BLACKENING OF STAINLESS STEEL

We are looking for information on a simple process for blackening of Type 420 stainless steel needles. The blackening should be permanent, corrosion-resistant and medically acceptable. Since the needles are small in size, the method should be capable of processing in bulk.


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Q. We are looking for information on a simple process for blackening of Type 420 stainless steel needles. The blackening should be permanent, corrosion-resistant and medically acceptable. Since the needles are small in size, the method should be capable of processing in bulk. M.B.

 

A. I am aware of one classic formulation that may work for you. The formulation and operating conditions are as follows:

Component/Operating Conditions

Concentration/Range

Sulfuric acid, 
(concentrated)

180 parts

Water

200 parts

Potassium dichromate

50 parts

Operating temperature

210°F

Two things to keep in mind are: 1) This solution at the temperature specified is rather nasty and must be treated with great care, and 2) Potassium dichromate is a hexavalent chromium compound and may not be acceptable in your facility.
Presumably, your parts could be finished using a basket to hold the needles. If this was my project I would consider using proprietary products. You can find a number of vendors for proprietary products by accessing

www.pfonline.com/plate/index.html and searching the vendor database for Blackening solutions.

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