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topic 3614

Anodising LM24 Die cast Alloy


Dear Friends ,

I want the following information regarding Anodising of LM24 Die cast Alloy.

Pls inform us the cleaning process and anodising process for clear anodising of above Alloy.

Presently we tried with regular sulfuric bath with caustic etch,we found gray colour coating instead clear,pls inform us.

Thank you,



You do not have a 'problem', what you have is a typical anodising response of Sulphuric acid anodising on LM24 castings.

LM24 has a 'high' Si (silicone)content. In simple terms when oxidation takes place the Si atoms oxidise as well as the Al atoms and the silicone oxide particles are incorporated within the anodic film.

However unlike Aluminium Oxide which is translucent (which gives you the 'clear' finish that you are familiar with) Silicone Oxide particles are not and they appear as black specks in the anodic film. When you get enough black specks within your clear film the overall effect is a grey finish rather than a silver one.

Good luck!

The only real advice I can give is to change to an alloy such as LM5 (which your casting supplier probably won't like dealing with because it is harder to cast) and keep the anodic film thickness to a minimum that satisfies the technical requirements.

Peter Hirst
- UK

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