The authoritative public forum
  for Metal Finishing since 1989
-----

Chromating

2001

What is your solutions for giving black (and olive, yellow) chromating coats?

Thank you,

Shahram Sepehri
- Tehran, Iran

/P>

---

The active material in chromate passivations is hexavalent chrome (Cr+6). (New processes are being implemented that use trivalent chrome Cr+3) It is deposited in an immersion bath and the resulting deposit is a very thin hydroscopic gel. Changes in composition and thickness give different colors. Details about this process like exact bath compositions are either patented or trade secrets. Specialty chemical suppliers spend a lot of money on researching these chemistries and they can not give up their competitive advantages for free.

Tim Neveau
Rochester Hills, Michigan
2001

---

What is meant by chromating and how is it done? (Actually I am a student and want to know in detail about this process)

Faizul Abbas Firozi
- Karachi, Pakistan

---

First of two simultaneous responses --  

Hi Faizul,

Chromating is a process that is used to produce a chromate conversion coating. Chromate coatings can be used as stand-alone metal pretreatments or can be used as a pre-paint treatment. Chromate coatings can be deposited on a wide range of metals. Thus, it is not simple to describe every type of chromating operation because conditions and parameters are dependent on the metal being chromated.

Most chromating operations contain at minimum the following sequence:

1. Clean
2. Water rinse
3. Chromate
4. Water rinse
5. Dry-off

The above is generic; a deoxidation step after cleaning can be used for some Al alloys, for example.

George Gorecki
- Naperville, Illinois

---

Second of two simultaneous responses --  

There is a good article in the 2002 Metal Finishing Guidebook that explains chromate conversion coatings. It is written by Fred W. Eppensteiner and Melvin R. Jenkins. The Metal Finishing Guidebook is published by Metal Finishing Magazine.

Tim Neveau
Rochester Hills, Michigan