Note: Descriptions are shown in the official language in which they were submitted.
86
Background of the Invention
This invention is related to the stripping of nickel-base
brazes and "hard surface" coatings from a variety of metal sub-
strates, and, more particularly, to a novel stripping composi-
tion and accompanying method for the selective removal of nickel-
base brazes and hard surface coatings, applied by plasma spray,
detonation gun, sputtering and vapor deposition techniques, to
a variety of metal surfaces, without damaging the underlying
substrate.
Description of the Prior Art
Selective metal stripping is one of the most common steps
in many industrial manufacturing processes. Usually, the strip~
ping is part of either a general overhaul, involving the re-
furbishing of a particular coating, or the reclaiming of a
defectively plated part. The overhauling process becomes
economically feasible when the particular product to be stripped
is particularly valuable, such as in the case of high performance
aircraft engine components. Also, defective plating will
usually occur in a certain percentage of plated parts, result-
ing from imperfections in the basis metal, improper cleaning,excessive porosity of the substrate which leads to bleeding out
of various cleaning and plating solutions, impurities, and
human error. The particular method of stripping will usually
depend upon the metal (or metals) to be stripped, the sub-
strate material of the basic part, waste disposal requirements,and profitability.
The prior art has employed a variety of both chemical and
mechanical methods in an effort to selectively remove protective
coatings from the underlying metal substrate, such as the coat-
ings and brazes that are frequently used in the aircraftindustry, and particularly in high performance jet engines.
Such methods, however, have proven unsatisfactory for a
variety of reasons. Low metal stripping rates, general
8;
ineffectiveness at low operating temperatures, -the handling
and disposal of toxic cyanide containing solutions, and the
environmental disposal of the spent fluids are problems arising
from known chemical methods. Damage to precision machined
parts such as jet engine stator assemblies~ particularly the
large dimensional changes which can result on the metal sub-
strate, as well as pit formation in the base metal caused by
pinholes in the hard coating are frequent results of using
pressure blasting abrasive tool methods, and electrolytic
stripping.
Hydrogen peroxide, H2O2, has been used as an oxidizing
agent, in both acid and alkaline solutions, for the selective
stripping and etching of metals; however, H2O2 has not been
effectively adapted for stripping nickel-base metals and
"hard surface" coatings, i.e., surfaces which are comprised
of intermetallic or cermet compositions, and are applied to
the surface to be protected by several high temperature flame
deposition -techniques, from metal substrates.
Nitric acid, HNO3, is a well known stripping agent; how-
ever, since it is extremely corrosive it has found infrequentuse when selectively stripping, since gentle treat~ent of the
underlying substrate is necessary.
Phosphorous-oxy acids, used herein to signlfy phosphorous
and oxygen containing substances which have a tendency to
release a proton in aqueous solution, have found limited usage
in the prior art; see, for example, U.S. Patents 3r607,398 and
4,128,~63; however, they have not been used in combination
with peroxide agents nor for selective stripping required during
the removal of nickel brazes and hard surface coatings.
Selenium compounds and halogen containing accelerators have
been used for the stripping of nickel from stainless steel
in HNO3 solutions; however, such solutions also oxldize and
corrode the steel substrate.
8~
Objects of the Invention
Accordingly, it is an object of this invention -to formu-
late an effective composition and accompanying method for the
selective stripping of nickel-base brazes and protective hard
surface coatings from metal substra-tes.
It is another object of this invention to formulate a
stripping solution which has a much more rapid coating removal
rate at lower operating temperatures than those of the prior
art.
It is still another object of this invent:ion to formulate
a stripping solution for surface coatings which is noncorrosive
to the metal substrate.
It is another object of this invention to strip flame
sprayed cemented metal carbides, nickel aluminides, nickel
graphite and molybdenum from metal substrates used in aircraft
englnes .
~ummary of the Invention
These and other objects of the invention have been
accomplished through the discovery of an aqueous composition
useful Eor the selective stripping of pro-tective hard surface
coatings and nickel-base brazes from metals, and particularly
high strength alloy substrates, comprising about 75-250
grams/liter (g/l) of a peroxide agent, i.e., usually hydrogen
peroxide, about either 50-200 g/l of nitric acid, or in the
alternative, about 0-200 g/l of a mixture containing at least
one phosphorous-oxy acid, with at least one such acid selected
from the group consisting of phosphoric acid, Nitrilotris
(methylene phosphonic acid)(NTPA); and hydroxyethane-l, 1-
diphosphonic acid (HEDPA). Both the nitric and phosphorous-
oxy acid peroxide solutions preferably include about 0-~0 g/l
of an accelerating agent selected from the group consisting of
chlorine and bromine containing ions and oxyanions, oxy
compounds of Group VI elements such as selenium, and various
mixtures thereof; about 0-5 g/l of an inhibiting agent selected
from the group consisting of amines, polyamines, amine oxides
5 and mixtures thereof, and about 0~5 g/l of a surfactant, such
as a fluorocarbon; the acidity of the stripping solution having
a pH value ranging from about -1 to about +0.5. The invention
further includes a method for the selective stripping of hard
surface coatings and nickel-base brazes from metal substrates,
comprising contacting the surface deposit with an aqueous
solution of the composition disclosed above~ and immersing
the surface deposit in the solution until the deposit is sub-
stantially removed from the metal substrate without damaging
the substrate, keeping the solution temperature during operation
between 20-55 C~ and preferably, about 35-40Co
Description of the Invention
A troublesome problem for the metal stripping art has been
to formulate a composition which can selectively strip even the
most difficult to remove coatings from metal substrates without
20 damaging the underlying surface to be stripped. The need for
such a stripping composition is particularly acute in the air-
craft industry, where it is frequently necessary to disassemble
and overhaul high performance aircraft engines which contain
many expensive, reusable parts, particularly the gas turbine
25 jet engines used, for example, in jet aircraft. The continual
development of differe.nt protective coatings enabling higher
temperature operation has necessitated new approaches to
selective stripping. Dur.ing such an overhaul, there exists
a need for a composition which can chemically refurbish these
worn or damaged parts, and the removal of brazes and hard
surface coatings is an essential step in this processO The
stripping agents used for such an overhaul must be very
selective, iOe., they must attack only the braze or hard
surface coating, while not attacking the underlying expensive
~4 ~
~2~
metal substrate. Although applicants do not wish to be bound
by theory, many of the coatings and brazes currently in use
contain quantities of nickel and/or cobalt, and it is believed
the dissolution of these metals promotes the loss of structural
integrity of the coating or braze. The stripping must also take
place at a practical, commercially viable rate, as opposed to
just being a laboratory curiosity. The problem is particularly
acute when one realizes that based on their chemical composition
alone, these coatings may normally be considered more resistant
to a chemical attack than the metal substrates they protect,
and, in fact, their purpose is to impart high temperature
corrosion protection and wear resistance to these metal sub-
strates.
Surprisingly, it has been found that pero~ide solutions in
combination with either nitric acid, or, in the alternative, a
class of mildly aggressive phosphorous-oxy acids, will remove
such nickel-base brazes and hard surface coatings, and yet be
inhibited from attac~ing the expensive machined metal sub-
strates. When nitric acid is the acid system of choice, which
occurs when high stripping rates are desired, HNO3 in amounts
ranging from about 50-200 grams/liter ~g/l), and mos-t prefer-
ably, about 75-150 g/l, in combination with a peroxide agent,
most preferably H2O2, in amounts of about 75-250 g/l, and
preferably incorporating an effective amount of accelerators,
inhibi-tors, and surfactants, ~ives excellent results. Brazes
which can be stripped from metal substrates by such an
EIN03-H202 based system include AMS 4779 (94% Ni, 3.5% Si,
1.8~B) from high alloy steel, AMS 4778 (Ni 92, Si 4.5, B 2.9
C O.06) from AMS 1422, AMS 1424 and AMS 1455 nickel-base alloys,
AMS 4777 (Ni 82, Si 4.5 Cr 7.0, B 3.1, Fe 3.0) from 410
Martensitic Stainless Steel (AMS 5504), and Nicrobraze LM
(Ni 82.5, Cr 7.0, Si 4.5, B 3.0, Fe 3.0) from S-tellite 31
(Co 57, Cr 26~ Ni 10, W 7, Carbon 0~5). Hard surface coatings
which have been stripped include Metcols 71NS and 71VF alloys
(Co 12, C 4, Fe 1, W bal) from 347 and 410 stainless steels,
Metco 307 (Ni 75, C 25) from Inconel 600 alloy, and Metco 450
--5--
(Al 4.5, Ni Bal) from 4130 steel, Hastelloy X and Inconel 600
substrates.
Surprisingly, it has also been found -that peroxide solutions
in combination with certain weakly aggressive phosphorous-oxy
acids will attack nickel-base brazes and hard surface coatings,
and yet be inhibited from attacking the metal substrate. Phos-
phorous-oxy acids which have been particularly effective are
phosphoric acid, nitrilo tris (methylene phosphoric acid)(NTPA),
and hydroxy-ethane-l, l-diphosphonic acid (HEDPA), although most
phosphonic acids, as well as many mixtures thereof, are believed
to be particularly adaptable to the stripping composition of
the invention. The precise amount of the phosphorous-oxy acid
depends upon the character of the substrate to be stripped, as
well as the coating composition. For example, phosphonic acids
NTPA or HEDPA, when mixed with phosphonic acid can range from
about 0-200 g/l when stripping from a stainless steel type
substrate, and preferably about 100 gjl, while the same acids,
when stripping from either titanium base or steel based
substrate should be present from about 0-lQ0 g/l and preferably~
in about 75 g/l. In the same solution phosphonic and phosphoric
acids can range from abou-t 0-200 g/lr and preferably, about
100 g/l.
The greatly preferred peroxide agent of choice is hydro-
gen peroxide, H2O2, both from the perspective of performance,
cost, availability, and environmental effect. But, other
peroxide agents, e.g. perborates, peroxydiphosphates, peroxy-
sulfates and the like, can also be used in place thereof.
In order to maximize stripping effectiveness on steel
surfaces, the peroxide and acid concentrations should be
balanced. The preferred formulation approach is to combine
a major amount of a weakly aggressive phosphorous-oxy acid
towards steel and/or titanium with a lesser amount of a more
aggressive acid. The order of decreasing acid aggressiveness
towards attacking carbon steel in peroxide solution of some
--6--
, ::
~2~
commonly available acids of particular intetest is as ollows:
N(cH2po3H2)2(cH2c~2o~ ~H3po~cH3c(OH)(po3H2)2 ~N(CH2PO3H2)3
If the peroxide oxidant concentration becomes exhaus-ted,
white or tan salt encrustations begin to orm on the attached
steel surface, indicating that the steel or titanium has
corroded. Hard surface coatings which have been successfully
stripped from metal substrates using phosphorous-oxy acid-
peroxide-based systems without damaging the substrate can be
seen from the following Table:
'a u~
U U~ o
C ~
o
H 0 - ~ ! C
a
r O ~ u.
2; 1~ ~ U~
H 00 .
C~
H
~ O;) ~ , ,_
~ 8 /\ ~ ~
Ul Q
H I I IC Q~
U~
m ~ ,~ O
U~ U~ X
O U~ r~ ~
.z Ul
o
O
o
.= a ~
U~
Z U U
o
-- o a~ d'
~L
~f ~
~q a) c _,
L ~ ~ 1
a, ~z; ~.
r~ _ J
r
--8--
~2~
Amounts ranging from 0-5 g/l of inhibiting a~ents can
significantly aid in preventing chemical attack on the metal
substrate particularly in the case of steel and titanium
substrates. In particular, those inhibitors selected from
the group of alkyl bis (2-hydroxy ethyl)-amines:
R-N(cH2cH2o~)2
diamines
N-(CH2)n~N(CH2CH2OH)2,
2 C 2
amine oxides:
N(C 2C 2 )2'
where R, represents an alkyl group, and mixtures of each,
including different Rls for each constituent such as the
mixture o~ aliphatic amines sold under the trade name
"Armohib 31", have been found to be ef~ective. However, alky-
polyethoxylated amines are ineffective, while mono, di and tri
(2-hydroxyethyl~ amines have been only marginally effective
toward low carbon stee~s, thus leading to the conclusion that
an alkyl group, as well as the 2-hydroxyethyl moiety, is
required. It is suspected that the pendant NH2 groups may
provide greater inhibiting ~ualities than the isoelectronic
OH ~mit. A nonexclusive list of inhibitors includes bis
(2-hydroxyethyl) oleyl amine, bis (2-hydroxyethyl) oct~decyl
amine, bis (2-hydroxyethyl) cocoa amine oxide, bis (2-hydroxy-
ethyl) tallow amine oxide, N, N', N' tris (2-hydroxyethyl)-N-
tallow-l, 3-diaminepropane, and imino-bis-propylamine.
Although applicants do not wish to be bound by theory, it is
believed that a stereochemical characteristic exhibited by
_g_
38~
these structurally similar compounds is involved in preventing
oxidant attack on the substrate. The preferred inhibitor, which
is commercially available, sold by Armak Chemicals of Chicago,
Illinois under -the name "Armohib 31" which is a registered
-trademark owned by Armak Chemicals of Chicago, Illinois, is an
aliphatic amine blend having a proprietary composition.
The addition of Group VI elements, particularly selenium
and tellurium containing compounds, and most preferably those
selenium compounds such as selenium dioxide, selenous or
selenic acids, their metal salts, and mixtures, act to accelerate
the removal of the coated deposit. The amounts added should
range from about 0-40 g/l, preferably towards the lower end
of the range. Halogen containing compounds, particularly such
chlorine and bromine ions and oxyanions, such as Cl , OCl ,
Cl03, Br03 , I , I2, IO 3, IO 4 and IO6 , as well as their
reaction products with peroxide should be effective accelera-
ting agents, particularly when used in conjunction with selenium
based accelerating agents. The preferred agents are selenium
dioxide and certain chlorine containing species. Fluorine
based ions, however, are not usually as effective as chlorine.
It is further preferred, although not essential, to add
small amounts of a surfactant, such as 0-150 mg/l of "DB-31"
a registered trademark owned by Dow Corning Corp., a silicone
based anti-foaming agent sold by Dow Corning Inc., when
"Zonyl FSN" a registered trademark owned by DuPont Company, a
fluorocarbon surfactant marketed by DuPont, or, instead, an
amine kased compound o~ -the type that are well known in the
art is included. These surfactants are desirable because they
are resistant to oxidation by the stripping solution, thereby
prolonging their life in the bath.
-- 10 --
..
:~2~8~
One significant advantage of the strippiny composition is
that effective strippin~ can be undertaken at significantly
lower operating temperatures than currently required by con-
ventional stripping agents. Stripping can be carried out
between 20-55C, and preferably, about 35-40C, whereas the
prior art alkaline solutions operated at a bath temperature of
about 60C. The acidity of the stripping bath is an important
parameter for effective stripping; the pH of the acidic
stripping solution must remain below about +1, and preferably,
below 0, for most effective operation. Thus, most carboxylic
acids cannot be used as the acid system since they are not
sufficiently acidic, as only strong or moderately strong acids
may be employed.
EXAMP~ I
The following composition:
g/l
Nitrilo tris (methylene phosphonic acid)
(NTPA) 82
Phosphoric Acid 62
Hydrogen Pero~ide 120
Armohib 31 (Inhibitor) 2.1
DB-31 (non ionic silicone defoamer) 75 mg/l
Water balance
was warmed to between 22-57C (aver. 35C) in order to strip
a titanium coupon (AMS 4911) of 7 cm X 4 cm dimension, having
a plasma spray coating of .3 mm Metco VF-NS, (87 WC, 12 CO,
1 Fe) on one side. The coupon was immersed in the bath for
5 hours, at which time the coating had been stripped from the
titanium surface. The coupon was subjected to a load of 300
lb/in in order to determine whether embrittlement had taken
place~ No crack formation was observed.
--11--
S
EXAMPLE II
Three titanium shafts coated with a 7~7 1/2 mil coating
of LW-llB (WC 88, Co 12) were immersed in the solution of
Example I, kept at a bath temperature o 38-43C., without any
solution agitation, and after three hours the coating had been
completely removed without dama~e to the underlying substrate.
EXAMPLE III
The following solution:
g/l
Nitrilo tris (methylene phosphonic acid)
(NTPA) 82
Phosphoric Acid 30
Hydro~en Peroxiae 120
Selenium Di.oxide
Armohib 31
Water balance
was mixed, and a 2 mil coating of LW-llB (WC 88, Co 12) on a
titanium tube was immersed into the solution. The bath was
maintained at 25-35C, wile the tube was frequentl~ agitated
during the stripping process. Aftex 1-1 1/2 hours the coating
had been stripped, whereupon the machined areas of the part
was observed to have retained thei.r original surface finish
throughout the entire process.
-12-
8~i~
EXAMPLES IV - V
The following solution:
g/l
NTPA 82
Hydroxyethane~ diphosphonic acid
(HEDPA) 6 2
Hydrogen Peroxide 120
Armohib 31 (Inhibitor) 2.3
DB-31 75 mg/l
Water balance
was used to strip WC-Co coatings from both a high alloy steel
and a Ti-64 substrate. Both metal coupons were separately
immersed in the stripp~r bath for about five hours, after which
time both specimens were removed from the solution. The WC-Co
coating had been s~stantially stripped withou~ damage to either
of the underlying substrates.
EXAMPLE VI
The following solution:
~/1
Nitric Acid 150
Hydrogen Peroxide 150
Selenium Dioxide 0.6
Zonyl FSN 0.29
Water balance
was mixed, after which a second stage nozzle guide vane,
fabricated from Mar-M-200 (lCb, 9Cr, lOCo, 2 Ti, 5Al, .14C,
12-5W, 2Hf, bal Ni~ + HF (PWA 1422 Directionally Solidified),
and having two covers brazed with AMS 4778 (Ni 92, Si 4~5,
B 2.9, C ~06), was immersed for five hours in the solution,
-13-
which was kept at an average temperature of 40C. Upon
removal from solu~ion, the braze had been completely stripped
so the covers could be removed from the vane by gentle prying.
No substrate metal attack occurred, but the pack. aluminide
coating had been partially removed.
EXAMPLE VII
The solution of Example VI was again mixed and used to
strip a jet engine part comprising a honeycomb lattice brazed
to a high alloy steel with AMS 4779A in a bath kept at an
10 average temperature o~ 43C. After 1 1/2 hours the honeycomb
structure separated cleanly from the steel backing.
EXAMPLE VIII
The following solution:
Volume
%
Nitric Acid (70 wt %) 20
Hydrogen Peroxide (50 wt %) 20
Water balance
was mixed, and an Inconel 600 coupon coated with 25 mils of
Metco 307 nickel-graphite was immersed in solution and stripped
within 3/4 hour at an average bath temperature of 43C.
EXAMPLE IX
The solution of Example VIII was mixed and an Inconel 600
coupon coated with a 25 mil layer of Metco 450 nickel aluminide
was next immersed into the solution, which was kept between
45-50C. Complete coating removal required 80 minutes.
-14-
.. ~
EXAMPLE X
The solution of Example VIII was mixed, and a 347 stain-
less steel shaft coated with a protective covering of 5 1/2 mils
of Metco 71 VF (Co 12, Fe 1, WC bal.) was then immersed and
stripped in the solution maintained at a temperature between
40-50C. After a quarter hour the coating had been completely
removed.
EXAMPLE XI
The following solution:
g/l
Nitric Acid 75
Hydrogen Peroxide 150
Selenium Dioxide
Zonyl FSN 0.5
Water balance
was warmed to between 30-38C, whereupon an Inconel X-750 stator
segment which had been stripped o its nickel electroplate was
immersed in the solution, which was gi~en intermittant
agitation during stripping. After 22 hours the AMS 4777
braze had been stripped (1.22 grams), whereupon the outer
shroud and foot could be separated rom the vane.
-15-
EXAMPLE XI I
The following solution:
Nitric acid 150 g/l
Hydrogen Peroxide 120 "
Selenium Dioxide 5 "
Zinc Chloride (50 wt %) 8 ml/l
Water balance
was warmed to 47C (over), whereupon a 410 stainless steel "T"
section brazed with PWA 996 was immersed for three hours, the
solution being subjected to mild mechanical agitation during
this time. Upon removal, -the braze was sufficiently dissolved
to permit separation of the joined parts. The cut edges of
the 410 steel remained shiny an~ unetched.
EXAMPLE XI I I
The following solution:
Nitric acid (50 wt %) 20 vol %
Hydrogen Peroxide (50 wt %) 25 vol %
Aluminum Chloride (AlC13 - 6H2O) 23 g/l
Zonyl FSN 0.1 vol %
Water balance
was warmed to 43C, whereupon a cobalt base stator assembly
fabricated with Nicrobraze LM (Ni 82-S, Cr 7, Si 4.5, B 3.0,
Fe 3.0) was immersed therein~ After 19 hours the braze had
been completely selectively stripped allowing separation of
the two segments.
Several significant improvements have been realized by
these novel compositions. The formulations can strip coatings
and brazes from a plurality of metal substrates at stripping
--16--
:~2~
rates measured in hours, as compared with days for prior art
alkaline solutions. Also, the lower opera-ting temperatures
of the solutions of the invention permit the usage of low
melting point was maskants. The peroxide reaction products
decompose after use to harmless reaction products/ H2O and 2'
as contrasted with many oxidants in current use which often
cannot be easily disposed of, and may pose environmental
hazards.
In accordance with the invention, a novel method for
stripping hard surface coatin~s and nickel-base brazes from
metal, particularly high strength alloys, has been devised
which comprises contacting the surface coating at a tempera-
ture of about 30-45C, and preferably about 35C, with an
aqueous solution of the composition described earlier, in an
amount sufficient to maintain -the acidity, i.e., the p~ of
the contacting solution at a value bet-.ween about -1 to 0.5,
and continuing the contacting of the surface coating with the
solution until the surface coating has been selectively
removed from the metal substrate.
During operation, it is preferred that the solution be
regularly agitated in order to produce the most efficient
strippin~ conditions. ~ither workpiece agi~ation or strong
mechanical solution agitation is satisfactory. Spray
applications may also be used.
It will be apparent to those skilled in the art that many
variations and modifications can be made to the specific
embodiments discussed above. All such departures from the
foregoing specification are considered to be within the scope
of the invention as defined by this specification and the
appended claims.
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