Note: Descriptions are shown in the official language in which they were submitted.
Q60
... .
M~INTAINING ~`ME EFFECTIVENESS OF A COATING COMPOSITION
. _ . _~ .
Field of the Invention
-; - -
This invention relates to the formation of
organic coa-tings on metallic surfaces, More specifical-
ly, this invention relates to the deposition on metallicsurfaces of organic coatings by contacting the metallic
- surfaces with an acidic aqueous coating solution contain-
ing dispersed particles o~ an organic coating-forming
material such as resin particles.
.
A re]atively recent development in the coat-
ing field is the pxovision of water-based coating .
compositions which are effective, without the aid of
electricity, in forming on metallic surfaces immersed
therein organic coatings that increase in thickness or
weight the lonyer the time the surfaces are immersed in
the composition. ~(For convenience, a coating
,' . ~ ' ~' . .
~ .,
for~ea from such'a composition is hereafter referred to as
"an organic coating which grows with time" or as an "autodeposited
coating".) Speaking ~enerally, compositions which are so
effective comprise acidic aqueous coating solutions having
dispersed therein particies of an organic material such as
resin particles. Autodeposited coatings are formecl from such
compositions as a result of their abilit~ to attack and dissolve
from the metallic surface meta], ions in amounts which cause the
particles to deposit on the surface in a manner such that there
is a continuous buildup of organic eoating on the surfa~e.
,, Coatings formed from such eompositions are distinctly
different from coatings formed by immersing the metallic sur-
faees in conventional latiees, that is, compositions compris-
ing solid resin particles dispersed in water. The weight or
thickness of a coating formed b~ immersing a metallic surface
in a conventional latex is not influenced by the time the
surf~ce is immersed in the latex. It is in the main influenced
by the amount of resin solids dispersed in the aqueous medium.
Coatings formed,from'the aforementioned recently
developed coating compositions are also distinctly dif~erent
from coatings formed from earlier known acidic aqueous coat-
ing solutlons'containing dispersed solid resin particles and ' '
relatively high amounts of wat'er soluble corrosi.on inhibit~rs,
such as compounds containing hexavalent chromium. The use of
25 relatively high amounts of corrosion inhibitors in such solu- ,',
tions deters attack of the metallic surface to an extent such :~
that resinous coatings which grow with time are not obtainecl.
Thus, resinous coatings formed by immersing metallic surfaces
in such compositions are like those formed from immersing the ~''
metallic surfaces in con~entional latices in that ~hey do not
grow with time.
.
~0E~1060
The use of the recently developed coating composi-
tions which produce coatings whicll grow with time offer a
number of advantages. For example, other factors held consiant,
they can ~e used to apply thicker organic coatings to the
metallic surface in a shorter period of time and in a one-step
operation. Also, the coating thickness can be controlled by
varying the in~ersion time of the metallic surface in the
coa~ing composition. In general, coatings which have improved
corrosion resistant properties and aesthetic appearance are
ohtainable. These are but a few of the advantages which flow
from the use of said compositions.
One type of composition which is capable of forming
autodeposit~d coatings includes dispersed resin solids, a
so~uble ferric-containing compound (for example, ferric fluoxide)
and acid (for example, HF) and has a pH within the range of
about 1.6 to about 5. Such a composition is the subject of
South African Patent No. 72/1146. It has been found that as
the composition is used continuously to coat metallic surfaces
immersed therein, coatings formed from the composition tend
to become thinner, the longer the use. Continued use of the
composition can result in the destabilization of the composi-
tion as manifested by the tendency of the dispersed solid
particles to flocculate, coagulate or gel throughout the com-
position. This happens notwithstanding that ingredients compris-
ing the composition are replenished, as needed, during use.
The present invention is directed to maintaining theeffectiveness of a composition of the type which forms auto-
deposited coatings on a metallic surface as the composition is
used to coat quantities of metal.
:
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1 0 8 ~ ~ 6 ~
Reported_Developments
U. S. Patent No. 3,791,~31, to Steinbrecher
and Hall, assigned to the same assignee as the present
invention, discloses that coating compositions capable
of for~ing on metallic surfaces immersed therein
autodeposited coatings tend to destabllize as a result
of the buildup of metal ions in the composition as it
is used continuously. The patent further discloses that
the stability of the compositiOn can be maintained by
-inhibiting or deterring the buildup in the composition
of the excessive amounts of metal ions which ~ause
it ~o: become unstable. Exemplary methods for accomplishing
this are disclosed to be removing the excessive amounts of
metal ions from the composition or converting them to
an innocuous for~, that is, a fonn in which they are
ineffective for destabilizing the composition. An example
of the f~rmer method is di,closed to be the use of
a precipitating agent toprecipitate the metal ions.
(See also U. S. Patent No. 3,839,097 which discloses
the use of an ion exchange material to remove metal
ions from the composition.) An example of the latter
method is disclosed to be including in the composition
a chelating agent which complexes the metal ions.
In U. S. patent No. 3,936,546, issued Feb. 3,
1976, in the name of the appl~cant herein, and assigned
to the same assignee as the present invention, it is
disclosed that the effectiveness of a coating composition
which tends to become inoperative as a result of the buildup
of metal ions can be prolonged by adding dispersing agent
thereto in an amount over and above that which wouId be
conventionally
4.
1:
- . . , . ., . , . . . . . ~
- - .
61~
added when replenishing the ingredients of the composition as
they are depleted during use.
U.S. Patent No. 3,709,743 discloses that metal ion
buildup in the composition has a destabilizing effect thereon
and causes coagulation of the composition when the buildup
becomes sufficiently high. The patent discloses further that
the metal ion buildup can be controlled by the use of methods
such as electrodeposition, precipitation, ion exchange, and
ultrafiltration.
The present invention is directed to a method ,or
maintaining the effectiveness of a coating composition which
is capable of forming autodeposited coatings as it is continu-
ously used, which method has certain advantages over other
methods such as those described above.
Summary of the Invention
In accordance with this invention, the effectiveness
of a coating composition of the type which forms autodeposited
coatings on a metallic surface immersed therein is maintained
as the composition is used b~ adding to the composition a
mater:ial which is effective in o~idizing metal ions which
tend to buildup in the composition and adversely affect the
coating capabilities of the composition or cause it to de-
stabllize.
Detailed Description of the Invention
The invention will be described in connection with
the use of an ~cidic aqueous coating composition of the type
,
'.
-5-
. , . . . ,, , ' .,
8 i 0 6 0
which is prepared ~.~ solid resin particles, FeF3, HF and
optionally pigment. (For example, see aforementioned South
A~rican Patent No. 72/1146.) It should be appreciated
that principles underlying the use of the present invention
have applicability to other types of compositions which
are capable of forming autodepo~i~ed coatings.
The aforementiQned composition comprises about 5
to about 550 g/l of resin solids, ferric fluoride in
an amoun~ equivalent to about 0.025 to about 3.5 g/l
ferric iron, preferably about Q.3 to about 1.6 g/l of ferric
i~on,and HF in an amount sufficient to impart to the
composition a pH within the range of about 1.6 to
about 5Ø Suitable pigments can be included in the compo-
sition. Examples of pigments that can be used are
carbon black, phtalocyanine blue, phtalocyanine green,
quinacridone red, Hansa yellow, and benzidine yellow. The
pigment should be added to the composition in an amount which
imparts to the coating the desired color and/or the
desired depth or degree of hue. It should be understood
that the specific amount used will be governed by the
specific pigment used and the color of coating desired.
A preferred composition for use in the practice
of the present invention is described in Canadian
patentapplic~tionSerial No. 248,915, filed March 26,
1976, in the name of Wilbur S. Hall, the applicant herein,
assigned to the same assignee as the preeent invention. The
preferred composition has a pH of about 1,6 to about
4 and is prepared from ~ter ferric fluoride in an amount
such that it contains the equivalent of about 0.5 to
about 3.5 g/l of ferric iron, about 0.2 to about 5 g/l of
HF, optionally, a pigment, for example, carbon black,
and about 50 to about
t ~
., , , ~
~181060
.
100 ~/1 of resin particles which ~re all Q~ substantiall~ the
same size and substantially chemically homo~eneous, that is,
each particl~ is comprised of thç same monomeric constituents
present in substantially the same proportions or resin particles
which are prepared by copolymerizing the follo~ing monomers:
1) about 25 to about 70, and preferably about
40 to about 65 wt. % of a conjugated diene
having, for example, 4 to about 9 carbon
atoms, such as butaæiene or isoprene;
- 2) about 5 to about 70, and preferably about
30 to about 65 wt. % of CH2=CHR, wherein
R is an aryl or a cyano group, for example,
~ styrene or acrylonitrile;
3) about 1 to about 50, and preferably about 3
to about 15 wt. ~ o~ ~ vinyl halide such as
vinyl chloride or vinylidene chloride; and .
4) about 0.5 to about l5c and preferably about
1 to about 4 wt. ~ of a monoethylenically
unsaturated monomer having a functional group
selected rom the class consisting of amide
and carboxylic groups, such as acrylamide,
,~ ~
methacrylamide, octyl acid maleate and mono-
ethylenically unsaturated monocarboxylic and
dicarboxylic acids having about 3 to about
12 carbon atoms, and pre~erably about 3 to
about S carbon atoms, such as, for example:
~ , . .
acrylic acid; cinnamic acid; methacrylic acid;
crotonic acid; itaconic acid; maleic acid;
.. . .
and fuMaric acid.
', ':,
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' " ~
~(J8~060
Tha resin is used most conveniently in the form of a latex
which can be prepa~ed according to available techniques.
A particularly preferred latex contains particles of
resin prepared from the aforementioned monomers, which particles
are chemically and physically homogeneous. As to the chemical
homogeneity of the particles, the ratio of the constituent
monomers is substantially the same from particle to particle.
As to physical homogeneity, the sizes of the particles are
relatively uniform. For example, with latices having an
average particle size in the range of about 1,000 to about
3,500A, the deviation in particle size from the average is a
maximum of about +200 A. Latices having such characteristics
are described, for example, in U.S. Patent Nos. 3,397,165 and
3,472,808. The resin particles of the preferred latex are
1~ prepared from st~r-ne, butadiene, vinylidene chlor-de and
methacrylic acid. In addition, the emulsifier content of the
preferred latex is about 1 to about 4% based on the resin
solids and comprises at least 90 wt. %, most preferably 100
wt. ~ of an anionic emulsifier such as a sulfonate, for ex-
~ ample, sodium dodecylbenzene sulfonate, or a sulfosuccinate,for example, sodium oleoyl isopropanolamide sulfosuccinate,
or a mixture thereof.
In forming pigmented black coatings, excellent re-
sults have been achieved by using a composition wherein the
resin particles are dispersed by an anionic emulsifier, for
example, a sulfonate, and wherein the black pigment is added
to the composition in the form of an aqueous dispersion o a
black pigment dispersed by a nonionic emulsifier, for example,
an ethoxylated alkyl phenol. Excellen~ results have been
:
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lL060
achieved by usin~ such an aqueous dispersion in an amount
such that the composition contains about 0.2 to about 3 g
of furnace black/100 g of resin solids.
Although the coa~ing composition can ~e contacted
with the metallic surface in a variety of ways, it is believed
that the most widely used method of contact will comprise im-
mersing the metallic surface in the coating composition at
room temperature. As mentioned above, the longer the metallic
surface is immersed in the coating composition, the greater
$he buildup in coating thickness. It is believed that for
most applications, desired coating thicknesses can be obtained
by immersing the metallic surface in ~he composition for a
period of time within the range of about 30 seconds to about
3 minutes. However, it should be understood that longer or
shorter peri ods Of time can be used.
Agitating the composition aids in maintaining it
uniform. Also, agitation of the composition is effective in
improving the uniformity of the coatings formed.
As mentioned above, as the composition is used to
coat quantities of metal, for example, ferriferous surfaces, it
tends to form thinner coatings and can eventually destabllize.
This occurs notwithstanding that ingredients comprising the
" ' ' jf
composition are replenished, as needed, during use.
With regard to replenishment, the solids, including
resin and pigment (if present) are depleted during use as they
deposit in the form of coatings on the metallic surfaces. In
general, the rate of coating formation is directly related to
the solids concentration. The higher the concentration of solids,
the heavier or thicker the coating formed, other factors held
constant. In some industrial operations, it is believed that
-
_ g_
.. .~ .... , _
~llD811)~
the amount of solids removed f.rom the composition as quantities
of metal are coated will be small, relative to the total solids
in the composition. In such an operation, the solids content
may vary over a wide range without significantly reducing the : .
thicknesses of the coatings formed. It is further noted that
pigment and resin particles may be consumed in the same propor-
tion as existed in a freshly formulated composition. In such
applications, these solids can be replenished in the form of a
mixture in which the desired ratio of pigment to resin is
present. The solids can be added to the composition period-
ically as needed or they can be added continuously at a rate
which will approximate the rate of depletion. The solids
content of the composition can be determined readily by vari-
ous methods. For example, a known volume of the used aqueous
coating compositicn can be p-petted into a container of known
weight and the water evaporated at elevated te,mperature. The
àmount of solids in the sample can then be determined by weigh-
ing and this compared to the amount desired in the composition.An appropriate amount of'solids can then be added to the com-
position for replenishing.
With regard to the HF ingredient, it is believedthat at least a portion of the HF dissociates to hydrogen ion
and fluoride ion. The hydrogen ion attacks and dissolves metal
from the surface of the article being coated and in the process
is reduced to hydrogen. As this occurs, additional quantities
of the HF are dissociated with the result that this ingredient
is depleted. Other factors held constant, it has been found ':
that as HF is depleted, the composition tends to form uneven
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- . - . . . ... . . .. .
- 11771-C
~081~60
coatings of reduced thickness and coatings which have a
tendency to sloughO Accordingly, this ingredient should be
replenished during use of the composition. The effective
amount of HF can be monitored by the use of available appara-
tus which directly or indirectly measures HF concentration. An
example of such an apparatus is described in U.S. Patent No.
3,329,587 and is sold under the trademark "Lineguard" Meter
101. By determining the effective concentration range of HF
in a satisfactorily working composition, amounts of HF can be
added to the composition to keep`its concentration in the
effective range, being guided by monitoring apparatus used to
track the HF concentration. In general, replenishing the HF
as needed will maintain the pH of the composition in the
desired range.
With respect to the FeF3, it is believed that this
ingredient in some way, not fully understood, either directly
or indirectly aids in the dissolution of the metallic surface
or directly is involved in the dissolution reaction. Accord-
ingly, this ingredient or source thereof is needed in the com-
position and if depleted, should be replenished. It has been
observed that there are operations where the ferric fluoride
concentration appears to remain substantially constant during
relatively long periods of continued use of the composition.
It is believed that this is attributable to the formation of
ferric iron in situ. By way of explanation, it is believed
that ferrous iron is formed in the composition from the dissolu-
tion of a ferriferous surface. It is believed that at least some
of the ferriferous iron is oxidized to the ferric form, for
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11771-C
--\
~ 8 1 0 6 ~
example, by oxygen in air which is in contact with the
surface of the composition. Thus, ferric iron is formed
in situ. As mentioned above, fluoride is generated in
the composition as a result of dissociation of the ~F and
is available to combine with the ferric ironO Although
formed in situ, it is noted that the ferric iron concen-
tration will be depleted as a result of drag out~
It should be noted further that although ferric
fluoride is added to the composition, it may exist therein
in various forms such as various complex fluorides and
free ferric and fluoride ionO Similarly, ferrous iron
which is generated may exist in various forms of complex
fluorides and as free ferrous ionO
Thus, as the coating composition is used to coat
quantities of metallic surfaces immersed therein, and as
the ingredients thereof are replenished, as needed and,
for example, as described above, quantities of ferrous
iron are released into the composltion as a result of the
chemical action of the composition on the metallic surface.
2D As this occurs, the composition tends to form thinner coat-
ings, and, if continuously used, the resin solids begin to
collect into larger particles, which may appear as grit-
like particles on the coated surface. With continued use,
the resin dispersion can breakO As mentioned above, some
of the ferrous iron may be oxidized to ferric ironO How-
ever, in an actively used composition, it appears that the
amount of ferrous iron released into the composition is so
great that air oxidation is not sufficient to deter the
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11771-C
~ 1~ 81 ~6 ~
buildup of excess quantities of ferrous iron which have
an adverse effect on the coating operation. (While the
ferrous iron, in whatever form present, has an adverse
effect on the composition, ferric iron in whatever ~orm
present, does not.) It is believed that excess quantities
of ferrous iron cause the dispersed particles to coagulate
throughout significant portions or the whole of the com-
position. It is believed also that the ferrous iron sig-
nificantly reduces the role the ~erric iron plays in the
formation of the coating. This is believed to account for
the tendency of the composition to form thinner coatings
as it is used and as the ferrous iron builds up in the
concentration.
The above problems can be alleviated or avoided by
adding to the composition, as needed, a material which
oxidizes ferrous iron to ferric iron.
In view of the numerous variables which are inherent
in the coating process, it is impractical, if not impossi-
ble, to state a numerical value at which the excess ferrous -
iron tends to create problems. The following factors can
have a bearing: the type of metal being coated; the specific
t~pe of organic coating-forming material comprising the
composition, the types and amounts of dispersing agents
in the composition; the rate of throughput of metallic
surfaces in the composition; the extent to which the com-
position dissolves the metallic surface; the age of the
composition; and the extent to which ferrous iron tends
to be air-oxidized. For this reason, it has been found
more expedient to make certain empirical determinations
respecting the operating characteristics of specific
;
11771-C
,-
~0 ~ 1 ~ 6 ~
processes, and then utilize these determina~ions as
guidelines for adjusting or controlling the amount of
excess ferrous iron which causes problems. Information
gathered from these empirical determinations can be used
to determine when ferrous iron begins to buildup in an
excessive amount and how much oxidizing agent should be
added to the composition.
One exemplary way of monitoring the buildup of
excessive amounts of ferrous iron is by measuring the
redox potential of the composition. It is noted that
in the aforedescribed composition one oxidation/reduction
couple is present. Any suitable instrument which is cap-
able of measuring a difference in potential in millivolts
can be used. For example, a potentionmeter hav~ng a
calomel cell, or other suitable reference cell, and a
platinum electrode can be used. The electrodes are con-
tacted with the composition and the electrical potential
between the two electrodes is measured. In the use of
such an apparatus it has been observed that as the com-
position is used and ferrous iron builds up, the milli-
volt reading of the instrument tends to fall. Accordingly,
for any particular application, the millivolt reading can
be related to the point at which coatings of unsuitable
thickness are formed. In effect, the millivolt reading
range of a satisfactorily operating co~osition can be
determined and steps taken to maintain the millivolt read-
ing within the desired range by adding to the composition
an oxidizing agent for the ferrous iron. As the oxidizing
agent is added, the millivolt reading rises.
11771-C
, ~
~08106Q
Any oxidizing agent which does not substantially
adversely affect the coating operation can be used. The
use of hydrogen peroxide is particularly preferred. It
rapidly performs its oxidizing function and forms water,
a constituent of the composition. The amount of oxidizing
agent to add will~ of course, depend on the amount of
ferrous iron that should be oxidized. This can be deter-
mined readily for any specific coating operation. Depend-
ing on the specific application involved, the oxidizing
agent can be added periodically or continuously.
After contact with the coating composition, the
coated metallic surface can be subjected to further pro-
cessing steps as are known. Such steps are described
briefly hereafter.
Water rinsing the coated surface after it has
been withdrawn from the composition, and before signifi-
cant drying takes place, is effective in removing there-
from residuals such as acid and other ingredients of the
bath that adhere to the coated surface. If such residuals
are allowed to remain on the coated surface, they may
change or adversely affect the quality of the coating.
For a specific application, a determination can be made
as to whether the residuals cause adverse effects which
are not tolerable. If they do, they should be removed,
for example, by water rinsing with tap or deionized
water. If they do not, this step of removing them can
be avoided.
If desired, the corrosion resistant properties of
the coated surface can be improved by contacting the
coated sur~ace with an acidic aqueous rinse solution con-
11771-C
, _,
~ 08 ~
taining hexavalent chromium. Such rinse solutions can
be prepared from chromium trioxide or a water soluble
dichromate or chromate salt, for example, ammonium,
sodium and potassium salts. There can also be used a
chromium composition obtained by treating a concentrated
aqueous solution of chromic acid with formaldehyde to
reduce a portion of the hexavalent chromium. This type
of rinse composition, which is described in U.S. Patent
No. 3,063,~77 to Schiffman, contains chromium in its
hexavalent state and reduced chromium in aqueous solution.
It has been reported also that the water and salt spray
resistance of a fused resinous coating can be improved
by contacting the unfused coating with a solution, pre-
ferably an aqueous solution, of phosphoric acid ~see ~.S.
Patent No. 3,647,567~. The recommended amount of phos-
phoric acid in the solution is about 0.25 to about 7
wt. % based on the total weight of the solution.
Following any rinse steps employed after the
coated surface is withdrawn from the composition, the
coating should be dried. Fusion of the resinous coat-
ing renders it continuous, thereby improving its resis-
tance to corrosion and adherence to the underlying
metallic surface.
The conditions under which the drying and/or
fusion operation is carried out depend somewhat upon
the type of resin employed. In general, heat will be
required to fuse the resin. The corrosion resistant
properties of coatings fused at elevated temperature have
been obser~ed to be better than coatings which have been
air dried. However, there are applications where air
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11771-C
0 8 1 0 6 0
dried coatings can be used satisfactorily. The fusion
of the coating should be carried out below temperatures
which cause the resinous coating to degrade. Exemplary
conditions used in fusing coatings produced according to
the present invention are temperatures within the range
of about 100 to about 200C for periods of time within
the range of about 10 to about 30 minutes, depending on
the mass of the coated part. Baking the coating for a
period of time until the metallic surface has reached the
temperature of the heated environment has been used
effectively.
-` ~0~1~6~
.
~Y~PL~S
.
Examples below are illustrative of the practice of
the present invention.
The basic ~oatin~ compositions used in all of the
examples were prepared by combining
Ingredients Amounts
latex containing about 54~ solids l90 g .
ferric fluoride 3 g
hydrofluoric acid 2.3 g
biack pigment dispersion ` 5 g
lO water to make l liter.
The resin of the latex used in the above composi-
tion comprised about 62% styrene, about 30~ butadiene, about
5Qo vinylidene cAloride and about 3% methacrylic acid. A
film.formed from the resin is soluble in refluxing chloro-
.
benzene to the extent of about 13~. That the resin is cross-
linked is indicated by its insolubility in Soxhlet extraction
with chlorobenzene. The water soluble content oE the latex
is about 2% based on the weiyht of dried resin, with the ~ ;
water soluble content comprising about lO~ sodium phosphate,
about 13~ sodium oleoyl isopropanolamide sulfosuccinate and
about 75% sodium dodecylbenzene sulfonate, the first mentioned
ingredient being a buffering agent used ln preparing the
latex, and the last ~ mentioned ingredients being emulsifiers.
The pH of the latex was about 7.8 and the surface tension
~5 thereof about 45-50 dynes/cm. The average particle size of
the resin was about 2,000 A.
The black pigment dispersion used in the above com-
position is an aqueous dispersion h~ving a total solids content
-18-
.
3l~ 6(1
~ of about 3G~ Carbon black comprises about 30% of the dis-
persion. It has a pH of about 10-11.5 and a specific gravity
of about 1.~7. The dispersion contains a nonionic dispersing
agent for the solids, and is sold under the trademark Aquablak
115.
The metallic surfaces coated in the composition were
unpolished cold rolled st,~el panels tQ-panels) 3" x 4". All
metallic surfaces were cleaned with a conventional alkali
cleaner and rinsed with water prior to being coated.
A potentiometer having a calomel cell and a platinum
electrode was used to monitor the buildup o~ ferrous iron in
the above composition as it ~Jas used to coat panelsO The mv
reading for the freshly prepared composition was about 350..
The panels were immersed in the composition for about 90 seconds
one after another. The following table shows the thicknesses
of coatings formed on the panels at various mv readings:
mv reading coating thickness in mils
219 1.25 - 1.4
107 1.0
64 0.75 - 0.8
As can be seen from the above table, as the redox potential of
the composition decreased (evidencing a buildup in the composi-
tion of ferrous iron), the used composition formed thinner coatings.
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- . . . - . . . -- - ~
In another series of tests, the amount of metal
dissolved from panels was determined periodically as panels
were coated in the composition. The composition dissolved
substantially the same amount of metal from various panels ~ -
S coated in the composition when the mv readings were within
the range of about 300 to about 400. (The amount of metal
dissolved was about 25 to about 30 mg/sq. ft. per minute.)
However, as additional panels were processed through the
composition, and at an mv reading of about 200, the metal
1~ dissolution rate dropped to somewhat below 20 mg/sq. ft. per
minute. As additional panels were processed through the
composition, the mv reading dropped to about 67 at which value
the amount of metal dissolved from the panel was 13.6 mg/sq.
ft. per minute. This is exemplary of how the buildup of
ferrous iron decreases the rate at which metal is dissolved
by the composition.
,
In another series of tests, metal panels were
processed through the composition, and at an mv reading of
47, it was observed that the surface of the composition was
loaded with grit-like particles. Such particles were also
present on the surface of a coated panel. Additional use of
the composition resulted in an mv reading of about 12, at
which value the resin dispersion of the composition was
broken.
2~ In another series of tests, it was observed that
excellent quality coatings of uniform thickness could be ~;
produced when the mv reading of the composition was about
300 or above. As the redox potential fell below about 300 mv,
-2~-
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10~ 60
the composi~lon began to form thinner coatings.. Accordingly,
a freshly prepared composition was used to coat panels. As
the redox potential of the composition approached 300 mv,
there was added to the composition a 30% aqueous solution of
hydrogen peroxide in an amount of about.004 ml. This raised
the redox potential of the composition to about 350.
Excellent quality coatings of uniform thickness were obtained !
as additional panels were coated in the composition.
.'' '"' " .
It should be appreciated from the above description
that the present invention has particular applicability to a
coating system of the following type. The composition includes
or is prepared from a metallic salt, the cation of which is
capable of existing in a lower valence form. The compositiol;
also includes an acid which, together ~:ith sa~d metallic salt,
is capable of dissolving metal from a metallic,,surface immersed
in said composition in amounts sufficient to cause tha dispersed
resin particles in the composition to deposit on the surface in
a manner such that a resihous coating which grows with time is
formed. In the coating-forming process, the cation o the
metallic salt is reduced to its lower valence form. The metallic
surface generates metal ions, a~ it is oxi.dized by the chemical
action of the acid and salt, which metal ions are capable of
being oxidized to a higher valence state and to a form which
does not affect the coating process adversely. The metal ions
are so oxidized by adding to the composition a material capable
of oxidizing them. :
-21-
',.
"' " ' ' '- - : :'.
