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Effects of silicon on the bluing process

Q. I have heard that Silicon has a detrimental effect on the bluing process. I'm interested in bluing some "Investment castings" e.g., 4130, 4140 alloys, which inherently have higher levels of silicon for foundry performance such as fluidity. Can anyone lend some information as to why this may be.

Thanks in advance.

Shawn Mckinney
- Ephrata, Pennsylvania, USA
2002

A. Yes, silicon has a detrimental effect in that it will not blacken the way the surrounding metal will. You must first activate the parts in a fluoride-containing acid (e.g., hydrofluoric) to remove the surface silicon, and then blacken. The blackening chemistry is not designed to react with silicon.

Dan Brewer
chemical process supplier - Gurnee, Illinois
2002

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