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Description  |
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FIELD OF THE INVENTION
This invention pertains to a process for producing iron phosphate coatings
on the surface of metals, particularly, ferrous metals and to compositions
for producing such coatings. The phosphate coating compositions are of the
alkali metal phosphate type, that is, coating compositions in which the
phosphate in the coating solution is present as dissolved alkali metal
phosphate and there is no dissolved metal which forms any substantial
portion of the coating. Such coating compositions are utilized primarily
in the fabricated metals industry where light-weight coatings are desired
in order to facilitate bending and working of the metal and where the
anti-corrosion requirements are less stringent. Phosphate coating
processes employing this type composition are generally operated at
temperatures in the range of about 140.degree. F. to 180.degree. F. at a
pH in the range of about 4.2 to about 5.8.
Phosphate coating compositions which are claimed to operate at even lower
temperatures are known but have not been found generally satisfactory.
Such coating solutions are usually applied by spray to develop coatings of
about 50 mg per sq. ft. in periods of time of about 1 to about 3 minutes.
Accelerators such as chlorates, bromates, nitrates and other oxidizing
agents are employed where heavier coating weights or shorter coating times
are desired.
Iron phosphate coatings are usually applied in five stage systems
comprising: (1) alkaline cleaning; (2) water rinse; (3) coating; (4) water
rinse; and, (5) final rinse with reduced chrome or partially reduced
chrome. Three stage coating systems are also in common use. In such
systems the cleaning and coating operations are combined into a single
step by incorporating cleaning agents into the phosphate coating bath.
Phosphate coating compositions which incorporate as cleaning agents
organic solvents, such as kerosence, tetrahydronapthalene, ethyl benzoate
and the like, along with emulsifying agents or surfactants and other
additives to control foaming or to handle specific cleaning problems
generally utilize reducing agents as accelerators. Hydroxylamine salts are
known for use in such combined cleaning and coating compositions. Where
such compositions are utilized to simultaneously clean and coat ferrous
surfaces, it is frequently the cleaning requirement that controls the
operative temperatures at which the bath is maintained for the spray
process. Generally, in order to achieve effective cleaning of the metal
surface, the temperature of the bath must be maintained at about
140.degree. F. or higher.
In recent years with increased cost of energy, there has been renewed
emphasis on the use of lower temperature processes for producing iron
phosphate conversion coatings. This has led to a search for better
accelerator systems for use with iron phosphate compositions to achieve
effective coatings with good corrosion resistance at lower temperatures.
Among the accelerators generally employed, the chlorates and bromates are
generally regarded as the preferred low temperature accelerators. These,
as well as the other oxidizing agents used as accelerators, are believed
to function by depolarizing hydrogen formed by the action of phosphoric
acid on the ferrous surface and to convert the ferrous ion passing into
solution to ferric ion in order to avoid rapid build-up of ferrous ion
with consequent loss in coating quality. The conversion of ferrous to
ferric ion results in the precipitation of the less soluble ferric form
which must be periodically removed as sludge.
The formation of the insoluble ferric salt causes further dissolution of
the acid phosphate in the coating solution resulting in even more vigorous
attack on the surface and a more rapid coating formation. The mode of
action of the reducing agent accelerators is not fully understood though
generally they are used to accelerate coating formation where the rapid
creation of sludge is a problem and must be minimized. Both the reducing
agents and oxidizing agents are used at about the same pH range; namely,
between about pH 4.5 and 5.8.
While the literature and patent references indicate relatively broad
operable pH ranges, in practice, it is desirable to maintain the pH of the
working bath within a very narrow pH range in order to obtain consistent
coating quality. The usual operating pH in practice is between about 5.0
and 5.5. At this pH the low temperature coating processes such as the
chlorate accelerated iron phosphates are not entirely reliable and bare
spots are often encountered in operation with such systems.
The oxidizing agents which are employed for low temperature iron phosphate
coating formation are not employed in combination with cleaning
compositions, since as noted previously, the cleaning requirement is
generally for higher temperatures and the oxidizing agents tend to
increase the sludge formation. Nor is it known to use a combination of
oxidizing agent and reducing agent accelerators though there are broad
statements concerning accelerator combinations in the literature.
PRIOR ART
The use of hydroxylamine, specifically, hydroxylamine hydrochloride as an
accelerator was first described in U.S. Pat. No. 2,702,768 and
compositions utilizing hydroxylamine accelerators are available,
particularly the clean and coat variety. Hydroxylamine phosphate used as a
reducing rinse following an acid phosphate coating is disclosed in U.S.
Pat. No. 2,928,762.
Oxidizing agents for use as accelerators in iron phosphate coating
compositions are well known. The use of chlorate to provide a light
coating at a temperature of about 160.degree. F. is disclosed in Canadian
Pat. No. 557,727. The use of bromate instead of chlorate enabling the bath
to be operated at a temperature as low as 125.degree. F. is disclosed in
British Pat. No. 884,954. U.S. Pat. No. 3,726,720 discloses a composition
and process for preparing iron phosphate coatings at even lower
temperatures, i.e. in the range of 90.degree. to 110.degree. F. However,
the coating composition for operation at these low temperatures
incorporates a fluorine complex in addition to oxidizing accelerators such
as nitrates, nitrites, chlorates or peroxides and combinations thereof
which may be used. Chlorate accelerated iron phosphate coatings are also
disclosed in British Pat. No. 714,321 which operates at about 140.degree.
F. and a pH in the range of 4.2 to 5.8 with the single example given
operating at a pH of 5.3.
The clean and coat iron phosphate compositions are disclosed, for example,
in U.S. Pat. No. 2,744,555 which uses a pH in the range of 3.5 to 5.8 at a
temperature of about 150.degree. to 170.degree. F. In the single example
given the pH is in the range of 5.3 and there is indication that when the
coating solution does not contain emulsified solvents, pH must be lower
than a pH of 4.5. Thus, whereas the use of hydroxylamine salts in iron
phosphate coating compositions is known and the use of oxidizing
accelerators for low temperature operation is also known, the use of an
oxidizing agent in combination with a reducing agent accelerator,
particularly, hydroxylamine sulfate was not previously known. The general
practice has been to select as accelerator either an oxidizing agent or
combination of oxidizing agents or else a reducing agent or combination of
reducing agents depending upon the particular requirement of the coating
process and the specific benefits provided by these separate types of
accelerators. The two have not been previously regarded as complementary.
It is therefore surprising to find that the combination, in accordance
with the present invention, provides the capability of producing improved
coatings or lower temperatures.
Accordingly, it is an object of this invention to provide a novel iron
phosphate accelerator combination of increased effectiveness for operation
at low temperatures. It is a further object of this invention to produce a
novel accelerated phosphatizing bath and concentrates for producing such
bath. It is yet a further object of this invention to provide a low
temperature process for producing relatively heavier iron phosphate
conversion coatings on ferrous surfaces to impart improved corrosion
resistance thereto. These and other objects will become more clear from
the description of the embodiments of this invention which follows.
SUMMARY OF THE INVENTION
A high quality iron phosphate conversion coating of moderate coating weight
is developed on ferrous metal surfaces by a spray applied sodium acid
phosphate solution employing a combination accelerator comprising
hydroxylamine sulfate and an oxidizing agent such as a chlorate or a
bromate. In one specific embodiment the preferred accelerator combination
is sodium chlorate and hydroxylamine sulfate in a ratio of about 4 parts
by weight of sodium chlorate to 1 part by weight of hydroxylamine sulfate.
The coatings are formed by spray application of a coating solution having
a sodium acid phosphate concentration of about 4% by weight and a total
chlorate/hydroxylamine accelerator concentration of about 0.4% by weight.
The solution is applied at a bath temperature of between 90.degree. F. and
130.degree. F. in a conventional five stage spray treatment employing an
alkaline cleaner and a partially reduced chrome final rinse. The treated
metal is preferably dried at a temperature of about 250.degree. F. to
about 350.degree. F. for a period of about 5 to about 10 minutes. The
conversion coatings produced at low temperatures in accordance with this
invention have unusually good salt spray resistance. Coatings of
equivalent coating weight and salt spray resistance cannot be obtained by
the use of either type accelerator alone.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The accelerated phosphatizing compositions of this invention contain as
essential ingredients an alkali metal phosphate usually the monosodium
acid phosphate prepared by combining phosphoric acid with soda ash, to
which is added an accelerator combination to provide chlorate or bromate
ion and hydroxylamine. The hydroxylamine is added to the composition as
hydroxylamine sulfate. The chlorate and bromate ion are preferably
supplied by adding the sodium salts to the solution. The essential
ingredients can be formulated as a premixed concentrate suitable for
making up the treatment bath by simply diluting with water, or one or more
of the ingredients may be added separately to the bath at make-up. The
composition is generally used for make-up of a bath to be applied in a
five-stage phosphatizing operation. It can also be used in a three stage
operation by combining cleaning ingredients into the phosphatizing bath.
In accordance with the invention, there is provided a single make-up
concentrate with shelf life of at least about one week which when diluted
to application concentration has an even longer shelf life, even if the
bath remains unworked. Optionally, the bath components can be provided in
two or more separate packages combined at make-up. Any suitable phosphate
can be utilized, particularly, the alkali metal phosphates. Sodium is
preferred because of its ready availability. Where the composition is to
be utilized in a three stage operation, there may be added to the
concentrate or to the bath at make-up suitable cleaner ingredients such as
surfactants, for example, the anionic, cationic or nonionic surfactants
and mixtures of such surfactants. The surfactants chosen should be
resistant to acid and oxidizing agents. Generally, if surfactant is
incorporated into the phosphate coating composition, it is present in
about 1 to 10% by weight of the concentrate and is preferably there in
sufficient amount to provide about 0.01% by weight preferably at least
about 0.05 to 0.1% by weight in the coating bath. Other components such as
sodium bisulfate etc. are also included.
The surfactant combination should include specifically a low foaming
surfactant and other ingredients which will be suited to the particular
metal substrate to be cleaned. Generally, however, the combined
accelerator of this invention is intended to be used in alkaline metal
phosphate treatments of the five stage type, that is, where the cleaning
operation precedes the phosphate conversion coating operation. In such
instances, the cleaners preferred are the strong alkaline cleaners which
produce a clean, oil, rust and scale free metal surface which provides
optimum coating results. Suitable cleaners are available for example as
concentrates containing about 30% by weight caustic soda. Silicate
cleaners may also be used. Among the surfactants that can be suitably
employed in the cleaning operation the nonionic surfactants are preferred.
Examples of suitable surfactants are those available under the brand names
of Makon, Plurafac, Tergitol, Triton and Surfonic. The coating can be
conveniently applied through conventional power spray equipment, though
dip, flow coat and other conventional application processes can also be
employed. The concentration of the coating composition in the bath will be
varied depending upon the type of application and particularly the contact
time. Where spray processes are employed, suitable compositions can be
formulated which enable the development of an excellent coating in one
minute contact time.
The bath once it has been made up is easily maintained by replenishing with
additional amounts of the same concentrate with addition of acid or
caustic to control alkalinity and acidity. The operation of the bath is
maintained by monitoring several control parameters, most important of
which are, the pH, alkalinity, the total acidity and free acidity. By free
acidity is meant the number of milliliters of 1/10 normal sodium hydroxide
(points) required to neutralize a 10 mil sample of the solution to a
bromo-phenol-blue end-point. By total acidity is meant the number of mils
of 1/10 normal sodium hydroxide (pointage) needed to neutralize a 10 mil
sample of the solution to a phenolphthalein end-point. By alkalinity is
meant the number of mils of 1/10 normal sulfuric acid or hydrochloric acid
(points) needed to titrate a 10 mil sample of the solution to a
brom-cresol-green end-point.
Generally, the coating baths used in the phosphatizing process of this
invention are made-up at the plant site by dissolving in water a premixed
concentrate with all of the bath components present. Such concentrates
contain sodium acid phosphate prepared by blending phosphoric acid with
the stoichiometric amount of sodium carbonate and adding the accelerators
and such optional ingredients as coloring agents and the like. Suitable
concentrates are formulated to contain between about 0.25 and 0.5 parts by
weight preferably about 0.4 parts by weight of sodium chlorate and between
about 0.05 and 0.25 part by weight preferably about 0.1 part by weight of
hydroxylamine sulfate for each part by weight of sodium acid phosphate. If
desired one or both of the accelerator components can be provided
separately and combinations of oxidizing agent accelerators particularly
chlorate and bromate can be used either by incorporating both into the
concentrate or adding either one or both separately.
Whether provided separately or added to the concentrate the accelerator
combination is used in amounts such that the ratio of hydroxylamine
sulfate to oxidizing agent accelerator is such that the coating bath
contains between about 0.2 and about 5 parts by weight of oxidizing agent,
preferably chlorate, for each part by weight of hydroxylamine sulfate.
Preferably the amount of chlorate or similar accelerator is used in an
amount greater than the amount of hydroxylamine sulfate and preferably in
a ratio of about 2 to 5, suitably 4, parts by weight of sodium chlorate
for each part by weight of hydroxylamine sulfate. The total amount of
chlorate and hydroxylamine should be such as to provide in the bath at
make-up a concentration of total accelerator of at least about 0.2% by
weight and preferably between about 0.4% and 0.8% by weight. Greater
amounts can be used though the advantages of substantially greater amounts
are not correspondingly greater. A typical coating bath utilizing the
accelerator combination of this invention is made up by adding to the bath
sufficient concentrate to provide about 5 to about 20 grams per liter of
sodium acid phosphate and preferably about 7.5 to about 15 grams per
liter; about 3.0 to about 12 grams per liter of sodium chlorate,
preferably about 5 to about 10 grams per liter; and about 0.75 to about 3
preferably about 1.5 to 2 grams per liter of hydroxylamine sulfate. The
bath can also be conveniently controlled by use of electronic control
devices, particularly, pH meters and redox potential meters. By way of
illustration a bath is make up by adding the ingredients either as a
preformulated concentrate or separately in amounts sufficient to provide
about 15 g/l of sodium acid phosphate; about 6 g/l of sodium chlorate and
about 1.5 g/l of hydroxylamine sulfate. The control parameters are
measured by titration and adjusted by addition of acid or caustic as
necessary. The essential control points for operating the bath are total
acidity, alkalinity, temperature, spray time and nozzle pressure. The
total acidity (pointage) should be maintained between about 4 to 12
preferably between about 6 and 8. If the total acidity drops below about
6, it can be adjusted upward by addition of more make-up concentrate. If
the total acidity is high, it will generally be decreased as the bath is
operated. The operating alkalinity range (points) is between about 0.1 and
about 1.0 and preferably between about 0.4 and about 0.7. Alkalinity is
controlled by addition of sufficient amounts of caustic if low or
phosphoric acid if high. The operating pH should be maintained between
about 4.5 and 5.5 and preferably between about 5.0 and 5.5. If the pH is
low, it can be raised by addition of sodium hydroxide or sodium carbonate
solution. If it is high it can be lowered by addition of phosphoric acid.
In making up the bath the control parameters and particularly pH may be
affected by the acidity or alkalinity of the water. It is preferred to use
potable water of about 7 pH, however tap water is generally adequate. Once
the bath is made up and adjusted it can be operated by maintaining the
level through addition of both components at the same concentration as at
make-up with periodic adjustments as necessary to maintain it within the
control parameters indicated. Since some sludge (insoluble iron salts) is
generated during processing, periodic shut down is required for sludge
removal.
The metal such as steel to be coated should be thoroughly cleaned by
treating in a first stage with a moderate to harsh alkaline cleaner
followed by water rinse; for exceptionally heavily soiled pieces a
detergent cleaner can be added in the cleaning stage. The cleaning stage
is preferably a power spray at about 20 psig nozzle pressure with the
cleaner solution at a temperature of about 140.degree. F. to 180.degree.
F. A typical cleaner that can be satisfactorily employed is one prepared
by dissolving in about 6 liters of water about 90 to 100 grams of a
cleaner composed of about 60% by weight tripolyphosphate; about 20% by
weight caustic soda; about 3 to 4% by weight gluconate and the remainder
surfactants such as Plurafac A-38, and Makon NF-12. A low temperature
cleaner (140.degree. F. or lower) is preferred in order to further
conserve energy.
Following the cleaner stage the metal is water rinsed before spray coating.
The coating stage uses conventional power-sprayer processing equipment
operating at 10 to 20 psig nozzle pressure for a contact time of 1 minute.
Shorter or longer contact times can be used depending upon the coating
weight desired. The concentration of the bath and temperature can also be
varied to change coating time. Generally the coating time for spray
application is between about 0.5 to 3 minutes. For dip application the
same coating time can be achieved by using approximately double
concentration. The temperature of the coating solution is maintained
between about 90.degree. F. and about 130.degree. F. preferably between
110.degree. F. and 120.degree. F.
The preferred operating temperature is about 110.degree. F. though the
process can be operated at temperatures as low as 90.degree. F. and less.
At the lower temperatures some loss of coating weight and coating quality
may occur unless spray time or concentration are increased. The
preparation of the metal surface before coating and other factors such as
concentration of other ingredients in the coating composition and the
ratio or amount of accelerator employed all affect the exact choice of
temperature which can be easily varied in practice. The accelerator
combination of this invention is ordinarily used in amounts such that the
chlorate or bromate or combination of chlorate and bromate together with
hydroxylamine is present in the coating bath at about 0.6% by weight
concentration. However, greater or lower amounts can be utilized.
Following the coating stage the metal is water rinsed in a conventional
spray treatment and then treated with a solution of a chromic acid or
partially reduced chrome. The final rinse may be heated to about
140.degree. to 160.degree. F. to facilitate drying. The final rinse may be
a reduced chrome of the type described in U.S. Pat. Nos. 3,189,489 and
3,063,877. Non-chrome final rinses can also be used. The final rinse is
usually at a pH of about 4 to 5. After the final rinse the metal can be
air dried before painting though it is preferably oven dried at a
temperature of 250.degree. F. to 350.degree. F. for about 5 to 10 minutes.
The coated surface provides excellent paint bonding and salt spray
resistance. The process is a relatively low sludge producing operation
apparently providing the reducing agent accelerator advantage of reduced
sludge and also the increased coating weight and shorter coating time of
an oxidizing agent accelerator with the added benefit of better salt spray
resistance. While the process has shown best results on mild steel it can
be used to coat various ferrous alloys particularly steel cabinets,
furniture, lighting fixtures, appliances and the like. Where the
phosphatized metal is to be painted with electrocoat, it is preferred to
add a final rinse with deionized water following the chrome rinse in the
five stage process.
While not intending to be bound by any particular theory, it is believed
that the particular effectiveness of the accelerator combination of this
invention is achieved from the increased rate of metal attack by
phosphoric acid in the bath, brought about by the presence of oxidizing
agents, while simultaneously achieving a valence balance between the
ferrous and ferric form of the iron ion present in the bath as a
consequence of the presence of the reducing agent, hydroxylamine sulfate.
It is believed that the use of the combination accelerator may affect the
ferric/ferrous ion equilibrium at the metal interface, favoring the
development of a higher quality coating. This would appear to be
consistent with the known composition of the iron phosphate coating which
is generally regarded to consist of a complex material containing among
other things, vivianite which contains iron in both valence states, see
for example "Theory and Practice of Phosphating" by Edward A. Rodzewich
from the Educational Series sponsored by the American Electroplaters
Society. In any event, the observed improvement in coating weight and
coating quality obtained by using the accelerator combination indicates a
synergistic effect, whether complementary or stepwise. The combination
allows for the development of a coating of significantly improved salt
spray resistance at a lower temperature than could be obtained with the
use of either accelerator alone. Coating weights in the range of 30 to 50
mg per ft.sup.2 are readily obtained at about 100.degree. F. in one minute
contact time at concentrations of about 4% by weight sodium acid phosphate
and about 0.6% by weight of the combined accelerator. The invention also
contemplates addition of the readily soluble hydroxylamine to any
conventional chlorate accelerated iron phosphate coating process.
Significant improvement in coating weight and salt spray resistance can be
expected in any chlorated accelerated iron phosphate operation by adding
hydroxylamine sulfate in a concentration of about 0.05 to 5% by weight in
the working bath. Similarly iron phosphate processes utilizing
hydroxylamine as accelerator can be improved by addition of chlorate or
bromate in accordance with this improved phosphatizing process.
The invention will be more fully understood from the following examples
which are given by way of illustrating the process and are not to be
considered as limiting the invention.
EXAMPLE 1
A concentrated phosphating composition containing a chlorate accelerator is
prepared by mixing the following:
______________________________________
lbs. % weight
______________________________________
Phosphoric Acid (25% by wgt)
2.942 27.45
Sodium carbonate 0.883 8.24
Chromic nitrate (color additive)
0.007 0.07
Sodium chlorate 1.242 11.59
Water 6.009 56.08
______________________________________
The sodium carbonate is slurried in one-half the formula amount of water
and carefully blended with phosphoric acid by slow addition to control the
effervescence. The remaining water, the chromic nitrate and sodium
chlorate, are mixed with stirring to produce a clear solution having a
specific gravity of 1.289.
The concentrate is utilized to prepare a phosphating bath at a
concentration of 4% by adding 320 ml. of concentrate to tap water to make
up an 8 liter bath. 45 ml. of a caustic soda solution, prepared by adding
2 lbs. of caustic to a gal. of water, is added to the bath. The control
parameters for the bath are measured and found to be as follows:
Free Acid 0.5
Total Acid 12.2
In view of the high total acid an additional 15 mls. of caustic soda
solution is added. The control parameters measured again are as follows:
Alkalinity 0.8
Total Acid 10.7
pH 5.25
Unpolished Q panels of cold rolled steel measuring 4".times.6" are cleaned
in a strong alkaline cleaner at about 160.degree. F. and treated with the
phosphate solution by spraying the solution heated to about 115.degree. F.
for a contact time of one minute using a nozzle pressure of about 20 psig
in a laboratory spray machine. The results are as shown in Table I below.
TABLE I
__________________________________________________________________________
Cleaner
Phosphate Coating
Temp Bath Temp
Alka-
Total Weight
Coating
Panel
(.degree. F.)
(.degree. F.)
linity
Acid
pH (mg/sq ft)
Appearance
__________________________________________________________________________
1 160 113 0.8 10.7
5.25
no coating
almost bare
2 162 115 1.0 10.2
5.46
" "
3 160 115 1.1 10.3
5.54
thin-almost
thin and
none powdery
4 160 115 1.3 10.2
5.64
thin bare spots
"windows"
*5 162 115 -- -- -- " "
6 162 115 -- -- -- " smaller and
less bare
area
7 160 112 1.3 10.3
5.56
" good coating
but thin
8 156 116 -- -- -- " "
9 152 117 -- -- -- 15.9 "
__________________________________________________________________________
*After panel #5 was treated an additional 2 ml. of caustic soda solution
was added to the bath.
To the above bath there is then added 12.5 grams of hydroxylamine sulfate
and 1 ml. of caustic soda to adjust pH. Additional panels are treated with
results as shown in Table II below.
TABLE II
__________________________________________________________________________
Hydroxyl-
Cleaner
Phosphate amine Sul-
Coating
Temp Bath Temp
Alka-
Total fate (% of
Weight
Coating
Panel
(.degree. F.)
(.degree. F.)
linity
Acid
pH formula)
(mg/sq ft)
Appearance
__________________________________________________________________________
1 1.2 12.3
5.29
3.0
2 176 114 1.4 12.2
5.40
3.0 24.3 very good
heavier
coating
3 168 116 " "
4 164 115 " "
*5 152 114 3.0 24.3 very good
heavier
coating
**6 1.2 14.0
5.15
6.0 "
***7 1.6 13.6
5.25
" "
8 167 115 1.8 13.5
5.32
" "
9 160 115 " "
10 158 115 " "
11 1.4 5.25
" "
12 160 110 1.1 14.5
5.04
" "
13 165 118 " "
__________________________________________________________________________
*After panel #5 was coated an additional 12.5 g. of hydroxylamine sulfate
was added to the bath.
**After each of panels 6 and 7 there was added 31/2 mls and 2 mls
respectively of caustic solution to reduce total acid.
***After each of panels 10 and 11 there was added 1 ml and 1.5 ml
respectively of phosphoric acid to reduce the alkalinity. The coatings
obtained with hydroxylamine sulfate additions to the phosphating bath
demonstrated exceptional salt spray resistance when treated in standard
salt spray tests for 96 and 168 hours.
EXAMPLE 2
A phosphating bath is prepared by dissolving in water a sufficient amount
of the chlorate accelerated concentrate described in Example 1 to provide
a bath having a concentration of 4% (vol/vol) which is then heated to
about 120.degree. F. 4".times.6" unpolished cold rolled steel Q panels are
used in each treatment. Three panels each are painted with Steelcase white
paint and three are painted with DuPont Refrigerator White after an iron
phosphate coating with % hydroxlamine sulfate as indicated in Table III
below. Prior to phosphatizing, the panels are cleaned with a strong
alkaline cleaner and water rinsed. The phosphate solution is then spray
applied for one minute contact time. After phosphatizing, the panels are
water rinsed and dip treated with a reduced chrome final rinse. Following
the final rinse the panels are dried at about 250.degree. F. for about
eight minutes. After cooling the panels are painted and the paint cured.
Each panel is then scribed vertically down to the bare metal. The scribed
panels are placed in a conventional salt spray cabinet and subjected to
standard salt spray test for 96 hours in the case of the Steelcase paint
and 168 hours in the case of the DuPont Refrigerator White. The results
are shown in Table III below. Failure is measured from the scribe and
given in 32.sup.nds of an inch. The results are for three panels in each
case.
TABLE III
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% Hydroxy-
Steelcase
DuPont Refrigerator
Coating
lamine Sul-
96 hour
White - 168 hr.
Treatment
Weight
fate pH salt spray
salt spray
__________________________________________________________________________
A 15.9 0 5.56
2 to 5;
3 to 4; 8 max.
4 average
B 24.3 3 5.40
trace to
trace to 1; 2 max.
1 max.
C 24.6 6 5.32
trace to
trace to 1; 3 max.
1;*1 panel
10
D 31.8 6 5.04
trace to 1;
trace to 1; 2 max.
2 max.
__________________________________________________________________________
*This panel showed results inconsistent with the others in the test and i
considered to be a "sport".
EXAMPLE 3
This example is for comparison of iron phosphate coatings prepared from the
combined accelerator of this invention and from a conventional clean and
coat iron phosphate using only hydroxylamine as an accelerator. The test
panels are first cleaned in a strong alkaline cleaner and the phosphating
composition is spray applied. After phosphatizing, each panel is given a
20 to 30 second dip in a partially reduced chrome final rinse. The clean
and coat treatment employed a bath prepared as a 1.5% (wt/vol)
concentration of a clean and coat phosphatizing formulation prepared by
combining 10.6 grams of hydroxylammonium sulfate with 109.4 grams of an
alkali phosphate composition having the following components.
______________________________________
Ingredient grams/kg
______________________________________
monosodium phosphate 704
disodium phosphate 118
Microcel A (calcium silicate)
26
Makon NF 12 10
Plurafac RA43 20
Petro 22 10
______________________________________
The treatment conditions and results are shown in Table IV which follows.
TABLE IV
__________________________________________________________________________
Spray
Coating
Temp.
Total Time
Weight
Coating
Panel (.degree. F.)
Acid
pH (min)
(mg/ft.sup.2)
Appearance
__________________________________________________________________________
One 4".times. 6"
unpolished characteristic
Q panel
118 9.8 5.55
1 20.1 blue, sl.
" 120 2 21.0 "
" 121 3 24.3 "
Six 4".times. 12"
cold rolled
steel panels
121 3 "
__________________________________________________________________________
The treatment according to this invention employed a bath prepared as a 4%
concentration of the chlorate accelerated phosphatizing composition
described in Example 1 and 0.12% by weight of hydroxylamine sulfate. The
results of these treatments are shown in Table V below.
TABLE V
__________________________________________________________________________
Spray
Coating
Temp
Total
Alkalin-
Time
Weight
Coating
Panel (.degree. F.)
Acid
ity pH (min)
mg/ft.sup.2
Appearance
__________________________________________________________________________
4".times. 6"
unposlished
Q panel
121 10.8
0.4 4.95
1 35.1 light blue
" 121 2 47.1 golden
" 122 3 55.5 violet gold
Six cold
rolled steel
panels
122 3 "
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EXAMPLE 4
A concentrate for making up coating baths with the accelerator combination
of this invention is prepared by combining the following ingredients.
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grams/liter
% by weight
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Phosphoric Acid (75% by wt)
346.23 26.64
Sodium Carbonate (light soda ash)
103.92 8.00
Chromic Nitrate (50%)
0.82 0.06
Sodium Chlorate 146.16 11.25
Hydroxylamine Sulfate
38.60 2.97
Water 707.16 54.41
1342.89 103.33
On combining the phosphoric acid
and sodium carbonate, there is a
loss of carbon dioxide
-43.19
1299.70 100.00
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The soda ash is first slurried in a portion of the formulation water and
the phosphoric acid is slowly added to the slurry to control the rate of
carbon dioxide evolution which results. The remaining ingredients are
added in the order indicated and the mixture is stirred until all
ingredients are dissolved. A 1% solution of the concentrate prepared in
this manner in deionized water has a pH of 2.70.+-.0.1 at 25.degree. C.
and Total Acid of 3.6, i.e., requires 3.5.+-.0.2 ml of 0.1 N sodium
hydroxide to titrate to pink with phenolphthalein, specific gravity at
60.degree. F. is 1.303.
The concentrate is sufficiently stable to permit storing for at least one
week. After one week the concentrate may react, generating a gas. To
prepare a coating solution the concentrate is simply added to water to
provide about 4% vol/vol of the concentrate in solution. The bath is
heated to about 90.degree. to 130.degree. F. and spray applied.
EXAMPLE 5
A coating bath prepared from the concentrate of Example 4 at 4% vol/vol was
held for one week without working. Thereafter cleaned test panels were
treated as shown in Table VI below.
TABLE VI
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Coating
Temp
Total Alkalin-
Time
Weight
Coating
Panel (.degree. F.)
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