A question about black oxidation

Hello sir. I have a question about black oxidation.
I work in a factory and we do black oxidation on some of our products. And recently we have problem in the black oxidation of some parts; it is a body consisting of 2 parts connected together: the first one takes good black oxidation and the other one is oxidized but not in a dark black color, it's like reddish black, but notice that the material of the two parts of the body is not the same -- both steel but different types.
Our formulation consists of the following:
caustic soda ⇦liquid caustic soda in bulk on Amazon [affil link] - sodium nitrite operating at 142 °C
So is there a way by which we can oxidize the reddish black part effectively?
Thanks a lot
Bye

Let me try to answer this question. On one hand, different material will lead to different color for black oxidation. The other hand, high temperature of plating solution usually caused to the problem for you asked.

Good Day

Kindly
1-check you material types plus hardness
2- check the balance of the blackening solution
3- You can try to remove the first time of blackening in dilute HCl after that make blackening again you will take an excellent result.

adv.