Chemical milling method for interiors of narrow tubes

A method for etching patterns on the interior surface of narrow metal tubes. A thin tube having a narrow internal diameter is dipped into a maskant solution to a depth equal to the distance from the tube end etching is to be stopped. A thixotropic maskant having high edge thickness along the dipped edge is required. The shaft is removed from the maskant and dried. Plural dips to increase maskant thickness may be used, if desired. Both ends of the tube may be dipped to localize the area to be etched away from both ends. The tube is placed in an etching solution which is agitated and replenished as desired. After a suitable period, the tube is removed from the etchant, rinsed, dried and the maskant peeled away. A sharp, even, etched edge results, with a smooth fillet and no leaks of etchant through the maskant. The quality of the etch will be greatly increased where an iron-nickel-chromium alloy such as 718 Inconel is being etched by maintaining the metal ion ratio of the etchant at from about 50 to 60 g/l for the iron ion and from about 35 to 49 g/l for the nickel ion, with the grams per liter of the nickel ion always less than the iron ion.

BACKGROUND OF THE INVENTION 
This invention relates in general to chemical etching of metals and, more 
specifically, to improvements in the etching of portions the interior 
walls of narrow metal tubes. 
Chemical etching processes in which a pattern of etchant-resistant maskant 
is placed over a metal surface, the surface is immersed in an etching 
solution which dissolves the exposed metal to the desired depth, the 
surface is removed and rinsed and the maskant is removed have long been 
known. This has been found to be very effective in producing precise 
patterns and of removing excess metal to reduce the weight of structures, 
especially those used in aerospace products. 
Weight saving is very important in aircraft to reduce the energy consumed 
in flight. Attempts have been made to chemically etch interior surfaces in 
aircraft engine shafts which require a high tolerance cylindrical exterior 
surface and greater strength near the ends, but less strength near the 
centers. Such tubular shafts have been dipped into a maskant (one or both 
ends) to leave a central area exposed. The tube is immersed in an etchant 
which etches the tube interior, significantly reducing weight. 
Unfortunately, this process has had a number of problems with very 
irregular edges on the etched area, poor fillets along the edges of the 
etched area and etch leaks through the maskant near the etched area. 
Attempts have been made to control this problem by inserting a scribing 
tool into the tube and scribing a circular line through the maskant at the 
desired etch area edge. The maskant overlapping into the area to be etched 
is removed and etching is accomplished. While successful in overcoming the 
edge irregularity and poor fillet problems, this method is very difficult 
to accomplish with very narrow tubes and requires great skill in scribing 
the line and removing the excess maskant without damaging the remaining 
maskant. 
With many metals, particularly with iron-nickel-chromium alloys, etching is 
especially difficult in the confines of a narrow tube, with uneven etching 
and channelling being likely to occur, producing a less than optimum 
etched surface. In aircraft engines or the like it is particularly 
important that clean etch lines with smooth fillets be produced to retain 
maximum tube strength together with maximum removal of unnecessary weight. 
Thus, there is a continuing need for improvements in methods for etching 
away unnecessary tube wall thickness in specific areas within tubes 
intended for aerospace applications. 
SUMMARY OF THE INVENTION 
The above noted problems, and others, are overcome by the method of this 
invention which, basically, comprises the steps of dipping at least one 
end of the tube into a maskant to the desired etch line, using a 
thixotropic maskant which is carefully selected to produce high edge 
thickness, drying the maskant, placing the tube in an etchant with 
suitable agitation and replenishment, removing the tube after sufficient 
time to produce the desired etching, then rinsing and drying the tube and 
peeling off the maskant. 
I have further found that the quality of the etch produced by the method 
can be further improved where the tube is formed from an 
iron-nickel-chromium alloy such as those sold under the "Inconel" 
trademark by the Huntington Alloy Products Division of the International 
Nickel Company by using an etchant having a metal ion ratio in which the 
iron ion is present in quantities of from about 50 to 60 g/l and the 
nickel ion is present in quantities of from about 35 to 49 g/l. It is 
important that the nickel ion grams per liter never exceed the iron ion 
ratio. During etching the nickel level will tend to increase and 
eventually exceed the iron level because the nickel is being dissolved at 
approximately three times the rate of the iron. Therefore, the ion 
concentrations must be monitored during etching and the iron ion 
concentration must be replenished as necessary.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to FIG. 1, there is seen an axial section through a portion 
of a tube 10 etched according to the prior art. The original thickness of 
tube 10 is seen in the lower portion of the figure. The tube has been 
dipped into a conventional maskant one or more times to a depth sufficient 
to bring the maskant surface up to a line 12. The maskant is then dried, 
leaving a coating 14 having a substantially uniform thickness over the 
inside wall 16 of tube 10 except for the upper approximately 2 to 4 inches 
where the coating dries in a drastic taper 18 resulting in a very thin 
coating near line 12. When tube 10 is etched in a conventional etchant, 
the inside wall thickness is etched away at 20 to the desired thickness. 
Apparently because of the very thin coating in taper 18, an unacceptable 
etch near line 12 results. Line 12 is irregular, resulting in an irregular 
line 12 and a rough, pitted fillet 22 which reduce tube strength due to 
stress concentrations in the irregularities. Also, etchant leaks through 
pinholes in coating 14 in the thin areas along taper 18, allowing 
undesired etched spots as seen at 24. 
Also with many metals and conventional etchants, uneven etching and 
channelling may occur in the etched area 20. These further reduce the 
strength of the etched wall area, requiring that less metal be etched away 
overall to retain the necessary wall strength and resulting in a heavier 
final tube. 
FIG. 2 shows an axial section through a tube etched in accordance with the 
method of this invention. Tube 110 is dipped into a maskant to line 112. 
The maskant is dried and the tube re-dipped one or more additional times 
to line 112, as desired. While the tube may be dipped any suitable number 
of times, I have obtained optimum results where the tube is dipped to 
exactly the same line 4 times, with at least partial drying of the maskant 
between dips. 
The maskant must be one which produces a high edge thickness rather than 
the taper 18 as seen in FIG. 1. Thickness in the immediate vicinity of the 
edge should be about 0.015 inch for best results. Just which factors 
produce the high edge thickness characteristic is not fully understood. It 
is likely to be a combination of thixotropic agent, maskant viscosity, 
resin and rubber content and similar factors. Also, since many maskant 
manufacturers tend to maintain their exact formulations as trade secrets, 
it is difficult to determine which factors produce an effective maskant 
for my purpose. 
The preferred maskants are mixtures of synthetic rubber, synthetic resin, 
filler, a thixotropic agent and solvent. Typically, the thixotropic 
characteristic can be obtained by the carefully controlled reaction of a 
small portion of a polyamide resin with an alkyd resin vehicle. 
A simple test may be used to determine whether a specific maskant will be 
effective in my method. A sheet of metal is dipped into the maskant to a 
marked line and dried, then re-dipped to the line and dried any additional 
times desired. The maskant coating is then scribed along a line 
perpendicular to the line and the maskant on one side of the scribed line 
is peeled away. The maskant coating along the scribed line is examined. If 
the thickness of the maskant about 0.25 inch from the dip line is at least 
about 80% of the thickness of the maskant about 6 inches from the line, 
the maskant should be satisfactory. I have obtained outstanding results 
with a maskant available from adcoat Incorporated under the designation 
"8J-100". That maskant easily passes the above test. It appears to be a 
high resin, high rubber maskant and has a viscosity of about 36 to 42 
seconds through a No. 5 Zahn cup. 
Once the desired maskant coating 114 is formed the tube is placed in an 
etchant bath for the required time, with agitation and replenishment as 
desired. Typically, the tube may be agitated in an apparatus of the sort 
described in Brimm's U.S. Pat. No. 4,137,118 or etchant may be pumped 
through the tube. 
Any etching solution suitable for the metal of which tube 110 is fabricated 
may be used. Typical etching solutions are described in U.S. Pat. Nos. 
3,039,909 (De Long), 3,108,919 (Snyder et al.) and 3,745,079 (Cowles et 
al.). 
An inside wall 116 is etched away above line 112, the desired width is 
reached in etched area 120. As etching progresses, coating 114 at line 112 
is undercut uniformly, forming a smooth fillet 122. There are no etchant 
leaks through pinholes, since pinholes are absent from the uniform 
thickness coating 114. Since the edge of coating 114 at line 112 is smooth 
and uniform, a smooth and uniform fillet 122 is formed. Upon completion of 
etching the tube is removed from the etchant and rinsed, with water or any 
other suitable liquid, to remove residual etchant. The maskant then may be 
easily peeled from the tube surface. 
As a further feature of my invention, I have found that a very smooth and 
uniform etched wall 120, with maximum weight removal consistant with wall 
strength, can be obtained by carefully controlling the metal ion content 
of the etchant when tube 110 is formed from an iron-nickel chromium alloy. 
Excellent results are obtained with 718 Inconel, an iron-chromium-nickel 
alloy from International Nickel which is highly desirable for aircraft 
engine applications. This alloy is, therefore, preferred for use with the 
method of this invention. I have found that very uniform etching with 
substantially no channelling can be obtained if the metal ion ratio is 
kept in the following ranges; Iron ion, about 50 to 60 g/l and Nickel ion, 
about 35 to 49 g/l, with the nickel quantity in grams per liter always 
less than the iron ion. 
Details of various etching parameters, such as concentration of active 
agent, temperature, rate of agitation, etc. will vary depending on the 
tube material, sizes, etc. One skilled in the art may select these process 
variables from any variety of handbooks, such as the Metals Handbook, 
available from the American Society for Metals, Metals Park, Ohio. 
In many cases, both ends of a tube will be masked and only a center portion 
will be etched. For example, a 58 inch tube having an outside diameter of 
two inches and an inside diameter of three quarters inch may be easily 
masked to etch a 27 inch length near the center of the tube. 
Other variations, ramifications and applications of this invention will 
occur to those skilled in the art upon reading this specification. Those 
are intended to be included within the scope of this invention, as defined 
in the appended claims.