U.S. patent number 3,654,001 [Application Number 04/739,648] was granted by the patent office on 1972-04-04 for process for etching beryllium.
This patent grant is currently assigned to North American Rockwell Corporation. Invention is credited to Howard G. Mann.
United States Patent |
3,654,001 |
Mann |
April 4, 1972 |
PROCESS FOR ETCHING BERYLLIUM
Abstract
A process for chemical milling of beryllium and high beryllium
alloys is described wherein the beryllium is subjected to an
aqueous solution of an acid selected from the group consisting of
sulfuric acid, oxalic acid, and ammonium acid fluoride plus
sufficient hexavalent chromium ion to prevent channeling. In a
preferred embodiment sulfuric acid is present in the range of 5-15
percent chromium ion is present in the range of 7-10 percent
CrO.sub.3 ; sodium nitrate is present up to about 1.3 percent; and
hydrofluoric acid is present up to about 2 percent.
Inventors: |
Mann; Howard G. (Seal Beach,
CA) |
Assignee: |
North American Rockwell
Corporation (N/A)
|
Family
ID: |
24973227 |
Appl.
No.: |
04/739,648 |
Filed: |
June 25, 1968 |
Current U.S.
Class: |
216/109;
252/79.2; 216/108; 252/79.3 |
Current CPC
Class: |
C23F
1/30 (20130101) |
Current International
Class: |
C23F
1/30 (20060101); C23F 1/10 (20060101); C23f
001/00 () |
Field of
Search: |
;156/18,20,7,8
;252/79.3,79.2,79.4 ;134/41 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
missel et al., "Chemical Milling of Beryllium,"Metal Finishing,
Vol. 63, No. 11, Nov. 1965, pages 69-71.
|
Primary Examiner: Goolkasian; John T.
Assistant Examiner: Gil; Joseph C.
Claims
What is claimed is:
1. A process for chemical milling beryllium and beryllium alloys
comprising:
subjecting the beryllium to an aqueous solution consisting
essentially of sulfuric acid in the range of about 5 to 15 percent
by weight;
hexavalent chromium in the range of about 7 to 10 percent by weight
CrO.sub.3 ;
nitrate ion in the range of about 1 to 1.3 percent by weight of
NaNO.sub.3 ;
hydrofluoric acid in the range up to about 2 percent by weight;
and
wherein said aqueous solution has a concentration of beryllium of
less than about 0.1 pound per gallon of solution and the etch rate
is in excess of about 5 mil/hour.
2. A process for chemical milling beryllium and beryllium alloys
comprising subjecting the beryllium to an aqueous solution of an
acid selected from the group consisting of sulfuric acid, oxalic
acid and ammonium acid fluoride in the range of 5 to 15 percent by
weight, hexavalent chromium in the range of 7 to 10 percent by
weight of CrO.sub.3, hydrofluoric acid in the range of up to 2
percent by weight and less than 0.1 pound of beryllium per
gallon.
3. A process as described in claim 2 wherein the temperature of
said aqueous solution is about 70.degree. F. to 140.degree. F.
4. A process as described in claim 3 wherein the etch rate is 5 to
15 mils per hour.
5. A process as described in claim 4 wherein said aqueous solution
further comprises nitrate ion in the range of up to about 2 percent
by weight of NaNO.sub.3.
6. An aqueous chemical milling solution consisting essentially of
sulfuric acid in the range of 5 to 15 percent by weight, hexavalent
chromium in the range of 7 to 10 percent by weight of CrO.sub.3,
nitrate ion in the range of 1 to 1.3 percent by weight of
NaNO.sub.3, hydrofluoric acid in the range up to 2 percent by
weight and less than 0.1 pound of beryllium per gallon.
7. An aqueous chemical milling solution consisting essentially of
an acid selected from the group consisting of sulfuric acid, oxalic
acid, and ammonium acid fluoride in the range of 5 to 15 percent by
weight, hexavalent chromium in the range of 7 to 10 percent by
weight of CrO.sub.3, hydrofluoric acid in the range of up to 2
percent by weight and less than 0.1 pound of beryllium per
gallon.
8. An aqueous chemical milling solution as described in claim 7 and
further including nitrate ion in the range of up to 2 percent by
weight of NaNO.sub.3.
Description
BACKGROUND
The CHEM-MILL (Registered Trademark of North American Rockwell
Corporation) selective etching process is a widely used industrial
technique wherein a portion of a structural part is masked with an
inert material and another portion of the part is exposed. The
exposed area is then subjected to action of an etching solution to
uniformly remove metal from the exposed area thereby changing the
geometry of a part. Beryllium is a structural material having
excellent high temperature properties, very low weight and
exceptionally high modulus of elasticity. These desirable
properties are offset by a relative difficulty in machining
beryllium by conventional mechanical techniques. It is therefore,
highly desirable to provide a means for chemical milling of
beryllium.
Beryllium is soluble in most mineral acids, however, this alone is
not sufficient for most chemical milling operations since it is
usually necessary that the finished part have a good surface finish
and that the part be uniformly etched in all areas. In addition it
is necessary to have an etch rate sufficiently fast to be
economical and yet sufficiently slow that carefully controlled
tolerances can be achieved. The ability to achieve these
characteristics in a chemical milling process is dependent on
selection of an etching bath having a composition suitable for a
material being etched. In beryllium, in particular, care must be
taken to avoid channeling or threading which are names given to a
phenomenom wherein the etch rate is higher at the boundary, and a
shallow swale is found at the boundary after etching.
Advantages of this invention will be obvious from the matter set
forth hereinafter or may be learned by practice of the invention,
the same being realized by means of the composition, processes and
steps pointed out hereinafter.
SUMMARY OF THE INVENTION
There is provided in practice of this invention according to a
preferred embodiment, a process for etching beryllium by subjecting
the surface to action of an aqueous solution of an acid selected
from the group consisting of sulfuric acid, oxalic acid, and
ammonium acid fluoride in a concentration sufficient to maintain a
chemical milling etch rate, and sufficient hexavalent chromium ion
to prevent channeling.
DISCLOSURE
Referring now in detail to the composition of the etching bath, it
is preferred that an acid selected from the group consisting of
sulfuric acid, oxalic acid, and ammonium acid fluoride be employed
in the range of from about 5-15 percent by weight. If less than
about 5 percent of acid is employed it is found that the etching is
unreasonably slow, if present at all; and above about 15 percent it
is found that uneven etching is obtained over a surface, with
resultant peaks or pits in the surface.
It is also preferred that hexavalent chromium be present in the
range of about 7-10 percent CrO.sub.3 to inhibit channeling. If
less than about 7 percent CrO.sub.3 is present channeling may occur
at the edges; further the etching action is excessively fast and
control of the tolerance of the part becomes difficult. In
addition, significant heating occurs during rapid etching and
uneven etching in other areas of the exposed surface may be
obtained. Excessive heating of the etchant solution may also cause
health hazards due to the toxic nature of beryllium compounds. It
is preferred that the concentration of CrO.sub.3 be less than about
10 percent since larger amounts of hexavalent chromium unduly
passivate the beryllium surface and reduce the etching rate below
any economically feasible level.
Another constituent of the etchant is hydrofluoric acid in a
concentration up to about 2 percent by weight. The presence of
hydrofluoric acid in the etching bath eliminates smut from the
surface thereby promoting more even etching and minimizing cleaning
operations subsequent to etching in the chemical milling bath. A
concentration of hydrofluoric acid higher than about 2 percent in
combination with the other acids may give an excessively high
etching rate for controlled chemical milling.
Another constituent of the etching bath is sodium nitrate in a
concentration up to about 2 percent. The sodium nitrate
substantially inhibits pitting of the beryllium surface during
chemical milling and when substantial amounts of beryllium are to
be removed a better surface finish is thereby obtained. When only
thin layers of beryllium are to be removed, the sodium nitrate can
be safely omitted from the etchant. The concentration is preferably
limited to about 2 percent sodium nitrate since higher
concentrations unduly inhibit etching for production
efficiency.
It is preferred to maintain the beryllium concentration in the
etchant bath at less than about 0.1 pound of beryllium per gallon
of solution since the dissolved beryllium inhibits etching to an
undue extent and chemical milling baths having a higher
concentration of beryllium are substantially unusable because of a
lowered etch rate.
If desired conventional acid compatible wetting agents in small
quantities can be included in the bath to assure good wetting and
smooth etched surfaces as is often done in chemical milling
baths.
A particularly preferred composition of chemical milling etchant
for beryllium comprises an aqueous solution having 5-15 percent by
weight of sulfuric acid, 1-1.3 percent sodium nitrate, hexavalent
chromium in the range of 7-10 percent CrO.sub.3 and hydrofluoric
acid in a concentration of about 2 percent. An etchant in this
range employed for chemical milling of beryllium is economical, and
uniformly provides a good surface finish, a satisfactory etch rate
and freedom from channeling, pitting, and smut.
It is preferred to employ a chemical milling bath for beryllium in
the temperature range from approximately room temperature to about
140.degree. F. with the temperature and concentration of the
etching bath being maintained in a range to give an etch rate in
the range of about 5 to about 15 mil per hour. (1 mil = 0.001
inch). An etchant having a (maximum) concentration of sulfuric acid
of about 15 percent in a (minimum) concentration of hexavalent
chromium of about 7 percent CrO.sub.3 at 140.degree. F. (maximum)
has an etch rate of about 1 mil per minute which is considered
excessive for chemical milling of beryllium for most applications
since close tolerance control is difficult to achieve with rates
this rapid. It will be apparent that either lower concentration of
sulfuric acid or lower temperatures or higher chromium ion
concentration will reduce the etch rate to the preferred range. It
is preferred that the etch rate be greater than about 5 mil per
hour for best economy since it is found that lower rates give no
substantial increase in the tolerance achievable, and further, a
tendency toward channeling at the edges of etched areas is noted.
The upper etch rate can be maintained at any value compatible with
personnel safety and which will give acceptable tolerances.
Generally, it is found preferable to maintain an etch rate less
than 15 mil/hour to maintain good dimensional control and provide
the best surface finish.
Suitable maskants for the beryllium in the described etching bath
include adhesively bonded lead tape, conventional vinyl type
maskants, conventional neoprene type maskants, and butyl-neoprene
type maskants such as described in U.S. Pat. No. 3,325,320 entitled
"Method of Selectively Etching Metals" by C. C. Shepherd and
assigned to North American Rockwell Corporation, assignee of this
application, any of which is suitable for use in the relatively low
temperature acid baths.
After chemical milling a chemical polish can be employed, if
desired, to enhance the appearance of the part. A substantial
luster is obtained by brief exposure to a solution comprising
approximately 90 percent phosphoric acid; 5 percent sulfuric acid
and 5 chromic acid.
In order to make about 61/2 gallons of etchant 1,050 grams of
CrO.sub.3 and 150 grams of NaNO.sub.3 were dissolved in about 23.8
liters of water and about 815 ml. of 98 percent sulfuric acid was
added. Sheets of beryllium were masked with a neoprene type maskant
and a grid of scribe lines was made in the maskant on one side of
the sheet. Alternate squares of the maskant were peeled to expose a
"checkerboard" pattern on the sheet with three-quarter-inch
squares. The sheet was immersed vertically in etchant at slightly
about room temperature to yield an etched depth of about 0.030
inch. The resultant surface was smooth and uniform and there was no
evidence of channeling.
Etchants of the compositions shown in Table I also provide good
smooth surface finishes, no channeling and a satisfactory etch
rate, the exact etch rate being determined by temperature of the
bath. In any of these compositions, the etch rate is increased
slightly by addition of up to 2 percent hydrofluoric acid, however,
the principal effect of hydrofluoric acid is elimination of smut
during etching.
TABLE I
H.sub.2 SO.sub.4 NH.sub.4 HF.sub.2 C.sub.2 H.sub.2 O.sub.4.sup. .
2H.sub.2 O CrO.sub.3 NaNO.sub.3 15% -- -- 9% 1-1/2% 5 -- 5% 7 -- 5
-- -- 8 2 -- 7-1/2% -- 10 2-1/2 10 -- -- 10 -- -- 5 -- 9 1-1/2 5 5
-- 9 1-1/2 9 -- -- 18 3 -- -- 12 7 1
it is to be understood that the above described examples are merely
illustrative of application of the principles of the invention.
Those skilled in the art may readily devise other variations that
will embody the principles of the invention. It is therefore to be
understood that within the scope of the appended claims the
invention may be practiced otherwise than as specifically
described.
* * * * *