U.S. patent number 3,658,662 [Application Number 04/811,267] was granted by the patent office on 1972-04-25 for corrosion resistant metallic plates particularly useful as support members for photo-lithographic plates and the like.
This patent grant is currently assigned to Durolith Corporation. Invention is credited to Edward A. Casson, Jr., Albro T. Gaul, Eugene L. Langlais, Gerald Shadlen, Eugene L. Vanaver.
United States Patent |
3,658,662 |
Casson, Jr. , et
al. |
April 25, 1972 |
**Please see images for:
( Certificate of Correction ) ** |
CORROSION RESISTANT METALLIC PLATES PARTICULARLY USEFUL AS SUPPORT
MEMBERS FOR PHOTO-LITHOGRAPHIC PLATES AND THE LIKE
Abstract
A process for electrolytically forming on a metallic element a
protective layer or film in an electrolyte consisting of an aqueous
solution of preferably sodium silicate or alternately of other
salts rendering the electrolyte substantially basic, the metallic
element constituting the anode in the process. The processed
metallic element has particular usefulness as a support member for
photolithographic printing plate, the electrolytically formed film
acting as a barrier layer preventing deterioration of the light
sensitive diazo resin, or the like, utilized as a photosensitive
coating on lithographic plates.
Inventors: |
Casson, Jr.; Edward A. (Easton,
MD), Gaul; Albro T. (Matawan, NJ), Langlais; Eugene
L. (Detroit, MI), Shadlen; Gerald (Arnold, MD),
Vanaver; Eugene L. (Dallas, TX) |
Assignee: |
Durolith Corporation (Easton,
MD)
|
Family
ID: |
25206063 |
Appl.
No.: |
04/811,267 |
Filed: |
January 21, 1969 |
Current U.S.
Class: |
205/323; 205/208;
430/159; 430/278.1; 430/158; 430/168 |
Current CPC
Class: |
B41N
1/08 (20130101); C25D 11/08 (20130101); C25D
9/04 (20130101); B41N 3/034 (20130101) |
Current International
Class: |
B41N
1/08 (20060101); C25D 11/08 (20060101); B41N
1/00 (20060101); B41N 3/03 (20060101); C25D
9/04 (20060101); C25D 9/00 (20060101); C25D
11/04 (20060101); C23b 009/00 () |
Field of
Search: |
;204/58,28 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
663,910 |
|
Aug 1938 |
|
DD |
|
342,256 |
|
Jan 1931 |
|
GB |
|
459,263 |
|
Jan 1937 |
|
GB |
|
Primary Examiner: Edmundson; F. C.
Claims
Having thus described the present invention, by way of several
examples of the methods for practicing the invention, what is
sought to be protected by United States Letters Patent is as
follows:
1. A method for electrolytically forming a layer on a surface of a
metallic element comprising disposing said metallic element in
contact with an electrolyte, disposing a conductive electrode in
contact with said electrolyte, electrically connecting said
metallic element and said conductive electrode to a supply of
electricity such that said metallic element is anodic and said
conductive electrode is cathodic for electrolytically forming on
said metallic element said layer which comprises anions of said
electrolyte reacted at the surface of the metallic element, said
method being characterized by said electrolyte being a basic
aqueous solution of sodium silicate containing from about 0.5
percent to about 37 percent per weight of sodium silicate, said
electrolyte being maintained at a temperature between 20.degree. C
and the boiling temperature of said electrolyte, said metallic
element and said conductive electrode being disposed in said
electrolyte in close proximity to each other, said metallic element
being in contact with said electrolyte for a duration of about 2 to
360 seconds and said supply of electricity being a source of direct
current at a voltage comprised between about 6 and 60 volts.
2. The method of claim 1 wherein the metallic element consists
principally of aluminum.
3. The method of claim 1 wherein the aqueous solution of sodium
silicate contains from about 0.5 percent to about 15 percent by
weight of sodium silicate.
4. The method of claim 1 wherein said voltage is comprised between
about 6 and 36 volts.
5. The method of claim 1 wherein the metallic element has at least
a surface which is grained, said surface being disposed opposite
the electrode in the electrolyte.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention belongs to the field of methods and processes
for forming on the surface of metallic elements a protective layer
which is corrosion resistant, which acts as a barrier layer
preventing spontaneous interreaction between the material of the
element and a coating disposed thereon, and which is endowed with
specific physical characteristics or qualities different from those
of the base material. Although products obtained by way of the
present invention have a general usefulness as a result of being
provided with a corrosion and electrical resistant surface film
they are particularly useful as support members for
photo-lithographic plates and the like.
The protective surface layer is obtained by an anodic electrolytic
process.
2. Description of the Prior Art
Photo-lithographic plates currently in use today often include a
metallic support member, having, for example, aluminum as its
principal component, a surface of which has been silicated by
chemical methods to provide a barrier layer which prevents
interreaction between the photosensitive diazonium salts, or other
photosensitive and non-photosensitive coatings, placed upon the
support member and the metal surface of the support member.
Silication of the metal surface provides a chemical pacification
which increases the shelf life of the lithographic plate,
facilitates the processing of the plate after exposure, and
improves the length of the printing run and the quality of print.
The barrier layer is obtained, according to the prior art, by
subjecting the metallic surface to the action of a solution of one
or several of a plurality of compounds, examples of which include
hydrolized cellulose ester, sodium phosphate glass, alkali metal
silicates, sodium metaborate, phosphomolybdate, sodium silicate,
silicomolybdate, water-soluble alkylated methylomelamine
formaldehyde, polyalkylene-polyamine-melamine-formaldehyde resins,
urea-formaldehyde resin plus polyamide, polyacrylic acid,
polymethacrylic acid, sodium salts of carboxymethylcellulose,
carboxymethyl-hydroxyethyl-cellulose, zirconium hexafluoride,
etc.
An often used solution in the prior art is an aqueous solution of
sodium silicate in which the metallic plate, forming the
lithographic plate support member, is dipped, or which is applied
to a surface of the plate. The solution is preferably heated before
dipping the plate therein or before applying to the surface of the
plate, and the plate surface is optionally washed with an acidic
medium in order to harden the silicated surface and neutralize any
alkali that may remain on the surface.
In addition to acting as a barrier layer between the metal of the
metallic plate and the diazo resin, the silicated surface forms a
hydrophilic surface which partially acts as an initial
water-carrying surface when the processed plate is placed in a
printing press. The hydrophilic surface thus formed is desirably
relatively insoluble in the fountain solutions used in a printing
press in order to prevent undercutting or hydration of the image
areas.
It has been postulated that the following reactions take place
during conventional silication of an aluminum surface:
1. The aluminum and the aluminum oxide at the surface of the plate
react with the solution according to the formulas:
A1 + 2OH .fwdarw. A10.sub.2 + H.sub.2 (a)
A1.sub.2 0.sub.3 + 2OH .fwdarw. 2A10.sub.2 + H.sub.2 O
2. Silication, simultaneously or consecutively, takes place at the
surface, according to the following formula:
A1 + A10.sub.2 + SiO.sub.3 .fwdarw. (A1.sub.2 SiO.sub.5)2x
The aluminum silicate surface layer thus formed is substantially
insoluble, although it may be dissolved to some extent in strong
reagents, and it has been postulated that it is in the form of
large super crystals having an endless chain-like structure as
follows:
However in addition to aluminum silicate, other compounds may be
formed and included in the surface layer, which often result in
differences in the qualities of the surface layer. Some of the
compounds that may be present in the film of aluminum silicate
include A1 (OH).sub.3, hydrated A1.sub.2 O.sub.3, and hydrated
sodium aluminum silicate, such as, for example, Na.sub.2 O.sup..
Al.sub.2 O.sub.3.sup.. 2SiO.sub.2.sup.. 6H.sub.2 O, could present
varied degrees of solubility in fountain solutions used on printing
presses. In addition, if varied cations such as Ca, Mg, etc., are
present, they may also form complex double silicates with the
aluminum, which may cause further loss in quality of the formed
layer.
Silication of aluminum plates by the processes of the prior art
requires control of the purity of the solution and of the process
variables as closely as feasible, such process variables being the
pH of the solution, the concentration of silicate, the temperature
of the solution, the duration of the operation, the amount of grain
of the plate, the plate surface cleanliness, the degreasing or
dismutting processes utilized, etc. If all the process variables
are closely controlled in the prior art processes, it is possible
to obtain silicated aluminum plates of acceptable quality for use
as support members for photo-lithographic plates. The most
important of the desirable qualities to be achieved consist in an
adequate chemically inert surface layer which does not deteriorate
with age and is uniform and well bonded to the aluminum base
material and which protects the aluminum surface in such manner
that it is prevented from interreacting with the acidic diazo resin
and will be only slowly etched by the acidic fountain solutions,
and in providing an appropriate anchorage for the light exposed
diazo resin which permits the developing lacquer to build up on the
image areas and to supply long lasting oleophilicity of the image
areas, thus insuring long runs of the plate in the printing press.
Such qualities are difficult to obtain in a repetitive manner by
way of the processes of the prior art.
The present invention, by contrast, by utilizing an electrolytic
process for forming an improved functional surface on aluminum
plates and other metallic elements permits to achieve consistent
and repetitive quality in the surface and permits to obtain a
surface greatly enhancing the quality of photo-lithographic plates
as compared to what is achieved by prior art methods.
SUMMARY OF THE INVENTION
The present invention provides an electrolytic process for forming
on the surface of a metallic plate, such as is generally used as a
support member for a coating of diazonium salts or the like in
photo-lithographic plates, a pacified, corrosion resistant,
hydrophilic surface layer greatly enhancing lithographic and
printing performances.
Although silicaton obtained by prior art methods provides a barrier
layer between the metallic plate and the diazonium salt compounds
or the like utilized as the photosensitive coating in
photo-lithographic plates, electrolytically formed surface layers
according to the present invention provide barrier layers which are
much improved as far as lithographic hardness, continuity and
uniformity of the layers or films is concerned. The electrolytic
process of the present invention also produce surface layers which
are intimately bonded to the underlaying materials, which have high
hydrophilic qualities and provide a practical improvement in the
fine grain of the plate surface. In addition, the electrolytically
formed surface layer has a much improved anchoring quality for
adhesion of the diazo resin thus reducing any tendency to image
failure and resulting in improved runs. The improved surface grain
and the increase in bonding quality of the electrolytically treated
surface also result in more retained diazo, more retained lacquer
and a more oleophilic image, leading to longer running and higher
quality press performances, as compared to conventional
lithographic plates.
Other advantages provided by surfaces obtained by the method of the
present invention to photo-lithographic plates, cylinders, rollers,
and other support members are less propensity to attack from the
printing press fountain solutions, less soluble film remaining on
the plate after rinsing, improved hydrophilic quality on the
surface, and a more compact film resulting in a lithographically
harder surface and less deterioration as a result of wear. The
hard, compact surface film or layer obtained by the present
invention on a metallic element, because of its corrosion resistant
characteristics, its bonding and anchoring qualities with respect
to a decorative or protective film which may subsequently be
applied thereto and its increase in electrical resistivity as
compared to the resistivity of the base material, results also in
providing articles having general usefulness in the industry.
These and other advantages and objects will become apparent to
those skilled in the art when the accompanying description of some
of the best modes contemplated for practicing the invention is read
in conjunction with the accompanying drawings wherein like
reference numerals refer to like or equivalent parts.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic representation of an example of arrangement
for practicing the electrolytic process of the present
invention;
FIG. 2 is a schematic representation of a modification of the
arrangement of FIG. 1;
FIG. 3 is a schematic representation of a further modification of
the arrangement of FIG. 1, illustrating a continuous line
process;
FIG. 4 is a schematic sectional view of a metallic plate having
been subjected to the process of the invention;
FIG. 5 is a schematic sectional view of the metallic plate of FIG.
4 provided with a coating of photo-sensitive material such as a
diazo resin or the like;
FIG. 6 is a chart representing the current flow as a function of
time in a typical example of operation according to the
electrolytic process of the present invention;
FIG. 7 is a chart representing a family of curves of the current
flow, at diverse electroyte concentrations, as a function of the
linear feet of metallic plate strip electrolytically processed
according to the arrangement of FIG. 3;
FIG. 8 is a schematic representation of another example of
arrangement for practicing the electrolytic process of the present
invention;
FIG. 9 is a schematic representation of a modification of the
arrangement of FIG. 8; and
FIG. 10 is a schematic representation of a further modification of
the arrangement of FIG. 8 showing a continuous line process.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In order to practice the present invention, a metallic element such
as a metallic plate 10, as shown in FIG. 1, is dipped in an
appropriate electrolyte 12, contained in a tank 14, in proximity to
an electrically conductive electrode 16. The metallic plate 10 is
connected to the positive terminal of a DC power supply 18, and the
electrode 16 is connected to the negative terminal of the power
supply, such that the metallic plate 10 is electrolytically anodic
and the conductive electrode 16 is electrolytically cathodic. The
conductive electrode 16 may be in the form of a solid metallic
plate, or in the shape of a grid or mesh made of the same material
as the metallic plate 10, or made of a dissimilar material.
The DC power supply 18 may be a bank of storage batteries, an AC-DC
dynamo-electric or static converter, an AC-DC rectifier or any
other convenient source of DC power. A pulsed DC current power
supply may be used, and it does not seem material whether the DC
voltage across the terminals of the power supply is constant and
steady or include an AC ripple. An AC power supply may be also
used, which is arranged to operate on that portion of the cycle
when the metallic element 10 is substantially anodic.
EXAMPLE I
Plates of 1100 aluminum, having an area of 25 sq. in. and 0.009 in.
thick were prepared by having a surface of a continuous web of the
aluminum material grained at a line speed of 12 feet per minute
using a sand slurry. The web was then cut so as to provide plates
of the indicated area. The plates were electrolytically silicated
according to the arrangement of FIG. 1, by dipping the plate in the
electrolyte at a predetermined distance from a cathode 16
consisting of a stainless steel grid, the grained surface of the
plate being disposed opposite the cathode. The spacing between the
plate and the cathode was three inches in a series of runs and 6
inches in another series of runs, and experiments were run with an
electrolyte solution consisting of an aqueous solution of diluted
"Star Brand" 42.degree. Baume sodium "silicate" defined as (1
Na.sub.2 O: 2.5 SiO.sub.2), sold by Philadelphia Quartz Co., the
concentration of "silicate" in the solution being equivalent to
1.56 percent of "silicate" by weight in a series of runs and 4.05
percent of "silicate" by weight in another series of runs, having a
pH of approximately 13 in both cases. The conditions of operations,
namely the voltage applied across the plate and cathode, the time
or duration of operation, the spacing between the plate and the
cathode, the concentration of silicate in the electrolyte, and the
temperature of the electrolyte are tabulated hereinafter together
with the relative quality rating of the samples.
It will be appreciated that percents of silicate by weight as
mentioned herein refer in each instance to the percent solids of
"solicate" as defined hereinbefore. ##SPC1##
After silication, the silicated surface of each sample was coated
with a conventional diazo resin, according to conventional methods
in the lithographic plate manufacturing industry. The diazo resin
used for all the tests mentioned herein was Diazo Resin No. 4,
manufactured by Fairmount Chemical Co. The sample plates were
exposed and developed by means of a one-step developer which
developed the image at the same time as it lacquered it.
The relative qualitative rating of the sample plates resulted from
lacquer "breadkdown" tests. After the first development of the
image, the one-step developer was reapplied so as to redissolve the
lacquer and relacquer the image. The procedure was repeated until
the image broke down and did not relacquer. In the "poor" category
were those sample plates which broke down at the first
redevelopment, which is the case for the lower quality
conventionally silicated plates silicated generally at low
temperature. The "fair" category includes sample plates which
withstood two or three redevelopments, which is generally
comparable to plates which are conventionally silicated at high
temperature. The "good" category includes sample plates which were
redeveloped five or more times, while the "excellent" category
includes plates which were even better. It will be appreciated that
the "breakdown" test utilized for the relative qualitative rating,
although commonly used in the lithography industry, is far from
being an objective or scientific test, dependent as it is upon the
human tester's technique and skill, but such a test when effected
by the same person upon a plurality of samples, permits to obtain a
substantially reliable relative rating.
Even the sample plates included in the "poor" category as far as
the breakdown tests were concerned yielded good quality images and
in some other aspect were superior to the average conventional
lithographic plates. The sample plates did not scum up and they did
not yield any black spots, which are common defects in
conventionally silicated plates.
Table I indicates that the best results are achieved with a
relatively high temperature of electrolyte and with a relatively
high voltage, in the neighborhood of 36 volts. With reduced
voltage, longer times in the electrolyte bath are required.
Tests were also run with an electrolyte having a concentration of
0.5 percent by weight or less. It was found that with such low
concentration of silicate in the electrolyte it becomes difficult,
if not impossible, to obtain a silicated layer in a reasonable
time. This may be due to the fact that the electrolyte does not
contain a high enough concentration of silicate or hydroxide anions
to react at the surface of the aluminum plate. When sufficient
silicate and hydroxide anions are present in the solution as a
result of utilizing higher silicate concentrations in the
electrolyte, the anions forced to the positively charged aluminum
plate are able to react to form a film which may be a complex
aluminum silicate. An increase in the voltage and in the
temperature of the electrolyte not only produce superior results
but permit shorter times in the electrolyte bath which are
advantageous in continuous coil manufacturing processes, as will be
hereinafter explained. Experimentally it was found that electrolyte
concentration between 0.5 percent and 15 percent by weight, applied
voltage between 6 and 60 volts DC, temperature of the electrolyte
between about 20.degree. C and the boiling temperature of the
electrolyte and time of immersion between 10 and 360 seconds yield
a good quality silicated layer on the plate.
Other concentrations of the electrolyte solution may be effectively
used, up to saturation, depending upon the particular silicate or
other salt used in the electrolyte and the temperature of the
bath.
High concentrations reduce immersion time requirements. For
example, in one pair of tests, immersion time was decreased from 60
seconds to 5 .sub.seconds by increasing the concentration from 1.95
to 3.75 percent. Very high concentrations, for example 37 percent
by weight of a 2.5 SiO.sub.2 /Na.sub.2 O ratio, have lower
electrical conductivity which must be taken into account. Very high
concentrations do react with the aluminum both before and after the
electrolytic treatment and should therefore be used with
appropriate care. Although the test results of Table I were
obtained with a silicate containing a SiO.sub.2 to Na.sub.2 O ratio
of 2.5, it is obvious that other ratios may be used. For example
silicate solutions having a SiO.sub.2 to Na.sub.2 O ratio of 2.65
and 2.84, made by Diamond Alkali Co., were successfully used.
Other voltages and temperatures than the preferred ranges
hereinbefore indicated may also be used, all of such variables
being readily determinable by a reasonably skilled operator and,
depending on the particular requirements, quality standards and
available equipment.
It has been determined that rinsing of the electrolytically treated
plate is desirable. Rinsing is relatively more difficult after long
immersion times or other process combinations which produce a
similar effect. It should however not be concluded that such
surfaces are inferior in quality and performance.
Other salts which may also be included in the electrolyte, in
addition to sodium silicate, include metal silicates, phosphates,
chromates, borates, vanadates, and molybdates. These and other
constituents when used alone or in combination in electrolyte
solutions, instead of sodium silicate, in practicing the present
invention, are propounded as accomplishing the same or equivalent
results in varying degrees of effectiveness.
It should be appreciated that the process of the invention differs
from anodization. Anodization utilizes acid electrolyte solutions
only as a current conductive medium and the anions in the
electrolyte serve no permanent role in the surface composition
obtained. In aluminum anodization, for example, it is sought to
obtain Al.sub.2 O.sub.3, even though SO.sub.4, or C.sub.2 O.sub.4
anions may be used in the acidic electrolyte. In the present
invention, the anions being displaced to the anodic plate appear to
become an integral part of the surface produced. Basic anodic
processes are not generally used. An example of a research study,
(Briggs et al., Trans. Faraday Soc., 51, 1433, (1955) 52 1272
(1956)), related to Nickel-Iron and Nickel-Cadmium battery
processes describe oxidation of Nickel in alkaline solutions.
The electrolytic process of the present invention preferably
utilizes a basic electrolyte and results in electrochemically
pacifying the surface such that the surface becomes resistant to
corrosion and dissolution and also produces a base film suitable
for anchorage. This is clearly demonstrated by electrolytically
forming a surface, as previously indicated, on an aluminum plate
according to the arrangement of FIG. 1, and in monitoring the
electrical current flowing through the electrolyzing circuit.
Keeping the voltage constant, the current flow as a function of
time follows the curve shown at FIG. 6. It can thus be seen that
after a predetermined period of time, of several seconds, the
current flowing through the electrolyte is reduced to a fraction of
the original current.
If it is desired for some applications, generally other than
photo-lithographic applications, to provide both surfaces of a
metallic element or plate with a passive silicated surface layer,
the arrangement of FIG. 2 may be used wherein the metallic plate 10
is disposed in the tank 14 containing an appropriate electrolyte 12
between two cathodes 16 and 16'.
Referring now to FIG. 3 there is schematically illustrated a
continuous electrolytic process for forming on a surface of a
continuous metallic web 20 a layer according to the present
invention. The web 20, made for example of aluminum foil which has
been preferably pregrained on a surface 22 thereof, is deflected by
means such as rollers 24, 26, and 28 into a tank 14 containing an
electrolyte 12, for example, a sodium silicate aqueous solution as
previously mentioned. By means of rollers 30, 32, and 34, and
rollers 31, 33, and 35, the continuous web 20 is caused to be
linearly displaced in the tank 14 in proximity to an electrode 16,
the grained surface of the web being opposite the electrode. In a
photo-lithographic plate manufacturing continuous process, the web
emerging from the tank 14 is fed by further rollers 36, 38, 40 to
rinsing and drying stations and to a diazo coating station, not
shown, and to a station, not shown, where the web is sectioned in
any appropriate lengths.
The electrode 16 is connected to the negative terminal of a DC
power supply 18 so as to be cathodic, while the continuous web 20
is rendered anodic by being connected to the positive terminal of
the DC power supply 18 by means such as a current conductive roller
42, or by any other appropriate means, including by way of example
but not limitation, brushes, sliding contacts, or the like.
EXAMPLE II
A web of 1100 aluminum, 291/2 in. in width, was silicated according
to the arrangement of FIG. 3 utilizing an electrolyte heated above
70.degree. C and consisting of an aqueous solution of sodium
silicate (1Na.sub.2 O:2.5SiO.sub.2) containing 3.10 percent by
weight of sodium silicate, the cathode being spaced 4 inches from
the moving web and the cathode extending 10 feet along the length
of the web. A voltage of 31 volts was used, and the aluminum web
was continuously pregrained at a line speed of 12 feet per minute
using a sand slurry. A total current of 240 amps flowed in the
electrical circuit at the beginning of the silication operation and
progressively reduced to 180 amps after 1145 linear feet of the web
had passed through the bath.
It seems that the decrease in current flowing through the
electrolyte is the combined result of a progressive reduction of
effective surface area of the web due to wear of the abrasive
particles in the slurry used for graining the surface thereof, and
due to an apparent depletion and/or contamination of the
electrolyte. Consequently, the decrease in current flowing through
the electrolyte may be used as a means for monitoring the effect of
surface area and electrolyte effectiveness in a continuous
manufacturing process.
The decrease in current as a linear function of the amount of
linear feet traveling through the electrolyte bath is represented
at FIG. 7 by curve 44 corresponding to an electrolyte concentration
of Cl. with an electrolyte concentration of C2, C2>C1, and in
the concentration range where increase in concentration results in
increased conductivity, the current flowing through the electrolyte
as a function of the linear feet of web passing through the
electrolyte is according to curve 46, while at still a higher
concentration C3, the current flow is according to curve 48.
An increase in the velocity of displacement of the web through the
electrolyte bath causes an increase of the current flowing through
the electrolyte, as tabulated in Table II.
---------------------------------------------------------------------------
table ii
ft/min Current Temperature Voltage (amp.) (.degree.C) (volts)
__________________________________________________________________________
9 174 83 30 12 184 83 30 15 194 83 30 18 205 83 30
__________________________________________________________________________
The results of Table II can be foreseen from the curve of FIG. 6
and from what has been hereinbefore explained, as the electrolytic
process of the present invention is partly self-limiting and
results in only a leak current flowing through the electrolyte as
soon as an appropriate silicated surface has been formed.
Experiments were conducted in which the metallic element 10 of FIG.
1 and the electrode 16 were connected to the terminals of a DC
power supply in such manner that the metallic element 10 was
connected to the negative terminal of the power supply so as to be
cathodic while the electrode 16 was connected to the positive
terminal of the power supply so as to be anodic, all other
conditions being the same as mentioned relatively to Example I
hereinbefore. Under such conditions, no surface layer having the
desirable properties was obtained on the metallic element 10.
As previously mentioned, it is immaterial whether the voltage
applied across the metallic element and the electrode has any AC
ripple. As a matter of fact, the principles of the present
invention apply to arrangements wherein a metallic element
connected to a terminal of an AC power supply is disposed in an
appropriate electrolyte bath in which is immersed another electrode
which may be either a dissimilar or a similar metallic element
connected to the other terminal of the AC power supply. On
application of an AC voltage, the metallic element is anodic for
approximately each half cycle of applied voltage. Such arrangement
is shown in FIG. 8 wherein a tank 14 contains an appropriate
electrolyte 12 in which is immersed a metallic element 10 connected
to a terminal of an AC power supply 18. An electrode formed by a
dissimilar or similar metallic element 10' is connected to the
other terminal of the power supply. The apparatus functions with
greater electrical efficiency when both metallic elements 10 and
10' are workpieces to be provided with a protective layer. If
element 10' is a dissimilar electrode, power is dissipated without
useful performance when such electrode is anodic with respect to
the workpiece, metallic element 10.
EXAMPLE III
Utilizing the arrangement of FIG. 8, metallic elements 10 and 10'
being both plates made of 1100 aluminum alloy were immersed in an
electrolyte consisting of an aqueous solution of 6.5 percent by
weight of sodium silicate solution of SiO.sub.2 : 2.5 Na.sub.2 O
maintained at a temperature of 25.degree. C. The two plates were
disposed 5 inches apart in the electrolyte and were connected
across an AC power supply providing a 60 cycle, 60 volts RMS
potential, for a duration of operation of 30 seconds. A surface
layer was formed on the opposing faces of both plates, such surface
layer having excellent properties, at least as good as the
properties obtained by the arrangement of FIG. 1 using a DC power
supply. The surface layer formed had a purplish blue color which
turned slightly greyer after rinsing with clear water. The surface
layers obtained on aluminum by the DC processes of the present
invention are also generally blue in coloration, although they lose
more of their coloration after rinsing.
In addition to permitting to obtain surface layers having qualities
at least equivalent to the layers obtained by way of the DC
electrolytic process of the present invention, the use of an AC
power supply has the added advantage of simplification of the power
supply, of allowing more flexibility in placement of the electrodes
and, in providing a process wherein both electrodes consist of
metallic elements whose surfaces are sought to be provided with
protective surface layers.
If it is desired to provide both faces of a metallic element with a
surface layer according to the present invention, utilizing an AC
power supply, the arrangement schematically shown in FIG. 9 may be
utilized. A plurality of metallic elements 10a, 10b, 10c, etc., are
electrically connected in parallel by means of a line 42 connected
to a terminal of an AC power supply 18. A plurality of similar
metallic elements 10a', 10b', 10c', etc., are connected in parallel
by means of a line 43 to the other terminal of the power supply. In
such manner, all the metallic elements with the exception, in the
arrangement of FIG. 9, of the extreme elements are provided on both
faces with a protective surface layer. It is obvious that, for
example, the tank 14 may be a circular tank of appropriate
dimensions such that an even number of plates are disposed in the
electrolyte in the tank, all the odd numbered plates being
connected in parallel to a common terminal of the power supply and
all the even numbered plates being connected in parallel to the
other terminal of the power supply. It will be appreciated that
such an arrangement may be automated with an appropriate fixture on
which the plates are mounted and which is dipped, after loading,
into the electrolyte tank, the power supply being turned on for the
appropriate time, then turned off, and the fixture removed from the
electrolyte.
EXAMPLE IV
Samples of 1100 aluminum having an area of 4 square inches, 0.009
in. thick and having a surface grain obtained by the method
mentioned with respect to Example I, were electrolytically treated
according to the arrangement of FIGS. 1 and 8 to establish a
comparison between the results achieved by the DC and AC processes
of the present invention. The "cathode" was stainless steel unless
otherwise indicated. The spacing between "cathode" and "anode" was
4 inches in a series of runs and 1 inch in another series of runs.
The temperature of the electrolyte solution was 26.degree. C and it
consisted of an aqueous solution of 6.5 percent by weight of sodium
silicate of the ratio 1Na.sub.2 O: 2.5SiO.sub.2. The duration of
the electrolytic operation was 40 seconds for one series of runs
and two seconds for another. Comparisons were made using DC, AC and
full wave (FW) rectified AC power supplies. These data as well as a
relative quality rating are tabulated hereinafter. ##SPC2##
The relative quality rating was obtained from the same breakdown
test described with respect to Example I herein. Table III
indicates that, at the electrolyte concentration indicated and for
40-second electrolytic operation durations, voltages within the
9-36 volt range either DC, AC, or full wave rectified can be used
to produce good to excellent plates at 26.degree. C. However, even
for 2-second durations, using 36 volts, good to excellent plates
can be produced using the indicated concentration, with little or
no difference between DC, AC and other wave form electrical
power.
EXAMPLE V
Plates of 1100 aluminum as described in Examples I and III were
electrolytically treated in an electrolyte consisting of an aqueous
solution of 6.5 percent by weight of sodium silicate of ratio 1
Na.sub.2 O: 2.5 SiO.sub.2 at 25.degree. C and at 75.degree. C, at
various AC voltages using aluminum as both electrodes for a series
of runs whose conditions of operation and results are tabulated in
Table IV, and platinum as one of the electrodes in another series
of runs. It was noted that the current drops rapidly when two
aluminum electrodes are used but not when a platinum electrode and
an aluminum electrode are used. Samples were produced at intervals
from 30 to 220 volts AC at 25.degree. C and at 75.degree. C for
times of 60 and 180 seconds. It is noted that the surface
coloration of these samples changed as the voltages were increased.
This change appears to be related to the thickness of the
electroformed surface layer. Also the electrical resistance of the
surface seems directly related to the voltage and time, with
increasing resistance and thickness resulting from increased
voltage and time. The resistance of the surface was measured by
placing two metal probes from an ohmmeter onto the surface of the
aluminum. Samples treated below 150 V AC showed conductive readings
even on a 1 ohm full scale position indicating a discontinuous or
delicately thin coating. Samples treated above 150 V AC started to
show resistive readings when the probes were gently laid on the
surface but conductive readings were observed when the probes were
pressed into the surface as if they were breaking through a
dielectric layer. This layer resistance read off the high
resistance side of the scale even with the meter switched to a full
scale 100,000 ohm position. The resistance noted is apparently
analogous to the type of insulating features generally associated
with electrical oxidation (anodization of aluminum) and suggestive
of a unique process for producing dielectrics for use in various
electrical applications which would compare favorably with
commercial methods. Examples of this potential use include
capacitors of the types used in the semiconductor industry wherein
this process offers advantages in uniformity and performance which
can be very important.
Some of the sample plates provided with a surface layer by way of
the electrolytic process of Example V were selected at random and
coated with a diazo resin layer and subjected to the "breakdown"
test referred to in Example I. All the samples tested were rated as
excellent. ##SPC3## -32'-
By utilizing an AC power supply a continuous line process has been
devised, schematically represented at FIG. 10, having two
continuous metallic webs, or strips of, for example, aluminum, as
shown at 20 and 20', arranged to be dipped into a tank 14
containing an appropriate electrolyte 12 by means of adequate
deflecting drive roller assemblies 24-26-28 and 24'-26'-28'
respectively. The two webs are displaced substantially parallel to
each other within the electrolyte by means of roller assemblies
30-32-34 and 31-33-35, and 30'14 32'14 34' and 31'-33'-35',
respectively. One of the webs, for example web 20 is connected by
means of an appropriate contact making current conductive roller 42
or any other appropriate means to a terminal of the AC power supply
18, while the other web 20' is connected by means of current
conductive roller 42', or any other appropriate means, to the other
terminal of the power supply. If one surface of each web is
grained, the grained surfaces are disposed opposite to each other.
The electrolyte compositions, concentrations and temperatures, the
distance between the webs while being translated within the
electrolyte, the duration of immersion of the webs are generally
quite alike such variables as used in the DC electrolytic process
of the invention, while the AC voltages (RMS) are preferably
slightly higher than the preferred DC voltages.
After passage through the electrolytic bath the metallic plate 10,
as shown schematically at FIG. 4, is provided with a DC or AC
electrolytically formed surface barrier layer 50, preferably only
on one surface thereof if the plate 10 is to be used, after coating
with an appropriate photo-sensitive material, in photo-lithography
and the like. It is obvious that with the arrangement of FIGS. 2
and 9 the metallic plates are generally provided with a layer on
both faces thereof and that a certain amount of the layer has been
formed also on the edges of the plates.
Electro-silicated metallic plates, in view of the electro-silicated
surface providing an electrically resistant and corrosion resistant
surface can find general applications in many industries.
Electro-silication of metallic surfaces may be used as a corrosion
inhibition step instead of or before applying paint, lacquer or the
like to a metallic surface.
When the electro-silicated plate has been treated according to any
one of the processes of the present invention for purpose of
providing a support member for a lithographic plate or the like,
the silicated surface 50, as shown at FIG. 5, is coated with a
diazo resin 52, or the like, the silicated layer 50 providing, as
previously mentioned, a good anchoring surface for the
photosensitive diazo material or the like and a generally
hydrophilic surface, substantially resistant to the attack of
fountain solutions when the plate, after processing, is placed in a
conventional printing process. The electro-silicated surface
described herein may be applied to a metallic element which has
sufficient rigidity to act as its own support, or an
electro-silicated surface may be applied to a thin metallic
element, such as aluminum foil, which is in turn bonded onto a
support structure.
* * * * *