of
Search: |
;134/3,41,28 ;117/49
;252/142,147 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Scovronek; Joseph
Assistant Examiner: Millman; D. G.
Claims
1. A process for the chemical surface treatment of metals which
comprises pickling the metal surface to be treated with an aqueous
solution containing HF and CrO.sub.3, applying said solution so
that at least about 50 percent by volume remains on the metal
surface, with no more than about 50 percent by volume being
returned to the main body of the treating solution, retaining from
about 0.1 to 6 grams/square meter HF and 0.6 to 3 grams/square
meter CrO.sub.3 on the metal surface and effecting pickling of the
metal surface to the extent of removing from about 0.1 to 2
grams
2. The process as claimed in claim 1 wherein at least about 80
percent by volume of the pickling solution is retained on the metal
surface and no more than about 20 percent by volume is returned to
the main body of the
3. The process as claimed in claim 2 wherein the treating solution
used has a maximum weight ratio of HF:CrO.sub.3 of 1, which
solution is applied so that after drying the solution on the
treated surface, the resulting coating contains from about 6 to 120
milligrams/square meter of hexavalent
4. The process as claimed in claim 2 wherein after the desired
pickling of the metal surface is obtained, the remainder of the
treating solution is removed and the surface is thereafter treated
with a second aqueous solution containing hexavalent chromium to
form a coating which contains from about 6 to 120 milligrams/square
meter of hexavalent chromium,
5. The process as claimed in claim 4 wherein the first treating
solution used has a minimum weight ratio of HF:CrO.sub.3 of 1.
Description
The present invention relates to an improved process for the
chemical surface treatment of steel, galvanized steel, zinc and
aluminum and more particularly relates to a process which is
suitable for the preparation of these materials for the application
of organic coatings.
It is known from Belgian Pat. No. 715,978 to pickle steel, zinc and
aluminum with a solution containing 0.2 to 3 g./l. CrO.sub.3 and
0.5 to 5 g./l. HF to such an extent that the metal quantity removed
by the pickling will be 0.2 to 2 g./m..sup.2, and subsequently, to
bring these materials in contact with an aqueous solution
containing hexavalent chromium, in quantities of preferably 0.6 to
6 g./l. and calculated as CrO.sub.3, and then to dry the materials.
The advantage of this process against the conventional processes is
that the steel, zinc and aluminum are treated with only one
solution and this will in no way impair the resistance
characteristics of the subsequently applied organic coating layers,
with respect to the specific surface treatment processes applied to
the particular type of metal. As in this process only one treatment
has to take place, unlike the conventional three phases of
treatment. Therefore, the investment costs of the apparatus and the
floor space requirements of the process are significantly
reduced.
On the other hand, a drawback of the foregoing process involves a
buildup of detrimental cations in the treatment solution. Thus a
buildup of Fe-III ions will increase the corrosion attack on the
steel and zinc surfaces. Furthermore, the corrosion resistance of
the subsequently applied organic coating layers, such as, for
example, lacquer coatings or polyvinyl chloride or polyvinyl
chloride films, will be impaired. This detrimental effect upon
corrosion resistance may be observed particularly with steel and
aluminum and to a lesser degree also with zinc. Finally, through
the increasing quantities of Fe-III, Al-II and Cr-III ions,
significant quantities of active fluoride complexes will be tied up
and therefore the pickling reaction will lose its efficiency. To
overcome this drawback of the known process, it has been proposed
that the buildup of these detrimental cations may be avoided by
artificially increasing the mechanical losses of the bath, or by
the utilization of a cation exchanger. Additionally, the
continuously developing hexavalent chromium containing solutions
and their subsequent antipollution treatment will require
additional expenditures and involves considerable difficulties.
Furthermore, the utilization of a cation exchanger is rather
difficult due to the fact, that the Fe-III, Al-III and Cr-III ions
in the pickling solution tend to give rise to only slightly
dissociated fluoride complexes. Consequently, removal of these ions
does not appear to be possible within economically feasible
limits.
It was found, however, that in the chemical surface treatment of
steel, galvanized steel, zinc and aluminum, particularly in the
form of continuous strips or bands or in sheet form, the foregoing
drawbacks may be avoided by pickling with a solution containing
hydrofluoric and chromic acids, and by drying upon said surfaces a
solution containing hexavalent chromium, through the application of
an aqueous solution to the metal surface, said solution containing
hydrofluoric and chromic acid, the application being carried out in
such a manner that at most 50 percent, but preferably only 20
percent of the utilized solution will be returned into the
treatment bath, and 0.1 to 6 g./m..sup.2 HF and 0.06 to 3
g./m..sup.2 CrO.sub.3 will remain on the metal surface in order to
give rise to a pickling effect of 0.1 to 2 g./m.sup.2.
The quantities of hydrofluoric acid and chromic acid required
according to the method of the present invention are brought into
contact with the metal surface in the form of an aqueous solution.
The quantity of the solution and the conditions of application are
so selected that at least 50 percent, but preferably 80 percent of
the solution applied to the metal surface will remain upon that
surface for the duration of the pickling reaction. In the
conventional immersion or spraying process after the reaction with
the metal surface more than 90 percent of the treatment solution
will be returned to the storage tank and reused for the next phase
of the process. The remainder stays on the metal surface and will
then be carried over into the rinsing tank. On the other hand,
according to the method of the present invention at most only 50
percent of the pickling solution which comes into contact with the
metal surface, but preferably only 20 percent of it will be
returned to the storage tank and reused for the next pickling. In
this manner any excessive buildup of detrimental cations will be
substantially avoided. The buildup will decrease with decreasing
quantities of solution which are recycled after the reaction.
The concentrations of hydrofluoric and chromic acid in the pickling
solution, according to the method of the present invention, are
desirably in the range of 5 to 30 g./l. HF and 3 to 15 g./l.
CrO.sub.3. Usually, 20 to 400 ml./m..sup.2 aqueous solution is
sufficient to ensure the application of the required HF and
CrO.sub.3 quantities to the metal surfaces. The lower quantities
are appropriately applied when using higher concentrations of
hydrofluoric-chromic acid solutions. These quantities, however,
generally increase when the HF and CrO.sub.3 concentrations are
decreasing toward the lower limit.
The concentrations of HF and CrO.sub.3 in the quantity of liquid
applied to the metal surfaces should be so selected that under
given temperature conditions and reaction times a pickling effect
or metal removal of 0.1 to 2 g./m..sup.2 will be ensured. Below 0.1
g./m..sup.2 the roughening of the metal surface will be
insufficient, therefore, adhesion difficulties may appear during
subsequent coating with organic materials. On the other hand,
pickling effects higher than 2 g./m..sup.2 will lead to the
deposition of pickling residues, particularly on steel and
aluminum, and this may be equally detrimental as far as the
adhesion of organic coating layers is concerned. At the same time
the consumption rate of the chemical ingredient will increase. With
a pickling effect in the range of 0.1 and 2 g./m..sup.2 the metal
surfaces will show a uniform roughening effect without any
noticeable layer formation or pickling residues.
The pickling effect is also a function of the composition of the
treatment solution, the temperature and the reaction time. The
influence of the composition of the treatment solution, and
therefore, the influence upon the pickling effect of the quantities
of HF and CrO.sub.3 applied to the metal surfaces may be seen from
tables 1-3. To obtain these values in every instance 60
ml./m..sup.2 of the various pickling solution compositions were
applied to steel sheet samples of UST 1405/m quality according to
German Industrial Standard (DIN) 1623, page 1, also to galvanized
steel samples with nonchromium treated surfaces and samples of
aluminum quality AlMg.sub.3. After 10 secs. of reaction time the
solution film is rinsed off with water and dried. The pickling
effect was determined from the weight difference of the sample
sheets before and after the treatment.
Raising the temperature will accelerate the pickling reaction.
Preferably the temperature of the pickling solution should be
between 15.degree. and 40.degree. C., even though the process may
be carried out at higher temperatures as well. The temperature,
however, should not exceed about 60.degree. C., as in this range it
is rather difficult to control the desired pickling effect.
##SPC1##
The pickling effect increases rapidly at first with increasing
reaction time, but after about 10 to 30 seconds it will usually
reach a limit which is a function of the composition of the
solution, its temperature and the type of the metal as well. The
increase of pickling effect with reaction time for steel is given
in table 4. These results were obtained on weighed steel samples
degreased with steam, and by applying 60 ml./m..sup.2 aqueous
solution at 20.degree. C., containing 5.3 g./l. CrO.sub.3 and 10
g./l. HF, corresponding to 0.32 g./m..sup.2 CrO.sub.3 and 0.60
g./m..sup.2 HF. Following the different extents of pickling times
the samples were rinsed with water then dried and reweighed.
The process according to the present invention may be carried out
by flooding, pouring or spraying as well as by roller application.
When using the flooding or pouring process usually a higher
recycling rate of the treatment solution from the metal surface
into the storage tank should be taken into account. The recycled
quantity, however, may be reduced by using less excess solution
quantities and by distributing the solution on the metal surface in
a uniform manner with rollers, squeezers or by some appropriate
blowing arrangement.
When using the so-called roll-coat process, then practically 100
percent of the applied pickling solution will remain on the metal
surface. This particular type of application should be carried out
preferably by reverse coating. Altering the revolutions of the
rollers or the relative speed of the application roller to the
material to be treated, the thickness of the liquid film may be
varied. The roll-coat process is less applicable to wavy metal
sheets, as in this instance the treatment solution may not be
applied uniformly. This may result in nonuniform pickling of the
material which, under certain conditions, may give rise to a
negative effect concerning the resistance characteristics of the
subsequently applied organic coating.
In consideration of the foregoing difficulties, it is most
appropriate to apply the solution to the metal surface by spraying.
The diameter, the efficiency and the number of jets as well as the
spraying pressure should be so selected that a uniform film will be
formed on the metal surface, without any substantial quantities of
solution passing by the treated material or running off. Generally,
the distance between the jets and the metal sheet should be 10 to
25 cm. In order to obtain improved uniformity of the sprayed liquid
film, several tubes provided with jets may be arranged in series,
the jets being located at angles which will ensure uniformity. The
reaction solution may be used directly from storage tank to metal
surface without any dilution. On the other hand, a correspondingly
higher concentrated solution may be injected into a water-fed
spraying system. Here, the blending ratio of water to concentrate
should be so adjusted that the solution which leaves the jets
should have the desired concentration of CrO.sub.3 and HF. This
particular method of application will permit considerable volume
reduction in the storage tank.
In the treatment of continuous strips or sheets the application of
the pickling solution may take place either on the upper or on the
lower surface of the materials or again simultaneously on both
sides. With simultaneous application the quantity of liquid applied
to the upper or the lower side of the metal may be identical or
not.
Whenever at least 80 percent of the applied solution remains on the
treated surface, no special control of the composition will be
required. On the other hand, when greater proportions are recycled
from the metal surface, then from time to time the CrO.sub.3 and
free acid contents should be controlled by means of conventional
analytical methods. The replenishment should be carried out
preferably with CrO.sub.3 to Cr (VI) consistency and with HF to
free acid consistency. The replenishing materials may be added to
the bath either individually or in premixed condition.
At the end of the pickling process, the excess solution remaining
on the metal surface may be removed by squeeze rollers, a blowing
arrangement or by some other appropriate devices, and the resultant
solution film is dried. The prerequisite for this particularly
advantageous method of operation is that the pickling solution
should contain maximum 1 part by weight of HF per every part by
weight of CrO.sub.3. It should be particularly noted, that in this
method of operation the two customary stages, i.e., the water rinse
and the after-rinse with CrO.sub.3 containing solutions will not be
required, this again represents considerable savings on
installations, floor space and expenditures otherwise required for
the treatment of chromic acid containing effluents. When using the
above-mentioned HF/CrO.sub.3 ratio, under the given application
conditions hexavalent chromium will be present in the pickling
solution which is applied to the metal surface. When using higher
HF and CrO.sub.3 contents in the pickling solution, it is
appropriate and particularly in the treatment of steel and zinc
surfaces, that the HF/CrO.sub.3 ratio should be adjusted to a value
below 0.9 in order to ensure that after the reaction sufficient
quantities of hexavalent chromium will remain on the surfaces.
According to the method of the present invention, the solution film
is removed from the metal surface to such an extent, e.g., by means
of rubber squeeze rollers, that after drying a coating will be
obtained which contains 6 to 120 mg./m..sup.2 hexavalent chromium,
calculated as CrO.sub.3.
According to another but similarly preferred embodiment of the
method according to the present invention, the HF/CrO.sub.3 ratio
in the pickling solution is adjusted to values above 1. In this
instance, in the treatment of steel and zinc surfaces under
appropriate experimental conditions, for example under extended
reaction times, and also in the treatment of aluminum, the total
chromic acid quantities applied to the metal surface are being used
up in the pickling reaction. The solution which remains after the
reaction may be then removed by squeezing and/or water rinsing. As
in this instance the solution does not contain any more hexavalent
chromium, this solution may be immediately neutralized without any
specific effluent treatment, therefore, the expenditures otherwise
required for a chromic acid reduction process will be saved.
After the removal of the residual solution film from the metal
surface or after rinsing with water, an aqueous solution containing
Cr (VI) is used for after-rinsing, whereby 6 to 120 mg./m..sup.2
hexavalent chromium, calculated as CrO.sub.3, will be applied to
the surface. The solution is then dried. The Cr(VI) content of the
solution calculated as CrO.sub.3, should be preferably between 0.6
and 6 g./l., where the higher contents are used preferably whenever
a squeezing process is employed to ensure the uniformity of the
adhering solution film. The hexavalent chromium may be added to the
bath for example in the form of chromic acid and/or zinc, calcium,
aluminum and chromium-III-chromate. As far as the after-rinse is
concerned, it is sufficient to bring the metal surface into contact
for only a few seconds with the Cr(VI) containing solution. The
temperature of the after-rinse may range from room temperature to
about 60.degree. C. Following the after-rinse, the liquid film
remaining on the metal surface is dried. This may be followed by
the application of lacquers, adhesives for plastic films and
such.
The pickling solution, which contains CrO.sub.3 and HF according to
the process of the present invention, should be applied as far as
possible to degreased surfaces. For degreasing the metal surfaces
such well-known aqueous solutions of alkali cleaning agents are
suitable which contain alkali hydroxides, alkali carbonates, alkali
ortho and condensed phosphates, alkali silicates, alkali borates,
complex-forming as well as surface active substances, and which may
be applied to the metal surfaces by immersion, flooding or spraying
and preferably at elevated temperatures. The degreasing process may
be omitted whenever the metal to be treated is substantially
grease-free, such as for example freshly galvanized steel. The
process according to the present invention is particularly suitable
for the treatment of continuous rolls and sheet metals.
Nevertheless, it is also possible to treat objects of complicated
shape such as for example auto body parts, refrigerator and washing
machine housings and such. In this instance, however, it is
appropriate to degrease the objects with low vaporizing solvents,
for example chlorinated hydrocarbons. The objects should be then
transported preferably into a chamber wherein an HF and CrO
containing aqueous solution is present in fine cloud dispersion. In
this manner, the metal surfaces become uniformly coated with the
pickling solution and the runoff of the solution is substantially
avoided.
In order that those skilled in the art may better understand the
method of the present invention, the following specific examples
are given:
EXAMPLE 1
A 300-mm. wide steel band is treated in the following manner in a
continuous arrangement:
a. Degreasing with an aqueous solution containing 3 g./l. NaOH 3
g./l. Na.sub.5 P.sub.3 O.sub.10, 0.2 g./l. organic complex builder
0.7 g./l. nonionic emulsifier, 14 seconds spraying at 70.degree.
C.;
b. 4 seconds spray-rinsing with cold water;
c. After-degreasing with an aqueous solution containing 1.3 g./l.
NaOH, 1.3 g./l. Na.sub.5 P.sub.3 O.sub.10, 0.08 g./l. organic
complex builder, 0.3 g./l. nonionic emulsifier, 14 seconds spraying
at 70.degree.C.; rinsing according to (b);
d. Pickling with a solution containing 5.3 g./l. CrO.sub.3 and 10
g./l. HF, by spraying with 7 atm. pressure through two series of
jets arranged above and below the steel band. The two jets are
positioned alongside each other at a distance of 150 mm., and the
distance of the jets from the steel band is 150 mm. The performance
of the jets at 7 atm. pressure is 0.1 l./min. Flat spray jets are
used with the angle of opening being 65.degree.. Under these
conditions, a liquid quantity of about 110 ml./m..sup.2 is applied
to the steel band, of this quantity about 105 ml./m..sup.2, i.e.,
95 percent remains on the surface. The solution film which contains
0.56 g. CrO.sub.3 and 1.05 g. HF per m..sup.2 of sheet surface, is
allowed to react for 8 secs. at 25.degree. C.
A separate analytical investigation of the solution film upon the
steel surface after 8 secs. of reaction time gave the following
values:
CrO.sub.3 not detectable Cr (III) 2.3 g./l. Total iron 5.5 g./l. Fe
(II) 1.1 g./l. Fe (III) 4.4 g./l.
e. 4 sec. spray rinse with cold water;
f. After-rinse with an aqueous solution containing 3.4 g./l.
CrO.sub.3, and 0.6 g./l. Cr (III), 7 secs. spraying at 45.degree.
C.;
g. Squeezing with a rubber roller;
h. Drying in air circulation oven, 15 secs. at 85.degree. C.
Following this treatment, the upper as well as the lower surface of
the steel band have indicated clean, uniform and mat pickling
effect. The content of hexavalent chromium was about 30 mg.
CrO.sub.3 per m..sup.2 of surface. Under the particular treatment
conditions described in the foregoing, the pickling effect was
evaluated on steel samples secured onto the steel band. These
samples were the quality UST 1405 m. according to German Industrial
Standard (DIN) 1623, page 1. The result was found to be in the
range of 0.5 and 0.6 g./m.sup.2.
EXAMPLE 2
Samples of steel, hot galvanized steel and aluminum (AlMg.sub.3)
were degreased and rinsed with water in the manner described in
example 1. Subsequently these samples were dipped for 1 sec. at
25.degree. C. into a solution containing 5.3 g./l. CrO.sub.3 and 10
g./l. HF. The HF/CrO.sub.3 ratio was 1.9. The 60 ml./m..sup.2
solution film adhering to the samples after their removal from the
solution was removed after an about 10 sec. reaction time, either
by rinsing with water or by squeezing with rubber rollers. After
the squeezing of the samples a solution film of 5 ml./m..sup.2
remained. On one portion of the samples this resulting solution
film was dried. On the steel, hot galvanized steel and aluminum
sheets, the presence of hexavalent chromium could not be
detected.
The remainder of the samples were dipped into a solution containing
3.4 g./l. CrO.sub.3 and 0.6 g./l. Cr (III), for 8 secs. at
55.degree. C. This solution was then squeezed off and the solution
film of about 5 ml./m..sup.2 was dried for 3 minutes in an oven at
80.degree. C. From the solution quantities remaining on the metal
surface and the CrO.sub.3 concentration, an about 15 mg./m..sup.2
CrO.sub.3 content in the layer was ascertained.
In parallel experiments samples were pickled under identical
conditions, however, with the exception that a 5.3 g./l. CrO.sub.3
and 4 g./l. HF containing pickling solution was used (HF/CrO.sub.3
ratio equals 0.76). The samples were squeezed with rubber rollers
and dried. The CrO.sub.3 content on the surface was found to be
about 10 mg./m.sup.2.
The samples were subsequently lacquered by using a conventional
lacquer of acrylate resin base. Following a storage period of 3
days, the samples were scratched and subjected to a salt-spray test
according to ASTM B 117-64. The results of this test are given in
table 5.
The data in this table clearly indicate that the corrosion
resistance will significantly decrease when omitting the
application of a chromate containing after-rinse when using an
HF/CrO.sub.3 ratio of 1.9, and thereby there will be no CrO.sub.3
present on the surface. On the other hand, with an HF/CrO.sub.3
ratio of 0.76 there will be still sufficient CrO.sub.3 quantities
present on the surface in order to produce satisfactory corrosion
protection even without an after-rinse with a CrO.sub.3 containing
solution. ##SPC2##
EXAMPLE 3
Samples of steel, hot galvanized steel and AlMg.sub.3 were
degreased and rinsed in the manner described in example 1. These
samples (series A) were pickled by an ordinary spray process, i.e.,
with a recycling ratio of over 90 percent, for 11 seconds at
25.degree. C., using a solution containing 0.5 g./l. CrO.sub.3 and
2.5 g./l. HF. Subsequently, the samples were rinsed, after rinsed
and dried in the manner described in example 1.
In the course of a throughput experiment a total of 400 m..sup.2
steel surface per 100 l. of pickling solution was put through the
bath. An ion exchange material was continuously added to the baths.
The volume of the bath and the exchange resin were in 10:1 ratio.
The pickling bath was replenished with CrO.sub.3 and HF to constant
CrO.sub.3 and free acid content.
After about 150 m..sup.2 steel surface throughput per 100 l. bath
solution, the ion exchange resin became saturated with Fe-III ions.
After the total throughput of 400 m..sup.2 per 100 l. bath, an
Fe-III content of 1.3 g./l. was found. At the same time, a second
series of samples of steel, hot galvanized steel and AlMg.sub.3
were pickled and after-treated in this bath in the manner described
for series A (Series B).
For comparison purposes further samples of steel, hot galvanized
steel and AlMg.sub.3 (series C) were subjected to a spray treatment
according to the process of the present invention. The experimental
conditions were identical to those described in example 1 under
treatment steps (a)-(h). The samples were arranged on a running
steel band.
Some of the samples in Series A-C were lacquered with the acrylate
resin lacquer customarily used for the lacquering of the steel
bands, and another portion of the samples after the application of
an adhesive were laminated with a hard-PVC-film.
The samples were prepared and tested in the scratched condition
according to ASTM B 117-64.
The results are presented in table 6 in comparison with the results
obtained from samples only degreased by steam and subsequently
lacquered or laminated with PVC films. ##SPC3##
* * * * *