Testing quality of blackening coating

A. Hi Prasad. We appended your inquiry to a thread where I have asserted that such questions are just very slightly 'backwards'. The better question is "what specification/standard should I be blackening in accord with?" Once you have made that decision/determination, the specification/standard will usualy tell you what is required of you in terms of testing the finish and the bath.

ISO 11408 is a specification and list of test methods, SAE AMS2485 is a black oxide standard, so are MIL-DTL-13924 [on DLA] and DIN 50938. Look them over, as well as perhaps Indian standards, then pick one if your customer hasn't (Nissan, GM, Boeing, and probably many other companies have their own black oxide specifications). Because, until you do that, your question can be thought of as "how should I test an unspecified finish for compliance with unspecified requirements?" :-)

Luck & Regards,

A. Hi Prasad. You cannot choose concentration and boiling temperature independently; they are locked together; the concentration determines the boiling temperature.

I am not a black oxide expert. I conveyed exactly one load of parts through a black oxide line in my whole career; I only know what I read in the papers. Still, 140-145 °C (284-293 °F) is the correct temperature, and it is not common to titrate with sulfuric acid, but rather to hold the boiling temperature constant, adding however much salt is needed to do that. So if it won't blacken in the correct temperature range, my guess has to be that your salts are not in correct proportion (maybe you do not have sufficient oxidizing agent -- nitrite/nitrate -- in the mix?), or there is some contaminant 'poisoning' the bath, slowing/stopping the reaction at 140-145 °C but not at the higher temperature.

• The Metal Finishing Guidebook suggests 80 oz/gal NaOH, 30 oz/gal potassium nitrite, 20 oz/gal potassium nitrate, at 295 °F.
• Robert Probert suggests in thread 19339, 80% NaOH, 10% sodium nitrite, 10% sodium nitrate ⇦ this on Amazon [affil link] at a concentration to cause 285 °F boiling temperature.
• Rod Henrickson, a gunsmith for decades, suggests in thread 1150, 80 oz/gal NaOH, 16 oz/gal sodium nitrate, with temperature 280-310 °F. He also says that contaminants, especially aluminum, can kill the bath.

Luck & Regards,

A. Hi Prasad. People express chemical concentrations lots of different ways, by volume to volume (ml per l), by weight to volume (grams per l), by percentage weight (80% NaOH, 10% sodium nitrite, 10% sodium nitrate), etc.

When one is doing a titration, things are often expressed in molecular weights because it's convenient and makes logical sense: for example, one molecular weight of sulfuric acid, 98 grams, (with its two H⁺) will neutralize two molecular weights of caustic soda (with its one OH^-).

Some black oxide formulations use cyanide, but it is not required in other formulations.

Luck & Regards,

A. I have tested blackened parts last up to 6 hours in a SST only. This is too fine to be rated as a conclusive go - no go test. I would rate a good jet black appearance as a better guide. If you want to go further, then cut a cross section and measure it out on a magnifying microscope.

A. If your blackening process is a black oxide process, the corrosion resistance is given by the post treatment applied over it, for example oil, wax, etc. The black oxide by itself has almost no protection value. General Motors tests these coatings in a Humidity Test, not SS test. Carlos

A. Hi cousin Jebaraj.

Although the responses to the original poster are valid in response to your question as well, the subject cannot actually start with "how to test an unspecified finish for compliance with unspecified requirements?"   :-)

Rather, the essential starting point must be "what specification are you blackening these parts in accord with?" If none, no one can authoritatively suggest applicable requirements.

Decide whether you are trying to do cold blackening, hot black oxiding, salt bath nitriding, or whatever and then find an ISO, ASTM, AMS, IS or some accepted standard for it. Then you've got a starting point. Good luck!

Regards,