Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
This invention relates to the coating of metals
to improve their use, particularly in resisting corrosion
as well as attack by chemicals.
Among the objects of the present invention is
the provision of novel coating methods and compositions, as
well as novel coated metals, that are simple to manufacture
and use and are highly effective.
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. SUMMARY OF THE INVENTION
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. 10The foregoing as well as additional objects
of the present invention will be more fully understood
.from the following description of several of its exempli-
fications.
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According to the present invention, ~lake forms of pro-
tective metals form particular}y effective protective coatings
for corrodible metals when partially diffused into the surface to
be protected or when combined with special binders or added over
other coatings.
As shown in U.S. Patents 3,248,251 granted April 25,
1966 and 3,787,305 granted January 22, 1974, powdered aluminum has
been suggested for use in applying protective layers over corro-
dible metals. The protection thus obtainable from layers of less
than about 1 or 1.5 milligrams per square centimeter, is greatly
improved if the aluminum coating is effectively continuous over
the surface being protected, a result that is obtained when leafing-
type aluminum particles are applied in amounts that permit the
individual aluminum flaXes to partially overlap each other over
the entire surface being protected. It is also helpful, as sug-
gested in Patent 3,787,305, to sub~ect the aluminum-coated ferrous
member to a temperature that causes at least a little bit of the
aluminum to diffuse into the ferrous surface.
~ eafing type aluminum particles can be made as described
'in U.S. Patent 2,312,098, and are generally characterized by the
presence of stearic acid or aluminum stearate or the like as a
very thin coating on the surface of each aluminum particle, a con- j
dition which makes it extremely difficult to disperse such aluminum
particles in water. A substantial amount of wetting agent will
effect a suitable dispersion, although it is easier to effect such
dispersions by also adding diethylene glycol or triethylene glycol ;
or ~ore ~ighly polymeric ethylene glycols having a molecular weight
up to about 9000, as described in U.S. Patent 3,318,716 granted
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May 9, 1967, or by adding glycerine. As shown in the last-mentioncd
patent, very effective dispersions of leafing-type aluminum can be
made from a concentrate that consists essentially of the leafing
aluminum, the polymeric ethylene glycol and a wetting agent, the
aluminum being present in an amount about 1/4to about 1-1/2 parts by
weight for every part of the polymeric ethylene glycol by weight,
and the wetting agent concentration from about 5% to about 25%
by weight of the concentrate. ',
The foregoing concentrate readily mixes with water in
all proportions to provide an aqueous dispersion of almost any
desi~ed aluminum content. Thus a diluted dispersion containing 5%
aluminum, 6% hexa-ethylene glycol and O.r/O para-n-octyl phenyl
ether of decaethylene glycol, is readily sprayed onto a stator
ring of a jet engine compressor to leave a coating weighing 0.5
milligram per square centimeter after drying in air to evaporate
most of the water. The stator thus coated is then heated in an
air oven until its temperature reaches 800F. The heating first
causes the glycol and wetting agent to be volatilized off leaving
a very adherent continuous and shiny coating that resembles polish-
ed aluminum and significantly adds to the corrosion resistance of
~the stator ring even if the heating temperature soeq no higher
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~,~ ' than 600F. The increase in corrosion resistance becomes more
,'significant when the heating carries the coating to temperatures
of about 900F, where some diffusion of the aluminum into the
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;ferrous surface of the stator begins. The rate of diffusion and
the degree of resulting corrosion resistance is further increased
by confining the coated stator in an atmosphere of gaseous aluminum
chloride while it iq at temperatures a~ove about 700F.
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me aluminum chloride atmosphere is conveniently provided by
a pack treatment as described in Example I of U.S. Patent # 3,958,046
(May 18, 1976) or Example I A (infra) of the present application, but with
no aluminum in the pack, However the stator ring containing the leafing
aluminum coating can merely be hung on a wire in a retort containing a
little energizer and no pack, and fired in this way in an otherwise inert
atmosphere.
Other aluminum halides such as aluminum bromide and aluminum
iodide also behave like aluminum chloride and indeed other well known
energizers for low temperature aluminum diffusion coatings can be used
instead of the aluminum halides with corresponding results, A list of
such energizers is given in U,S~ Patent No, 3,948,687,
Instead of adding the energizer to the atmosphere in which
the heating is effected, they can be added to the dispersion from which
~, the metal flake is applied, Thus the above-mentioned diluted
dispersion of aluminum deposits on 410 stainless steel or on plain carbon
steel an adherent 0,1 milligram per square oentimeter coatingr after
heating in air to about 600 or 700F for as little as 20 seconds,
Other ammonium halides and similar high temperature or low temperature -
aluminizing energizers that are driven off at 600 to 1000F can be used
to give adherent protective films weighing as much as 4 milligrams per
- square centimeter when so heated, For this purpose the energizer content
should be no greater than about 80~ and at least 1%! of the weight of
, the flaked metal. Larger quantities can be used but merely require more
time and heat energy to be driven off. Some fluorides can cause
significant attack of the aluminum or of the substrate, and are best avoided
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or used in a high spc~d opcration in which the time they contact
these metals is restrictcd to minimize thc dcletcrious e~fccts of
such attac~.
The adhesion of 0.1 to 4 milligrams per square ccntimcter
layer of a flaXed metal such as aluminum is also improved by
incorporating in the layer chromic acid, or a compoùnd such as
ammonium chromate or dichromate which is a salt of chromic acid
with a volatile base, or magnesium chromate or dichromate, or
water-soluble chromates or dichromates of other divalent metals.
Dissolving in the aluminum dispersion an amount of ammonium dichro-
mate 2Yo by weight of the aluminum gives a sharp increase-in adhe-
sion upon heating of a dried 4 milligram per square centimeter
layer of such composition on pl~in carbon steel to 700F for 5
minutes. A 5% addition of the dichromate to the dispersion renders
such a heat-treated coating completely resistant to wiping off.
Similar results are obtained when the heating is at 600 to 1000F
for as little as 20 seconds, and as much as 20 to 30 minutes,
although not m~ch i~ gained by prolonging the heating beyond about
1 minute. The ammonium dichromate content can be as high as 8~/o
of the weight of the metal, but above this level the corrosion
resistance tends to drop off.
The dichromates are used in amounts corresponding to the
foregoing amounts of ammonium dichromate, the chromates in amounts
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about 1/5 greater, and chromic acid in amounts about 1/5 less.
Each of these chromium compounds improves the aluminum coating so
that it provides good salt spray resistance to plain carbon steel
on which such a coating is applicd. When such coat'ng is covered
by any other corrosion-resisting top coating, such as thosc
described in the working examples, e~ceptionally good corrosion
resistance is impartcd, ~vcn to plain carbon steels.
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! ~he lcafing-type of aluminum particlcs or other protec-
tive metal used in thc above connection are preferably from about
i~50 to about 250 microns in maximum size although other sizes can
also be used.
Ij Non-ionic wetting agents are preferred for dispersing
"the aluminum inasmuch as such wetting agents are more readily
driven off by high temperatures. However other types of wetting
i agents, including those that are not driven off or not completely
driven off at 600 to 900F or 1000F, can be used. Making the
aluminum coatings heavier than about 4 milligrams per square
centimeter does not add anything slgnificant to the corrosion
resistance, and as little as 0.1 milligram per square centimeter
j is helpul although at least about 0.3 milligram per square
centimeter, and better still 1 to 2 milligrams per square centi-
meter is preferred.
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Inasmuch as ferrous metals, such as plain carbon steel,
- l~cast iron, low alloy steels, stainless steels and-other chromium-
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¦ containing steels begin to oxidize on their surface at the
~ temperatures used for the heating of the flake layer, it is help- i
- j ful but not essential to conduct that heating in a non-oxidizing
atmpsphere. When the energizer used in such heating step is
incorporated in the layer of flaked metal, the volatilization
of the energizer along with the volatiliz~tion of any suspending
~;agent present in the dispersion from which the flaked metal
deposits, provides an atmosphere of reduced oxidation potential,
, and heat treatments that only extend for half a minute or less
,~need no further oxidation-preventing precautions. 8Owever, if
desired, the hoating can be effected in a closed chamber as by
batch heating an opened coil of coated metai sheet or wire placed '
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in the closed ch~mber which then has its atmosphere flushcd out
with argon, hydrogen or nitrogen, or by continuously passing a
continuous strip or wire of the coated metal into and out of the
chamber through close-fitting slots or holes in the chamber walls,
while continuously introducing into the chamber a small stream of
protective gas that maXes up for losses of gas through the slots
or holes.
When the metal flakes to be applied are aluminum, the
benefits of a non-oxidizing atmosphere are not significant and an
atmosphere of ambient air is entirely adequate.
It is not essential to have the polyglycol present in
the flake dispersions in the foregoing proportions, or at all,
although such presence is helpful. Reducing its concentration
leaves less of it in the layer of metal flakes, so that less has to
be driven off or converted to innocuous residue by the heating
operation. Without the polyglycol, the dispersions require frequent
agitation and then coatings applied by spraying from such
dispersions tend to be of non-uniform thic~ness.
The application of a leafing-type aluminum coating
particularly improves the corrosion resistance of plain carbon
steel or other ferrous surfaces that contain less than 1%
chromium, when such surfaces have a diffusion coating of aluminum.
The adhesion promotion obtained with the energizer treatments
described ab~ve is accompanied by a little diffusion of the
aluminum into the substrate, and in addition the aluminum coatings
thus formed are highly conductive to electricity as well as highly
p~otective.
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' The leafing aluminum coating also improves the corrosion
resistance of a coating obtained from mixturcs of aluminum
particles with phosphoric acid, chromic acid and maqnesium,
aluminum, calcium or zinc salts of these, as described in Patent
3,248,251. Thus substituting the leafing aluminum, along with
sufficient wetting agent and with or without the polymeric ethylene
glycol, for the spherical aluminum in the formulations described
in that patent contributes a significant increase in corrosion
resistance, particularly in cured layers weighing not more than
about 1 milligram per square centimeter. In such mixtures firing
of an aluminum-containing coating does not effect significant
diffusion of aluminum into a ferrous substrate so long as the
firing temperature is not over 1000F. Above that temperature the
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firing tends to adversely affect ferrous metals, particularly
those used in jet engine compressor sections.
Another feature of the use of leafing-type aluminum
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is the improved appearance that the workpieces are given. Sub-
stituting this type of aluminum for that shown in the composition
of Example I in Patent 3,248,251 with the help of the foregoing
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polyglycol-wetting agent formulation, not only gives a product
having somewhat better corrosion resistance, but with a bright
aluminum sheen. During the heating of the new compositions to
cure them, fumes are given off indicating that the polyglycol and
the wetting agent are being volatilized away, and no significant
reduction of the hexavalent chromium to trivalent condition seems
to taXe place.
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The foregoing improvements in corrosion resistance and in
. appearance are also obtained when the last-mentioned coating is covered
by a similar coating, even one that does not contain metallic aluminum.
Such top coatings are described in U S, Patents No 3,948~687 and No.
3,048,689. However multiple coating layers each of which contains
metallic aluminum are very effective, particularly when each layer
weighs between 0.1 and 0.5 milligrams per square centimeter.
As shown in the aforementioned applications, the proportions
of the ingredients in the ehromic acid-phosphorie acid-salt coating
mixture ean range as follows:
Chro~ate ion 0 2 to 1, preferably 0.4 to 0,8
mols per liter
Phosphate ion 0 7 to 4, preferably 1.5 to 3.5
mols per liter
Magnesium ion 0.4 to 1,7~ preferably 0,9 to
1.4 mols per liter
! Polytetrafluoroethylene 2 to 14, preferably 3 to 10
. resin grams per liter
The magnesium ion ean be replaced by any of the other ions referred to
above, in the same eoneentrations,
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; Instead of directly applying such an overlying coating whether
or not it contains metallic aluminum, it can be applied afte~ an
intervening coating of colloidal alumina or the like weighing about
0.1 to about 1 milligram per ~quare centimeter, also as described
in the aforementioned U.S. Patents No, 3,948,687 and No. 3,048,689
. with increases in corrosion resistance as described in those patents,With or without such an intervening coat, the final cured article has
a golden sheen, contributed by the presence of chro.mate, that is
extremely attractive and quite a&erent, The presence of poly-
: 10 tetrafluoroethylene particles in the phosphoric acid-chrGmic
acid-salt mixtures of either or both of such layers is also helpful and
doe s not detract from the golden appearance, Such presence in a top
coating m~kes that coating very smooth and slippery without detracting
. significantly from the coating hardness, These coating co.mbinations
with or without the intervening coating of colloidal particles are
. most effective in increasing the corrosion resistance of chromaum-freeand chromium-containing ferrous substrates that have aluminum-
diffused surfaces,
Indeed they also have this desirable effect on buIk
aluminum such as aluminum sheets, foil and bars, as well as on
titanium, On aluminum substrates such coatings adhere exceptionally
~ well and withstand severe deformation of the surfaces to which they
: are applied. However the gold color contributed by the foregoing top
coatings that are free of metallic aluminum, is not provided when
metallic aluminum is included in those top coating
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formulations. The intervening coatings of colloidal alumina andthe like are not heavy enough to obscure the metallic appearance
of the substrate and accordin~ly do not adversely affect the
appearance. On the oth~r hand those intervening layers improve
the wettability of the aluminum-containing surface by the top
coating. Some alumina dispersions are acid and tend to attack a
layer of aluminum on which they are applied. To alleviate this
situation the alumina dispersion used can be neutral or even some-
what alkaline, or the acidity of such dispersion can be so low in
strength and the dispersion so rapidly applied and dried that any
attack is immaterial, or a little chromic ac~d added to the dis-
persion to protect the aluminum.
The following are examples of the production of gold-
colored hi~hly attractive and very corrosion-resistant steel and
aluminum products.
EXAMPLE I
A. Into each of four plain carbon steel retort cups 2
feet wide and 14 inches high is poured a powder pack consisting
of 2~ aluminum by weight and 8~o alumina, both minus 325 mesh
and uniformly mixed together. After the retort bottoms are
covered with about 1/2 inch of powder, jet engine compressor
blades made of AISI 410 stainless steel are laid over the powder
layer, the vanes being spaced about 1/8 inch apart. This layer
of blades is thcn covered with more pow'er till the powdcr is
about 1/2 inch above the vane tops, and another layer of blades
is then laid down and the layering repeated until the entire
packing is 12-1/2 inches deep in cach retort. More pack powder is
then added to each retort to assure there i9 about 1 inch of
powder above thc tops of the topmost blades,follow~ng which thcre
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is sprin~led over each a very thin stratum of crystalline
AlC13 61;2O in an amount weighing 0.6" of thc total powder weight.
The retorts are then filled to their tops with additional pack
powder, and they are stacked one above the other on the floor of
a gas-fired bell furnace. The stacking does not seal any of the
retorts shut. The top of the furnace equipped with gas inlet and
outlet flush lines is lowered over the stack and sealed against
the furnace floor, and a slow flow of argon gas is passed through
the furnace interior to start flushing out the air within it.
After the argon purge hydrogen is substituted for the argon, and
is introduced at a rate that permits it to be burned with a small
flame as it emerges from the end of the outlet tube. Only a very
low flow rate is necessary, about 10 to 15 standard cubic feet
per hour.
The heating of the furnace is started at a rate of about
1.5F per minute, as measured by thermocouples in each retort and
connected to external meters, and when the thermocouples reach
300F the flow of hydrogen can be reduced so that the outlet
flame is very tiny. At this point the hydrogen inflow can be
less than 10 standard cubic feet per hour.
As the heating-up continues, the temperatures indicated
by the thermocouples increase uniformly and gradually and chemical
vapors begin to appear in the burning outlet gas. By the time
the thermocouple temperatures reach about 450F the discharge of
chemical vapors has subsided, the gas flow continuing till the
temperatures reach 875F where the furnace hea~ing is set to hold.
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After 16 hours a~ 87SF che furnace heating is tcr-
minated and the furnace ~ermitted to cool until the thermocouplc
temperaeures reach 300F. The atmosphere in the furnace is then
purged by switching the inflow gas to argon or nitrogen and thc
furnace shell then removed from the retorts, permitting the
retorts to cool further in air. The contents of the retorts are
then poured out, washed, dried and finally lightly blasted with
fine glass ~articles propelled by an air stream supplied at 5
to 10 pounds per square inch, giving an aluminum pick-up of 4.0
milligrams per square centimeter of ferrous surface.
~_.' B, On the lightly blasted aluminized surface there
is sprayed with an air-propelled spray, a uniform very thin layer
from an aqueous dispersion of
3.5% CrO3;
2.4% MgO;
11% H3P04i
5.7% leafing aluminum;
6`8% polyethylene glycol having an average mole-
-~ cular weigkt of 300 and in which the glycols-
range from pentamethylene glycol through
, hep;amethylene glycol; and
:; 0,8% para-isononyl phenylether of dodecaethylene
. glycol;
: ~ ~ all percentages bein~ ~yjweight.
.~ . The sprayed blades are then air d~sr~and baked at 700F in an: air oven for 30 minutes to give a coating weight from this
spray of 0.7 milligram per square centimeter of ferrous surface.
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C. The blades coated in steps A and B have their
;coated surfaces given a spray coating of colloidal alumina dis-
persed in a 20 concentration by weight in water to which a
little HCl is added to bring thc pH down to about 4. A very
fine spray is used to leave a light coating which after drying
in air weighs 0.5 milligram per square cen~imeter.
D. The blades with the air-dried coatings are then
j given a top spray coating from an aqueous dispersion of
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5.8% CrO3:
4% MgO:
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~ 0.5% polytetrafluoroethylene particles about 1
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this spray being such that upon air drying in an oven and then
baking at 700F for 30 minutes in an air oven, the final coating
weighs 0.5 milligram per square centimeter.
, EXAMPLE II
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` The coating steps of Example I are repeated but this
time the worXpieces are SAE 1010 steel, the diffusion pack peak
. temperature is 800F, the aluminum picked up in the diffusion
step is 7-1/2 milligrams per square centimeter, the baking in
',- steps B and D is at 900F, and the coat weight applied in step B
is 0.9 milligram per square centimeter.
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~PLE III
Discs of low alloy steel containing 0.5% chromium and
0.0~/~ carbon as the only significant alloy ingredients, which
discs are used to hold jet engine compressor blades, are given
the coating treatment of Example I, this time the diffusion
coating pack being held at a peaX temperature of 900F, the
aluminum picked up in the diffusion being about 8 milligrams per
square centimeter; the coating step B is followed by a light
blasting with very fine glass microspheres about 5 microns in
diameter propelled by an air stream from a blast supplied at 5
pounds per square inch gauge, and care being taken to make sure
that no significant a unt of the leafing aluminum in this coating
is removed during such blasting.
EXAMPLE IV
Sheets of 18-8 stainless steel are given the coating
sequence of steps B, C and D of Example I except that the sheets
with coating B are baked at 800F for 30 minutes and after such
baking that coating weighs 1 milligram per square centimeter.
''Coating D is also baked at 800F for 30 minutes with its weight
being 0.7 milligram per square centimeter.
EXAMPLE V
Plates of Type 3S aluminum were coated by the sequence
of steps B, C and D of Example I, and the coated plates had a
gold sheen of very,attractive appearance.
EXAMPLE VI
Titanium sheets coated by the steps B, C and D of
Example I were also colored with a golden shecn.
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E~YP~E VII
Coupons of 410 stainless stecl are dipped in thc coatin~
mixture of Example I B, modified by the addition of 0.4~,' fine
teflon particles from an aqueous teflon dispersion. The coupons
were removed from the coating mixture and heated to volatilize
off the wetting agents, taking care to remove the aqueous material
running down to the lower edges of the coupons and to thus keep
the coating more uniform in thickness. After the volatilization
is completed the coupons are quenched in ~ater and have a very
'smooth .and slippery surface. -
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Essentially the same results are obtained in Example Ias well as in Examples II and III when anhydrous aluminum
chloride, bromide or iodide, or hydrated aluminum bromide or
iodide is used in place of the hydrated chloride energizer, with
the aluminum content of the pacX ranging from 10~/o down to 2%.
For aluminum diffusion effected below 900F it is preferred that
at least 4% aluminum be in the pack. As disclosed in Serial No.
304,220, a convenient amount of hydrated energizer is rom 3 to
6 grams for a 6-1/2 pound pack when the pack is first broken in
as well as when the broken-in pack is subsequently used for
coating.
Instead of using aluminum of relatively pure composi-
tion such aluminum can be an alloy containing significant quanti-
ties of beneficial ingredients such as silicon. A content of 12%
silicon will, by way of example, improve the resistance to high
temperature oxidation of ferrous metals subjected to diffusion
coating by such an alloy.
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~ he very effcctive proeection imparted to ferrous mctals
containing less than 1~ chromium, such as plain carbon and low
alloy steels does not require more than a single layer of thc
chromic acid-phosphoric acid-salt-aluminum mixture, whcn prcccdcd
by an aluminum diffusion treatment. This is illustrated by thc
~ollowing examples.
EX~tPLE VIII
Panels of SAE 1010 steel are given the diffusion coating
treatment of step A in Example I, but using anhydrous AlC13 ener-
gizer. ~he diffusion coating weighed 7 milligrams per square centi-
meter, and it was then coated by spraying-on an aqueous dispersion
containing the chromic-acid-phosphoric acid-salt aluminum mix in
;th~ following proportions:
1.25 moles per liter P04
0.68 moles per liter M~+~
0.38 moles per liter CrO4
64.5 grams per liter Aluminum
77.0 grams per liter of the polyethylene glycol of
Example II, and
10.0 grams per liter of para-isooctyl phenyl ether
of tetradecaethylene glycol
The sprayed-on layer was dried and heated in an air oven to 900F
for 25 minutes to give a 1 milligram per square centimeter coating
weight.
The thus-coated panel withstood 10 cycles of alternately
heating to 900F for 6 hours in air followed by 16 hours exposure
to a 5% salt-spray at 95F without showing attack of base metal,
and substantially no attack nor spalling of the coating. Even
better results are produced whcn the dried and ov~n-heated coating
ls covered by another layer preferably just liXe the spraycd-on
layer but not containing the metallic aluminum. A porous alumina
barrier betwcen thcse two layers givcs still further improvcmcnt.
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SLmilar results are obtained when aluminum ions replacc
ehe magnesium ions, as well as when the baking is ac 700F and
the bakcd coating lightly blasted with very fine glass micro-
spheres aboue 25 microns in diameter impelled by air blasted at
a pressure of 5 pounds per square inch. Also the use of hydra~ed
energizer durin~ the diffusion coating ?.roduces the same results
as the use of anhydrous energizer.
In the aluminum-containing coating mixtures the con-
centration of the leafing-type aluminum particles can range from
about 30 to about 150 grams per liter of mixture, and the re-
maining ingredients can have the concentration ranges given supra.
The water in these compositions can also be replaced in whole or
in part by the polyethylene glycols or by any other inert liquid
in which the ingredients can be dispersed and sprayed. For com-
binations in which only a single chromic acid-phosphoric acid-
salt layer is used such layers can advantageously weigh as much
as 1.5 milligrams per square centimeter. However even such.a
layer containing the leafin~ type aluminum of the present inven-
tion and weighing only 1 m'lligram per square centimeter or as
much as two milligrams per square centimeter imparts excellent
corrosion resistance to plain carbon ant low alloy steels as
well as other ferrous metals containing less than 1% chromium,
when applied over an aluminum diffusion coating on the metal.
This corrosion resistance is even fureher increased when the
layer containing the leafing type aluminum has its electrical
conductivity increàsed as by heating to 900F or higher; or
by li~htly blasting it with finc non-corroding particlcs such as
glass or ground walnue shells or the like; or by heating it in
ehe presence of ammonium chloride or ochcr energizers as
described abovc.
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m us panels of steel containing 0.05% carbon and 0,3% titanium
as the only material alloying metal, show unusually high resistance to
salt spray corrosion when covered by an aluminum diffusion coat having
an aluminum pick-up of 6.5 milligrams per square centimeter, over which
is applied the phosphoric acid-chromic acid-salt-alumin,um co,ating of
Example IX but baked at 70QF and then given a light blasting wi.th fine -
glass micro-spheres in a 5 psi air streamt the blasting removing about
0.1 milligram of the kaked coating per square centimeter,
Although the thus protected panels show splendid corrosion
resistance, their coated surfaces tend to tUXn whi,te or greX after
long exposure to salt spray~ indicating that the aluminum in the top
layer is being attacked very slowly, This whitening or greying can
proceed for a considerable time before the steel is attackedl even
where the coating is scratched through to the base ~etal~ ~owever
the whitening or greying can be greatly slo~ed by covering the
phosphoric acid-chromic acid-salt-aluminum layer with a top c,oating
such as the combination of an air-dried colloidal alumina layer
weighing 0~1 to 1 milligram per square centimeter and an overlying
baked phosphoric acid-chromic acid-salt-Teflon~layer~ weighing 0.2
to 1 milligram per square centimeter. It is Preferred that the
combination of layers on the aluminum diffused surface weigh not
more than about 2 milligrams per square centimeter.
-20-
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10759~0
:
.
A, pointed out above, the coatings containing the leafing
type of aluminum in accordance with the present invention arc
electrically conductive to an appreciable degree when they
have been subjected to baking of at least 900F or when they
; have been burnished as by means of the fine glass blasting, or
when they are heated to at least about 600F in the presence of
an energizer or an ammonium chromate or chromic acid. The greater
their electrical conductivity, the greater the corrosion resistance
they impart, particularly to ferrous substrates. These coatings
' are also smoother and more effective in thinner layers than com-
parable coatings containing granular aluminum as described in
Patents 3,248,251 and 3,787,305)and thus much more suitable for use
` an air foils, particularly of turbines.
! A silver flake coating adheres well to ferrous metal
- when heated to 600 to 700F in the absence of energizers. Indeed
- , the application of energizers to silver flake coatings detracts
... ~.
from their effectiveness. On the other hand tin flake does
not disperse very well and tin flake coatings are best heated
' to ~00F or higher, without energizers.
Although Patent 3,248,251 suggests coatings as thin as
0.5 mil, the commercial coating formulation derived from it is
., ,
;marXeted with instructions to apply it in thicknesses of at
least 1.5 mil. Those instructions also suggest that those
1:
coatings be glass blasted and/or baked to 1000F or higher for
,'best results. However superior results with the 3,248,251
.
coatings are obtained when there is applied over such coatings a
` very thin layer of flake aluminum. Thus a 410 stainless steel
'' ` ~,'' ` , . '~
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10759~0
jet engine compressor blade coated with a 2 mil thick layer of
the oven-dried ~550F) commercial mixture containing granular
aluminum and corresponding to ~xample 2 of Patent 3,248,251,
gave much better protection after there was sprayed over the
,
oven-dried coating a 0.4 milligram per square centimeter layer
of flake aluminum from a ~/O suspension in an aqueous solution
of only 2.4% hepta-ethylene glycol and l/3% p-nonyl-phenoxy
octadecaethoxy ethanol, and the thus-coated blade again baked
dry at 550 or 600F. The degree of improvement thus contributed
iby the additional flaking aluminum layer is approximately the
., ,~ :.
same as that obtained by baking at 600F without that additional
layer and then glass-blasting the first coating layer.
Continuous aluminum coatings as thin as 0.1 milligrams
per square centimeter, and even thinner, are particularly
, ~ :
desirable for coating titanium riv~ts used in aircrafts or
~spacecraft or similar eguipment. In such combinations, the
~aluminum is conveniently applied in the form of flakes, as
indicated above, and the adhesion of the flakes is improved by
the dif,fusion type heat treatment with or without an energizer,
'or by incorporating an ammonium chromate with the aluminum
flakes and heating to produce the bonding acton, which is '~
!also described above.
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. .
Other protective metals in flake or leafing form can
be used in place of or in addi~ion to the flake aluminum to also
increase the resistance of corrodible metals to o~idaeion and
the like, particularly at high temperatures. E~amples of such
pro~ective leafing metals are nickel, tin, stainless steels of
all kinds, and silver. U.S. Patent 3,709,439 describes the
making of such flakes. Ohromium-bearing or austenitic stainless
steels are especially effective. Indeed flake type 304 or type
316 stainless stèel when mixed with from about half to about
twice its weight of flake aluminum, gives better protection
against alkaline attack than flake aluminum alone- Alloys of
nickel and aluminum such as NiAl, and alloys of iron or silicon
with aluminum are also suitable for use in flake form in place
of the aluminum flake to provide improved protection in accordance
with the present invention. Mixtures of flaked metals can be
applied as coating, and heated to cause the mixed metals to
alloy with each other as well as with the substrate, and NiAl
is readily formed in this way to make a very effective high-
temperature-resistant coating.
While the dispersions containing flaked metal, with
or without the chromate, phosphate and salts, can be applied by
dipping, they are preferably applied by spraying or roller
coating. In general such flake-metal-~ontaining coatings shou~d
be baked at a temperature of at least 550F for a few minutes,
and preferably for at least 30 minutes at as high a temperature
as the coated combination will withstand, to provide best results.
Alloying of che metals in the coating with each other and with the
metal substrate is also desirably effected in the manner described
in U~S. Patent 3,720,537. Aluminized or tin coatet piain carbon
.
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:
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.` ,.
qteel ~s well as seeel coated wi~h a mix~ure of nluminum and
ein is convenien~ly made in this way, as by spraying sheec s~cl
unwinding from a coil, with a dispersion of ~he flaked me~al,
then passing the thus coated metal sheet through gas flames to
heae it to 700-750F for 30 seconds, after which ehe sheet is
cooled and recoiled.
Dispersions of flake metal, such as aluminum, also
containing polytetrafluoroethylene, deposit coatings ehat are
not only very decorative by reason of the metallic sheen, but
they are also quite hydrophobic and smooth and especially low in
. frictiona~ resistance to sliding with respect to other objects.
Such coatin~s are also quite adherent to plain carbon steel or
stainless steel or aluminum or aluminized substrates, even when
cured at temperatures as low as 500F. or barely sufficient to
drive off the dispersing and suspending agents. Top coatings
- of the foregoing chromate-phosphate-salt mixtures or oth~r con-
version coatings can be applied over such a metal-tetrafluoro-
ethylene initial coaeing. For best resistance to corrosion the
teflon content should be held down to not over about 1% of the
~ ~ .
final coating, about 0.5% being particularly effective. Where the
teflon-containing coating is heated to about 900F or higher the
teflon coneent can before heating be as high as about 2%
' w~thout detracting too much from the corrosion resistance.
, : -
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)75980
e foregoing top coatings of chromic acid-phosphoric
acid-salt formulations are also helpful when applied over aluminum
diffusion coatings that are produced by the inhibited diffusion
processes described in U.S. Patents 3,257,230, 3,690,934, and
3,867,184. m ose processes are generally conducted at
temperatures well above 1100F with cobalt- and nickel-based
superalloys, but can also be conducted with ferrous substrates at
lower temperatures, particularly to diffuse less aluminum.
In such inhibited diffusion it is desirable to use
extremely fine particles of pre-fired alloys such as alloys of
aluminum and chrominum, or of aluminum, chrcmium and silicon.
Particle sizes of from about 1 to about 10 microns are particularly
suitable.
The separate step of pre-firing the chromium and
aluminum mixture can be avoided by directly preparing such a mixture
in finely divided form. To this end the magnesothermic reduction
of chrcmium compoNnds such as Cr203 as described in French Patent
1,123,326 and its Addition Patent 70,936, can be modified by
oombining an appropriate quantity of alumina with the chromium
oompound, and such combination mixed and subjected to the
magnesothermic reduction as described in those patents. This
simultaneous reduction takes place at about the same temperatures
and times as is shown for the reduction of the chromium compound
alone and with the same equipment, producing a chromium-aluminum
alloy having a particle size of about 1 micron. Residual magnesium
as well as magnesium oxides present in the reduced material is
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removed by treatment with an excess of dilute nitric acid having
a spccific gravity of about 1.12 to about 1.26. Such acid will
not attacX chromium-aluminum alloys having as little as 16%
chromium by weight, but will readily dissolve metallic magnesium
as well as magnesium oxide. Crushing the alloy to a fine powder
helps the acid dissolve all the magnesium rapidly. It is not
essential to remove any magnesium oxide present in the reduced
mixture inasmuch as this compound is essentially inert during a
coating operation and does not tend to sinter or adhere to the
workpieces being coated or to the other ingredients of the coating
pack. Where the hot magnesothermic reaction mixture has its
~; c `
vapor flushed out at high temperatures to flush out the relative-
ly volatile magnesium metal remaining after the reduction is
completed, t,he crude reaction product can after crushing be direct-
ly used for diffusion coating. Where nitric acid washing is
carried out, the washed material is rinsed with water, preferably
to neutrality, filtered and dried before use.
Magnesothermic reduction can also be used in the same
J ~ way to directly produce chr~mium-silicon, chromium-aluminum-
silicon, chromium-aluminum-iron, molybdenum-silicon and tungsten-
silicon alloys in the extremely finely divided form so highly
desirable for diffusion coating workpieces. Silica makes a con-
venient source of silicon for such purposes and can be directly
substituted for or added to the mi~ture being reduced without
materially changing the reduction rate or temperature. The finely
divided alloys can also be produced by magnesothermically reducing
chrom1um, iron, molybdenum or tungsten oxid~s or oth~r compounds
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; i 1075~80
of these metals in the presence of aluminum and/or silicon in
elemcntal form. During sueh reduction the aluminum and/or silicon
alloys with the metallic chromium, iron, molybdenum and tungsten
as it is formed.
The following is an example of the dual reduction
techni~ue:
EX~MPLE IX
1392 grams of magnesium metal were placed in a plain
carbon steel retort cup 8 inches in diameter and 7 inches deep,
the retort uncovered with an inverted outer inconel retort and
the combination heated in a furnace under an argon atmosphere
to 1700F where it was held for 25 minutes to melt the magnesium.
The molten metal was then permitted to cool, still under argon,
to room temperature, when the covering retort was removed, and
replaced after 104 grams powdered A12O3 and 500 grams powdered
Cr2O3 poured over the solidified magnesium. The combination was
again heated under argon, this time to 1825F for 8 hours, and
cooled.
A powdery reaction product remained in the retort. It
was removed from the retort, treated with excess 2 ~ HNO3 until
there was no further reaction evident, and then washed to neutra-
lity with water. The resulting material was a chromium-aluminum
intermetallic in the form of particles averaging about 1 micron
in size. ~t analyzed 81~2-~o chromium ana 16.6% aluminym by weight,
its yield being 91%. When mixed with alumina and ammonium chloride
it gave very good aluminum diffusion coatings in the process of
Canadian Patent 806~618~ in place of the mixture of chromium and
aluminum there suggested.
1075980
Similar results are obtained when the preliminary melting
of the magnesium is not effected, and where the inter-metallic is
used for diffusion coating steels at lower temperatures, Cther
intermetallics similarly made and used have the follcwing analyses:
a) 45,5% Al
54,5% Cr
b) 44,1% Cr
47,7% Fe
8,5% Al
c) 74,5% Cr
7,0% Al
8,5% Si
Alloy c) contains some unreduced oxide~ but it still is very effective
for use in the inhibited diffusion process.
Another type of aluminum diffusion coating over which the
~ top coatings described a~ove can be appliedr is a pack diffusion
';r aluminizing in which the aluminizing is inhibited by cobalt, m is
. diffusion coating is illustrated by the following example: -
':
' EX~LE: X
. . .
A pack was made up in parts by weight of
cobalt 30
;` aluminum 14
'~ alumina (calcined~ 56
:
: . NH4Cl 1/2
::
.. Each of the foregoing ingredients was a 250 to 360 mesh pcwder. m e
` mixture was thoroughly blended and then packed in a plain carbon
steel retort along with nickel blow tips used for blowqng
incandescent light bulbs~
-~8-
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m e re-tort thus loaded was loosely covered and a larger retort lowered
over it as illustrated in U.S, Patent 3,764,371, Using a hydrogen-
bathed atmosphere between the retorts, as also described in 3,764,371,
the packed material was heated to 1975 where it was maintained for
20 hours. After cooling the treated workpieces and lightly glass-
blasting them, all showed an aluminized case frcm about 4 to about
6 mils thick.
~ hen the same treatment is applied to U-700 nickel base
blades for the hot section of a jet engine, somewhat thinner diffusion
coating cases are produced, While leafing aluminum coatings can be
applied over the foregoing diffusion aluminized blades, even without
such top coating the aluminized blades shcw exoeptional resistance
to oxidation and sulfidation. In general the resistance to these
. .: .
effects of the U-700 alloy blades is a little better than the
resistance of such blades aluminized in a chrominum-inhibited
diffusion coating pack. This improvement seems to be due to the
` introduction of some cobalt into the case from the pack, and is
not shown by the cobalt-base substrates. Other nickel-based
- superalloys such as B-1900, Rene 62, MAR-M 200 do show this improved
20 resistance when so coated.
Nickel can also be used for inhibiting aluminum diffusion
as for example into chromized dispersion-strengthened nickel in the
manner described in Patent 3,785,854, and will then make a product of
outstanding oxidation resistance, particulæly with a little chromium
also present in the pack A very suitable aluminum pack in which a
large amount of nickel and a small amount of chromium lowers the
diffusion effectiveness is disclosed by M S, Seltzer, B. A, Wilcox
and J. Stringer in Metallurgical Transactions, September 1972, pages
2391-2401 The Seltzer et al pack (600 g. alumina, 82 g. Ni, 17 g.
Al, 10,5 g. Cr, 6 g NaCl and 6 g, urea)
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I,at 2210F for 32 hours in a hydrogen or argon-washed atmosphere
i deposits on chromized dispersion-strengthened nickel an aluminized~
¦'layer having an aluminum pick-up of approximately 4 milligrams per¦
square centimeter, a surface aluminum content of only about 4 to
about 6%, and a case depth of about 4 mils. This product with-
stands about l90 hours of thermal cycling at 2500F,
¦; Instead of applying the Seltzer et al pack treatment J
i directly to a chromized dispersion-strengthened nickel, it can
be more effectively applied to a chromized dispersion-strengthened !
ilnickel that is first given an uninhibited aluminizing with a
¦ipick-up of 1.5 to 4 milligrams of aluminum per sauare centi-
,meters, and the resulting aluminum-containing surface layer
stripped off in accordance with U. S. Patent 3,622,391. The
iresulting re-a~uminiz~ material shows no failure after 190 hours
, of thermal cycling at 2200~F, and about one-seventh the weight ?
loss of the comparable product produced without the intervening
¦,aluminizing and stripping.
¦ The ingredients of the Seltzer et al aluminizing pack
! can be varied plus or minus 20/~ from the amounts given above
without significantly detracting from its effectiveness. The
¦metallic ingredients of the pack do not have to be pre-alloyed,
¦but the pack should be given a break-in heat treatment prior to
use.
l,i
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107980
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In connection with Example X, the cobalt and alum~num
atom proportions can vary from about 0.4:1 to about 1:0.9 to
obtain the advantages of th~t example: outside these ranges the
case thicXnesscs produced are substantially smaller and not as
desirable. Best results seem to be provided in the Co:Al range
0.8:1 to 1:0.9. The addition to the pack of about 0.1 atom
chromium for every atom of aluminum increases the case thicknesses
that are produced and increasing the chromium content to about
0.5 atom for every atom of aluminum further increases the thick-
nesses. The addition of the chromium also reduces the amount of
oxide inclusion otherwise found in the aluminized cases, but
those inclusions are not particularly harmful even when in the
amounts formed in the absence of chromium.
The foregoing results with the Co-Al pack compositions
are obtained when these two metals are the only ingredients of
the pack, as well as when they are diluted with alumina, kaolin
or other inert diluent to the point that the diluent is 90% of
the pack. Also the NH4Cl can be replaced with any other energizer
such as ~H4Br, NH4I, NH4HF2, I2 and the like, and the coating
temperature varied from 1100F to about 2200F. The atmosphere
~n the pack can be bathed with argon or other inert gas instead
of nitrogen, or can be unbathed as by using a so-called glass
sealed retort as described in U. 5. Patent 3,010,856 granted
November 28, 1961.
Moreover these Co-Al packs produce very high quality
diffusion coatings without requiring a preliminary break-in heat.
ey are accordlngly slmp r to prepare and use.
30a.
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1C~75980
A feature of the present invention is that the flake or
leafing aluminum or other metal forms coatings in which the
individual metal flakes overlap each other so that the coatings
are of continuous character with fewer skips or holes. This
improves the protective action of the coatings, Fven without
such a flake metal overcoat the diffusion coatings from a Cb-Al
coating pack either containing or free of chramium, are very
effective to protect iron, stainless steel, nickel and brass as
well as bronze members that are used to shape molten glass, DV
Minox bronze having the following composition is an example of such
a metal that is well protected by such treatments:
. Copper 63.5 - 68,5%
Nickel 15.5 - 17,5%
Zinc 8 - 10 %
. Aluminum 6,5 - 8.5%
Iron 1.0% maximum
Lead plus Tin 0,1% maximum
This same alloy is also very well protected by pack aluminizing at
800 - 900F for 20 hours in a simple diffusion coating pack such as
described in the above-mentioned U,S, Patents 3,764,373, or
3,785,854. Diffusion coating cases about 4 mils thick are
thus obtained and make excellent protective coatings for incan-
descent lamp glass-blowing molds rade from this alloy.
.
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While so-called conversion coatings make very effective tcp
coatings over the metal-containing coatings of the present invention,
a particularly desirable conversion coating for use at temperatures
of about 450F or belcw, is of the chrcmate-fluoride-iron-cyanide type.
mis is illustrated by the following examples.
EXAMPLE XI
. _ _
Jet engine compressor blades of 410 stainless steel were
.
- aluminized as described in Example I A, and were then conversion
coated by dipping in a bath of
5.6 grams CrO3
1.3 grams NH4HF2
6 7 grams K3Fe (CN)6
1.2 grams H3EO3
diluted to one liter.
me bath was kept at 80F, and after one minute the dipped
blades were withdrawn from the bath and rinsed in tap water. m e
- resulting blades have improved lives when kept from heating up above
450F, as compared to the same blades without the conversion coating.
m e boric acid doe sn't add much to the foregoing bath and
can be omitted without materially reducing its effectiveness. m e
- ferricyanide and bifluoride can have different alkali metals as
cations and the bifluoride can be replaced by fluoride wit~out
- noticeable changes in results. Ih general the CrO3 content can range
from about 0.01 to about 0 5 mols per liter, the fluoA de ion between
about 0.005 and about 0.2 ls per liter and the
.~
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` ` 1075980
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- ferricyanid~ also betwcen about 0.005 and about 0.2 mols per
liter. Best results with boric acid has the borate ion in a
concentration from about 0.005 and about 0.1 mols per litcr.
The bath temperature can vary from about 40F to its boiling
point at atmospheric pressure.
,The leafing aluminum or other leafing metal of the
coatings of the present invention are generally applied in such
small thicknesses that they do not significantly change the di-
.: .. ..
'' , mensions of the substrate being coated. Even where dimensional
accuracy is of very close tolerance, as in the roots and but-
tresses of jet engine blades and vanes, such coatings can be
applied over the entire substrate in the form of coatings 0.1 mil
- thick or thinner. Close tolerances will generally first require
masking of the substrate to keep the diffusion coating from de-
, position inasmuch as diffusion coatings generally add about 0.3
' mil to the dimensions of the material coated. Suitable maskingtechniques are described in Patent 3,801,357, but a preferred
' technique is exemplified as follows:
'' EXAMPLE XII
- ' A set of uncoated jet engine blades of IN-100 alloy
.. I . . I
was cleaned and their roots immersed in a stirred'slurry of 100 g '
I . !
;~ ~ powdered masking mix in a solution of 3 g. pol~ethylmethacrylate)
.; I , .
i resin in 100 g. chloroform. The masking mix was Ni3Al containing '
2% Cr, this combination then being diluted with equal weight of
alumina. The dipped blades were removed from the suspension and
air-dried for five minutes. The resulting blades had their roots !
coated with a layer ranging from about 300 to 800 milligrams per
;. :
- square centimeter of masking powder and resin.
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The blades with thc dried coating were thcn dippcd for
iia few seconds in a 5~O weight dispersion of powdercd nicXel in
, ~ i j
the same resin solution. After withdrawing the blades werc again
air-dried, and both coatings weighed from aoout S00 to about
' 1200 milligrams per square centimeter. They were then packed in
! a prefired diffusion aluminizing pack having the following
,composition in parts by weight: ;
Al (minus 325 mesh) 10
i Cr (about 10 micron particle size) 40
Al 23 S
~H4C1 0.3
,into which additional NH4Cl was blended to bring its concentration
to the designated value. The pacXed assembly was heated in a
retort as in Example I A to 1900F where it was kept for 5 hours.
Upon cooling down and opening the retort, the aluminizing pack
could be sucked out to the point that the blades could then be
individually pulled out from the pack. The blades carried a hard ,
i shell of the originally applied masking layers that appeared
sintered in place and did not readily crumble. ~t was a simple
matter to remove all the blades from the pack along with all the
~asking mixture, leaving the remainder of the pack reusable
without further separation.
he hard shell of masking mix could be broken off with
¦, an easy hammer blow and the cleanly masked blades thus recovered t
without damaging or even endangering them. Combined masking
!. coatings which together weighed only about 300 to 2000 milligram~ ,
per sq,centimeter were satisfactory for this purpose. Any of the ,-
other masking aluminide compositions of Patent 3,801,357 can be
I uscd in the first masking layer in place of the Ni3Al to makc the
- ! hard shcll of the prescnt invention.
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On the othcr hand when using gum tragacanth or bcntonite
and only one masking layer, as described in Patent 3,801,357, the
high temperatures generally cause such layer to crack so that
good masking is not obtained unless the masking coating weighs
about 5 or more grams per square centimeter. Moreover such
cracked coatings also disintegrate readily upon removal of the
workpieces from the pack, and crumbled pieces of the masking laycr
wind up in the recovered pack. Such pieces must be arduously
separated or the entire pack scrapped.
Other acrylic resins such as poly(ethylethacrylate),
poly(methylacrylate), poly(butylacrylate), poly(acrylic acid),
aad other thermoplastic resins such as carboxy methyl cellulose,
cellulose nitrate, ethyl cellulose, and even polyethylene, poly-
propylene, polystyrene and poly(vinyl chloride) can be used in
place of the poly(ethylmethacrylate) but with results that are
not as good. Readily volatilizable solvents such as those Doiling
below 100C are preferred as solvents for the resin.
Obviously many modifications and variations of the pre-
sent invention are possible in the light of the above teachings.
It i9, therefore, to be understood that within the scope of the
appended claims the invention may be practiced otherwise than as
speclfically described.
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