Metal finishing Q&As since 1989

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HELP WITH BLACK OXIDIZING

2000

But, I need to get a good finish (bright and smooth black or blue black). In order to get this good finish, I'm thinking of two possibilities:

1-The best alternative is that perhaps some additive can be added into the molten salt bath, in order to get the finish I am looking.

2-If it is not possible to get the adequate finish with this, I'm think in l add one more process, for black oxidising the nails. I know that this can be achieve with hot alkaline baths of NaOH, but I'm new in this, and don't know much about this. Somebody can help me with some information about this( explanation about the process, or a right formulation, like temp., time, composition of the bath)

Thanks a lot in advance.

VICTOR MENDOZA
- LIMA-PERU

Victor -

To achieve a smooth & shiny black finish, you must first start out with a smooth & shiny part. Black oxide is a conversion coating and WILL NOT cover up any cosmetic blemishes. By the same token, it will leave a part as shiny as it started without changing any dimensions. As far as any additives, that won't help and in fact may hurt the formation of the black oxide.

The general parameters are to run the bath at 285 degrees F, and to have the parts in the tank for 5-20 minutes. If you use a powdered product, you'll probably be adding 5.5 - 6 pounds of salt per gallon of water.

Dan Brewer
chemical process supplier - Gurnee, Illinois
2000

The blacking bath consists of mainly NaOH plus NaNO₃ and NaNO₂ boiling at 140 deg C. Occasional addition of extra nitrite gives better colour. If you use an alkaline deruster as the preclean stage (100 g/l NaOH + Sodium gluconate and/or triethanolamine) you will get greater tolerance to blacking bath changes.

Roger Bridger
- Croydon, UK
2000