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The black oxide process and gun bluing
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April 25, 2014
Q. We regularly do blackodising on Mild Steel and High Tensile Steel Nuts and bolt. We use a ratio of mixing chemicals to prepare bath as follows:
(1) caustic soda
[affil links]
80%
(2) Sodium Nitrite 10% &
(3) Sodium Nitrate 10%.
WE ARE NOT ABLE TO ACHIEVE A CONSISTENT RESULT. Sometimes the fasteners are getting red rust like colour.
A) Is there a online chemical test where we can know the balance of the bath?
B)Can anyone specifically tell me what are the functions of all the above chemicals?
C) why are the nut bolts getting red in colour?
Any serious reply will be of great help to us.
Thanks
Chintan
- Bhavnagar, Gujarat, India
A. Hi Chintin. As you see, we appended your inquiry to a thread where experts like Rod Henrickson have explained the process in excellent detail. I urge you to carefully study his comments as my guess is that operational details rather than the precise ratio of chemicals is the origin of your inconsistencies. Good luck.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
May 2014
Black oxide boils at 260 ° no matter what we do
Q. Hi -
I was reading this thread, it's pretty interesting. We're having an issue with one of our blackening tanks: it won't get to 305 degrees, no matter how we adjust the solution; it's boiling around 260, tops. Does the water get deionized over time or something? There are also a few inches of sludge/slurry at the bottom that I'm going to have cleaned today but obviously want to know what the issue is. It's gas-fired and I don't think lack of BTU's is the issue.
- Middletown Connecticut USA
June 2, 2014
|
A. Peter,  Blake Kneedler Feather Hollow Eng. - Stockton, California June 3, 2014 June 3, 2014 Boiling point is directly related to the concentration of dissolved chemicals.  Jeffrey Holmes, CEF Spartanburg, South Carolina |
What blackening process do OEMs use
Q. Being heavily involved in vintage auto restoration, can anyone tell me the specific type blackening used by OEM suppliers for automotive bolts and parts? Thanks.
orrin tucker- YOUNGSTOWN OHIO
December 16, 2015
A. Hi Orrin. The hot black oxide process first described by Ken Lemke and Rod Henrickson is surely what OEMs specify for general "blackened" parts. But there are probably occasions where more corrosion resistance is required, so zinc plating with black chromate is specified.
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
December 2015
Q. My parts are not full blackened. There are spots on the surface. Guide me please.
Osama Farooq- rawalpindi Pakistan
February 27, 2017
? Hi Hi Osama,
Please post what kind of parts you are blackening, what they are made of, what your sequence of operation is, and the parameters of your black oxide tank (temperature, boiling?, time, concentrations, etc.) People can't tell you what you might be doing wrong until you tell them what you are doing. Thanks!
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
March 2017
March 4, 2017
Q. Process sequence is as follows:
1. degreasing
2. pickling
3. blackening
4. chromic acid dipping
5. oiling
I am using 650 g/L of sodium hydroxide and 400 g/l of sodium nitrite. My solution starts to boil at about 140 °C. I have heard that black oxidation solution starts rising when it boils, but in my case only bubbles are formed on the surface of the solution.
The time I am giving to the parts is about 1 hour. I have tried doing it on quite a few types of steels but no success.
Osama Farooq [returning]- rawalpindi Pakistan
A. Hi again. All I know if what I read in the textbooks and forums ... but are you sure you're using sodium nitrite, not Sodium Nitrate [affil links]? Rod Henrickson says he uses only sodium nitrate (2 pounds/ gallon = 240 g/l), and some textbooks suggest a mix of nitrite and nitrate, but I'm not aware of using sodium nitrite without sodium nitrate, as it sounds to me like you would not have sufficient oxidation power.
I don't understand what you mean when you say that you're supposed to see "black oxidation solution starts rising" but it's not happening -- but the solution must be boiling.
The process should only take a few minutes, not an hour.
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
March 2017
! I have got to the root of the problem actually I was giving way more time to the parts after pickling. It resulted in the oxidation of the surface of the parts hence they were not being properly black oxidised.
Sodium nitrite can be used as I am using it and it has also passed the oxalic acid spot test.
- rawalpindi, Pakistan
March 7, 2017
Thanks for the feedback, Osama. Good to know that sodium nitrite works.
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
March 2017
October 30, 2017
A. Osama, delete the chromic acid it will destroy the black oxide finish immediately, If you must pickle the metal use the following in this order:
degrease with hot caustic solution
rinse
Pickle with acid (light sandblasting will produce the same effect and a lot safer) Pickle if you must, then rinse with Boiling water then into the bluing solution when the black oxide finish is a good deep color remove into a hot water rinse then into a hot soluble oil bath, be careful of soluble oil concentrations to avoid fire.
The old masters used a 300 grit finish and then blued the part, some applications call for use of a buffing wheel and abrasive wax; care and skill is needed with these devices they will hollow around screw holes.
Chromic acid will dissolve flesh and I think should be avoided at all costs.
Read Rod Henrickson's comments -- he is 100% correct in all that he has said. If You read and digest his comments there is little else to know in the short term.
You do not often get such detailed information offered freely!
- Plainland Qld Australia
A. This is in reply to Vaughn's comment the other day:
If following the MIL-SPEC, a chromic acid dip will not destroy the black oxide finish. It calls for a final rinse that is 8 oz per 100 gallons of water. Even though the pH is quite low, the concentration is too weak to attack the finish. Minimum of 30 seconds at elevated temperatures will help corrosion resistance, not destroy it.
- Indian Orchard, Massachusetts
November 1, 2017
Thanks Jameson. According to MIL-DTL-13924
[on DLA] you are correct.
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
November 2017
Q. Sometimes we use glycerol to remove high iron content from the bath. Are there any alternatives?
Phahad hashim- Dubai, UAE
June 18, 2017
Q. My situation: I don't know how to prepare for blacking can you explain the blacking process and chemical ratio?
Manoj Kumar VnShop employee - Bangalore, Karnataka, India
July 25, 2018
A. Hi Manoj. We added your inquiry to a thread that answers your question. Feel free to add any followups or requests for clarification on anything you didn't follow. Good luck.
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
May 2020
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