Note: Descriptions are shown in the official language in which they were submitted.
431
The present application is related to applicant's
Canadian application Serial No. 261,461 filed September 17,
1976.
The present invention relates to the coating of
metals to increase their resistance to corrosion and other
chemical attacks.
Among the objects of the present invention is the
provision of novel coating techniques and compositions for
use therewith, as well as novel coated products, all suit-
able for comrnercial operations.
The foregoing as well as additional objects of
the present invention will be more fully understood from the
following description of several of its exemplifications.
Where the dimensional accuracy of a work-piece is
of a high order, such as on the root of a rotor blade that
is to be securely received in a socket, it is frequently
impractical to permit aluminizing of that root because the
added aluminum increases the root's dimensions. Such
masking problems arise more frequentlY with the superalloy
components in the hot section of a turbine engine, where
aluminizing is more widely practiced.
A particularly desirable masking technique
according to the present invention, involves the coating
of the portions to be masked with at least one layer of a
dispersion of finely divided depletion-reducing masking
solid and resin in a volatile solvent, evaporating off
sufficient so1vent to set the coating, applying over the
set coating at least one stratum of finely divided non-
contaminating solid particles that upon aluminizing become
coherently held together to form a secure masking sheath.
The sheath-forrning layer or one or more of the
strata which constitute this layer, preferably have Cr203,
~"
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nickel or mi~tures of these -two, as the particles that
become coherently united by the aluminizing. Neither of
these materials contaminate superalloy or stainless steel
work-pieces inasmuch as orlly chromium or nickel can be
introduced into the work-pieces from these sources, and
these two metals are already present in the work-pieces.
Chromium and nickel are also notconsidered contaminants
for low alloy steels, particularly those ferrous alloys
containing at least 1% chromium.
The Cr2O3 and nickel particles are so actively
effective that they can be diluted with as much as twice
their weight of alumina or other inert filler, withou-t losing
their sheath-forming ability. While they can be used in
undiluted form, it is preferred to dilute these aluminizable
ingredients with some filler, at least about half as much
filler as active material, by weight. Such dilution dimin-
ishes the amount of material that can consume the aluminizing
aluminum, and also reduces the masking cost. ~orever,
undiluted coatings of Cr2O3 and resin tend to crack on drying.
For best masking of those aluminizing diffusions
that are conducted a-t extremely high temperatures, e.g.,
1900F or higher, it is helpful to have a three-layer
masking combination in which the work-piece-con-tacting
layer is of the depletion-reducing type, the next layer of
the Cr ~ 3 type, and the outermos-t layer of the nickel type.
The masking features of the present invention are
brought out in the following examples:
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~X~M~L~ 1
A number of hot section ~irst stage jet engine
blades made of ~-1900 nic]cel-base superalloy had their
roots dipped in the following mixture:
Powdered Ni3Al having 20 to 100
micron particle size111 g.
Powdered chromium having 20 to
100 micron particle size 3.4 g.
Powdered alumina having 20 to 100
micron particle size111 g.
Poly(ethylmethacrylate)9 g.
Methyl chloroform 123 g.
The resin is first dissolved in the methyl chloroform, and
the remaining ingredients then added with stirring to form
a uniform suspension that does not settle very rapidly.
A single dip coating treatment with the suspension
at room temperature, about 60 to about 80F, deposits a
layer weighing about 130 milligrams per square centimeter
after the methyl chloroform solven-t is permitted to evapor-
ate. Only a fraction of a minute is needed to complete such
a coating, and i-t is helpful to repeat the dip several times
until the combined coatings weigh about 500 grams per s~uare
centimeter. Dipping a previously dipped coating in the dis-
persion does not remove any significant portion of -the
previous coating, particularly if the previous coating had
been permitted to dry at room temperature for at least abou-t
1/2 minute.
After three or more dips as above, the thus-coa-ted
blades are dipped in the following coating mixture:
Po~Jdered nickel, 20 to 100 micron
particle size 175 g.
Powdered alumina, 20 to 100 micron
particle size 175 g.
Poly(ethylmethacrylate)~.7 g.
~5ethyl chloroform 62.1 g.
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3~L
This dipping is repeated two more tirnesr wi,th
interve~ lg dryincJs at least about 1/2 minute lony each,
to build up the la-tter coa-ting -to abou-t 500 milli,grams per
square centimeter. The blades were then promptly inser-ted
in a pre-fired diffusion aluminizing pack having the
following composition in parts by weigh-t.
Powde:~ed aluminum, 20 to 60 micron
particle size lO
Powdered chromium, abou-t 10 micron
particle size 40
Powdered alurnina, 20 to 60 micron
par-ticle size 50
Powdered N~I~C] 0.3
into which additional NH4Cl was blended to bring its con-
centration to the 0.3~ value and make up for pre-firing
volatilïzation. All of the blades were completely covered
by the pack, and the mass was held in a diffusion coating
retort. Diffusion coating was then carried out with a 6
hour hold at 1875F in the manner described in U.S. Paten-t
3,785,354 ~Alloy Sur:faces Co., Inc., January 15, 1974). The
blades are then removed frorl-the cooled retort and carry a
hard shell-like sheath or crust where the roots had been
covered with the masking dips. These crusts are quite
adherent and coheren-t, so that tlle a:Luminizing pack is not
materially contarlinated by the masking l,ayers, and can be
reused for additional aluminizing without further precautions.
The crusts are fairly bri-ttle and can be readily
removed from the b]ades by light blows of a llammer or e~en
a wood club, or by blas-t:;,ng wi-tll an air-prop~,l,led stre-.rn of
nickel shot. The crust fragrnen-ts are discarded leaving the
blade roots showing no aluminizing, and the balance of the
blades wi-th a 3 mil alurninized case.
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The sh~.!ll or crus-t fo:rma-tion is due to the fac-t
tha-t the nickel powder i.n the outermost maskiny layer under-
goes so much aluminizing that these powder particles grow
together. The dilution of -the nickel with as much as four-
thirds i.ts weigh-t of inert material such as alumina does not
prevent such growing together, and neither is it preven-ted
by the presence of the resin in the dipped masking composi-
tion. Such resin is completely driven off during the
initial portion of the diffusion coating heat, bu-t the
relatively small amount of such resin would not significantly
affect the results even if it were to survive the diffusion
heat or were carbonized by that heat.
The diluted Ni3Al in -the lower layers of masking
does not aluminize sufficiently to cause shell or crust
formation, even -though -those layers also contain a small
amount of chromium that by itself would form a shell. Any
metal-containing layer in con-tact with a work-piece should
contain at least 25% inert non-metal such as the alumina,
or ~aolin, to assure tha-t the metal of the layer does not
sinter to the work-piece, and such dilu-tion also keeps the
Ni3Al from forming a shell or crus-t.
On the other hand, o-ther aluminizable powders
that are essentially inert to the work-piece but :Eorm shells,
can he used in place of or in addi-tion to the nickel powder
in the shell-forming layers. Cr2O3 is another such shell-
forming material, apparen-tly undergoing some conversion to
aluminized metallic chromium. Being less experlsive -than
nickel, Cr2O3 is particulArly desirable for use where masking
expenses are -to be minirnized. Mixtures of nickel and Cr2O3
can be used with an effec-tiveness correspondillc3 to -that of
each individuall~ and i.ndeed a smal~L amoun-t, such as 5%, of
Ni3Al. can be mi~ed with the nickel or the Cr2O3 wi-tllout
de-tracting si.ynif:ican-tly from the resul.-ts.
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~ - 5 -
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~11 diluen-ts can be omitted from the shell--
forming lay~rs, if desired, but this makes it more important
to be sure that at least the minimum effective amount of
shell-formillg layer is applied. In undiluted condition
only about 100 milligrams per square cen-timeter of nickel
or Cr2O3 is needed, and the presence of the resin adds so
little to the shell-forming layer that :it has no si.gnificant
effect on the amount of shell-forming layer needed. Layers
deposited from undiluted Cr2O3 suspended in a resin solution,
tend to crack on drying, whereas there is no.such cracking
when the Cr2O3 i.s diluted with at leas-t about 1/~ its weight
of A12O3 or other diluen-t.
The resin in the outer layers acts to keep the
masking layers frorn rubbing off onto or into the diffusion
coating pack during the packing. Thus, a stratumof nickel
powder, with or without alumina, can be applied over the
Ni3Al-chromium-resin lower masking layers as by ro-tating
the blades coated with those masking Layers in a falling
stream of powdered ni.ckel so that falling powder particles
adhere to the resin-containi.ng lower masking layers This
is, however, not nearly as simple as the application of the
outer masking layers by dipping, its uniformity is not as
good, and some of the fallin~ powder so adhered tends to rub
off when the work-pieces are handled and when the diffusion
coating pack is poured over them.
After the first dip or two to apply resi.n-
containing masking layexs, some or all subsequen-t ~ips can
be effected in r~sin-free suspensions of the coating mater-
ials in a solvent -that dissolves the resin in the previously
applied layers. Inasmuch as coating suspensions en-tirely
free of res.in are more difficult to main-tain uniform, a
little resin or other viscosity-increasi.ng r.laterial can be
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added to sucll dispersions -to reduce the settliny rate of
the clispersed powcters.
It is also helpEul to use a combination of shell-
forming masking layers in which sorne are oE the type that
depend on the presence of nickel powder, and others are of
the type that depend on the presence of Cr2O3 powder. Thus,
it is particularly des:irable for aluminizings that are
effected at about 2000E' or higher, Eor the shell-forming
combination of layers to have the lo~ermost such layers
based on Cr2O3 and built up to a-t least about 300 milligrams
per square centimeter, while the uppermost are based on
nickel powder and are also built up to at least about 300
milligrams per square centimeter.
While other resins and solvents can be used to
make masking layers of the present invention, the acrylic
resins are preferred and poly(ethylmethacrylate) is parti-
cularly preferred because it gives such good dip coatings
and clean product-s. Methyl chloroform is also a par-ticul-
arly preferred solvent inasmuch as it has -the dcsired solvent
action combined with good evaporation charac-teris-tics and
low use hazard. Some acrylic copolymers are not suEficiently
soluble in methyl chloroform, and for such polymers acetone
or methyl ethyl ketone or me-thylene chloride can be used
as the solvent. Any o-ther resin that forms a cohesive film
can be used, even wax, so long as it does not contribute
contamination as by silicon present in silicon~resins.
The composi-tion of the depletion-reducing maskiny
mi~ture can vary in the manner descrihed in U~.S. Patent
3,801,357 (Alloy Surfaces Co., Inc., April 2, 1974). Thus,
the aluminide can be nickel or cobal-t aluminide containil-lg
hetween 1~3 and 3/4 atom of aluminum for every atom of
nickel or cobalt, -the inert particles can range from about
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3.1~
1/~1 to about 2~3 of this mix-ture by wei~ht, and the chrorFIium
COntellt can range from about 1/4 -to abou-t 3~ of -this mixture
by weig}lt.
The outermost or shea-th-forming layer of the
masking combination makes a very effective retaining or
securinc3 means tha-t assures the locking of other types of
layers beneath it onto the workpiece surface through the
coating heat. The same securing action can be used to hold
a slurry coating layer instead of a masking layer onto the
wor~piece, instead of the masking layer. This is illustrated
in the following example and makes it unnecessary to have the
workpiece embedded in a coating pack.
EX~MPLE 2
A 5-foo-t length of steam generator high pressure
*
tubing of Croloy alloy (1.9 to 2.6~ Cr, 0.97 to 1.13% Mo,
0.15~ C, balance essentially iron) having a 3/4 inch bore
and a 1/2 inch wall was thoroughly cleaned inside and out,
and had its bore filled with a chromizing pack composed of
a previously broken-in mixture of
10o chromium powder the particle sizes
of which range from about 10 to
abou-t 20 microns, and
90% tabular alumina granules having a
- particle size ranging from about
100 to about 300 microns
to which mixture was added 1/2% NH4Cl granules as an activator.
The breaking-in was effected by a mixtuxe of the foregoing
three ingredients in a re-tort in the absence of a workpiece,
to 1800 - 1850F for 10 hours under hydrogen. The tube so
filled had its ends capped w:ith 1010 sceel caps frictionally
fi-tted over the tube ends so as to provide semi-gas-tight
covers.
The outside surface of the tube was then painted
wi-~h the followincJ layers in succession, dr~ving the first
layer for a few rninutes before applying -che second.
~~ * ~ra~
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first layer - oO0 grams of a pre-viously broken-in rn:ix-ture
of ~5~j Cr, ~56 alumina and 10o Al, to which
l/250 N~I~C1 is added beEor~e as well as afler
break-in as described in the aforementioned
U.S. Patent 3,801,357, suspended in 200 cc of
methyl chloroform solution containins 7 weight
per cen-t of a copolymer of 70% ethyl and 30%
methyl me-thacrylates, 0.5 weigh-t per cent
s-tearic acid and 0.1 weight per cen-t cetyl
ether of decaethoxy ethanol. Four applica-
tions oE this mixture are made with inter-
vening drying to build -the non-volatile
coa-ting to 1200 milligrams per square centi-
me-ter of tube surface.
second layer - 600 grams of a mixture of 68.5% Ni powder
and 31.5~ Al powder, dispersed in 150 cc of
above methyl chloroform solution. The metals
of this slurry were no-t pre-fired, and only
-two applications oE -this sl~lrry was made to
provide a non-vola-tile coa-ting weigh-t of
abou-t half -tha-t of the first layer.
The tube so prepared was placed in a tubular
retort of a diffusion coa-ting furnace assembly having inlet
and ou-tle-t connections fora hydro~en-blanke-ting as in U.S.
Pa-tent 3,801,357 and then subjected to a diffusion coa-ting
hea-t of 1800F for- 10 hoursO Af-ter cooldown at -the end of
the hea-t, the tube end caps we L" removed, ti1e pack in the
tube bore poured ou-t, and -the sheath around th_ exterior
of the -tube broken off and removed. The in-terior of -the
tube was effective~ly chromized wi-th a ccise 1.8 to 2.3 mils
thick, and the outside of the tube aluminized with a case
~ about 2~ mils thick.
jb~c,
1~443~
The chromized case included an outer por-tion
about 0.3 mil thick rich in chromium carbide, and an inner
portion of columnar chromized structure. This case is
particularly effective in reducing erosion of the internal
tube surface by rapidly moving high pressure steam.
The aluminized outer surface prolongs the life of
the tube in a coal- or oil-fired furnace where it is sub-
jected to combustion atmospheres at temperatures as high
as about 1000F.
The internal pack is a highly fluent composition
that is easily poured into place before the heat, and is
readily removed afterwards, using a minimum of mechanical
poking and the like. Such a pack is particularly desirable
for packing of cramped recesses in the interior of work-
pieces, such as in the narrow bores described above, or in
hollow jet engine blades, or the like.
The noted fluenc~- is brought about because the
alumina granules, which are crushed from alumina which has
been melted and solidified, are quite fluent and show a
flow angle of about 45 degrees. This is the angle of
incline (measured from the horizontal) of a cone made by
pouring a stream of the granules onto one spot to build up
a cone. The fluency can be increased by selecting aluminas
or other inert particles having an even smaller flow angle.
Thus, alumina microspheres having particles sized about 100
to about 500 microns are exceptionally fluent. Tabular
alumina, which can be made by sintering alumina powder and
then crushing, is also fluent, and is preferred because it
tamps in place better and then during the diffusion coating
does not shrink from the surface against which it is tamped.
The pack need only have about 1/2 its volume of
any of the foregoing fluent materials. Thus, the chromium
---l
~ ,~
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part:icles hy themselves need nol be fluent at all, and
will provide a sui-table f]uent pack when J/2 of -tl~e pack
is eonstituted by thc f1uent yranules or microspheres.
Similarly non-fluellt alumilla or o-ther non-flllen-t inert
material ean be present in the paci with or without non--
fluent ehromium powder, withou-t detracting significantly
from the flueney provided by the foregoing volume of
fluent material.
The fluen-t diffusion coating packs are also
highly beneficial for use in chromizing bent small-bore
tubing. Thus, for some steam-genexating arrangements
lengths of steam-generating tubes have one end bent back
180 degrees to form eane-shapecL uni-ts whieh ean have
their ends welded to additional lengths to form a eon-
tinuous furnace tube assembly. The paeking of the bores
of sueh "eanes" for diffusion coating, and the subsequen-t
paek removal af-ter eoating, is grea-tly simplified by the
use of fluent pae~s.
Any ferrous base steam boiler tubing is improved
by the foregoing diffusion eoatincJs. Plain earbon and low
alloy steels wil:L form -the ehromium earbide stratum with
unclerlying eolumnar ehromized ease, and will show very little
abrading :Loss when used with high pressure steam. Sueh
abrasion by the steam eauses the steam to earry along with
it the abraded partieles and they ean damage the vanes and
blades of turbines driven by the steam. For sueh pressur-
iæed use the boiler tubing should have a wall -thie~ness of
at leas-t about 3~15 incll.
The eY-ternal surEaces of bo:~er tubing are also made
more resistant to corrosion as a result of the a1uminizing or
the chromizing describecl above, even -the minor ehromizillg
effected when only the interior oi such a tube is pac~ed
with -the di.L fusion c~!romiziny mix.
. ~
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.~s pointed out, -the shell-forming layer used -to
hold a slurry coa-ting in place where there is notmuch of an
aluminizing or chromizing atmosphere, contains non-
contaminating materials that sinter together under coating
conditions. Thus, nickel and aluminum powders smaller than
about 500 microns in size and in an atom proportion from
about 1 : 0.9 to ahout 1 : 1.1 are very effec-tive. Chromium
can be substituted for the nickel in such a mixture. Because
of the non-contamlnating character, a cliffusion coating pack
will not be ruined in the even-t a small. amount of the masking
materials should inadver-tently become mixed into it.
Fluent diffusion coating packs are also very help-
ful even when they are used without insertion into cramped
spaces. Thus, in diffusion coating of the outer surfaces
OL turbine vanes or blades in the standard arrangemen-ts
such as shown in U.S. Patent 3,76~,371 (Alloy Surfaces Co.,
Inc., October 9, 1973) or in glass-sealed retorts as in
U.S. Patent 2,~44,273 (G. A. Samuel, July 22, 195~), a
fluent pack is simpler to load into a retort, simpler to
embed the work in and far easier to remove from the retort
after the coating heat is completed. Forsuch use the fluent
particles can have repose angles of as much as 55 degrees,
and can constitute as little as about ~0~ of the pack by
volume, although the greater fluency of the packs having
at least half of the particles with the ~5 degree repose
angle, are preferred.
:[t is not necessary to diffusion coatthe
external sur:Eaces of SUcil tubes or to diEfusioIl coat those
external sur:Eaces wi-th aluminum. The internal bores of
such tubes can be chromized for instance withou-t the use of
a covering layer on -their exterior. Such chromizing in the
retor-t arrangement described in Example 2 will produce a
jb/cz - 12 -
L3~!L
dis-tinct outer chromized case in addition to the above-
described internal case, but -the ou-ter case will generally
no-t show the carbide stra-tum and will have less chromium
pick-up.
The foregoing inner and outer coatinys can be
applied to tubes as long as ~0 fee-t or more in length,
whether those -tubes be s-traight or cane-shaped. E'or such
long tubes it is desirable to efEect the diffusion coating
uniformly along the tube lengths, and to -this end the
diffusion coating temperature along the length o~ the tube
can be closely controlled. Thus, a plurality of the tubes
each wi.th its bore packed and its ends capped can be
inserted lengthwise in a tubular open-ended .retort suEfi-
ciently narrow to make a snug con-tainer for the tubes. O:ne
or more o-f the tubes is also fitted with a number of thermo-
couples on its ex-terior and distributed along its length.
The retort so loaded can then be inserted in a furnace
having a number of gas burner rings spaced along the
retort's length, each ring encircling the re.tort. Only
one end o-f the retort need be open, and that end can be
sealed with a head that has a number of passageways. Some
of these passageways are used for the passage of thermo
couple connection wires, and two can be used as inlet and
outlet respectively, :Eor a bathing gas such as hydrogen.
The operation of the burner rings is then adjusted
~ to bring al.l -the thermocouples -to the desired temperature.
Shoulcl there be a temperature variation among the thermo-
couples, individual burners can be further adjusted. In
; this way a cool therrnocouple can have its temperature
brough-t up by turning up the nearest burner or burners,
and conversely a hot one can have its temperature reduced
by turning down the nearest burner or burners. These
jb/ ~ 13 -
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control functions can be effected manually or automatically
to main-tain the temperature of the packed tubes within a
range of abou-t 25 F along their en-tire leng-ths.
Cane-shaped tubes can be handled the same way.
Also an argon washing atmosphere can be maintained through-
out most or all of the diffusion coating of Example 2 with-
out signi icantly different results. Because of its expense,
the flow of argon should be reduced to a very small rate by
back pressuring it equivalent -to 1 to 2 inches of wa-ter.
Alternatively, that diffusîon can be effected in a glass~
sealed retort with an autogeneously generated atmosphere.
Fluent diffusion coating packs are a:Lso helpful
for use w~ith the shell-forming masking arrangements described
above. Such coating packs do not have to be mechanically
worked to loosen them after a coating heat is completed,
and there is accordingly less danger of damage to the
masking shell.
Obviously, many modifica-tions and variations of
the present invention are possible in the ligh-t of the
above teachings. It is therefore to be understood that
within the scope oE the appended claims the invention may
be practiced otherwise than as specifically descrihed.
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