Acid spray machining technique chemically mills aluminum alloy panels without immersing them in an etchant. The spray does not require artificial heating to initiate the etching process

Dufour, G.

English

NASA Technical Reports Server

November 1967
	 Brief 67-10463 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Acid Spray Technique Mills Aluminum Alloy Materials Without Immersion 
The problem: 
To develop a process for chemically milling alumi-
num alloy panels without immersing them in an 
etchant. The alkaline milling method presently used 
requires complete immersion in an etching bath. This 
is particularly cumbersome when milling large compo-
nents. 
The solution: 
An acid spray machining technique, which allows 
large aluminum alloy parts to be chemically milled 
economically and without the need for large immer-
sion baths. 
How it's done: 
Sample panels, constructed from 2219-T37 and 
2014-T651 aluminum alloy, were chemically milled to 
depths of 0.500 inch using the spray technique. Across 
the surface of the 9-inch by 9-inch 2219-T37 speci-
mens, the overall standard deviation in depth was 
±0.007 inch with a range of ±0.012 inch. The 2014-
T651 specimens showed less depth variation. By 
moving the specimens with respect to the spray, the 
uniformity of depth improved even more. The spray 
solution contains hydrochloric acid, acetic acid, and 
sodium dichromate. 
To chemically mill the aluminum parts, 35 gallons 
of solution was sprayed over the surface of each 
sample. Maximum initial etch rates as high as 0.008 
inch/minute were obtained, with the surface rough-
ness within the allowable 125 rms. The average maxi-
mum etch rate was somewhat lower, however, because 
an average run is 30 minutes and the solution degen-
érates during this time. At the end of the run, the etch 
rate had dropped to approximately 0.001 inch/minute. 
Thus, the average maximum rates over the 30-minute 
period were 0.0047±0.0019 inch/minute for alloy 
2219-T37, and 0.0040±0.0018 inch/minute for alloy 
2014-T65 1.
The spray etching solution became essentially 
depleted after approximately 1.5 ounces of aluminum 
per gallon of sOlution had been removed from the 
samples. This results from using up the sodium 
dichromate and reducing the acid content. Replenish-
ment of the solution components showed various 
degrees of restoring initial solution activity, but the 
total effect was generally inconclusive. 
Notes: 
I. The acid spray does not require artificial heating to 
initiate the etching process. It is probable, however, 
that the etch rate increases with temperature up to 
about 120°F. 
2. Provisions should be made to draw the chemical 
solution alternately from two reservoirs, so that 
fresh solution is made ready as the solution in use 
is being depleted by the chemical reaction. 
3. The ratio of the average undercut at a single edge 
to the depth of cut averaged about 0.6. The speci-
mens etched in a horizontal position (sprayed from 
the underside) had an undercut ratio of 0.54 ±0.08, 
which was lower than those etched at 45° or 90° 
from the horizontal. The mean ratio at these angles 
was 0.7 or greater. 
4. Inquiries concerning this innovation may be di-
rected to:
Technology Utilization Officer 
Marshall Space Flight Center 
Huntsville, Alabama 35812 
Reference: B67-10463 
Patent status: 
No patent action is contemplated by NASA. 
Source: George Dufour 
of Lockheed Missiles and Space Co. 
under contract to 
Marshall Space Flight Center 
(M FS-12500) 
Category 03 
This document was prepared under the sponsorship of the National	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States 	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States 	 will be free from privately owned rights.
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Acid spray technique mills aluminum alloy materials without immersion

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