The practice of chemically reducing aluminum structure for weight saving has been discussed in this publication recently, along with the danger of explosive decompression at the site of such milling. (See Aviation Safety Journal, “Too Much Metal Shaved Out of Airplanes?” and “Metal Fatigue Led to Fuselage Rupture”)
Herewith, a good, brief explanation of the process:
Why chemical milling? Chemical milling, the removal of metal by chemical attack of alkaline or acid solution is routine for specialized reductions in thickness. For complex large surface areas in which uniform metal removal is required, chemical milling is often the most economical method. The process is used extensively to etch preformed aerospace parts to obtain maximum strength to weight ratios. Integrally stiffened aluminum wing and fuselage sections are chemically milled to obtain an optimum cross section and minimum skin thickness.
Source: Aluminum & Aluminum Alloys by J.R. Davis & Associates, ASM International Handbook Committee, p. 11.