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

How to get reddish colour in Zinc passivation

Im into manufacturing Cargo restraint products, lashings and cargo bars. Now the problem arises that after electroplating the zinc we have yellow passivation(chrome) and after drying the colour is not uniformly red. the microns and salt spray tests are Ok but the material is not uniform. Can I get uniform red color in all material even if it of different batches plated on different days?

Saurabh Khandelwal
Tie downs and lashings - Kanpur ,UP, India
2005

---

2005

Hello Saurabh,

Some of my customers do claim that the darker the yellow passivation the longer the parts last in a SST till first formation of white rust.

I personally am inclined to nod along only if I didn't know that it is next to impossible to passivate all components uniformly in a process where the control parameters are so tight. Variation in the iridescent finish is normal and generally acceptable since even the lighter shades meet all corrosion requirements easily.

Why not fix limit samples?

Khozem Vahaanwala
Saify Ind

Bengaluru, Karnataka, India

---

2005

Saurabh

In addition to what Mr Vahanwala has said..Chromating color consistency is easy if you follow these points...

1) Operational ph is a factor. Follow suppliers data sheet.

2) bath size is V V important. If its too small you consume a large percentage per pass.

Use a large 200 to 400 liter passivate bath for consistency.

3) Room temp has different meaning in summer and winter. maintain an average 30 centigrade with heaters in winter.

4) pH of rinses. Too high a pH will cause uneven color. Use water thats neutral pH at say 7.

5) Nitric acid predip must be replenished every 4 hours especially if you use a cyanide or alkaline solution.

This applies to acid zinc too.

6) Avoid use of hard water for Chromates.

This set of points is quite useful and some will apply to your application.

Regards

Asif Nurie [deceased]
- New Delhi, India
With deep regret we sadly advise that Asif passed away on Jan 24, 2016