Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to the surface cleaning of
copper base alloys and provides a rapid and effective two-
stage process for removing surface oxide deposits formed on
the metal, as during annealing treatments. In the case of
copper alloys contalning aluminum and other readily oxidizable
metals such as iron, cobalt, nickel, zinc or silicon, a
resistant metal oxide surface film is formed during annealing
treatments at elevated temperatures, applied for a period of
time sufficient to relieve strains imposed during mechanical
operations, as in the rolling to metal ingots to elongated
strip of reduced thickness or for the fabrication of metal
parts and articles. Such surface coatings of oxlde are
produced even when the anneallng is carried out in the presen-
ce of a protective atmosphere, such as may be prepared by the
partial combustion of a hydrocarbon, because the active metals
present in the alloy will react with oxygen that is present
in the free state or as moisture or as an oxide of carbon
to form refractory oxides. When various metal oxides are
thus formed, they often tend to combine with each other to
form complex metal oxides, such as spinels, of refractory
nature, which at times are extremely resistant to removal
by conventional cleaning methods. Unless such surface oxide
films are thoroughly removed, however, subsequent surface
finishing treatments, such as soldering, electroplating, or
pressure bonding with other metals, will generally yield
unsatisfactory results because of the resulting poor adhesion.
While numerous proposals have been made in the past
for single and multi-stage cleaning treatments, they have
generally proved ineffective for the successful cleaning of
copper alloys containing aluminum and other metals tending to
form refractory oxides in thick surface layers. Although some
of these procedures mayj if prolonged, finally result in the
removal of such layers, an undesirable extent of pitting and
etching of the metal surface is found to occur simultaneously
during such extended treatments, which may render them
unsuitable for the intended purpose. Such unsatisfactory
results characterize attempts to remove thick refractory oxide
layers from various copper alloys containing aluminum and
other active metals by single treatments with aqueous pickling
solutions, such as aqueous solutions of sulfuric acid, sulfu-
ric acid and alkali dichromate, sulfuric acid and ferric
sulfate, as described on Pages 308-309 of "The Chemical
; Formulary", Vol. IX, 1951, Chemical Publishing Co.~ Inc.
Brooklyn, ~.Y.
Accordingly, it is the principal object of this
invention to provide a rapid and effective treatment for the
removal of refractory complex metal oxide films, even of
unusually high thickness, from the surface of copper base
alloys.
- A further object of the invention is to provide
a process that is readily applied and is effective for such
removal without causing harmful surface etching or pitting of
the metal.
In accordance with the present invention 9 there is
provided a process for cleaning the surface of a copper base
alloy having a surface layer of thermally formed complex metal
oxide of a refractory nature, comprising immersing the surface
in an aqueous alkaline solution having a pH of 11 to 14 and
heated to a temperature of about 40C to its boiling point for
2-
a period of two seconds to not more than ten minutes, and
then immersing the surface in 0.5 to 3 N. aqueous solution
of ferric sulfate containing no added mineral acid at a
temperature of 25 to 95C for a period of two seconds to not
more than ten minutes.
The process of the invention involves the combina-
tion of the following two-step treatrnent, neither step alone
being effective to produce a clean metal surface satisfactory
for soldering or other surface treating process. The oxide-
coated copper-base article or strip is first immersed in a
stirred aqueous alkaline solution, as of sodium, potassium or
lithium hydroxide, having a pH above 10, for at least two
seconds at a temperature of about 40C to the boiling point,
and, after draining and rinsing with water, is then submerged
in a stirred aqueous solution of ferric sulfate, or similar
ferric salt of a mineral acid containing no added mineral
acid, at a ferric salt concentration of 0.5 to 3.0 ~, for at
least two seconds, preferably for five to sixty seconds, at
a temperature of 25 to 95C, prefera~ly at 60 to 90C.
~ Each step should be followed by draining the solution from
the article or subjecting it to blowing with air streams,
and removing the residual solution by rinsing with waterO
The need for such improved cleaning process arose
during the use of the duplex process of U.S. Patent 3,646,946
issued March 7, 1972, consisting of first immersing annealed
metal in a hot alkaline solution and then treating with a hot
solution of mineral acid, preferably a 12% by volume aqueous
sulfuric acid solution. Effective results were attained
thereby in removing complex oxide films and producing a
surface displaying excellent solderability in a variety of
f~
annealed copper base alloys, such as Cu, Fe and Al; Cu, Al,
Si, Co and P, and C.D.A. Alloy 688 containing 22.7% Zn, 3.4%
Al, 0.4% Co, and balance e~sentially Cu. The above duplex
treatment accomplished the substantially complete removal of
complex
~i,,,
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333 ~ 5~
oxide film ln annealed metal shown ~y capacitance measure-
ments to have sur~ace oxide films up to about 105 A in
thickne s. However~ when the latter alloy was annealed
under more severe conditions and having surface oxide
films up to 140 to 160 A in thickness, such as resulted
from bell annealing treatments whlch were prolonged or
carried out in a less ~rotective atmosphere, ~he above
duplex cleaning process displayed subs~antiall~ decreased
effectiveness. Thereb~, the need for an improved cleaning
procedure ~rose as an urgent pro~lem, which has now been
solved through the provision o~ the novel second step of
the duplex treatment described below.
- DE~AILED DESCRIPTION
It was theorized that a solution having proper oxida-
tive power and solvent action on complex oxide films might
be found through measurements o~ the rate of weight loss
of the basis metal therein. This mode of attack has been
successful ln establishing an effective second treatment,
although the generality o~ such parallel behavior is
quite uncert in and unproved. A study o~ weight loss
measurements was carried out in the above Alloy 688 at
50C and 6~C immersed for periods o~ lO, 20, and 30
seconds in a l N. aqueous solution of ~erric sul~ate
containing no added acid, and comparison solutions con-
taining 1 N. ~erric sulfate and added sul~uric acid at con-
centrations o~ l N., 3 N., and 5 N. At each temperature,
the loss in weight per unit area, expressed as micrograms
per square centimeter, wa3 linear with time, in seconds,
and ~ielded the values for dissolving rate listed in
Table I.
34~ 901
TA~LE I
Solution ~i'ss~l~i'n~_R'at'eCu' grams ~e'r 'cm2'~er sec.
'50'C ''65C
1 N.Fe2(S04)3 45- 62.8
1 N.~e2Cso4)3 ~
1 N.H2S04 44.4 58.0
1 N.Fe2(S04)3 ~
3 N H2S4 41.3 53.8
1 N.Fe2~S04)3 ~
5 N~2S4 36.4 48.3
These results sho~ed quite unexpectedly that the
aqueous ~erric sulfate solution with no added acid, at
both temperatures, dissolves the metal at a higher rate
than any of the solutions containing added acid, and
further~ that the dissolving rate at both temperatures
decreases as the concentration of added acid is increased.
It was subsequently shown in duplex cleaning treat-
ments of annealed metal having a surface oxide film up to
160 A thick, wherein the treatment consisted o~ a Lirst
immersion in a hot alkaline solution7 followed by a second
lmmersion in a hot ferric sulfate solution containing no
added acid, that the removal o~ oxide film was accomplished
effectively, as substantiated by the applicable tests,
including the solderability test. The cleaning action is
as thorough and complete, or better, than when added sul-
furic acid is present in the ferric sul~ate solution,
thereby accomplishing at least equivalent resul~s more
~ convenlentl~y and at decreased expense.
: The initial treatment with hot alkaline solution is
carried out at a temperature o~ about 40C to the boiling
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~ 9015
point and preferablyg at a temperature of about 70C to
the ~oiling point, the solution having a pH abo~Je 10,
preferably 11 to 14. The solutlon is preferably of
caustic soda, but other alkali hydroxldes as of potassium
or lithium, or mixtures may be used within the above-stated
pH range. Immersion of the copper alloy should be for
at least two seconds and prefera~ly, for five to six~y
seconds. The treatment time may be prolonged, particularly
at temperatures near the lower lim~t of the abo~e range,
but generally should ~e for ten minutes or less, as longer
times usually provide no added advantages. Optimum
treating times and conditions may be determined in ac-
cordance with the particular alloy and the results desired.
The second step of the duplex treatment is best ef-
fected by immersion of the copper alloy strip or article,
after the alkaline solution has been drained therefrom
and rinsed with water9 in a hot 0.5 to 3 N., best 1.25 to 2 N.,
solution of ferric sulfate, without added acld at a tem-
perature of about 25 to 95C,~est 65 to 90C, for at least
two seconds, and preferably for five to sixty seconds, and
generally for not over ten minutes.
Ferric sulfate for such treatment may be replaced in
whole or in part by other soluble ferric salts, as for
example b~y ferric ammonium sulfate or ferric nitrate,
generally with the obtainment of equivalent results in
t~e obtainment of clean and solderable copper alloy sur-
faces. It may oe noted that aqueous solutions of ferric
sulfate, typical of soluble ferric salts of strong acids,
display a pH of about 1 at 0.5 N. solution, decreasing~
close to linearly, to about 0.5 at 2 N. and to about 0.35
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901
at 3 N.
The efficacy of a duplex treatment in accordance
with t~is invention, as above outlined, i5 illustrated in
the following spec fic example, contrasting its successful
results wit~ the ineffective cleaning provided by the
process taught in U.S. Patent 3,646,946, the closest prior
art duplex treatment.
EXAMPLE I
Sheets of C.D. Alloy 688 Ccontaining 22.7% Zn, 3.4%
Al, 0.4% Co, and balance essentially Cu) were bell an-
nealed at about 600C in a closed furnace containing an
~; atmosphere produced by the partial combustion of hydro-
carbons to produce two lots of annealed sheet, the first
having a surface oxide layer ranging 95 to 105 A in
thickness and the second having a sur~ace oxide la~er
ranging 140 to 160 A ~n thickness.
Samples of the two lots were given a first i.~mersion
~reatment in boiling caustlc soda solution having a pE OL
14 for twenty seconds, drained, and washed with water.
Samples of the two lots, treated as above, were then
sub~ected to immersion for twenty seconds in one o~ the
following three solutions:
(l) 1.5 N. aqueous ferric sulfate solution, con-
taining no added acid, at 65~C.
(2) 12% b~ vol. aqueous sulfuric acid solution at
65C.
(3) 12% b~y vol. aqueous sulfuric acid solution con-
taining 4 oz. per gallon o~ sodium dichromate.
~ollowing the twentJ second immersion, the samples
were drained, washed with water, and dried.
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7~3~ ~ 9015-~3
Solderability tests applied to the treated samples
of the first lot (95 to 105 ~ oxide layer~ revealed that
those treated ~ith solution Cl~ and (3) were solderable,
while those treated ~ith solution (2) could not be soldered
ef~ectively .
Solderability tests applied to the treated samples
o~ the second lot C140 to 160 A oxide layer) revealed that
those ~reated with solution Cl ) ~ere solderaole, while
those treated with solutions (2) and (3) could not be
ef~ectively soldered.
Furthermore, capacitance tests on the above-treated
samples revealed that the oxide layer had been substan-
tially completely removed from the samples whlch dis-
played acceptable solderability ~Lot 1, treated with (1)
or (3)] and ~Lot 2, treated with (1)]. The samples which
were not solderaole ~Lot 1, tre~ted wi~h (2)] and ~Lot 2,
treated with (2~ and ~3)] were determined to have retained
about 20 to 30% of the initial oxide layer.
Thus, the only duplex treatment which was completely
success~ul in the above example was the one utllizing for
the second treatment the immersion in aqueous ferric
sul~ate solution with no added acid.
The above test for solderability correspcnds to that
described in Report W 72 - 51.2, American Societ~ for
Metals, Metals Par~, Ohio. The sample is immersed in a
rosin ~lux solution, then Yertically dipped into a oO Sn:
40 Pb molten solder at 230C, held in the bath ~or five
seconds, ~,~ith~rawn, and examined a~ter cooling. The
coating is rated according to appearance, rang~ng ~rom
"ldeal't ~or a ~orlgh~, smooth deposit of uni~orm thickness
to that in which there has been no solder adherence to
the metal surf~ce.
The capacitance test referred to above has been described
by J. J. McMullen and M. J. Pryor in "First International
Congress in Metallic Corrosion", pages 53-54, 1961 (Butter~orth's,
London), as amplified by Beck, Heine, Keir, van Rooyen, and
Pryor in 'iInternational Journal of Corrosion Science", Vol. 2,
pages 136 and 144-145~ 1962.
The mechanism of the duplex cleaning treatment of this
invention is believed to differ in a significant way from
the mechanism of the duplex cleaning treatment of U.S. Patent
3,646,946. The first treatment in the a~ueous alkaline
solution is believed to hydrolyze the refractory oxides to
convert them to a gelatinous condition. In ~he patented
process the immersion in the mineral acid solution serves
to dissolve the copper oxides and to help break up the
refractory oxides without significant metal removal. ~n the
process of the present invention the immersion in the ferric
sulfate solution provides a cleaning action through removal
of underlying metal beneath the oxide which undermines the
oxide layer. The gelatinous condition of the oxide layer
from the alkaline treatment allows the ferric sulfate solution
to penetrate to the metal surface to provide the metal
removal.
It has been observed for certain silicon containing
copper alloys that an oxide penetration of from about 6 to
10 microns occurs at the grain boundaries. The prior art
duplex treatment is not effective to remove the oxides at
the grain boundaries because of insufficient removal of the
ad~acent metal. The process of this invention is highly
~015
f'B.j ~
,~ ,
effective ~or removing such grain boundary oxides because it
attacks the metal surface. The process of this invention is
designed to remove oxides which are insoluble or resistant
to normal pickling agents including ferric sulfate.
The effect of added mineral acid on the potency of the
ferric sulfate cleaning bath as demonstrated in Table I has
also been verified as applied to samples with an oxide
coating. Therefore~ it is an important aspect of this
invention that the ferric sulfate bath have no added mineral
acid to thereby provide improved cleaning efficiency.
This invention may be embodied in other forms or carried
out in varied ways without departing from the spirit or
essential characteristics thereof. The present embodiment
is therefore to be considered as illustrative, the scope
` of the invention being indicated by the appended claims, and
all changes and variations which come within the meaning and
range of equivalency are intended to be embraced therein.
3o
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