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Trouble with AISI 430 hot blackening

May 13, 2009

Hi,
I'm working in a manufacturing company in Italy producing brass and steel items with a large number of surface finishing for fashion industry. One of these surface treatment is hot blackening of stainless steel. We started this week to conduce hot blackening internally, but we sold in these years a large amount of blackened SS items, realized by a subcontractor. During the first run of our new oxidation plan I noted that a nice and compact black oxide layer covers the AISI 304 items, but no oxidation occurs on AISI 430 items.
We conduce blackening process in a 130 °C (266 °F), 560 g/l caustic soda ⇦liquid caustic soda in bulk on Amazon [affil link] /inorganic nitrates using a commercial product following their operating specs. We activate the metal surface with HCl 37%. What is the reason of this difference between two different SS? And, mainly, how can I blacken AISI 430 using this commercial product and obtaining the same result as with 304 SS?
Thanks to all and please excuse me for some mistake in my English!

Francesco Di Addezio
engineer - Colonnella, Teramo, Italy

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June 2, 2009

Last time I recommended a product I got censored but no one seems to have any suggestions or wants to help so I'll put my neck in the noose. What the hell!
I had a running battle for years bluing stainless steel. Your best bet is a product called DuLite. And it's fussy as hell to use. I found I had to run the operating temp at close to 300 Fahrenheit to 310 Fahrenheit.(Very dangerous as the bath will burn and become useless at 330). Cleanliness is next to godliness where bluing stainless is concerned. Clean first in solvent. lacquer thinner ⇦this on eBay or Amazon [affil links] Warning! highly flammable! or Varsol at the least. I don't know what your laws are like in your country and I hate to recommend it because its nasty stuff but Carbon tetrachloride (in Canada tetrachloromethane) is the best stuff. We need permits coming out of our, well you know! Just to think the word. And it is a hazardous as they claim. But it does work. After that boil it in soap and water. Dish soap seems to be the best but laundry soap is a close second. Then boil the parts in clean water for ¬ hour and rinse with cold water.
Clean material is rule number one. Rule number two is high temperature. Rule number three is shocking the material. Pull the work piece out of the salt bath and immerse it in cold water. Place it back in the bath and let it heat back up. Shock it four or five times or until it blues. If all ells fails, try polluting the work piece. De-grease some 00 00 steel wool ⇦this on eBay or Amazon [affil links] and buff the daylights out of the work piece and then put it in the bath. The last one is an old gun plumbers trick. It seams that once the stainless is polluted with chrome molybdenum it magically will take a blue. From a chemical standpoint I don't have a clue why the last one works. I'm just a gun plumber but I know it does work. It's been done to death! Hope this helps Rod :o)

Rod Henrickson
gunsmith - Edmonton, Alberta, Canada

Hi, Rod. Thanks for the technical suggestions.

Yes, brand recommendations are discouraged in all professional journals for good reasons. This is even more important on a website, where submissions can be from people whose vested interests are not known.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
June 1, 2009