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Testing quality of blackening coating



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Current postings:

July 15, 2021 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. How can we test blackodising chemical from the bath? Is there any standard process for that?

Prasad Mohan kasar
- Pune maharashtra
^


July 2021

A. Hi Prasad. We appended your inquiry to a thread where I have asserted that such questions are just 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 tell you what is required of you in terms of testing the finish and the bath.

ISO 11408 [affil. link to spec at Techstreet] is a specification and list of test methods, SAE AMS2485 [affil. link to spec at Techstreet] is a black oxide standard, so are MIL-DTL-13924 [link is to free spec at Defense Logistics Agency / dla.mil] and DIN 50938 [affil. link to spec at Techstreet]. 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,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
^


January 26, 2022

Q. I really Appreciated your response Mr.Ted Mooney. I have read all your responses regarding which you gave on other surface coating and finishing process -- it's been very helpful for grasping more knowledge in this field.

I have read the Indian Standards and also U.S. Military standards. I have realized that there is not any standard (titration) process that we can use to check the concentration of the blackodising bath (for example, we do the titration process for phosphating bath in total acidity/free acidity).

For now I am doing 1N sulfuric acid titration process which gives me the reading somewhere around 7.8 to 8.5 (7.5 x 1 x 114 = 889 -- 889 to 969 grams/liter) but the issue I am facing is the temperature goes somewhere 160 °C (320 °F). When I dilute the chemical with water to desired temperature, 140-145 °C (284-293 °F) the reading comes somewhere around 3.2-ish (3.2 x 1 x 114 = 364.8 grams/liter) which gives no blackening result.

I did not face any problem while doing blackodising at this temperature (160 °C)

But how can we come to required temperature and required concentrations together?

Thank you for your response. waiting for reply.

Prasad Mohan Kasar
- Pune, Maharashtra
^


January 2022

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 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,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
^


January 29, 2022

Thank you for guiding me this far. I will definitely try.

Just out of curiosity How to determine (%) percentage in chemical field? Is it based on molecular weight or just calculated on weight. Because I have done small research on black oxide it is mentioned the PERCENTAGE BY WEIGHT (what is the meaning of that)? Will you elaborate a little?

Sorry to bother you this far. Whenever I read something about chemicals somewhere it make me more curious then before in field.

I also have seen some people using sodium cyanide in the bath is that really necessary?

Prasad Mohan Kasar
- Pune, Maharashtra
^


January 2022

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,

pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
^




Closely related historical postings, oldest first:

2002

Q. We are using blackening process for our components (forgings). I want to know whether salt spray test is applicable for checking the quality of blackening. If not what is the test to be conducted to check the quality of blackening.

R KUMAR
- India
^


2002

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.

Khozem Vahaanwala
Khozem Vahaanwala
Saify Ind
supporting advertiser
Bengaluru, Karnataka, India
saify logo
^


2002

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

Carlos Cielak
plating shop - Mexico City, DF, Mexico
^


July 17, 2009

Q. Hi to all,
How to test the surface treatment of Blackening coating.
Is there any procedure for testing?

Jebaraj
medical - Bangalore, Karnataka
^


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,

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

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