Process description: Chemical milling

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Chemical milling

Chemical milling is a process used to remove metal on workpieces by dissolution in a caustic or acid bath without an external source of energy (mechanical or electrical). Metal is removed off the whole surface of the part dipped in the milling bath provided this surface is in contact with the solution. Areas to remain are masked. The speed of the chemical dissolution is low, processing 0.5 to 3 mm/h compared with mechanical processes. The thickness removed is almost constant all over the surface, but with a roughness created which varies with the process used.

The most important uses are on aluminium alloys for the aeronautical and aerospace industry. It is generally used on sheets that have been previously curved and where it is necessary to save weight. Chemical milling can also be used in titanium alloys, stainless steel and some special alloys with a nickel, cobalt or magnesium base.

Process chemistries used are:

  • for aluminium: caustic soda with additives (sodium gluconate, sodium sulphide)
  • for titanium: hydrofluoric acid (20-50 g/l) with nitric acid (50-70 g/l)
  • for stainless steel: hydrochloric acid with nitric and phosphoric acid

The main control of the bath:

  • concentration of the bath: from 100 to 150 g/l of free NaOH
  • temperature: 80°C for older processes, types up to 110°C for high speed processes
  • concentration of dissolved metals: 70 to 90 g/l Al
  • composition of the alloy chemically milled

All these parameters affect the speed of processing, the thickness regularly and roughness of the finish.

Generally, the entire surface is not milled. In order to prevent milling on some areas it is necessary to mask those areas with protecting layers such as neoprene. The usual technique is to mask the whole part and then remove the mask from the areas to be milled. The mask is cut by scalpel or laser beam.

A typical process sequence is:

  • solvent degreasing
  • alkaline etching
  • rinsing
  • pickling (e.g. sulphochromic pickling)
  • rinsing
  • drying
  • masking
  • removal of mask on surface to be treated
  • attack
  • rinsing
  • surface passivation (e.g. sulphochromic or nitric acid)
  • rinsing
  • removal of mask
  • quality control


Source: BAT Surface Treatment of Metals and Plastic, Aug. 2006.



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